AUTOMATED MATERIAL IDENTIFICATION SYSTEMS BASIC AND TUTORIALS

When construction materials arrive at CIC job sites, they are identified at the unloading area, and the job site inventory database in the central computer is updated. CIC requires tight control on inventory and integrated operation of automated equipment.

Further, all construction materials must be tracked from the time of their arrival at the job site to their final position in the finished facility. Such tracking of construction materials may be done by employing automated identification systems.

There are two means of tracking construction materials: direct and indirect. Direct tracking involves identifying a construction material by a unique code on its surface. This method of tracking can be employed with the use of large prefabricated components.

Indirect tracking involves identifying construction material by a unique code on the material handling equipment. This method of tracking can be employed for tracking bulk materials such as paints [Rembold et al., 1985]. Select automatic identification systems for construction materials are described below.

Bar Coding
The U.S. Department of Defense (DOD) was the first organization to implement bar coding technology. The Joint Steering Group for Logistics Applications of Automated Marking and Reading Symbols (LOGMARS) spearheaded the DOD’s effort in the implementation of bar coding technology. The symbology of bar codes conveys information through the placement of wide or narrow dark bars that create narrow or wide white bars.

With the rise of the LOGMARS project, code 39 (also called “3 of 9” coding) has become a standard for bar coding. To date, most construction bar code applications have used the code 39 symbology [Teicholz and Orr, 1987; Bell and McCullough, 1988].

Laser beams and magnetic foil code readers are two basic technologies available for reading bar codes. Lasers offer the ability to read bar codes that move rapidly. Magnetic code readers are among the most reliable identification systems. It is possible to transmit the code without direct contact between the code reader and the write head on the code carrier. When the workpiece passes the read head, the code is identified by the code reader [Teicholz and Orr, 1987; Rembold et al., 1985].

Voice Recognition
Voice recognition provides computers the capability of recognizing spoken words, translating them into character strings, and sending these strings to the central processing unit (CPU) of a computer. The objective of voice recognition is to obtain an input pattern of voice waveforms and classify it as one of a set of words, phrases, or sentences.

This requires two steps: (1) analyze the voice signal to extract certain features and characteristics sequentially in time and (2) compare the sequence of features with the machine knowledge of a voice, and apply a decision rule to arrive at a transcription of the spoken command [Stukhart and Berry, 1992].

Vision Systems
A vision system takes a two-dimensional picture by either the vector or the matrix method. The picture is divided into individual grid elements called pixels. From the varying gray levels of these pixels, the binary information needed for determining the picture parameters is extracted. This information allows the system, in essence, to see and recognize objects.

The vector method is the only method that yields a high picture resolution with currently available cameras. The vector method involves taking picture vectors of the scanned object and storing them at constant time intervals. After the entire cycle is completed, a preprocessor evaluates the recomposed picture information and extracts the parameters of interest [Rembold et al., 1985].

CAD APPLICATIONS IN CIVIL ENGINEERING BASIC AND TUTORIALS


AutoCAD
AutoCAD is the most widely used CAD software in civil engineering applications. In an effort toward computer-integrated construction (CIC), researchers have developed a link between AutoCAD and a knowledge-based planning program [Cherneff et al., 1991].

CATIA
CATIA is a three-dimensional solid modeling software marketed by IBM Corporation. Stone & Webster Engineering Corporation, in cooperation with IBM, developed an integrated database for engineering, design, construction, and facilities management. The system uses the DB2 relational database management system and the CATIA computer-aided-design software system [Reinschmidt et al., 1991].

Walkthrough™
Bechtel Corporation developed a three-dimensional simulation system called Walkthrough to aid in marketing, planning, and scheduling of construction projects. Walkthrough was developed to replace the use of plastic models as a design tool [Cleveland and Francisco, 1988].

It was designed to allow users to interact with a three-dimensional computer model as they would with a plastic model. The system uses three-dimensional, real-time animation that lets the user visually move through the computer model and observe visual objects.

Graphics of the system are presented such that objects are recognizable to  users not accustomed to typical CAD images. This includes the use of multiple colors and shading.

Walkthrough uses a Silicon Graphics IRIS workstation with specialized processors facilitating the high speed graphics required for real-time animation. This visualization and simulation system supports files from IGDS (Intergraph CAD system) and 3DM [Morad et al., 1992].

Object-Oriented CAD Model
An object-oriented CAD model for the design of concrete structures that uses EUROCODE2, a European standard for concrete structures, has been developed by German researchers. The primitive instancing solid-modeling technique was employed in the development of this object-oriented model [Reymendt and Worner, 1993].

A committee, entitled “NEW TECCMAR,” formed under the Japanese construction ministry, developed a three-dimensional finite-element method (FEM) program with an extended graphical interface to analyze general buildings [Horning and Kinura, 1993].

FIXED CONSTRUCTION AUTOMATION EXAMPLE BASIC INFORMATION


Fixed construction automation is useful in mass production or prefabrication of building components such as:

1. Reinforcing steel
2. Structural steel
3. Exterior building components (e.g., masonry, granite stone, precast concrete)

Automated Rebar Prefabrication System
The automated rebar prefabrication system places reinforcing bars for concrete slab construction. The system consists of a NEC PC98000XL high-resolution-mode personal computer that uses AutoCAD™ DBASE III Plus™, and BASIC™ software.

