Building codes usually classify a building in accordance with the fire zone in which it is located, the type of occupancy, and the type of construction, which is an indication of the fire protection offered.

The fire zone in which a building is located may be determined from the community’s fire-district zoning map. The building code specifies the types of construction and occupancy groups permitted or prohibited in each fire zone.

The occupancy group to which a building official assigns a building depends on the use to which the building is put.

Typical classifications include one- and two-story dwellings; apartment buildings, hotels, dormitories; industrial buildings with noncombustible, combustible, or hazardous contents; schools; hospitals and nursing homes; and places of assembly, such as theaters, concert halls, auditoriums and stadiums.

Type of construction of a building is determined, in general, by the fire ratings assigned to its components. A code usually establishes two major categories: combustible and noncombustible construction.

The combustible type may be subdivided in accordance with the fire protection afforded major structural components and the rate at which they will burn; for example, heavy timber construction is considered slow-burning.

The noncombustible type may be subdivided in accordance with the fire-resistive characteristics of components.

Building codes may set allowable floor areas for fire-protection purposes. The limitations depend on occupancy group and type of construction. The purpose is to delay or prevent spread of fire over large portions of the building.

For the same reason, building codes also may restrict building height and number of stories. In addition, to permit rapid and orderly egress in emergencies, such as fire, codes limit the occupant load, or number of persons allowed in a building or room. In accordance with permitted occupant loads, codes indicate the number of exits of adequate capacity and fire protection that must be provided.


Twentieth-century architecture was influenced by a single analogy coined by the great French architect, Le Corbusier. He proposed that ‘the building is a machine for living in’. This is very far from the truth. The mistake, at its heart, is that a machine is an inanimate object that can be turned on and off and operates only at the whim of its controller.

A building is very different because, although it is true that it can be controlled by its occupants, the driving force that acts upon the building to create comfort and shelter is the climate and its weather, neither of which can be controlled, predicted or turned on and off.

Machines are fixed, static objects, amenable to scientific assessment. Buildings are part of a complex interaction between people, the buildings themselves, the climate and the environment. The view that buildings are fixed also fits well with certain types of scientific analysis, of daylight factors, energy flows, Uvalues, mechanical ventilation and so on.

But this mechanistic view finds the more dynamic parts of the system (temperature, natural ventilation, passive cooling and all the multitude of human interactions) very difficult to model and, therefore, to understand. In houses it is often these ‘difficult’ parts of the system that change a house into a home, and the building intoa delight.

Considerations of daylight, energy, thermal insulation and the use of machinery, of course, cannot be avoided – but because we can calculate them does not mean that they are our only concern. If we could see heat, as the thermal imagining camera does, we would probably treat building very differently. We would know exactly where we need to put a bit more insulation or place a sun shade, which sun shade to use or which corner of the room is cold and needs a little attention.

We have to design for the invisible as well as the visible and so how is this to be done? Buildings have been traditionally designed using accepted premises (propositions that are adopted after reasoning) as well as, of course, on premises (the building and adjuncts set forth at the beginning of a building deed). Three principles on which all building should be based are:

1 design for a climate;
2 design for the environment;
3 design for time, be it day or night, a season or the lifetime of a building and design a building that will adapt over time.

Humans have been building on these premises for millennia and have evolved house types around the world that are well suited to particular climates, environments and societies. This was done by learning from experience, and with the benefit of repetitive tools and processes that help designers and builders through the
complex range of tasks necessary to actually put a building together.

One tool of the imagination that is often used when starting a design is the analogy. An analogy is used where two forms may not look alike but they function in the same way, just as Le Corbusier described a building as a ‘machine for living in’.


Margin comprises three components: indirect costs, company-wide costs, and profit.

Determining Indirect, or Distributable, Costs
The techniques used to calculate indirect costs (often called indirects) resemble those used to calculate direct costs .

Parametric Technique. The indirects calculated by this technique may be expressed in many ways, for example, as a percentage of the direct cost of a project, as a percentage of the labor cost, or as a function of the distance to the site and the volume of the construction materials that must be moved there. For a warehouse, for instance, the cost of indirects is often taken to be either one-third the labor cost or 15% of the total cost.

Unit-Price Technique. To determine indirects by the unit-price technique, the estimator proceeds as follows: The various project activities not associated with a specific physical item are determined. Examples of such activities are project management, payroll, cleanup, waste disposal, and provision of temporary structures.

These activities are quantified in various ways: monthly rate, linear feet, cubic yards, and the like. For each of the activities, the estimator multiplies the unit price by the unit quantity to obtain activity cost. The total cost of indirects is the sum of the products.

