Low-slope roofs can have slopes as minor as 1⁄8 inch per 12 inches. These roofs employ a waterproof roofing system and are found primarily on commercial structures.

A low-slope roof system generally consists of a roof membrane, insulation, and one of a number of surfacing options. To control the application and improve the quality of low-slope roofing, a variety of specifications and procedures apply to the assembly of the roofing components.

These specifications and procedures are generally accepted and used throughout the United States. Roofing systems that meet these specifications normally can be expected to give satisfactory service for many years.

Climatic conditions and available materials dictate regional low-slope procedures, which can vary greatly in different parts of the country. Low slope roofs are essentially a custom product. They are designed for a specific building, at a specific location, and manufactured on the jobsite.

Membrane Components
Low-slope membranes are composed of at least three elements: waterproofing, reinforcement, and surfacing. Some materials within the membrane might perform more than one function. The waterproofing agent is the most important element within the roof membrane.

In BUR and modified bitumen roofing (MBR), the waterproofing agent is bitumen. In single-ply roofing, the waterproofing agent is synthetic rubber or plastic.

The reinforcement element provides stability to the roof membrane; it holds the waterproofing agent in place and provides tensile strength. In BUR, reinforcement is typically provided by organic or glass-fiber roofing felts. In MBR, the reinforcement is generally glass-fiber felt or polyester scrim, which is fabricated into the finished sheet by the manufacturer.

Polyester and other woven fabrics are used as reinforcements for elastomeric and plastomeric, single-ply membranes. Some singleply membranes do not require reinforcement because the waterproofing material is inherently stable.

The surfacing materials protect the waterproofing and reinforcement elements from the direct effects of sunlight and weather exposure. They also provide other properties, such as fire resistance, traffic and hail protection, and reflectivity.

Some single-ply membranes are self- or factory-surfaced. Aggregate, which is field-applied, and mineral granules, which are usually factory-applied, are the most common types of surfacing materials. Smooth-surfaced coatings, however, are increasing in popularity.

Membrane Classifications
Low-slope roof membranes can usually be grouped, or classified, into the general categories reviewed below. There are, however, hybrid systems that might not fit into a category, or that might be appropriate in several categories.

BUR, which uses asphalt or coal tar products, is by far the oldest of the modern commercial roofing methods. Many commercial buildings in this country have BUR roofs. The large number of 20-, 30-, and even 40-year-old BUR roofs that are still sound attests to the system’s durability and popularity.

Roofing materials continue to evolve, however, and improvements are continually being made to asphalt and coal tar pitch, the basic bitumen components of BUR. Asphalt tends to be more popular with most roofers than coal tar.

Since the first MBR membranes were manufactured in the United States in the late 1970s, they have become one of the roofing industry’s fastest-growing materials. The popularity and specification of MBR membranes has increased steadily for more than two decades. Contractors have found the materials easy to use and easily inspected. MBR systems provide a time-tested, high-performance, reliable roof.

Since they first appeared in the 1950s, single-ply materials have become increasingly popular in the United States. Whether imported from Europe or produced domestically, these high-tech products have proven themselves in a wide variety of climates during more than three decades of use.


The major distinctions between architects and engineers run along generalist and specialist lines. The generalists are ultimately responsible for the overall planning.

It is for this reason that an architect is generally employed as the prime professional by a client. On some special projects, such as dams, power plants, wastewater treatment, and research or industrial installations, where one of the engineering specialties becomes the predominant feature, a client may select an engineering professional or an E/A firm to assume responsibility for design and construction and taken on the lead role.

On certain projects, it is the unique and imaginative contribution of the engineer that may make the most significant total impact on the architectural design.

The overall strength of a dynamic, exposed structure, the sophistication of complex lighting systems, or the quiet efficiency of a well-designed mechanical system may prove to be the major source of the client’s pride in a facility. In any circumstance, the responsibilities of the professional engineer for competence and contribution are just as important to the project as those of the

Engineers, for example, play a major role in intelligent building system design, which involves mechanical-electrical systems. However, a building’s intelligence is also measured by the way it responds to people, both on the inside and outside.

The systems of the building must meet the functional needs of the occupants as well as respect the human response to temperature, humidity, airflow, noise, light, and air quality. To achieve the multifaceted goals, an intelligent building requires an intelligent design process with respect to design and system formulation as well as efficient and coordinated execution of design and technical documentation within the management structure.

An intelligent building begins with intelligent architecture—the shape, the building enclosure, and the way the building appears and functions. Optimal building solutions can be achieved through a design process that explores and compares varying architectural and engineering options in concert.

Sophisticated visualization and analytical tools using three-dimensional computer modeling techniques permit architects and engineers to rapidly evaluate numerous alternatives. Options can be carefully studied both visually and from a performance standpoint, identifying energy and life-cycle cost impact. This enables visualization and technical evaluation of multiple schemes early in the design phase, setting the basis for an intelligent building.

In all cases, the architect’s or engineer’s legal responsibilities to the client remain firm. The prime professional is fully responsible for the services delivered. The consultants, in turn, are responsible to the architect or engineer with whom they contract.

Following this principle, the architect or engineer is responsible to clients for performance of each consultant. Consequently, it is wise for architects and engineers to evaluate their expertise in supervising others before retaining consultants in other areas of responsibility.


Lubricants are not generally regarded as being corrosive, and in order to appreciate how corrosion can occur in lubricant systems it is necessary to understand something of the nature of lubricants. Once, lubricants were almost exclusively animal or vegetable oils or fats, but modern requirements in the way of volume and special properties have made petroleum the main source of supply. In volume, lubricants now represent about 2% of all petroleum products; in value, considerably more.

There are many hundreds of different varieties of lubricants, many of them tailored to meet particular requirements. Lubricating greases are solid or semi-solid lubricants made by thickening lubricating oils with soaps, clays, silica gel or other thickening agents. Synthetic lubricants, which will operate over a very wide range of temperature, have been developed mainly for aviation gas-turbine engines.

These are generally carboxylic esters and are very expensive products. The main function of most lubricants is to reduce friction and wear between moving surfaces and to abstract heat. They also have to remove debris from the contact area, e.g. combustion products in an engine cylinder, swarf in metal-cutting operations.

Mineral lubricants may be distillates or residues derived from the vacuum distillation of a primary distillate with a boiling point range above that of gas oi1’*2*T3.h ey are mixtures of hydrocarbons containing more than about 20 carbon atoms per molecule, and range from thin, easily flowing ‘spindle’ oils to thick ‘cylinder’oils.

For hydrocarbons having the same number of carbon atoms per molecule, the higher the proportion of carbon to hydrogen, the more viscous the oil and the lower the viscosity index.

Distillate lubricating oils can be conveniently divided into three groups -low viscosity index oils (LVI oils), medium viscosity index oils (MVI oils) and high viscosity index oils (HVI oils). LVI oils are made from naphthenic distillates, with low wax contents so that costly dewaxing is not required.

MVI oils are produced from both naphthenic and paraffinic distillates; the paraffinic distillates have to be dewaxed. HVI oils are prepared by the solvent extraction and dewaxing of paraffnic distillates. Solvent extraction is a physical process which removes the undesirable constituents, thereby improving viscosity index and the oxidation and colour stability.

White oils are obtained by the more drastic refining of low viscosity lubricating oil distillates to remove unsaturated compounds and constituents that impart colour, odour and taste. They are usually solvent extracted and then repeatedly treated with strong sulphuric acid or oleum and alkali, and finally ‘clay’-treated to remove surface-active compounds.