The information regarding number, spacing, grade and dimension, and bending shapes of rebars is found from the database generated from an AutoCAD file. This information is used by an automatic assembly system to fabricate the rebar units.

The assembly system consists of two vehicles and a steel rebar arrangement support base. Of the two vehicles, one moves in the longitudinal direction and the other in the transverse direction. The longitudinally moving vehicle carries the rebars forward until it reaches the preset position.

Then, it moves backward and places the rebars one by one at preset intervals on the support base. Upon completion of placement of the rebars by the longitudinally moving vehicle, the transversely moving vehicle places the rebars in a similar manner. The mesh unit formed by such a placement of rebars is tied together automatically [Miyatake and Kangari, 1993].

Automated Brick Masonry
The automated brick masonry system, is designed to spread mortar and place bricks for masonry wall construction. The system consists of:

1. Mortar-spreading module
2. Brick-laying station

The controls of the system are centered around three personal computers responsible for:

1. Collecting and storing date in real time
2. Interfacing a stepping-motor controller and a robot controller
3. Controlling the mortar-spreading robot

A Lord 15/50 force-torque sensor is used to determine the placing force of each brick. The system is provided with an integrated control structure that includes a conveyor for handling the masonry bricks [Bernold et al., 1992].

Fully Automated Masonry Plant
The fully automated masonry plant is designed to produce different brick types with the production capacity of 300 m2 wall elements per shift. The system consists of several components: a master computer, a database server, a file server, stone cutters, masonry robots, pallet rotation systems, refinement systems, storage systems, transversal platforms, a disposition management system, an inventory management system, and a CAD system.

Two individual brick types can be managed in parallel by unloading the gripper and the cutter-system consisting of two stone saws. By conveyer systems, stone units and fitting stones are transported to the masonry robot system.

The masonry robots move two bricks at each cycle to the growing wall after a mortar robot puts a layer of mortar on it. A pallet rotation system carries the wall to the drying chamber. After 48 hours, the wall is transported to destacking stations to group the wall elements of the same order. Finally, grouped wall elements are transported to the construction site [Hanser, 1999].

Automated Stone Cutting
The purpose of the automated stone-cutting facility is to precut stone elements for exterior wall facings. The facility consists of the following subsystems:

1. Raw materials storage
2. Loading
3. Primary workstation
4. Detail workstation
5. Inspection station
6. End-product inventory

A special lifting device has been provided for automated materials handling. The boom’s rigidity enables the computation of exact location and orientation of the hook. Designs for the pallets, the primary saw table, the vacuum lift assembly, and the detail workstation have also been proposed [Bernold et al., 1992].

ALLOWABLE STRESSES FOR LUMBER BASIC INFORMATION AND TUTORIALS


The National Design Specification for Wood Construction (NDS) (AF&PA, 1997) makes comprehensive recommendations for engineered uses of stress-graded lumber. Stress values for all commercially available species groups and grades of lumber produced in the U.S. are tabulated in the NDS.

The moduli of elasticity for all species groups and grades are also included in these tables. These tabulated values of stresses and moduli of elasticity are called base design values. They are modified by applying adjustment factors to give allowable stresses for the graded lumber.

The adjustment factors reduce (or in some cases increase) the base design stress values to account for specific conditions of use that affect the behavior of the lumber. A list of these adjustment factors and a discussion of their use follows.

Load Duration — CD
The stress level that wood will safely sustain is inversely proportional to the duration that the stress is applied. That is, stress applied for a very short time (e.g., an impact load) can have a higher value than stress applied for a longer duration and still be safely carried by a wood member. This characteristic of wood is accounted for in determining allowable stresses by using a load duration factor, CD.

The load duration factor varies from 20 for an impact load (duration equal to one second) to 0.9 for a permanent load (duration longer than 10 years). ACI Committee 347 recommends that for concrete formwork, a load duration factor appropriate for a load of 7 days should be used. This corresponds to a value for CD of 1.25.

ACI Committee 347 says this load duration factor should only be applied to concrete forms intended for limited reuse. No precise definition of limited reuse is given by the ACI committee, but the no increase for duration of load should be used for concrete forms designed to be reused a high number of cycles.

Moisture — CM
Wood is affected by moisture content higher than about 19%. Higher moisture content significantly softens the wood fibers and makes it less stiff and less able to carry stresses. The reduction in allowable strength depends on the type of stress (e.g., shear stress is affected less than perpendicular to grain compressive stress) and the grade of the lumber.

Size — CF
Research on lumber allowable stresses has shown that as cross-sectional size increases, allowable stresses are reduced. A size factor, CF, is used to increase base design values for different sizes of lumber.

Repetitive Members — Cr
The NDS allows bending stresses to be increased for beams that share their loads with other beams. The increased allowable stress is referred to as a repetitive member stress. For a beam to qualify as a repetitive member, it must be one of at least three members spaced no further apart than two feet and joined by a load-distributing element such as plywood sheathing.