Crew Development Technique. To determine the cost of the indirects by this technique, the estimator proceeds as follows: The various project activities not associated\ with a specific physical item are determined. Next, the estimator identifies the specific personnel needed (project manager, project engineer, payroll clerks) to perform these activities and determines their starting and ending dates and salaries.

Then, the estimator computes total personnel costs. After that, the estimator identifies the specific facilities and services needed, the length of time they are required, and the cost of each and calculates the total cost of these facilities and services. The total cost of indirects is the sum of all the preceding costs.

Determining Company-Wide Costs and Profit
Company-wide costs and profit, sometimes called gross margin, are usually lumped together for calculation purposes. Gross margin is generally a function of market conditions. Specifically, it depends on locale, state of the industry and economy, and type of discipline involved, such as mechanical, electrical, or structural.

To calculate gross margin, the estimator normally consults standard handbooks that give gross margin as a percent of project cost for various geographic areas and industries. The estimator also obtains from periodicals the market price for specific work.

Then, the information obtained from the various sources is combined. As an example, consider the case of a general contractor preparing a bid for a project in a geographic region where the company has not had recent experience.

At the time that the estimate is prepared, the contractor knows the direct and indirect costs but not the gross margin. To estimate this item, the estimator selects from handbooks published annually the gross margin, percent of total cost, for projects of the type to be constructed and for the region in which the building site is located.

Then, the estimator computes the dollar amount of the gross margin by multiplying the selected percentage by the previously calculated project cost and adds the product to that cost to obtain the total price for the project.

To validate this result, the estimator examines reports of recent bids for similar projects and compares appropriate bids with the price obtained from the use of handbooks. Then, the estimator adjusts the gross margin accordingly.


The purpose of rock mass classification is to establish the quality of a particular rock mass (or part of a rock mass) by assigning rating values to a set of rock parameters. Webster’s dictionary defines ’classification’ as “the act of classifying or forming into a class or classes, so as to bring together those beings or things which most resemble each other, and to separate those that differ”.

This definition immediately highlights two main issues in rock mass classification: the purpose of the classification has to be established and the method of classification has to be commensurate with the purpose.

For example, if we only used the uniaxial compressive strength of the intact rock and the fracture frequency of the rock mass, we could generate a rock mass classification scheme for characterizing sections of rock in a tunnel as shown in Table 12.1.

Table 12.1 Illustrative simple rock mass classification scheme
Parameter Ratings, R
Uniaxial compressive strength, a,
Fracture frequency, h Ifh54/m, R = 1 If h 4/m, R =2
If a, 3 100 MPa, R = A If a, < 100 Ma, R = B

On the basis of this scheme, all rock masses must then be one of the categories, AI, A2, B1, B2. We could call this a Rock Index and assign the words ’Good’ to AI, ’Fair’ to A2 and BI, and ’Poor’ to B2. But what is the purpose of this classification?

Perhaps, the Rock Index would indicate the excavatability and stability of the rock masses in each category. If so, is the classification the best one for that purpose?

There are four main steps in the development of any rock mass classification scheme:
1. decide on the objective of the rock mass classification scheme;
2. decide on the parameters to be used, their ranges and ratings;
3. decide on the algebra to be used for the rock index (e.g. do we simply select values from a table, do we add rating values together, do we multiply ratings together, or something else?); and
4. calibrate the rock index value against the objective.

The advantage of using a rock mass classification scheme is that it is a simple and effective way of representing rock mass quality and of encapsulating precedent practice. The disadvantage is that one cannot use it for a different objective or in significantly new circumstances.

The rock mass classifications that have been developed to date follow this basic approach, but include more parameters and use a greater number of classes than the simple 'good', 'fair', 'poor' example we gave above.

For example, by adding a third parameter to the classification given in Table 12.1, 'thickness of the layers', and using more rating values (Vervoort and de Wit, 1997'), a useful rock index for rock dredging has been developed. By judicious choice of the relevant parameters, such rock mass classification schemes can be a powerful tool for rock engineering.

The two main classification systems, Rock Mass Rating and Tunnelling Quality Index (XMR and Q), have both been widely applied and there is now a large database of projects where they have been used as the main indicator of rock stabilization requirements in rock tunnelling.

The systems provide a coherent method of using precedent practice experience and can now be linked to numerical analysis approaches.

With all schemes, the key issues are the objective of the classification system, choice of the optimal parameters, assigning numerical ratings to parameter values, the algebraic manipulation of the parameter ratings, and drawing conclusions from the mean and variation of the overall rock quality index values.