Acid and clay treating is expensive and is being superseded by hydrofinishing, a catalytic hydrogenation
treatment. The residues from the vacuum distillation can also be refined to provide very viscous lubricants. The residues from paraffinic base oils are generally solvent extracted and dewaxed. The main use of these products (bright stocks) is as blending components for heavy lubricants.

Thus residues from naphthenic base oils, which are also used as blending components for heavy lubricants, are normally not extracted. The performance characteristics of a lubricating oil depend on its origin and on the refining processes employed, and in order to ensure consistent properties these are varied as little as possible. Some aero-engine builders insist on a complete re-evaluation of a lubricant, costing many thousands of pounds, whenever there is a change of source (crude) or refining process.


This article is important for both the service provider and the client.

Management of the building process is best performed by the individuals educated and trained in the profession, that is, architects and engineers. While the laws of various states and foreign countries differ, they are consistent relative to the registration requirements for practicing architecture.

No individual may legally indicate to the public that he or she is entitled to practice as an architect without a professional certificate of registration as an architect registered in the locale in which the project is to be constructed.

This individual is the registered architect. In addition to the requirements for individual practice of architecture, most states and countries require a certificate of registration for a single practitioner and a certificate of authorization for an entity such as a corporation or partnership to conduct business in that locale.

An architect is a person who is qualified by education, training, experience, and examination and who is registered under the laws of the locale to practice architecture there. The practice of architecture within the meaning and intent of the law includes:

Offering or furnishing of professional services such as environmental analysis, feasibility studies, programming, planning, and aesthetic and structural design Preparation of construction documents, consisting of drawings and specifications, and other documents required in the construction process

Administration of construction contracts and project representation in connection with the construction of building projects or addition to, alteration of, or restoration of buildings or parts of building

All documents intended for use in construction are required to be prepared and administered in accordance with the standards of reasonable skill and diligence of the profession. Care must be taken to reflect the requirements of country and state statutes and county and municipal building ordinances.

Inasmuch as architects are licensed for the protection of the public health, safety, and welfare, documents prepared by architects must be of such quality and scope and be so administered as to conform to professional standards.

Nothing contained in the law is intended to prevent drafters, students, project representatives, and other employees of those lawfully practicing as registered architects from acting under the instruction, control, or supervision of their employers, or to prevent employment of project representatives from acting under the immediate personal supervision of the registered architect who prepared the construction documents.


After being processed, quicklime can generate many varieties of lime, such as quicklime powder, hydrated lime powder, lime cream, and lime paste. And different varieties have different purposes.

1. Lime Powder
Lime powder can be made into silicate products mixed with materials containing silicon. With water, pulverized lime can be molded by being mixed with fiber materials (such as glass fiber) or lightweight aggregate. Then, it can be carbonized artificially with carbon dioxide for carbonized lime board.

Carbonized lime board has a good processing property, suitable for the non-load-bearing inner partition and ceiling. Mixed with a certain percentage of clay, pulverized lime can generate limestone soil.

Triple-combined soil can be generated by mixing lime powder with clay, gravel, and slag. Lime soil and triple-combined soil are mainly used for foundation, bedding cushion, and roadbed.

2. Lime Paste
The aged lime paste or hydrated lime can turns into lime milk, diluted with water, as paint of internal and external walls and ceilings; if mixed with a certain amount of sand or cement and sand, it can be prepared into lime mortar or compound mortar for masonry or finishing; it can be used to paint inner walls or ceilings by being mixed with paper pulp and hemp fiber.

3. Storage of Lime
Quicklime will absorb the water and carbon dioxide in the air, generate calcium carbonate powder and lose cohesive force. Thus, when stored on construction site, quicklime should not be exposed to moisture, not be more, and not stay for a long time.

Moreover, the aging of lime will release a great amount of heat, so quicklime and inflammable matter should be stored separately in order to avoid fire. Usually quicklime should be stabilized immediately and the storage period should be changed into aging period.


Comparison with Simple Spans.
Simple-span girder or truss construction normally falls within the range of the shortest spans used up to a maximum of about 800 ft. Either true arches under favorable conditions or tied arches under all conditions are competitive within the range of 200 to 800 ft.

(There will be small difference in cost between these two types within this span range.) With increasing emphasis on appearance of bridges, arches are generally selected rather than simple-span construction, except for short spans for which beams or girders may be used.

Comparison with Cantilever or Continuous Trusses.
The normal range for cantilever or continuous-truss construction is on the order of 500 to 1800 ft for main spans. More likely, a top limit is about 1500 ft. Tied arches are competitive for spans within the range of 500 to 1000 ft.

True arches are competitive, if foundation conditions are favorable, for spans from 500 ft to the maximum for the other types. The relative economy of arches, however, is enhanced where site conditions make possible use of relatively short-span construction over the areas covered by the end spans of the continuous or cantilever trusses.

The economic situation is approximately this: For three-span continuous or cantilever layouts arranged for the greatest economy, the cost per foot will be nearly equal for end and central spans. If a tied or true arch is substituted for the central span, the cost per foot may be more than the average for the cantilever or continuous types.

If, however, relatively short spans are substituted for the end spans of these types, the cost per foot over the length of those spans is materially reduced. Hence, for a combination of short spans and a long arch span, the overall cost between end piers may be less than for the other types. In any case, the cost differential should not be large.

Comparison with Cable-Stayed and Suspension Bridges.
Such structures normally are not used for spans of less than 500 ft. Above 3000 ft, suspension bridges are probably the most practical solution. In the shorter spans, self-anchored construction is likely to be more economical than independent anchorages.

Arches are competitive in cost with the self-anchored suspension type or similar functional type with cable-stayed girders or trusses. There has been little use of suspension bridges for spans under 1000 ft, except for some self-anchored spans.

For spans above 1000 ft, it is not possible to make any general statement of comparative costs. Each site requires a specific study of alternative designs.


Like building codes, zoning codes are established under the police powers of the state, to protect the health, welfare, and safety of the public. Zoning, however, primarily regulates land use by controlling types of occupancy of buildings, building height, and density and activity of population in specific parts of a jurisdiction.

Zoning codes are usually developed by a planning commission and administered by the commission or a building department. Land-use controls adopted by the local planning commission for current application are indicated on a zoning map.

It divides the jurisdiction into districts, shows the type of occupancy, such as commercial, industrial, or residential, permitted in each district, and notes limitations on building height and bulk and on population density in each district.

The planning commission usually also prepares a master plan as a guide to the growth of the jurisdiction. A future land-use plan is an important part of the master plan. The commission’s objective is to steer changes in the zoning map in the direction of the future land-use plan.

The commission, however, is not required to adhere rigidly to the plans for the future. As conditions warrant, the commission may grant variances from any of the regulations.

In addition, the planning commission may establish land subdivision regulations, to control development of large parcels of land. While the local zoning map specifies minimum lot area for a building and minimum frontage a lot may have along a street, subdivision regulations, in contrast, specify the level of improvements to be installed in new land-development projects.

These regulations contain criteria for location, grade, width, and type of pavement of streets, length of blocks, open spaces to be provided, and right of way for utilities.

A jurisdiction may also be divided into fire zones in accordance with population density and probable degree of danger from fire. The fire-zone map indicates the limitations on types of construction that the zoning map would otherwise permit.

In the vicinity of airports, zoning may be applied to maintain obstruction-free approach zones for aircraft and to provide noise-attenuating distances around the airports. Airport zoning limits building heights in accordance with distance from the airport.