When these three requirements are met, the allowable bending stress can be increased by 15%. This corresponds to a value for Cr of 1.15. Repetitive member stresses may be appropriate for some formwork components. Because the intent of allowing increased stress for repetitive flexural members is to take advantage of the load sharing provided by continuity, gang panels assembled securely by bolting or nailing and intended for multiple reuse would seem to qualify for this increase.

ACI Committee 347 specifies that they should not be used where the bending stresses have already been increased by 25% for short duration loads.

Perpendicular to Grain Compression — Cb
Allowable perpendicular to grain bearing stress at the ends of a beam may be adjusted for length of bearing according to: lb is the length of bearing parallel to grain.

Horizontal Shear Constant — CH
Shear stress in lumber beams used as components of concrete forms is usually highest at the ends of the members. For beams having limited end defects (e.g., splits, checks, cracks), the values of allowable shear stress can be increased. This is done by using a shear constant CH that depends on the size of end defects and varies from 1 to 2.

Temperature — CT
Sustained high temperatures adversely affect some properties of wood. It is unusual for concrete forms to be exposed to temperatures high enough to require the use of a temperature adjustment factor. For temperatures in excess of 100°F, the stresses and moduli should be adjusted using CT.

Stablity — CP
Like all columns, wood shores will safely carry axial loads in inverse proportion to their effective slenderness. The more slender a wood shore is, the less load it will support because of the increased influence of buckling. Prior to the 1997 edition of the NDS, wood columns were divided into three categories (short, intermediate, and long) according to their slenderness.

Allowable stresses and loads were then found using three different formulas — one for each category. Beginning with the 1997 NDS, allowable loads for all wood columns are found using a stability adjustment factor, CP, that reduces the base stress to account for the buckling tendency of the column. It is no longer necessary to divide wood shores into three categories to find allowable loads.

GOALS OF PROJECT MANAGEMENT BASIC INFORMATION

Regardless of the project, most construction teams have the same performance goals:

Cost — Complete the project within the cost budget, including the budgeted costs of all change orders.

Time — Complete the project by the scheduled completion date or within the allowance for work days.

Quality — Perform all work on the project, meeting or exceeding the project plans and specifications.

Safety— Complete the project with zero lost-time accidents.

Conflict— Resolve disputes at the lowest practical level and have zero disputes.

Project startup— Successfully start up the completed project (by the owner) with zero rework.

Basic Functions of Construction Engineering
The activities involved in the construction engineering for projects include the following basic functions:

Cost engineering —The cost estimating, cost accounting, and cost-control activities related to a
project, plus the development of cost databases.

Project planning and scheduling —The development of initial project plans and schedules, project monitoring and updating, and the development of as-built project schedules.

Equipment planning and management — The selection of needed equipment for projects, productivity planning to accomplish the project with the selected equipment in the required project schedule and estimate, and the management of the equipment fleet.

Design of temporary structures — The design of temporary structures required for the construction of the project, such as concrete formwork, scaffolding, shoring, and bracing.

Contract management — The management of the activities of the project to comply with contract provisions and document contract changes and to minimize contract disputes.

Human resource management — The selection, training, and supervision of the personnel needed to complete the project work within schedule.

Project safety — The establishment of safe working practices and conditions for the project, the communication of these safety requirements to all project personnel, the maintenance of safety records, and the enforcement of these requirements.

CONSTRUCTION CONTRACT BASIC DEFINITION AND TUTORIALS

Construction projects are done under a variety of contract arrangements for each of the parties involved. They range from a single contract for a single element of the project to a single contract for the whole project, including the financing, design, construction, and operation of the facility. Typical contract types include lump sum, unit price, cost plus, and construction management.

These contract systems can be used with either the competitive bidding process or with negotiated processes. A contract system becoming more popular with owners is design-build, in which all of the responsibilities can be placed with one party for the owner to deal with.

Each type of contract impacts the roles and responsibilities of each of the parties on a project. It also impacts the management functions to be carried out by the contractor on the project, especially the cost engineering function.

A major development in business relationships in the construction industry is partnering. Partnering is an approach to conducting business that confronts the economic and technological challenges in industry in the 21st century.

This new approach focuses on making long-term commitments with mutual goals for all parties involved to achieve mutual success. It requires changing traditional relationships to a shared culture without regard to normal organizational boundaries.

Participants seek to avoid the adversarial problems typical for many business ventures. Most of all, a relationship must be based upon trust. Although partnering in its pure form relates to a long-term business relationship for multiple projects, many single project partnering relationships have been developed, primarily for public owner projects.

Partnering is an excellent vehicle to attain improved quality on construction projects and to avoid serious conflicts. Partnering is not to be construed as a legal partnership with the associated joint liability. Great care should be taken to make this point clear to all parties involved in a partnering relationship.

Partnering is not a quick fix or panacea to be applied to all relationships. It requires total commitment, proper conditions, and the right chemistry between organizations for it to thrive and prosper.

The relationship is based upon trust, dedication to common goals, and an understanding of each other’s individual expectations and values. The partnering concept is intended to accentuate the strength of each partner and will be unable to overcome fundamental company weaknesses; in fact, weaknesses may be
magnified.