Most estimators begin their career doing quantity takeoff; as they develop experience and judgment, they develop into estimators. A list of the abilities most important to the success of an estimator follows, but it should be more than simply read through. Any weaknesses affect the estimator’s ability to produce complete and accurate estimates.

If individuals lack any of these abilities, they must (1) be able to admit it and (2) begin to acquire the abilities they lack. Those with construction experience, who are subsequently trained as estimators, are often most successful in this field.

To be able to do quantity takeoffs, the estimator must

1. Be able to read and quantify plans.

2. Have knowledge of mathematics and a keen understanding of geometry.Most measurements and computations are made in linear feet, square feet, square yards, cubic feet, and cubic yards. The quantities are usually multiplied by a unit price to calculate material costs.

3. Have the patience and ability to do careful, thorough work.

4. Be computer literate and use computer takeoff programs such as On-Screen Takeoff or Paydirt.

To be an estimator, an individual needs to go a step further. He or she must

1. Be able, from looking at the drawings, to visualize the project through its various phases of construction. In addition, an estimator must be able to foresee problems, such as the placement of equipment or material storage, then develop a solution and determine its estimated cost.

2. Have enough construction experience to possess a good knowledge of job conditions, including methods of handling materials on the job, the most economical methods of construction, and labor productivity. With this experience, the estimator will be able to visualize the construction of the project and thus get the most accurate estimate on paper.

3. Have sufficient knowledge of labor operations and productivity to thus convert them into costs on a project. The estimator must understand how much work can be accomplished under given conditions by given crafts. Experience in construction and a study of projects that have been completed are required to develop this ability.

4. Be able to keep a database of information on costs of all kinds, including those of labor, material, project
overhead, and equipment, as well as knowledge of the
availability of all the required items.

5. Be computer literate and know how to manipulate and build various databases and use spreadsheet programs and other estimating software.

6. Be able to meet bid deadlines and still remain calm. Even in the rush of last-minute phone calls and the competitive feeling that seems to electrify the atmosphere just before the bids are due, estimators must “keep their cool.”

7. Have good writing and presentation skills. With more bids being awarded to the best bid, rather than the lowest bid, being able to communicate what your company has to offer, what is included in the bid, and selling your services is very important.

It is also important to communicate to the project superintendent what is included in the bid, how the estimator planned to construct the project, and any potential pitfalls.

People cannot be taught experience and judgment, but they can be taught an acceptable method of preparing an estimate, items to include in the estimate, calculations required, and how to make them. They can also be warned against possible errors and alerted to certain problems and dangers, but the practical experience and use of good judgment required cannot be taught and must be obtained over time.

How closely the estimated cost will agree with the actual cost depends, to a large extent, on the estimators’ skill and judgment. Their skill enables them to use accurate estimating methods, while their judgment enables them to visualize the construction of the project throughout the stages of construction.


Estimating the cost of a proposed construction project is a very complex process containing many variable factors. This is not a skill that is easily acquired. Proper study, training and experience are needed to become proficient in this area of engineering.

There are several categories that can have significant impacts on project costs. The estimator should be aware of them and properly evaluate their effects, prior to finalizing the cost estimate. Refer to the following:

1) Similar Projects: The best references are similar projects. Refer to their final cost items and related expenses as a sound basis. Experience with similar projects is invaluable.

2) Material Costs: Obtain reliable costs for materials and supplies, plus shipping charges, prior to commencing tabulation.

3) Wage Rates: Determine if the project will mandate state or federal wage rates. Also, check if local wage rates are required. It is mandatory to factor this into the estimate.

4) Site Conditions: Project site conditions that can increase construction costs are: poor soil conditions, wetlands, contaminated materials, conflicting utilities (buried pipe, cables, overhead lines, etc.), environmentally sensitivity area, ground water, river or stream crossings, heavy traffic, buried storage tanks, archaeological sites, endangered species habitat and similar existing conditions.

5) Inflation Factor: The presence of inflation is always a factor that can be extremely variable. When utilizing previous, similar projects as a primary basis for estimating, consider the Construction Cost Index as published in the Engineering News Record. This nationwide tabulation of the construction industry has been continuously recorded for decades.

6) Bid Timing: The timing of the bid opening can have a significant impact on obtaining a low bid. Seasonal variations in construction activity and conflicts with other bid openings are critical factors.

7) Project Schedule: The construction schedule can certainly affect the cost. If the project requires too aggressive of a time frame, generally the price increases, especially if there is a significant liquidated damages condition for failure to complete within a specified deadline.