Compared with other binding materials, building gypsum has the following characteristics:

1. Fast Setting and Hardening
The setting time of building gypsum changes with the calcination temperature, grinding rate and impurity content. Generally, mixed with water, its initial setting needs just a few minutes at room temperature, and its final setting is also within 30min.

Under the natural dry indoor conditions, total hardening needs about one week. The setting time can be adjusted according to requirements.

If the time needs to be postponed, delayed coagulant can be added to reduce the solubility and the solution rate of building gypsum, such as sulfite alcohol wastewater, bone glue activated by borax or lime, hide glue, and protein glue; if it needs to be accelerated, accelerator can be added, such as sodium chloride, silicon sodium fluoride, sodium sulfate, and magnesium sulfate.

2. Micro-expansion
In the hardening process, the volume of building gypsum just expands a little, and there won’t be any cracks. Thus, it can be used alone without any extenders, and can also be casted into construction members and decorative patterns with accurate size and smooth and compact surface.

3. Big Porosity
After hardening, the porosity of building gypsum can reach 50%-60%, so its products are light, insulating, and sound-absorbing. But these products have low strength and large water absorption due to big porosity.

4. Poor Water Resistance
Building gypsum has low softening coefficient (about 0.2-0.3) and poor water resistance. Absorbing water, it.wil1 break up with the freeze of water. Thus, its water resistance and frost resistance are poor, not used outdoors.

5. Good Fire Resistance
The main component of building gypsum after hardcning is CaS04*2H20. When it contacts with fire, the evaporation of crystal water will absorb heat and generate anhydrous gypsum which has good thermal insulation. The thicker its products are, the better their fire resistance will be.

6. Large Plastic Deformation
Gypsum and its products have an obvious performance of plastic deformation. Creep becomes more serious especially under bending load. Thus, it is not used for load-bearing structures normally. If it is used, some necessary measures need to be taken


While low-slope roofs are generally limited to flat-roof styles and are seldom found on residential structures, steep-roof styles vary greatly.

Of the steep-roof styles, the gable roof is the most common. It has a high point, or ridge, at or near the center of the house or wing that extends from one end wall to the other.

The roof slopes downward from the ridge in both directions. This roof style gets its name from the gable, which is the triangular section of end wall between the rafter plate and the roof ridge.

The roof on one side of the ridge is usually the same size and slope as the roof on the other side. The gable roof of the saltbox house is an exception.

An architecture common in New England, the saltbox has different slopes and slopes of different lengths. A hip roof also has a ridge, but the ridge does not extend from one end of the roof to the other.

The lower edge of the roof, or eave, is at a constant height and the roof slopes downward to the eaves on all sides. The point where two roof surfaces meet at an outside corner is called a hip. The junction where two roof surfaces meet at an inside corner is called a valley.

A shed roof slopes in only one direction, like half a gable roof. The roof has no ridge and the walls that support the rafters are different heights. The shed roof has several variations. One is the butterfly roof, where two shed roofs slope toward a low point over the middle of the house.

In another variation, two shed roofs slope upward from the eaves, but do not meet at a ridge. The wall between the two roofs is called a clerestory, and is often filled with windows to let light into the interior
of the house.

A gambrel, or barn roof, has double slopes: one pair of gentle slopes and one pair of steep slopes. Like a gable roof, the gambrel roof slopes in both directions from a center ridge. At a point about halfway between ridge and eave, however, the roof slope becomes much steeper.

In effect, the lower slope replaces the upper exterior walls of a two-story house. It is common to add projections through the roof, called dormers, for light and ventilation.

Just as a gambrel roof is like a gable roof with two different slopes, a mansard roof is like a hip roof. From a shorter ridge, the roof drops in two distinct slopes to eaves that are the same height all the way around the structure.

Up to 40 percent of the building is roof with the mansard roof design. In addition to typical residential applications, mansard roofs are often used for apartment complexes, commercial buildings, and even institutions such as schools.


The building team should make it standard practice to have the output of the various disciplines checked at the end of each design step and especially before incorporation in the contract documents.

Checking of the work of each discipline should be performed by a competent practitioner of that discipline other than the original designer and reviewed by principals and other senior professionals.

Checkers should seek to ensure that calculations, drawings, and specifications are free of errors, omissions, and conflicts between building components.

For projects that are complicated, unique, or likely to have serious effects if failure should occur, the client or the building team may find it advisable to request a peer review of critical elements of the project or of the whole project.

In such cases, the review should be conducted by professionals with expertise equal to or greater than that of the original designers, that is, by peers; and they should be independent of the building team, whether part of the same firm or an outside organization.

The review should be paid for by the organization that requests it. The scope may include investigation of site conditions, applicable codes and governmental regulations, environmental impact, design assumptions, calculations, drawings, specifications, alternative designs, constructibility, and conformance with the building program.

The peers should not be considered competitors or replacements of the original designers, and there should be a high level of respect and communication between both groups. A report of the results of the review should be submitted to the authorizing agency and the leader of the building team.

(‘‘The Peer Review Manual,’’ American Consulting Engineers Council, 1015 15th St., NW, Washington, D.C. 20005, and ‘‘Peer Review, a Program Guide for Members of the Association of Soil and Foundation Engineers,’’ ASFE, Silver Spring, MD.)


A specialty contractor or subcontractor is a separate contractor hired by the prime contractor to perform certain portions of the work. The amount of work that the prime contractor will subcontract varies from project to project.

Some federal and state regulations limit the proportion of a project that may be subcontracted, but this is rarely the case in private work. There are advantages and disadvantages to using specialty contractors.

Trades such as plumbing, electrical, and heating and air-conditioning have a tradition of being performed by specialty contractors, due to their specialized nature and licensing requirements. However, specialty contractors can now be found who are capable of performing every aspect of the construction project.

Contractors today can construct entire projects without having any direct-hire craft personnel. The use of specialty contractors has gained popularity as a means to reduce risk and overhead; however, the contractor gives up a substantial amount of control when subcontracting the entire project.

If specialty contractors are to be used, the contractor must be certain to notify them early in the bidding period so that they have time to prepare a complete, accurate proposal. If rushed, the specialty contractor tends to bid high just for protection against what might have been missed.

The use of specialty contractors can be economical, but estimates still must be done for each portion of work. Even if the estimator intends to subcontract the work, an estimate of the work should be prepared. It is possible that the estimator will not receive proposals for a project before the bid date and will have to use an estimated cost of the work in totaling the proposal.

All subcontractors’ proposals are compared with the estimator’s price; it is important that a subcontractor’s price is neither too high nor too low. If either situation exists, the estimator should call the subcontractor and discuss the proposal with him.

The specialty contractor’s proposal is often phoned, faxed, or e-mailed into the general contractor’s office at the last minute because of the subcontractor’s fear that the contractor will tell other subcontractors the proposal price and encourage lower bids. This practice is commonly referred to as bid peddling or bid shopping and is highly unethical and should be discouraged.

To prevent bid shopping, specialty contractors submit their final price only minutes before the bids close, which leads to confusion and makes it difficult for the estimator to analyze all bids carefully. This confusion is compounded by specialty contractors who submit unsolicited bids. These bids come from specialty contractors who were not contacted or invited to submit a bid, but who find out which contractors are bidding the project and submit a bid.

Since these companies are not prequalified, there is an element of risk associated with accepting one of these bids. On the other hand, not using low bids from unsolicited subcontractors places the contractor at a price disadvantage.