Expected benefits include improved efficiency and cost effectiveness, increased opportunity for innovation, and the continuous improvement of quality products and services. It can be used by either large or small businesses, and it can be used for either large or small projects.

Relationships can develop among all participants in construction: owner-contractor, owner-supplier, contractor-supplier, contractor-contractor. (Contractor refers to either a design firm or a construction company.)

QUALITY INSPECTION ON CIVIL CONSTRUCTION PROJECT BASIC AND TUTORIALS

Prior to the Industrial Revolution, items were produced by an individual craftsman, who was responsible for material procurement, production, inspection, and sales. In case any quality problems arose, the customer would take up issues directly with the producer.

The Industrial Revolution provided the climate for continuous quality improvement. In the late 19th century, Fredrick Taylor’s system of Scientific Management was born. It provided the backup for the early development of quality management through inspection.

At the time when goods were produced individually by craftsmen, they inspected their own work at every stage of production and discarded faulty items. When production increased with the development of technology, scientific management was born out of a need for standardization rather than craftsmanship.

This approach required each job to be broken down into its component tasks. Individual workers were trained to carry out these limited tasks, making craftsmen redundant in many areas of production. The craftsmen’s tasks were divided among many workers.

This also resulted in mass production at lower cost, and the concept of standardization started resulting in interchangeability of similar types of bits and pieces of product assemblies. One result of this was a power shift away from workers and toward management.

With this change in the method of production, inspection of the finished product became the norm rather than inspection at every stage. This resulted in wastage because defective goods were not detected early enough in the production process.

Wastage added costs that were reflected either in the price paid by the consumer or in reduced profits. Due to the competitive nature of the market, there was pressure on manufacturers to reduce the price for consumers, which in turn required cheaper input prices and lower production costs.

In many industries, emphasis was placed on automation to try to reduce the costly mistakes generated by workers. Automation led to greater standardization, with many designs incorporating interchanges of parts. The production of arms for the 1914–1918 war accelerated this process.

An inspection is a specific examination, testing, and formal evaluation exercise and overall appraisal of a process, product, or service to ascertain if it conforms to established requirements. It involves measurements, tests, and gauges applied to certain characteristics in regard to an object or an activity.

The results are usually compared to specified requirements and standards for determining whether the item or activity is in line with the target. Inspections are usually nondestructive. Some of the nondestructive methods of inspection are

• Visual
• Liquid dyed penetrant
• Magnetic particle
• Radiography
• Ultrasonic
• Eddy current
• Acoustic emission
• Thermography

The degree to which inspection can be successful is limited by the established requirements. Inspection accuracy depends on

1. Level of human error
2. Accuracy of the instruments
3. Completeness of the inspection planning

Human errors in inspection are mainly due to

• Technique errors
• Inadvertent errors
• Conscious errors
• Communication errors

Most construction projects specify that all the contracted works are subject to inspection by the owner/consultant/owner’s representative.

CONSTRUCTION PROJECT QUALITY DEFINITION

The definition of quality for construction projects is different from that of manufacturing or services industries as the product is not repetitive but a unique piece of work with specific requirements. Quality in construction projects is not only the quality of product and equipment used in the construction of a facility but the total management approach to complete the facility.

The quality of construction depends mainly upon the control of construction, which is the primary responsibility of the contractor.

Quality in manufacturing is spread over a series of processes. Material and labor are input into these processes out of which a product is obtained. The output is monitored by inspection and testing at various stages of production.

Any nonconforming product is identified as repaired, reworked, or scrapped, and proper steps are taken to eliminate problem causes. Statistical process control methods are used to reduce the variability and increase the efficiency of the process.
However, in construction projects, the scenario is not the same. If anything goes wrong, the nonconforming work is very difficult to rectify, and remedial action is sometimes not possible.

The authors of Quality in the Constructed Project (2000) by the American Society of Civil Engineers (ASCE) have defined quality as the fulfillment of project responsibilities in the delivery of products and services in a manner that meets or exceeds the stated requirements and expectations of the owner, design professional, and constructor.

Responsibilities refer to the tasks that a participant is expected to perform to accomplish the project activities as specified by contractual agreement and applicable laws and licensing requirements, codes, prevailing industry standards, and regulatory guidelines. Requirements are what a team member expects or needs to receive during and after his or her participation in a project. (p. xv) Chung (1999) states, “Quality may mean different things to different people.

Some take it to represent customer satisfaction, others interpret it as compliance with contractual requirements, yet others equate it to attainment of prescribed standards” (p. 3). As regards quality of construction, he furtherstates, “Quality of construction is even more difficult to define.

First of all, the product is usually not a repetitive unit but a unique piece of work with specific characteristics. Secondly, the needs to be satisfied include not only those of the client but also the expectations of the community into which the completed building will integrate.

The construction cost and time of delivery are also important characteristics of quality” (p. 3). Based on the foregoing, the quality of construction projects can be defined as follows:

Construction project quality is the fulfillment of the owner’s needs per defined scope of works within a budget and specified schedule to satisfy the owner’s/user’s requirements.

STEEL JOIST FLOORS BASIC INFORMATION AND TUTORIALS

The lightest floor system in common use is the open-web steel joist construction. It is popular for all types of light occupancies, principally because of initial low cost.