Conversely, if the award notice is beyond a reasonable time and the notice to proceed is indefinite, the contractors fear inflation of material costs and may have other projects that have priority. Therefore, most bidders will inflate their bids to protect against these conditions. Any time beyond 60 days may result in higher bids.

8) Quality of Plans & Specifications: There is no substitute for well-prepared plans and specifications. It is extremely important that every detail and component of the design be properly executed and fully described. Any vague wording or poorly drawn plan not only causes confusion, but places doubt in the contractor’s mind which generally results in a higher bid.

9) Reputation of Engineer: If the project engineer or engineering firm has a good sound professional reputation with contractors, it is reflected in reasonably priced bids. If a contractor is comfortable working with a particular engineer, or engineering firm, the project runs smoother and therefore is more cost-effective.

10) Granting Agency: If a granting agency is involved in funding a portion of the project, contractors will take this into consideration when preparing their bids. Some granting agencies have considerable additional paperwork that is not normally required in a non-funded project. Sometimes this expected extra paperwork elevates the bid.

11) Regulatory Requirements: Sometimes there are conditions in regulatory agency approvals that will be costly to perform. Therefore, to be completely aboveboard with potential bidders, it is strongly recommended that copies of all regulatory approvals be contained in all bidding documents.

12) Insurance Requirements: General insurance requirements, such as performance bond, payment bond and contractors general liability are normal costs of doing business. However, there are special projects that require additional coverage. Railroad crossings are a prime example. Insurance premiums for these supplemental policies add to the project cost and must be considered up front.

13) Size of Project: The size and complexity of a project determines if local contractors have the capacity to execute the work. The larger and more intricate the proposed project is, the more it will potentially attract the attention of a broader number of prospective bidders. This is good for competition, but may increase mobilization costs.

14) Locale of Work Site: The locale of the proposed work can be a significant component in developing a realistic cost estimate. A rural setting usually has a limited labor force skilled in the construction trades. Therefore, the contractor must import tradesmen and generally pay per diem expenses; i.e., out-of-town lodging and related costs. Additionally, remote settings increase the charges for material shipment.

15) Value Engineering: Some agencies mandate that multi-million dollar projects perform a value engineering review, prior to finalizing the design or commencing the bidding process. Therefore, the estimator should be aware of this factor early in the process.

16) Contingency: The rule-of-thumb has historically added a 10% contingency on the construction total to cover those unforeseen costs that crop up as a project evolves. During times of high inflation or the limited amount of key construction materials and supplies, it is wise to increase the contingency to 15% or 20% for a more realistic estimate and provide a safety factor.

17) Supplemental Studies & Investigations: As stated in Item 4, some project sites will require special studies and/or investigations. Costs for this special work should be included in the initial cost estimate to avoid future surprises.

18) Judgement: In the final analysis, the best component of a good cost estimate is the art of practicing sound technical judgement. This factor is acquired by experience and the mentoring of senior personnel.


Structural steel material conforming to one of the following ASTM specifications is approved for use under this Specification:

(1) Hot-rolled structural shapes
ASTM A529/A529M
ASTM A572/A572M
ASTM A588/A588M
ASTM A709/A709M
ASTM A913/A913M
ASTM A992/ A992M

(2) Structural tubing

(3) Pipe
ASTM A53/A53M, Gr. B

(4) Plates
ASTM A242/A242M
ASTM A283/A283M
ASTM A514/A514M
ASTM A529/A529M
ASTM A572/A572M
ASTM A588/A588M
ASTM A709/A709M
ASTM A852/A852M
ASTM A1011/A1011M

(5) Bars
ASTM A529/A529M
ASTM A572/A572M
ASTM A709/A709M

(6) Sheets


Scarcity of good building land will often necessitate building on areas of fill. A variety of materials can be found in filled sites, ranging from quarry and mining waste to household and industrial refuse. Sites filled with refuse can give rise to problems of internal combustion, methane gas and other toxic chemicals; therefore building on these should be avoided whenever possible.

If the fill is fairly shallow then the most sensible option is to use piled foundations. The augured pile described earlier is often not suitable in fill if large stones and rubble are likely to be encountered and an alternative method is to use a driven pile.

One option is to use a driven pile made up of individual hollow pre-cast concrete sections, typically 300 400mm diameter. Using a special crane the pile is driven down into the ground adding extra sections as necessary. It has reached its correct depth when repeated hammer blows only produce minimal downward movement of the pile; this is known as a ‘set’ and is specified by an engineer.