In checking subcontractor proposals, note especially what is included and what is left out. Each subsequent proposal may add or delete items. Often the proposals set up certain conditions, such as use of water, heat, or hoisting facilities. The estimator must compare each proposal and select the one that is the most economical.

All costs must be included somewhere. If the subcontractor does not include an item in the proposal, it must be considered elsewhere. A tricky task for the prime contractor is the comparison of the individual subcontractor’s price quotes.

Throughout the estimating process, the prime contractor should be communicating with the specific subcontractors concerning the fact that they will submit a price quote and what scope of work is to be included within that quote. However, subcontractors will include items that they were not asked to bid and will exclude items that they were asked to bid.

A “bid tabulation” or “bid tab” is used to equalize the scope between subcontractors so that the most advantageous subcontractor’s bid can be included in the prime contractor’s bid.


The compositional specifications for wrought aluminium alloys are now internationally agreed throughout Europe, Australia, Japan and the USA. The system involves a four-digit description of the alloy and is now specified in the UK as BS EN 573, 1995.

Registration of wrought alloys is administered by the Aluminum Association in Washington, DC. International agreement on temper designations has been achieved, and the standards agreed for the European Union, the Euro-Norms, are replacing the former British Standards.

Thus BS EN 515. 1995 specifies in more detail the temper designations to be used for wrought alloys in the UK. At present, there is no Euro-Norm for cast alloys and the old temper designations are still used for cast alloys.

In the following tables the four-digit system is used, wherever possible, for wrought materials.

Alloy designation system for wrought aluminium
The first of the four digits in the designation indicates the alloy group according to the major alloying elements, as follow:

1XXX aluminium of 99.0% minimum purity and higher
2XXX copper
3XXX manganese
4XXX silicon
5XXX magnesium
6XXX magnesium and silicon
7XXX zinc
8XXX other element, incl. lithium
9XXX unused

1XXX Group:
In this group the last two digits indicate the minimum aluminium percentage.
Thus 1099 indicates aluminium with a minimum purity of 99.99%. The second digit indicates modifications in impurity or alloying element limits. 0 signifies unalloyed aluminium and integers 1 to 9 are allocated to specific additions.

2XXX-8XXX Groups:
In these groups the last two digits are simply used to identify the different alloys in the groups and have no special significance. The second digit indicates alloy modifications, zero being allotted to the original alloy.

National variations of existing compositions are indicated by a letter after the numerical designation, allotted in alphabetical sequence, starting with A for the first national variation registered.


For matters relevant to estimating and costs, the best source of information is your historical data. These figures allow for the pricing of the project to match how the company actually performs its construction.

This information takes into account the talent and training of the craft personnel and the management abilities of the field staff personnel. In addition, it integrates the construction companies’ practices and methodologies.

This is why a careful, accurate accounting system combined with accuracy in field reports is so important. If all of the information relating to the job is tracked and analyzed, it will be available for future reference.

Computerized cost accounting systems are very helpful in gathering this information and making it readily available for future reference. See Construction Accounting and Financial Management by Steven J. Peterson for more information on managing construction accounting systems.

There are several “guides to construction cost” manuals available; however, a word of extreme caution is offered regarding the use of these manuals. They are only guides; the figures should rarely be used to prepare an actual estimate.

The manuals may be used as a guide in checking current prices and should enable the estimator to follow a more uniform system and save valuable time. The actual pricing in the manuals is most appropriately used in helping architects check approximate current prices and facilitate their preliminary estimate.

In addition to these printed guides, many of these companies provide electronic databases that can be utilized by estimating software packages. However, the same caution needs to be observed as with the printed version.

These databases represent an average of the methodologies of a few contractors. There is no simple way to convert this generalized information to match the specifics of the construction companies’ methodologies.


Basically, the two bidding procedures by which the contractor gets to build a project for owners are as follows:

1. Competitive bidding
2. Negotiated bidding

Competitive bidding involves each contractor submitting a lump-sum bid or a proposal in competition with other contractors to build the project. The project may be awarded based on the price or best value.

When the project is awarded based on the price, the lowest lump-sum bidder is awarded the contract to build the project as long as the bid form and proper procedures have been followed and this bidder is able to attain the required bonds and insurance.

When the project is awarded based upon the best value, the proposals from the contractors are rated based on specified criteria with each criterion given a certain percentage of the possible points. The criteria may include review of the capabilities of the assigned project team, the company’s capabilities and its approach to the project (including schedule), proposed innovation, method of mitigating risk, and price.

The price is often withheld from the reviewers until the other criteria have been evaluated to prevent the price from affecting the ratings of the other criteria. Most commonly, the bids must be delivered to the person or place specified by a time stated in the instruction to bidders.

The basic underlying difference between negotiated work and competitive bidding is that the parties arrive at a mutually agreed upon price, terms and conditions, and contractual relationship. This arrangement often entails negotiations back and forth on virtually all aspects of the project, such as materials used, sizes, finishes, and other items that affect the price of the project.

Owners may negotiate with as many contractors as they wish. This type of bidding is often used when owners know which contractor they would like to build the project, in which case competitive bidding would waste time.

The biggest disadvantage of this arrangement is that the contractor may not feel the need to work quite as hard to get the lowest possible prices as when a competitive bidding process is used.


The first step in understanding earthquake risk is to dissect the earthquake risk or loss process into its constituent steps. Earthquake risk begins with the occurrence of the earthquake, which results in a number of earthquake hazards.

The most fundamental of these hazards is faulting, that is, the surface expression of the differential movement of blocks of the Earth’s crust. Faulting can be a simple “mole-track” lateral movement, or a major vertical scarp, or may not even be visible.

In most cases, faulting is typically a long narrow feature, and therefore affects a relatively small fraction of the total affected structures and persons. Affecting a much greater number of structures and persons is shaking, which is typically the primary hazard due to earthquakes.

Depending on the earthquake, liquefaction, other forms of ground failure, tsunamis, or other types of hazards may be significant agents of damage. For various reasons, many buildings, portions of the infrastructure, and other structures cannot fully resist these hazards, and sustain some degree of damage.

Primary damage can vary from minor cracking to total collapse. Some building types are more vulnerable than others, but even when a building sustains no structural damage, the contents of the building may be severely damaged.

For certain occupancies, such as hospitals or emergency services dispatch centers, this damage to contents (laboratories, specialized machinery, communication equipment, etc.) can be very important. Additionally, these various kinds of primary damage can lead to other secondary forms of hazard and damage, such as releases of hazardous materials, major fires, or flooding.

Damage results in loss.
Primary loss can take many forms — life loss or injury is the primary concern, but financial loss and loss of function are also of major concern. The likelihood of sustaining a loss is termed risk . Primary losses lead to secondary forms of loss, such as loss of revenues resulting from business interruption and loss of market share and/or reputation.


In Canada, parts of Europe, and on most road construction projects in the United States, the estimated quantities of materials required on the project are determined by a professional quantity surveyor or engineer and provided to the interested bidders on the project.

This is often referred to as a unit price bid. In this method of bidding, the contractors are all bidding based on the same quantities, and the estimator spends time developing the unit prices. For example, the bid may be $47.32 per cubic yard (cy) of concrete.

Because all of the contractors are bidding on the same quantities, they will work on keeping the cost of purchasing and installing the materials as low as possible.

As the project is built, the actual number of units required is checked against the original number of units on which the estimates were made. For example, the original quantity survey called for 715 linear feet (lf) of concrete curbing.