Many types of open-web joists are available. Some employ bars in their makeup, while others are entirely of rolled shapes; they all conform to standards and good practice specifications promulgated by the Steel Joist Institute and the American Institute of Steel Construction.

All joists conform to the standard loading tables and carry the same size designation so that designers need only indicate on project drawings the standard marking without reference to manufacturer, just as for a steel beam or column section.

Satisfactory joists construction is assured by adhering to SJI and AISC recommendations. Joists generally are spaced 2 ft c to c. They should be adequately braced (with bridging) during construction to prevent rotation or buckling, and to avoid ‘‘springy’’ floors, they should be carefully selected to provide sufficient depth.

This system has many advantages: Falsework is eliminated. Joists are easily handled, erected, and connected to supporting beams—usually by tack welding.

Temporary coverage and working platforms are quickly placed. The open space between joists, and through the webs, may be utilized for ducts, cables, light fixtures, and piping. A thin floor slab may be cast on steel lath, corrugated-steel sheets, or wire-reinforced paper lath laid on top of the joists. A plaster ceiling may be suspended or attached directly to the bottom flange of the joists.

Lightweight beams, or so-called ‘‘junior’’ beams, are also used in the same manner as open-web joists, and with the same advantages and economy, except that the solid webs do not allow as much freedom in installation of utilities.

Beams may be spaced according to their safe load capacity; 3- and 4-ft spacings are common. As a type, therefore, the lightweight-steel-beam floor is intermediate between concrete arches and open-web joists.

STRUCTURAL STEEL FABRICATION BASIC INFORMATION

When considering fabrication, as well as erection of the fabricated product, the designer must taken into account contractual matters, work by others on the construction team, schedule implications of the design, and quality assurance matters.

Fortunately, there are well established aids for these considerations. Contractual questions such as what constitutes structural steel, procedures for preparing and approving the shop detail drawings, and standard fabrication procedures and tolerances are all addressed in the AISC’s Code of Standard Practice.

Insights on economical connection details and the impact of material selection on mill material deliveries are generally available from the fabricator’s engineering staff. These engineers are also able to comment on unique erection questions.

Quality assurance questions fall into two categories, fabrication operations and field operations. Today, sound quality control procedures are in place in most fabrication shops through an AISC program which prequalifies fabricators.

There are three levels of qualification: I, II and III, with Level III being the most demanding. Fabricators with either a Level I or Level II certification are suitable for almost all building work. Most engineers incorporate the AISC’s Code of Standard Practice in their project specification.

Shop Detail Drawings
Detail drawings are prepared by the fabricator to delineate to his work force the fabrication requirements. Because each shop has certain differences in equipment and/or procedures, the fabricator develops details which, when matched with his processes, are the most economical.

To accomplish this end, the design drawings need to be complete, showing all structural steel requirements, and should include design information on the forces acting at connections. Designers should avoid specifying deck openings and beam penetrations through notes on the drawings. This is a frequent cause of extra costs on fabrication contracts.

Fabrication Processes
Mill material is cut to length by sawing, shearing, or flame cutting. Columns may also be milled to their final length. Holes for fasteners are drilled or punched. Punched and reamed holes are seldom used in building construction. Cuts for weld preparation, web openings, and dimensional clearances are flame cut.

AISC guidelines for each of these processes are associated with the AISC’s fabricator prequalification program. Welding for building construction is performed in accordance with the provisions of the AWS Structural Welding Code, D1.1. Most requirements can be satisfied using pre-qualified welding procedures.

PRECAST AND PRESTRESSED DECK SLABS BASIC AND TUTORIALS


The deck is usually the first element in a bridge to deteriorate and to require funds for rehabilitation. In situations where traffic volumes are high, it is often necessary to rehabilitate or replace the deck in sections during off-peak periods.

Because of the time required for site-cast concrete to cure, a number of replacement strategies have been developed using prefabricated deck slabs (Issa et al., 1995a,b). Most of the systems involve a transverse segment (Figure 33.11) connected to the supporting beams with a rapid-curing polymer or hydraulic cement concrete.


FIGURE 33.11 Prestressed deck slabs. (From Sprinkel, M.M., Prefabricated Bridge Elements and Systems, NCHRP Synthesis 119, Transportation Research Board, Washington, D.C., 1985.)


Shear transfer between adjacent slabs is achieved through the use of grouted keyways, site-cast concrete, and post-tensioning. Composite action is achieved through the use of studs on steel beams that extend into voided areas in the slabs that are then filled with polymer or hydraulic cement concrete.

Precast deck slabs can behave in a full-composite manner when connected to steel stringers with studs and epoxy mortar and when keyways are grouted with epoxy mortar (Osegueda et al., 1989).

An earlier study identified some suitable connection details and concluded that the deck slabs are more economical than site cast concrete because of the structural efficiency provided by post-tensioning and prestressing and because of the reduced construction time (Berger, 1983).

Improved connection details for the use of panels on steel beams and prestressed concrete beams have been developed (Tadros and Baishya, 1998).