As the fill naturally consolidates over the years there may be a downward force on the piles due to the friction of the ground against the pile sides. This ‘down-drag’ is rarely even and the resulting differential movement can cause cracking of the building.

In practice it is difficult to sleeve the whole length of a pile and several manufacturers prefer to coat the pile sections with a bituminous compound during manufacture. Under slow rates of strain the bituminous compound acts as a viscous fluid and reduces the down-drag (or up-lift) on the pile.

Deeper fill is best dealt with by the use of rafts and, as explained earlier, the raft spreads the load from the walls over the whole ground floor area. Some movement is to be expected and it is therefore essential to make sure that the services which enter or leave the property have flexible connectors immediately adjacent to the external wall.

Rafts, when designed for poor-quality ground, or ground where subsidence is expected, can be very expensive and have to be designed by structural engineers. However, they are a fast form of construction with minimal excavation and are sometimes also used on soft clays as an alternative to the reinforced wide strip foundation.

The pictures show a simple raft foundation formed from 150mm reinforced concrete slab and a more complicated raft foundation with downstand beams.

Occasionally it is possible to provide some form of ground treatment and use traditional strip foundations. On very large housing sites this can be cheaper than the use of rafts or piling. There are a variety of methods which attempt to increase the stability and bearing capacity of the ground. One method, called vibro compaction, involves the use of a crane-mounted poker which is driven into the ground.

A spinning eccentric weight inside the poker causes it to vibrate and this helps to compact the surrounding ground. The poker is then slowly extracted from the ground at the same time as sand is pumped through the poker to fill the void.

The operation is then repeated at 2 or 3m intervals to form a regular grid across the site. Vibro-replacement is another ground treatment; a treatment more suitable for cohesive soils. A special poker or piling rig forms a grid of stone columns in the ground, at the same time compressing the surrounding soil and increasing its density.

The stone piles act as weak columns transferring the building loads to a firmer strata. An alternative form of ground treatment is called dynamic compaction. This sounds a very grand title but in fact just involves dropping a weight of several tonnes on to the ground from a crane. It is not suitable if there are existing buildings in the immediate vicinity.


Provided certain simple rules are followed good concrete can be achieved by methods varying from the ‘bucket and spade’ hand-labour method to use of the most sophisticated weigh-batching and mixing plant. The following shows the principal matters that should receive the resident engineer’s attention.

First, choose good aggregates. The best guide is to use well-known local aggregates that have been and are being used satisfactorily on other jobs elsewhere. A reputable supplier will be able to name many jobs where his aggregate has been used, and the resident engineer will not be over-cautious if he visits one or two of these where the concrete is exposed to view.

When the aggregates are being delivered on the job (not just the first few loads, but the loads when the supply has really got going), random loads as delivered should be examined. Handfuls of aggregate should be taken up and examined in detail, looking for small balls of clay, soft spongy stones, flaky stones, pieces of brick, soft shale, crumbly bits of sandstone, and whether clay or dirt is left on the hands after returning the handful.

If the engineer finds more than one or two pieces of weak stone, or more than a single small piece of clay from a few handfuls, he should request the contractor to bring this to the notice of the supplier. He need not reject the load out of hand, but it will do no harm to let the supplier know the aggregates are being watched.

If a load contains numerous weak stones or several pieces of clay, it should be rejected. Diagnosing whether an aggregate is likely to give rise to alkali-silica reaction (which can cause expansion and disruption of concrete in a few years in the presence of moisture) requires specialist knowledge.

The most practical approach for the engineer is to ask the supplier if his aggregate has been tested for this; if not, structures built some years previously with the aggregate should be checked for signs of cracking due to alkali-silica reaction. Guidance and precautions are set out in certain publications (References 1 and 2), but if it is proposed to use an aggregate not used before, the site staff should refer the problem to the engineer.

Second, choose tested cement. The same principle applies to cement as with the choice of aggregates; find the supplier of cement to other jobs and request a recent test certificate. Troubles can start when imported cement has to be used or cement from a variety of suppliers.

Overseas it is not unusual for a small contractor to buy his cement a few bags at a time from the local bazaar. Testing such cement on site before any concrete is placed in an important part of a structure is essential. BS 12 provides methods for testing the compressive strengths of 1:3 mortar cubes or 1:2:4 concrete cubes but, if this is difficult to arrange, the flexural test mentioned in Section 19.3 can be applied on site.

Third, ensure reasonably graded aggregates. In delivery and stockpiling of coarse aggregate there is a tendency for the mix to segregate, the larger material remaining on top. Care has to be taken to ensure that certain batches are not made up from all the coarsest material and others from most of the fines.