If 722 lf were actually installed, then the contractor would be paid for the additional 7 lf. If 706 lf were used, then the owner would pay only for the 706 lf installed and not the 715 lf in the original quantity survey.

This type of adjustment is quite common. When errors do occur and there is a large difference between the original quantity survey and the actual number of units, an adjustment to the unit price is made. Small adjustments are usually made at the same unit rate as the contractor bid.

Large errors may require that the unit price be renegotiated. If the contractor is aware of potential discrepancies between the estimated quantities and those that will be required, the contractor may price his or her bid to take advantage of this situation.

With a belief that the estimated quantities are low, the contractor may reduce his or her unit price to be the low bidder. If the assumption is true, the contractor has the potential to make the same profit by distributing the project overhead over a greater number of units.


What Are The Classification Of Surveying?

Surveying may be classified on the following basis:
(i) Nature of the survey field
(ii) Object of survey
(iii) Instruments used and
(iv) The methods employed.

Classification Based on Nature of Survey Field
On this basis survey may be classified as land survey, marine or hydraulic survey and astronomical survey.

Land Survey. It involves measurement of various objects on land. This type of survey may be further classified as given below:

(a) Topographic Survey: It is meant for plotting natural features like rivers, lakes, forests and hills as well as man made features like roads, railways, towns, villages and canals.

(b) Cadestal Survey: It is for marking the boundaries of municipalities, villages, talukas, districts, states etc. The survey made to mark properties of individuals also come under this category.

(c) City Survey: The survey made in connection with the construction of streets, water supply and sewage lines fall under this category.

Marine or Hydrographic Survey. Survey conducted to find depth of water at various points in bodies of water like sea, river and lakes fall under this category. Finding depth of water at specified points is known as sounding.

Astronomical Survey. Observations made to heavenly bodies like sun, stars etc., to locate absolute positions of points on the earth and for the purpose of calculating local time is known as astronomical survey.

Classification Based on Object of Survey
On the basis of object of survey the classification can be as engineering survey, military survey, mines survey, geological survey and archeological survey.

(a) Engineering Survey: The objective of this type of survey is to collect data for designing civil engineering projects like roads, railways, irrigation, water supply and sewage disposals. These surveys are further sub-divided into:

Reconnaissance Survey for determining feasibility and estimation of the scheme.

Preliminary Survey for collecting more information to estimate the cost of the project, and

Location Survey to set the work on the ground.

(b) Military Survey: This survey is meant for working out plans of strategic importance.

(c) Mines Survey: This is used for exploring mineral wealth.

(d) Geological Survey: This survey is for finding different strata in the earth’s crust.

(e) Archeological Survey: This survey is for unearthing relics of antiquity.

Classification Based on Instruments Used
Based on the instruments used, surveying may be classified as:

(i) Chain survey
(ii) Compass survey
(iii) Plane table survey
(iv) Theodolite survey
(v) Tacheometric survey
(vi) Modern survey using electronic distance meters and total station
(vii) Photographic and Aerial survey

The survey is taught to students mainly based on this classification.

Classification Based on Methods Employed
On this basis surveying is classified as triangulation and traversing.
(i) Triangulation: In this method control points are established through a network of triangles.
(ii) Traversing: In this scheme of establishing control points consists of a series of connected points established through linear and angular measurements. If the last line meets the starting point it is called as closed traverse. If it does not meet, it is known as open traverse.


What Is Surveying and What Are The Objects And Uses Of Surveying?

Surveying is the art of making measurements of objects on, above or beneath the ground to show their relative positions on paper. The relative position required is either horizontal, or vertical, or both.

Less precisely the term Surveying is used to the measurement of objects in their horizontal positions. Measurements to deteremine their relative vertical positions is known as levelling.

As stated in the definition, object of surveying is to show relative positions of various objects of an area on paper and produce plan or map of that area. Various uses of surveying are listed below:

(i) Plans prepared to record property lines of private, public and government lands help in avoiding unnecessary controversies.

(ii) Maps prepared for marking boundaries of countries, states, districts etc., avoid disputes.

(iii) Locality plans help in identifying location of houses and offices in the area.

(iv) Road maps help travellers and tourist.

(v) Topographic maps showing natural features like rivers, streams, hills, forests help in planning irrigation projects and flood control measures.

(vi) For planning and estimating project works like roads, bridges, railways, airports, water supply and waste water disposal surveying is required.

(vii) Marine and hydrographic survey helps in planning navigation routes and harbours.

(viii) Military survey is required for strategic planning.

(ix) Mine surveys are required for exploring minearl wealth.

(x) Geological surveys are necessary for determining different strata in the earth crust so that proper location is found for reservoirs.

(xi) Archeological surveys are useful for unearthing relics of antiquity.

(xii) Astronomical survey helps in the study of movements of planets and for calculating local and standard times.


What Are Cambers?

Camber is a curvature built into a member or structure so that when it is loaded, it deflects to a desired shape. Camber, when required, might be for dead load only, dead load and partial live load, or dead load and full live load. The decision to camber and how much to camber is one made by the designer.

Rolled beams are generally cambered cold in a machine designed for the purpose, in a large press, known as a bulldozer or gag press, through the use of heat, or a combination of mechanically applied stress and heat.

In a cambering machine, the beam is run through a multiple set of hydraulically controlled rollers and the curvature is induced in a continuous operation. In a gag press, the beam is inched along and given an incremental bend at many points.

There are a variety of specific techniques used to heat-camber beams but in all of them, the side to be shortened is heated with an oxygen-fed torch.

As the part is heated, it tries to elongate. But because it is restrained by unheated material, the heated part with reduced yield stress is forced to upset (increase inelastically in thickness) to relieve its compressive stress.

Since the increase in thickness is inelastic, the part will not return to its original thickness on cooling. When the part is allowed to cool, therefore, it must shorten to return to its original volume. The heated flange therefore experiences a net shortening that produces the camber.

Heat cambering is generally slow and expensive and is typically used in sections larger than the capacity of available equipment. Heat can also be used to straighten or eliminate warping from parts. Some of these procedures are quite complex and intuitive, demanding experience on the part of the operator.

Experience has shown that the residual stresses remaining in a beam after cambering are little different from those due to differential cooling rates of the elements of the shape after it has been produced by hot rolling. Note that allowable design stresses are based to some extent on the fact that residual stresses virtually always exist.

Plate girders usually are cambered by cutting the web plate to the cambered shape before the flanges are attached.

Large bridge and roof trusses are cambered by fabricating the members to lengths that will yield the desired camber when the trusses are assembled. For example, each compression member is fabricated to its geometric (loaded) length plus the calculated axial deformation under load. Similarly, each tension member is fabricated to its geometric length minus the axial deformation.


What Is Electroslag Welding? What is Electrogas Welding?

Electroslag welding (ESW) produces fusion with a molten slag that melts filler metal and the surfaces of the base metal. The weld pool is shielded by this molten slag, which moves along the entire cross section of the joint as welding progresses.

The electrically conductive slag is maintained in a molten condition by its resistance to an electric current that flows between the electrode and the base metal. The process is started much like the submerged-arc process by striking an electric arc beneath a layer of granular flux.

When a sufficiently thick layer of hot molten slag is formed, arc action stops. The current then passes from the electrode to the base metal through the conductive slag. At this point, the process ceases to be an arc welding process and becomes the electroslag process.

Heat generated by resistance to flow of current through the molten slag and weld puddle is sufficient to melt the edges at the joint and the tip of the welding electrode. The temperature of the molten metal is in the range of 3500 deg F.