More recently, a special loop bar reinforcement detail has been developed to provide live load distribution across transverse and longitudinal joints (see FHWA, 2004). A new full-depth precast prestressed concrete bridge deck slab system has been developed that includes stemmed slabs, transverse grouted joints, longitudinal post-tensioning, and welded threaded and headless studs (Tadros and Baishya, 1998).

The deck slabs are thinner and lighter than a conventional deck and can be constructed faster. Prestressed deck slabs typically have been used on major bridge deck replacement projects (Figure 33.12) such as the Woodrow Wilson Bridge (Lutz and Scalia, 1984).


FIGURE 33.12 Prestressed post-tensioned deck slabs were installed at night to replace the deck of the Woodrow Wilson Bridge.


Also, most replacements have involved the use of transverse slabs. The decks on the George Washington Memorial Parkway were replaced using precast longitudinally post-tensioned transverse deck slabs (Jakovich and Alvarez, 2002).

A latex-modified concrete overlay was placed over the slabs. The truss spans of the deck on I-95 in Richmond, Virginia, were recently replaced with night lane closures using the full-depth transverse deck slabs (Figure 33.13).


FIGURE 33.13 Special loop bar connection detail for deck slabs. (From FHA, Prefabricated Bridge Elements and Systems in Japan and Europe, Summary Report, International Technology Exchange Programs, Federal Highway Administration, Washington, D.C., 2004)


The slabs were also used to replace the deck on Route 50 in Fairfax County, Virginia (Babaei et al., 2001). The Virginia Department of Transportation first used transverse precast deck slabs to replace a deck on Route 235 over Dogue Creek in Fairfax County in 1981 (Sprinkel, 1982).

Longitudinal slabs were successfully used to rehabilitate the Freemont Street Bridge (Smyers, 1984), and a new bridge was built in Thailand (Zeyher, 2003).

Longitudinal, partial-depth, or full-depth deck slabs that that are precast on one or more concrete or steel beams have also been used successfully (FHWA, 2004). The superstructure elements are set next to each other and are typically connected by transverse post-tensioning in the deck and diaphragms between the beams.

Keyways in the deck are grouted. The deck on I-95 in Richmond, Virginia, was recently replaced with night lane closures using the full-depth deck slabs on steel beam superstructure elements. When partial depth deck superstructure elements are set next to each other, reinforced site-cast concrete facilitates the connection of the elements.

CONCRETE CRACKS CAUSES AND TYPES BASIC INFORMATION

What causes concrete cracks? Several factors, when combined, can lead to restraint cracks in two-way reinforced-concrete slabs. Concrete slabs tend to shorten, and structurally stiff elements such as walls, elevator and stairwell cores, and columns can restrain the slab.

When the tensile stress exceeds the tensile strength of the concrete, a restraint crack occurs (ACI Committee 224, 1997). Depending on many factors, including the stiffness of the restraining elements and the length of the slab spans, multiple restraint shrinkage cracks may form.

The specific factors that cause shortening of concrete slabs include:
• Shrinkage of concrete
• Creep of concrete due to sustained loads (including precompression)
• Elastic shortening (prestressed slabs only)
• Fall in temperature


For a typical parking structure in Southern California with 70% ambient humidity and a moderate temperature variation of 40°F, the contributions of the above factors to slab shortening are as given in Figure 35.1 and Table 35.1. It is noteworthy that two thirds of slab shortening is typically due to concrete shrinkage.


Axial creep and elastic shortening, which are the only direct consequences of post-tensioning, contribute about one sixth of the total shortening.

To appreciate the magnitude of shortenings that are likely to occur in a post-tensioned slab, consider the example shown in Figure 35.2. For the 200 × 100-ft slab shown, the shortenings (if free to take place) are estimated to be 0.8 in. per 100 ft of slab length.

Obviously, this shortening cannot materialize in most cases, because the slabs are commonly tied to supporting structural elements. The interaction of the slab with its restraining structural elements is the crucial factor in the formation of cracks.

Referring to the breakdown of shortenings in Figure 35.2, only 18% of the calculated shortening is due to post-tensioning.

FIGURE 35.2 Reflected ceiling view of slabs: (a) post-tensioned slab; (b) reinforced concrete slab.

The balance is common to nonprestressed as well as post-tensioned slabs. This shows that little difference exists between post-tensioned and nonprestressed slabs as far as crack initiation is concerned; however, crack propagation is fundamentally different between the two types of slabs.


Prominent characteristics of cracks in unbonded post-tensioned slabs as compared to regular reinforced concrete are the following:

• Cracks are fewer in number; instead of a multitude of hairline cracks, fewer cracks form.
• Cracks are generally wider; they are spaced farther apart and generally extend deeper into the slab.

In regular reinforced concrete, the spacing between cracks is of the order of slab depth, whereas in post tensioned slabs it is more related to the span length and the overall dimensions of the slabs. In most cases, crack spacing is more than one quarter of the shorter slab span.

• Cracks are normally longer and continuous, and continuous cracks may extend over one span and beyond. In nonprestressed concrete, cracks are generally shorter in length.

• Cracks commonly do not coincide with locations of maximum moments. Restraining cracks do not necessarily develop at the bottom of midspan or the top of supports where the bending moments are maximum.