Crushed rock often has a considerable amount of dust in it, although this does not normally present a problem one does not want a batch made up mostly from dust and fines taken from the bottom of a stockpile.

Fourth, use washed aggregates. Unwashed aggregates suitable for concreting are rare: they are usually comprised of crushed clean homogeneous rock. Sometimes a river sand is supplied unwashed – it being assumed that the sand has already been ‘washed’ by the river.

This should not be accepted as a fact, since a river also carries silts and clays. Sea-bed or beach sands must be washed in fresh water to remove the salt from them.

Fifth, achieve the right workability. Mechanical mixers are seldom at fault with regard to mixing, and hand mixing can also be quite satisfactory; but it is the water content of a mix that requires the most vigilant attention. The site engineer should never let ‘slop’ be produced.

Although the slump test and the compacting factor test are useful in defining the degree of stiffness of a mix, in practice judging the water content of a mix ‘by eye’ is both necessary and possible.

The right sort of mix should look stiff as it comes out of the mixer or when turned over by hand on mixing boards. It should stand as a ‘heap’ and not as a ‘pool’ of concrete. When a shovel is thrust into such a pile, the shovelcut should remain open for some minutes.

Such a mix will look quite different after it is discharged and worked into some wet concrete already placed. As soon as it is worked with shovels or vibrated, it will settle and appear to flow into and become part of the previously placed concrete.

The same characteristic makes it possible to judge the water content by noticing what happens if the freshly mixed concrete is carried in a dumper hopper to the point of discharge. The ‘heap’ of stiff concrete discharged from the mixer to the dumper hopper will appear to change to a pool of concrete as the dumper bumps its way round the usual site roads.

When the dumper hopper is tipped, however, the concrete discharged should again appear stiff. But if, in transport, the concrete slops as a semi-fluid over the side of the dumper hopper, this shows too much water has been added.

A simple density test on freshly mixed concrete may assist in finding if the mix has too much water.

Sixth, ram the concrete well in place. Properly shovelled, rodded, or vibrated, the concrete should be seen to fill the corners of shuttering and to easily wrap around the reinforcing bars. When hand shovelling or rodding is adopted, it is scarcely possible to over-compact the concrete.

But when mechanical vibrators are used the vibration should not be so prolonged as to produce a watery mix on the surface. Vibrators of the poker immersion type should be kept moving slowly in and out of the concrete.

They should not be withdrawn quickly or they may leave an unfilled hole in the concrete; nor should they be left vibrating continuously in one location. Where vibrators are used, it is necessary for the contractor also to have available suitable hand rammers in case the vibrators break down in the middle of a pour.

Seventh, ensure the mix has sufficient cement in it. Normally contractors will use a little more cement than is theoretically necessary and this is helpful since batches of concrete vary.

But if a contractor becomes too keen on cutting the cement to the bare minimum, a number of the cube crushing tests may fail to reach the required strength, and much delay may be caused by conducting the investigations required to seek out the cause.


Studies of earthquake effects on cranes are few, and code development in this area is in its infancy. Generally, permanent installations such as bridge cranes and port cranes can be subjected to seismic analysis using the same principles as those used for other fixed structures.

A decision to analyze a crane seismically should be based on the degree of risk as weighed against potential consequences of a loss. Risk may be assessed by study of earthquake maps. In areas of low or moderate earthquake risk, seismic study may be demanded only for the most-sensitive applications, such as nuclear work.

In adopting a philosophy for earthquake resistance, the crane analyst or designer might consider one or more of three risk mitigation levels, or limit states.

1. The earthquake design does not cause structural damage to the crane. All stresses remain in the elastic range. The crane should remain serviceable.

2. The design earthquake may result in some damage that could be readily repaired and the crane restored. Failure may occur in components that are not part of the main force-resisting system. Component failures cannot put workers or the public at risk, and significant collateral damage to surroundings is not permissible.

3. Controlled ductile yielding may result in the complete functional loss of the crane, which would be replaced, but the avoidance of a catastrophic failure leaves the public, workers, and surroundings protected.

A designer might choose to calibrate the design to only one of these states or, alternatively, consider associating each of them with a different magnitude earthquake event. As all three imply a high level of life protection, this decision would be based only on economic considerations.

Except for those few industrial cranes that are in near-constant use, probability favors the premise that the device will be unloaded if an earthquake should occur. Nearly all cranes used in both general industry and construction will have a substantial load on the hook only a small percentage of the calendar year.