The liquid metal coming from the filler wire and the molten base metal collect in a pool beneath the slag and slowly solidify to form the weld. During welding, since no arc exists, no spattering or intense arc flash occurs.

Because of the large volume of molten slag and weld metal produced in electroslag welding, the process is generally used for welding in the vertical position. The parts to be welded are assembled with a gap 1 to 1 1⁄4 in wide. Edges of the joint need only be cut squarely, by either machine or flame.

Water-cooled copper shoes are attached on each side of the joint to retain the molten metal and slag pool and to act as a mold to cool and shape the weld surfaces. The copper shoes automatically slide upward on the base-metal surfaces as welding progresses.

Preheating of the base metal is usually not necessary in the ordinary sense. Since the major portion of the heat of welding is transferred into the joint base metal, preheating is accomplished without additional effort.

Electrogas welding (EGW) is similar to electroslag welding in that both are automatic processes suitable only for welding in the vertical position. Both utilize vertically traveling, water-cooled shoes to contain and shape the weld surface. The electrogas process differs in that once an arc is established between the electrode and the base metal, it is continuously maintained.

The shielding function is performed by helium, argon, carbon dioxide, or mixtures of these gases continuously fed into the weld area. The flux core of the electrode provides deoxidizing and slagging materials for cleansing the weld metal.

The surfaces to be joined, preheated by the shielding gas, are brought to the proper temperature for complete fusion by contact with the molten slag. The molten slag flows toward the copper shoes and forms a protective coating between the shoes and the faces of the weld. As weld metal is deposited, the copper shoes, forming a weld pocket of uniform depth, are carried continuously upward.

The electrogas process can be used for joining material from 1⁄2 to more than 2 in thick. The process cannot be used on heat-treated material without subsequent heat treatment. AWS and other specifications prohibit the use of EGW for welding quenched-and-tempered steel or for welding dynamically loaded structural members subject to tensile stresses or to reversal of stress.


Shielded metal arc welding (SMAW) produces coalescence, or fusion, by the heat of an electric arc struck between a coated metal electrode and the material being joined, or base metal. The electrode supplies filler metal for making the weld, gas for shielding the molten metal, and flux for refining this metal.

This process is commonly known also as manual, hand, or stick welding. Pressure is not used on the parts to be joined. When an arc is struck between the electrode and the base metal, the intense heat forms a small molten pool on the surface of the base metal.

The arc also decomposes the electrode coating and melts the metal at the tip of the electrode. The electron stream carries this metal in the form of fine globules across the gap and deposits and mixes it into the molten pool on the surface of the base metal. (Since deposition of electrode material does not depend on gravity, arc welding is feasible in various positions, including overhead.)

The decomposed coating of the electrode forms a gas shield around the molten metal that prevents contact with the air and absorption of impurities. In addition, the electrode coating promotes electrical conduction across the arc, helps stabilize the arc, adds flux, slag-forming materials, to the molten pool to refine the metal, and provides materials for controlling the shape of the weld.

In some cases, the coating also adds alloying elements. As the arc moves along, the molten metal left behind solidifies in a homogeneous deposit, or weld. The electric power used with shielded metal arc welding may be direct or alternating current. With direct current, either straight or reverse polarity may be used.

For straight polarity, the base metal is the positive pole and the electrode is the negative pole of the welding arc. For reverse polarity, the base metal is the negative pole and the electrode is the positive\ pole.

Electrical equipment with a welding-current rating of 400 to 500 A is usually used for structural steel fabrication. The power source may be portable, but the need for moving it is minimized by connecting it to the electrode holder with relatively long cables.

The size of electrode (core wire diameter) depends primarily on joint detail and welding position. Electrode sizes of 1⁄8, 5⁄32, 3⁄16, 7⁄32, 1⁄4, and 5⁄16 in are commonly used. Small-size electrodes are 14 in long, and the larger sizes are 18 in long.

Deposition rate of the weld metal depends primarily on welding current. Hence use of the largest electrode and welding current consistent with good practice is advantageous.

About 57 to 68% of the gross weight of the welding electrodes results in weld metal. The remainder is attributed to spatter, coating, and stub-end losses.

Shielded metal arc welding is widely used for manual welding of low-carbon steels, such as A36, and HSLA steels, such as A572 and A588. Though stainless steels, high-alloy steels, and nonferrous metals can be welded with this process, they are more readily welded with the gas metal arc process.


What Is Plane Surveying? How Plane Surveying Works?

In the past, most survey work depended on triangulation from known fixed points using a theodolite and this may still be a suitable method for smaller sites. Again it is necessary to ensure the instrument is in good condition and that its base is truly horizontal.

Readings taken on both faces of the instrument may reduce residual errors. Setting out by taping along a line given by the theodolite may also still be the clearest way of providing centre lines or points, particularly for regular structure layouts such as building columns.

The appropriate time for this is when blinding concrete has been placed to column and wall foundations. The base line, which is either the centre line of the building, or a line parallel to it but clear of the building, should have been set out previously by end pegs sited well clear of the work.

It is usual to work from co-ordinates along this base line from some fixed zero point, and measuring right angle distances out from them. In this way lines of walls and column centres can be marked on the blinding concrete.

Distances may be measured by steel or fibreglass tape pulled horizontally, so it is a great convenience if the site is level. If not a plumb bob has to be used to transfer distances. Distance co-ordinates along the base line from the zero peg are set out, using the steel tape and marking a pencil line across the peg.

The theodolite is set out over the pencil line, and its position is adjusted laterally so that it transits accurately on the two outermost base line marks. The plumb bob on the theodolite gives the mark for the co-ordinate point, a round headed nail being inserted on this point.

Distances at right angles to the base line are then set out with theodolite and steel tape. The advantage of this method is that the theodolite can sight down into column bases which are usually set deeper than the general formation level. For the assistance of bricklayers and formwork carpenters, sight boards can be provided, with the cross-arm fixed at a given level above formation level and with saw cuts exactly on the lines of sight to be used.

A builder’s line can then be fixed through such saw cuts. An alternative to the foregoing is to set out two base lines at right angles to each other and use theodolite right angle settings from these to give centres for such column bases, etc.

The introduction of EDM equipment has, however, meant that accurate distance and angle measurements can now be made from a single point set up. The instruments work by measuring the time of a wave in travelling from the transmitter to a reflector and back.

Readings may be automatically repeated to improve accuracy. Built-in or add-on equipment allows for automatic data logging, reduction of distances to horizontal and vertical components and for downloading to a computer.

Accuracy over short distances is good. Over longer distances corrections may need to be made for atmospheric conditions which vary from the manufacturers’ setting. The improved accuracy available has meant that setting out on site or general survey work is often done by some form of traversing. By this method the position of two known points is extended by noting the angle to a third point and its distance from the instrument set up over one of the points. Extended traverses should be closed onto another known point to check for errors.

Even with EDM equipment, setting out of regular structures is probably best done using a marked baseline as described above. The equipment also has major advantages in ground surveying since the location and elevation of any point in the area to be surveyed can usually be determined directly from just one or two positions of the instrument.

Data from the instrument can then be downloaded into a computer and with the use of appropriate software, contoured plans of the area can be produced for design or for earthworks measurement purposes.

Acertain amount of planning is necessary to produce the best results by ensuring a regular grid of locations is used for targeting and that any individual feature, such as sharp changes in slope are picked up. As an alternative ranging poles can be used to set out a rough grid and readings at say 20 m intervals between these should give sufficient coverage for accurate plotting.