• Cracks occur at axially weak locations. Axially weak regions are typically found at construction joints, pour strips, cold joints, paths with reduced discontinuities in slab, and, finally, where precompression is reduced either due to termination of tendons or friction losses in tendons.

Figure 35.2 compares typical crack patterns on the soffit of an interior panel of a two-way slab construction. For the regular reinforced-concrete structure, the shrinkage cracks are shown coinciding with the locations of maximum tension.

Unbonded post-tensioned slabs generally exhibit poorer cracking performance as a result of lesser bonded reinforcement, which mobilizes the concrete in the immediate vicinity of a crack. Hence, a series of large slab segments separated by wide cracks rather than well-distributed small cracks is produced unless either the unbonded post-tensioning is accompanied by a sufficient nonprestressed reinforcement or inplane restraining actions are present that result in a similar improvement of the crack distribution.

Examples of common cracks in slabs, columns, and walls due to restrained movement are illustrated below. Due to the variety of member types and geometry and the array of crack initiation factors, it is imperative that each concrete member be reviewed individually and as part of the overall framing system during the design detailing process.

Concentrated load application and vulnerable member joint conditions may require a very localized review of concrete detailing. On the other hand, the overall framing layout may cause indirect load transfer due to geometry or member incompatibility, resulting in concrete cracking based on overall behavior of the framing system.

WATER QUALITY LEGISLATION AND REGULATIONS IN THE UNITED STATES BASIC INFORMATION


Federal legislation for water-related activities in regard to transportation has been around since 1899 when the Rivers and Harbors Act was passed (Title 23 of the U.S. Code).

This law, amended by the Department of Transportation Act of 1966, requires the U.S. Coast Guard to approve the plans for construction of any bridge over navigable waters.

Accordingly, the required process, generally referred to as a Section 9 Permit from the applicable portion of the act, protects navigation activities from being affected by other transportation modes.

In 1972, Section 404 was added to the Federal Water Pollution Control Act. This required a permit (called a 404 permit) from the U.S. Army Corps of Engineers for any filling, dredging, or realignment of a waterway.

For smaller projects that do not pass established threshold limits, a general permit may be issued.

The Federal Water Pollution Act was changed in 1977 and issued as the Clean Water Act. This act reflected the desire to protect water quality and regulated the discharge of storm water from transportation facilities. Also included in this law was the option for the Corps of Engineers to transfer 404 permitting to the states.

The 404 permitting process also includes required assessments of potential wetland impacts. The amount of wetlands affected, the productivity (especially as related to endangered or protected species), overall relationship to regional ecosystems, and potential enhancements during the design of the project must all be considered.

Executive Order 11990, “Protection of Wetlands,” issued in 1977, required a public-oriented process to mitigate losses or damage to wetlands as well as to preserve and enhance natural or beneficial values. This has led to a policy of wetlands being avoided and replacement required if destruction occurs.

The Federal Highway Administration has released guidelines to help during this phase of the project.56 In addition, FHWA has also released a memorandum entitled “Funding for Establishment of Wetland Mitigation Banks” on October 24, 1994, to help state DOTs meet requirements when wetlands must be taken and replaced.

FOUNDATION EXCAVATION BASIC AND TUTORIALS


There are many different types of excavations performed during the construction of a project. For example, soil may be excavated from the cut or borrow area and then used as fill.

Another example is the excavation of a shear key or buttress that will be used to stabilize a slope or landslide. Other examples of excavations are as follows:

1. Footing Excavations. This type of service involves measuring the dimension of geotechnical elements (such as the depth and width of footings) to make sure that they conform to the requirements of the construction plans. This service is often performed at the same time as the field observation to confirm bearing conditions.

2. Excavation of Piers. As with the excavation of footings, the geotechnical engineer may be required to confirm embedment depths and bearing conditions for piers. Figure 1 presents typical steps in the construction of a drilled pier.


FIGURE 6.44 Typical steps in the construction of a drilled pier: (a) dry augering through self-supporting cohesive soil; (b) augering through water bearing cohesionless soil with aid of slurry; (c) setting the casing.


3. Open Excavations. An open excavation is defined as an excavation that has stable and unsupported side slopes.

4. Braced Excavations. A braced excavation is defined as an excavation where the sides are supported by retaining structures. Figure 6.45 shows common types of retaining systems and braced excavations.


Common types of retaining systems and braced excavations. (From NAVFAC DM-7.2, 1982.)

SPACE TRUSSES BASICS AND TUTORIALS


A space truss is the three-dimensional counterpart of the plane truss described in the three previous articles. The idealized space truss consists of rigid links connected at their ends by ball-and-socket joints.


Whereas a triangle of pin-connected bars forms the basic non collapsible unit for the plane truss, a space truss, on the other hand, requires six bars joined at their ends to form the edges of a tetrahedron as the basic noncollapsible unit.

In Fig. 4/13a the two bars AD and BD joined at D require a third support CD to keep the triangle ADB from rotating about AB. In Fig. 4/13b the supporting base is replaced by three more bars AB, BC, and AC to form a tetrahedron not dependent on the foundation for its own rigidity.