With few exceptions, then, earthquakes might reasonably be evaluated only for out-of-service consideration. Though earthquakes are dynamic events, the simplest methods of seismic analysis make use of equivalent static loads.

These methodsare suited to areas of low or moderate seismicity or for structures that are relatively simple in their response to excitation. Other methods in the toolboxes of seismic engineers may be applied for more complex situations or where the level of risk warrants the investment.

A freestanding tower crane may respond well to moderate earthquakes because its long period of oscillation will not resonate with the higher-frequency ground motion. However, the crane could be at risk in a severe earthquake due to base shear or from vertical acceleration acting on the counterweights.

In some soils, liquefaction could pose a risk. On a tower crane that is secured to a building, tied to the outside or mounted within, the interaction with the building can lead to higher seismic loads compared to those expected for standard freestanding erections.

Generalized assessment of earthquake risks for a mobile crane can be difficult because a typical machine changes location frequently and its boom disposition changes constantly. Overall exposure should not be great. There could be vulnerability under certain conditions, however.

For example, ground acceleration might induce an unloaded machine with a short boom at a high angle to tip backward.

Perhaps the most studied seismic event with respect to cranes was the Kobe, Japan, earthquake of 1995. Documented failures included

• Total collapse due to foundation failures probably caused by liquifaction
• Overstress of towers from horizontal shear, with resulting diagonal and connection failures
• Leg tension failures due to overturning moment on pillar cranes and tower cranes
• A bridge crane girder lifting off its supports
• Overturning of a rail-mounted gantry crane

In 2002, two tower cranes toppled from the 60th floor of a steelframe building under construction in Taipei, Taiwan, during a moderately severe earthquake. The failures were not caused directly by the earthquake, but rather by the cranes oscillating in resonance with the building.


The ACI 318 Building Code contains the following restrictions:

1. All bars must be bent without heating, except as permitted by the engineer.

2. Bars partly embedded in hardened concrete may not be bent without permission of the engineer.

3. No welding of crossing bars (tack welding) is permitted without the approval of the engineer.

4. For unusual bends, heating may be permitted because bars bend more easily when heated.

If not embedded in thin sections of concrete, heating the bars to a maximum temperature of 1500 F facilitates bending, usually without damage to the bars or splitting of the concrete.

If partly embedded bars are to be bent, heating controlled within these limits, plus the provision of a round fulcrum for the bend to avoid a sharp kink in the bar, are essential.

Tack welding creates a metallurgical notch effect, seriously weakening the bars. If different size bars are tacked together, the notch effect is aggravated in the larger bar.

Tack welding therefore should never be permitted at a point where bars are to be fully stressed, and never for the assembly of ties or spirals to column verticals or stirrups to main beam bars.

When large, preassembled reinforcement units are desired, the engineer can plan the tack welding necessary as a supplement to wire ties at points of low stress or to added bars not required in the design.


In general mechanical properties of structural materials improve with increasing rate of load application. For low-carbon steel, for example, yield strength, ultimate strength, and ductility rise with increasing rate of strain.

Modulus of elasticity in the elastic range, however, is unchanged. For concrete, the dynamic ultimate strength in compression may be much greater than the static strength.

Since the improvement depends on the material and the rate of strain, values to use in dynamic analysis and design should be determined by tests approximating the loading conditions anticipated.

Under many repetitions of loading, though, a member or connection between members may fail because of ‘‘fatigue’’ at a stress smaller than the yield point of the material. In general, there is little apparent deformation at the start of a fatigue failure.

A crack forms at a point of high stress concentration. As the stress is repeated, the crack slowly spreads, until the member ruptures without measurable yielding. Though the material may be ductile, the fracture looks brittle.

Some materials (generally those with a well-defined yield point) have what is known as an endurance limit. This is the maximum unit stress that can be repeated, through a definite range, an indefinite number of times without causing structural damage.

Generally, when no range is specified, the endurance limit is intended for a cycle in which the stress is varied between tension and compression stresses of equal value.

A range of stress may be resolved into two components—a steady, or mean, stress and an alternating stress. The endurance limit sometimes is defined as the maximum value of the alternating stress that can be superimposed on the steady stress an indefinitely large number of times without causing fracture.

Design of members to resist repeated loading cannot be executed with the certainty with which members can be designed to resist static loading. Stress concentrations may be present for a wide variety of reasons, and it is not practicable to calculate their intensities.

But sometimes it is possible to improve the fatigue strength of a material or to reduce the magnitude of a stress concentration below the minimum value that will cause fatigue failure.