The problems between main and sub-contractors were one of the areas to benefit most from Part II of the UK Government’s Housing Grants, Construction and Regeneration Act 1996 (see Section 1.6). The introduction of adjudication under that act to deal with disputes has at least allowed sub-contractors to press their claims to an earlier conclusion, and to challenge any withholding of payment by the contractor.

The Act requires payment terms to be stated and regular payments made. It prohibits ‘pay when paid’ clauses, and requires the contractor to issue a detailed ‘withholding notice’ if he seeks to hold back payment. These measures have eased the cash flow problems of sub-contractors.

Also most standard forms of sub-contract now contain provision for payment of interest on delayed payments, but this may not be very effective because a sub-contractor may not claim interest for fear the contractor might not as a consequence give him any further work.

The Civil Engineering Contractors Association (CECA) has issued a Form of Sub-contract ‘for use in conjunction with the ICE conditions of contract.’ Contractors are, of course, not obliged to use this form and many use one of their own devising or modify the standard form.

The provisions of the CECA sub-contract illustrate the many matters which such a sub-contract has to cover and the difficulty of trying to provide rights to the sub-contractor without putting the main contractor at risk under his contract.

Provisions of the CECA sub-contract, apart from defining the work, timing and duration of the sub-contractor’s input, require the sub-contract to set out the division of risks as between contractor and sub-contractor.

It defines procedures and methods of valuing variations made by the engineer and confirmed by the contractor, or made by the contractor; and sets out procedures for notification and payment for ‘unforeseen conditions’ or other claim matters. It also stipulates requirements for insurances and so on.

Many of the provisions are similar in terms to the ICE conditions applying to the contractor, and are thus passed on to the sub-contractor in respect of his work. The subcontractor is ‘deemed to have full knowledge of the provisions of the main contract’ and the contractor must give him a copy of it (without the prices) if the sub-contractor requests it.

Of particular importance is Clause 3 of the CECA sub-contract which requires the sub-contractor to carry out his work so as to avoid causing a breach of the main contract by the contractor. He has to indemnify the contractor ‘against all claims, demands, proceedings, damages, costs and expenses made against or incurred by the contractor by reason of any breach by the subcontractor of the sub-contract.’

But a sub-contractor undertaking a small value contract may find it impossible to accept this clause. If he fails to complete his work on time and this could possibly cause a delay to the whole project, he might be liable to pay many thousands of pounds to the contractor – far in excess of the value of his sub-contract.

A further problem for the engineer is that, if a dispute arises between the contractor and his sub-contractor as to who is responsible for some defective work, the defect can remain uncorrected until the dispute is resolved. If a defect is found after the sub-contractor has left site and he is believed or known to be responsible for it, the contractor may not be able to get the sub-contractor back to site to remedy the defect, or to pay for its repair.

To guard against this, the contractor may therefore hold back full payment to the sub-contractor for many months until a certificate of completion for the whole works is issued. This will cause another dispute between contractor and sub-contractor.

The development of sub-contracting in civil engineering has therefore brought both advantages and disadvantages. However, problems rarely arise if the contractor can use sub-contractors he has worked with before whose work has proved satisfactory and he treats them fairly.


What Is Sub Contracting?

Many civil engineering contractors now use sub-contractors to do much of their work. Most conditions of contract permit a contractor to sub-let work of a specialist nature; but the ICE conditions of contract have gone further and permit the contractor to sub-contract any part of the work (but not the whole of the work), subject only to notifying the engineer of the work sub-contracted and the name of the sub-contractor appointed to undertake it.

The contractor does not have to notify any labour-only sub-contracts he uses. The engineer can object, with reasons, to the appointment of a sub-contractor, but otherwise has no rights in connection with such sub-contracts, except that he can require removal of a sub-contractor who proves incompetent or negligent, or does not conform to safety requirements.

Under FIDIC conditions for overseas work, sub-contracting requires the engineer’s prior sanction. In building work there has long been a trend to pass the majority of work to sub-contractors who specialize in various trades, and the same has now occurred in civil engineering where many operations are ‘packaged up’ and sub-let.

Thus sub-contracts may be let for excavation, formwork, reinforcement supplied and erected, and concreting. The advantage to the contractor is that this reduces the staff he needs on site and his capital outlay on plant and equipment. He can use sub-contractors with proven experience and does not have to take on a range of temporary labour whose quality may be variable.

The contractor retains responsibility for the quality and correctness of work and, of course, has to plan and co-ordinate the sub-contract inputs, and often supply any necessary materials.

But if much of the work is sub-contracted, the contractor’s or agent’s main input to a project may be that of dealing with the sub-contracts and controlling their financial outcome, so these matters may take priority over dealing with any engineering problems which arise.

The contractor may therefore tend to leave a sub-contractor to solve any problems he encounters, on the basis that these are his risks under his sub-contract and it is up to him to deal with them. But the sub-contractor may think otherwise, so a dispute arises as each considers the other responsible for any extra cost or delays caused.

Frequent disputes have also arisen in recent years when any default or presumed default by a sub contractor has resulted in the contractor withholding payment to him. Late payment by contractors to sub-contractors is another widespread source of complaint by sub-contractors, but remedies are difficult to devise.

The sub-contracts are private contracts whose terms are unknown to the engineer and the employer, so they cannot interfere in any such dispute. The engineer has only power to protect nominated sub contractors, i.e. subcontractors he directs the contractor to use.


As a building rises the vertical alignment must also be controlled. This can be done by extending building centre lines at right angles to each other out to fixed points clear of the structure.

These lines can then be projected up the building and marked, allowing accurate measurements from these marks at each floor. Alternatively an optical plumb can be used to project a fixed point up through openings in the floors of the building so as to provide a set of reference points at each level.

The standard of setting out for tunnels must be high using carefully calibrated equipment, precise application and double checking everything. An accurate tunnel baseline is first set out on the surface using the methods described above. Transference of this below ground can be done by direct sighting down a shaft if the shaft is sufficiently large to allow this without distortion of sight-lines on the theodolite.

With smaller shafts, plumbing down may be used. A frame is needed either side of the shaft to hold the top ends of the plumb-lines and to allow adjustment to bring them exactly on the baseline. The plumb-line used should be of stainless steel wire, straight and unkinked, and the bob of a special type is held in a bath of oil to damp out any motion.

By this means the tunnel line is reproduced at the bottom of the shaft and can be rechecked as the tunnel proceeds. Many tunnels are nowadays controlled by lasers, the laser gun being set up on a known line parallel to the centre line for the tunnel and aimed at a target.

Where a tunnelling machine is used, the operator can adjust the direction of movement of the machine to keep it on target so that the tunnel is driven in the right direction. For other methods of tunnelling, target marks can be set on the soffit of rings, the tunnel direction being kept on line by adjusting the excavation and packing out any tunnel rings to keep on the proper line.

Lasers are also used in many other situations, usually for controlling construction rather than for original setting out since their accuracy for this may not be good enough. The laser beam gives a straight line at whatever slope or level is required, and so can be used for aligning forms for road pavements or even laying large pipes to a given gradient. For the latter, the laser is positioned at the start of a line of pipes and focused on the required base line.

As each new pipe is fitted into the pipeline a target is placed in the invert of the open end of the pipe, using a spirit level to find the bottom point, and the pipe is adjusted in line and level until the target falls on the laser beam. Bedding and surround to the pipe are then placed to fix the pipe in position.

Rotating lasers are also widely used and once set up give a constant reference plane at a known level. Use of a staff fitted with a reflector allows spot levels to be obtained anywhere in the area covered by the laser. Earthmoving equipment fitted with appropriate sensors can also be operated to control the level of excavation or filling with minimum input other than by the machine operator.


What Are The Usual Problem In Concrete Finishing?

The skill required by carpenters to make and erect form work for concrete is seldom fully appreciated. The formwork must remain ‘true to line and level’ despite substantial loading from the wet concrete. Column and wall faces have to be strictly vertical, and beam soffits strictly level, or any departure will be easily visible by eye.

Formwork for concrete which is to remain exposed to view has to be planned and built as carefully as if it were a permanent feature of the building. Many methods have been tried to make the appearance of exposed concrete attractive: but any of them can be ruined by honeycombing, a bad construction joint, or by subsequent weathering revealing that one pour of concrete has not been identical with adjacent pours, or that the amount of vibration used in compacting one panel has been different from that used in others.

If concrete has to remain exposed to public view, then the resident engineer should endeavour to agree with the contractor what is the most suitable method for achieving the finish required if the specification or drawings do not give exact guidance on the matter. The problem is that if, through lack of detailed attention, a ‘mishap’ on the exposed surface is revealed when the formwork is struck, it is virtually impossible to rectify it.

Sometimes rendering the whole surface is the only acceptable remedy. Where concrete will not remain exposed to view, minor discrepancies can be accepted. ‘Fins’ of concrete caused by the mix leaking through butt joints in the formwork should be knocked off. Shallow honeycombing should be chiselled out, and a chase cut along any defective construction joint.

The cut-out area or chase should be washed, brushed with a thick cement grout, and then filled with a dryish mortar mix. This rectifying work should be done as soon as possible so the mortar mix has a better chance of bonding to the ‘green’ concrete.

Shrinkage cracking of concrete is a common experience. The shrinkage of concrete due to drying is of the order of 0.2–0.5mm/m for the first 28 days. Subsequently concrete may expand slightly when wet and shrink on drying.

The coefficient of temperature expansion or contraction is very much smaller, of the order of 0.007mm/m per degree centigrade of change. Rich concrete mixtures tend to shrink more than lean mixes. The use of large aggregate, such as 40 mm instead of 20 mm, helps to minimize shrinkage. To avoid cracking of concrete due to shrinkage, wall lengths of concrete should be limited to about 9 m if restrained at the base or ends.

Heavy foundations to a wall should not be allowed to stand and dry out for a long period before the wall is erected, because the wall concrete bonding to the base may be unable to shrink without cracking. Concrete is more elastic than is commonly appreciated, for example the unrestrained top of a 300 mm diameter reinforced concrete column 4m high can be made to oscillate through nearly 1 cm by push of the hand.


What Are The Types Of Suspension Bridges?

Several arrangements of suspension bridges are illustrated in Fig. 1. The main cable is continuous, over saddles at the pylons, or towers, from anchorage to anchorage.  

FIGURE 15.9 Suspension-bridge arrangements. (a) One suspended span, with pin-ended stiffening truss. (b) Three suspended spans, with pin-ended stiffening trusses. (c) Three suspended spans, with continuous stiffening truss. (d ) Multispan bridge, with pin-ended stiffening trusses. (e) Self-anchored suspension bridge.

When the main cable in the side spans does not support the bridge deck (side spans independently supported by piers), that portion of the cable from the saddle to the anchorage is virtually straight and is referred to as a straight backstay.

This is also true in the case shown in Fig. 1a where there are no side spans. Figure 1d represents a multispan bridge. This type is not considered efficient, because its flexibility distributes an undesirable portion of the load onto the stiffening trusses and may make horizontal ties necessary at the tops of the pylons.

Ties were used on several French multispan suspension bridges of the nineteenth century. However, it is doubtful whether tied towers would be esthetically acceptable to the general public. Another approach to multispan suspension bridges is that used for the San Francisco–Oakland Bay Bridge (Fig. 2). It is essentially composed of two three-span suspension bridges placed end to end.

This system has the disadvantage of requiring three piers in the central portion of the structure where water depths are likely to be a maximum. Suspension bridges may also be classified by type of cable anchorage, external or internal. Most suspension bridges are externally anchored (earth-anchored) to a massive external anchorage (Fig. 1a to d).

In some bridges, however, the ends of the main cables of a suspension bridge are attached to the stiffening trusses, as a result of which the structure becomes self-anchored (Fig. 1e). It does not require external anchorages.

The stiffening trusses of a self-anchored bridge must be designed to take the compression induced by the cables. The cables are attached to the stiffening trusses over a support that resists the vertical component of cable tension. The vertical upward component may relieve or even exceed the dead-load reaction at the end support. If a net uplift occurs, a pendulum link tie-down should be provided at the end support.

Self-anchored suspension bridges are suitable for short to moderate spans (400 to 1,000 ft) where foundation conditions do not permit external anchorages. Such conditions include poor foundation bearing strata and loss of weight due to anchorage submergence. Typical examples of self-anchored suspension bridges are the Paseo Bridge at Kansas City, with a main span of 616 ft, and the former Cologne-Mu¨lheim Bridge (1929) with a 1,033-ft span.

Another type of suspension bridge is referred to as a bridle-chord bridge. Called by Germans Zu¨gelgurtbru¨cke, these structures are typified by the bridge over the Rhine River at Ruhrort-Homberg (Fig. 15.11), erected in 1953, and the one at Krefeld-Urdingen, erected in 1950.  

It is a special class of bridge, intermediate between the suspension and cable-stayed types and having some of the characteristics of both. The main cables are curved but not continuous between towers. Each cable extends from the tower to a span, as in a cable stayed bridge. The span, however, also is suspended from the cables at relatively short intervals over the length of the cables, as in suspension bridges.

A distinction to be made between some early suspension bridges and modern suspension bridges involves the position of the main cables in profile at midspan with respect to the stiffening trusses. In early suspension bridges, the bottom of the main cables at maximum sag penetrated the top chord of the stiffening trusses and continued down to the bottom chord.

Because of the design theory available at the time, the depth of the stiffening trusses was relatively large, as much as 1⁄40 of the span. Inasmuch as the height of the pylons is determined by the sag of the cables and clearance required under the stiffening trusses, moving the midspan location of the cables from the bottom chord to the top chord increases the pylon height by the depth of the stiffening trusses.

In modern suspension bridges, stiffening trusses are much shallower than those used in earlier bridges and the increase in pylon height due to midspan location of the cables is not substantial (as compared with the effect in the Williamsburg Bridge in New York City where the depth of the stiffening trusses is 25% of the main-cable sag).

Although most suspension bridges employ vertical suspender cables to support the stiffening trusses or the deck structural framing directly, a few suspension bridges, for example, the Severn Bridge in England and the Bosporus Bridge in Turkey, have inclined or diagonal suspenders.

In the vertical-suspender system, the main cables are incapable of resisting shears resulting from external loading. Instead, the shears are resisted by the stiffening girders or by displacement of the main cables. In bridges with inclined suspenders, however, a truss action is developed, enabling the suspenders to resist shear.

(Since the cables can support loads only in tension, design of such bridges should ensure that there always is a residual tension in the suspenders; that is, the magnitude of the compression generated by live-load shears should be less than the dead-load tension.) A further advantage of the inclined suspenders is the damping properties of the system with respect to aerodynamic oscillations.