In general, avoid design details that cause severe stress concentrations or poor stress distribution. Provide gradual changes in section. Eliminate sharp corners and notches.

Do not use details that create high localized constraint. Locate unavoidable stress raisers at points where fatigue conditions are the least severe. Place connections at points where stress is low and fatigue conditions are not severe.

Provide structures with multiple load paths or redundant members, so that a fatigue crack in any one of the several primary members is not likely to cause collapse of the entire structure.

Fatigue strength of a material may be improved by cold-working the material in the region of stress concentration, by thermal processes, or by prestressing it in such a way as to introduce favorable internal stresses.

Where fatigue stresses are unusually severe, special materials may have to be selected with high energy absorption and notch toughness.


Escalators, or powered stairs, are used when it is necessary to move large numbers of people from floor to floor. They provide continuous movement of persons and can thus remedy traffic conditions that are not readily addressed by elevators.

Escalators should be viewed as preferred transportation systems whenever heavy traffic volumes are expected between relatively few floors. Escalators are used to connect airport terminals, parking garages, sports facilities, shopping malls, and numerous mixed-use facilities.

Although escalators generally are used in straight sections, spiral escalators also are available. Although expensive due to manufacturing complexities, they offer distinct advantages to both the designer and user because of their unique semicircular plan form.

An escalator resembles a powered ramp in construction. The major difference is that a powered ramp has a continuous treadway for carrying passengers, whereas the treadway of an escalator consists of a series of moving steps.

As for a powered ramp, the installation of powered stairs should conform with the requirements of the ‘‘American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks,’’ ANSI A17.1.


An escalator consists of articulated, grooved treads and risers attached to a continuous chain moved by a driving machine and supported by a steel truss framework.

The installation also includes a handrail on each side of the steps that moves at the same speed as the steps; balustrades, or guards, that enclose the steps on each side and support the handrails; brakes; control devices; and threshold plates at the entrance to and the exit from the treadway.

The purpose of the threshold plates is to facilitate smooth passage of passengers between the treadway and landing.

The plates are equipped with a comb, or teeth, that mesh with and are set into grooves in the treadway in the direction of travel, so as to provide firm footing and to minimize the chance that items become trapped between treadway and the landing.

Each step is formed by a grooved tread portion connected to a curved and grooved riser. The tread and riser assembly is either a single die-cast piece or is assembled to a frame.

Both are suspended on resilient rollers whose axles are connected to the step chain that moves the steps. The step rollers ride on a set of tracks attached to the trussed framework. The tracks are shaped to allow the step tread to remain horizontal throughout its exposed travel.


All estimates should be reviewed by all responsible parties at every stage. An estimate review should begin with a survey of the verbal description of the work, including all or most of the following: scope statement, assumptions, clarifications, qualifications, and exclusions.

As an example, the estimate is to be reviewed for a warehouse to be built in an urban area as part of a redevelopment project. The scope statement should specify the location, refer to design drawings and specifications, and list applicable building codes.

The assumptions might include such data as the number of persons who will work in the warehouse. This is an indication of the number of restrooms and fixtures needed, which can be listed as a clarification. If the price quotes are valid for 90 days, this should be stated as a qualification.

Handling and disposing of any existing hazardous material found on the site might be listed as an exclusion. This warehouse description may be reasonably complete from the viewpoint of designer and contractor and may be accurately priced.

But because of the assumption regarding the number of occupants, it may not be suitable from the viewpoint of the intended users. The exclusion regarding hazardous materials may result in unacceptable financial exposure for the client.

Issues such as these need to be addressed. The client may decide that the prospective tenants, or users, may employ more persons than the number assumed. Hence, either the estimate will have to allocate more money for rest rooms or the client will have to give the tenants an allowance to enable them to build the rest rooms they desire.

The client may also decide that an analysis of soil samples may be necessary before any construction is done to determine the extent of contamination, if any, and cost of cleanup.

Bearing these issues in mind, the parties should now review the quantitative part of the estimate. This review should comprise the following:

A summary of the key quantities involved; for example, floor area, tons of steel, cubic yards of concrete.

As a cross check, a list the key quantities—by discipline if the estimate has been prepared with the industry approach or by industry if the estimate has been prepared by the discipline approach.

A summary of the project, by industry or discipline.

At each step of the review, changes may be made, as required. After all parties agree to all parts of the estimate, it can be considered final.

At this stage, the designer should be satisfied that enough money has been allocated to carry out the project. The client should have a clear idea as to what the project will entail and how much it will cost.