We may form a new rigid unit to extend the structure with three additional concurrent bars whose ends are attached to three fixed joints on the existing structure. Thus, in Fig. 4/13c the bars AF, BF, and CF are attached to the foundation and therefore fix point F in space.

Likewise point H is fixed in space by the bars AH, DH, and CH. The three additional bars CG, FG, and HG are attached to the three fixed points C, F, and H and therefore fix G in space. The fixed point E is similarly created.

We see now that the structure is entirely rigid. The two applied loads shown will result in forces in all of the members. A space truss formed in this way is called a simple space truss.

Ideally there must be point support, such as that given by a balland- socket joint, at the connections of a space truss to prevent bending in the members. As in riveted and welded connections for plane trusses, if the centerlines of joined members intersect at a point, we can justify the assumption of two-force members under simple tension and compression.

SOLUBLE GLASS CIVIL ENGINEERING APPLICATION BASIC AND TUTORIALS


1. Acid-proof Material
Soluble glass can be used as binding material to prepare acid-proof plaster, acid-proof mortar, and acid-proof concrete which are commonly used in anti-acid projects.

2. Heat-resistant Material
Soluble glass has a good heat resistance that can bear a certain high temperature and its strength does not increase. Thus, it can be made into heat-resistant concrcte and mortar.

3. Coating
Soluble glass solution can be used to paint building materials or immerging porous materials. It can enhance the density and strength of materials and increase their resistance to weathering when infiltrating into the materials.

But the solution can not be used to paint or immerge gypsum products because soluble glass can react with gypsum to generate sodium sulfate crystals which will expand in pores and destroy the gypsum products.

4. Grouting Material
Soluble glass solution and calcium chloride solution are injected into soil alternately, and the two solutions will cause chemical reaction to precipitate silicate gel which can cement or fill the pores of soil and prevent the infiltration of water to increase the density and strength of soil.

5. Water-proof Plugging Material
Soluble glass solution mixed with sand or cement can make setting and hardening occur quickly, for repairing or plugging structures. Moreover, mixed with various alum solutions, soluble glass can be used as water-proof agent for cement mortar or concrete.

BUILDING GYPSUM CHARACTERISTICS BASIC AND TUTORIALS


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 will 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.

PILE FOUNDATION DESIGN NATURE OF LOADING BASIC INFORMATION


Usual. Usual loads refer to conditions which are related to the primary function of a structure and can be reasonably expected to occur during the economic service life. The loading effects may be of either a long term, constant or an intermittent, repetitive nature.

Pile allowable loads and stresses should include a conservative safety factor for such conditions. The pile foundation layout should be designed to be most efficient for these loads.

Unusual. Unusual loads refer to construction, operation or maintenance conditions which are of relatively short duration or infrequent occurrence. Risks associated with injuries or property losses can be reliably controlled by specifying the sequence or duration of activities, and/or by monitoring performance.

Only minor cosmetic damage to the structure may occur during these conditions. Lower factors of safety may be used for such loadings, or overstress factors may be applied to the allowables for these loads. A less efficient pile layout is acceptable for these conditions.

Extreme. Extreme loads refer to events which are highly improbable and can be regarded as emergency conditions. Such events may be associated with major accidents involving impacts or explosions and natural disasters due to earthquakes or hurricanes which have a frequency of occurrence that greatly exceeds the economic service life of the structure.

Extreme loadings may also result from a combination of unusual loading effects. The basic design concept for normal loading conditions should be efficiently adapted to accommodate extreme loading effects without experiencing a catastrophic failure.

Extreme loadings may cause significant structural damage which partially impairs the operational functions and requires major rehabilitation or replacement of the structure. The behavior of pile foundations during extreme seismic events is a phenomenon which is not fully understood at present.

The existing general approach is to investigate the effects of earthquake loading at sites in seismic Zones 1 or 2 by applying psuedostatic forces to the structure and using appropriate subgrade parameters.

In Zones 3 or 4 a dynamic analysis of the pile group is appropriate. Selection of minimum safety factors for extreme seismic events must be consistent with the seismologic technique used to estimate the earthquake magnitude. Designing for pile ductility in high risk seismic regions is very important because it is very difficult to assess pile damage after earthquakes and the potential repair costs are very large.

AMERICAN CONCRETE INSTITUTE (ACI) - BASIC INFORMATION


American Concrete Institute (ACI)
PO Box 9094
Farmington Hills, MI 48333
Tel. # (248) 848-3700
Homepage: http://www.aci-int.net/

Founded in 1905, the American Concrete I nstitute (ACI) has grown into a chartered society with over 20,000 members worldwide. The ACI is a technical and educational nonprofit society dedicated to improving the design, construction, manufacture, and maintenance of concrete structures.

Among ACI’s 20,000 members are structural designers, architects, civil engineers, educators, contractors, concrete craftsmen and technicians, representatives of materials suppliers, students, testing laboratories, and manufacturers from around the world. The 83 national and international chapters provide the membership with opportunities to netw ork with their peers and keep in tune with the activities of ACI International.

Membership
Membership is open to individuals who wo rk directly in, have an association with, or have an interest in concrete. All members are encouraged to participate in the activities of the ACI International, which include involvement on voluntary technical committees that develop ACI codes, standards, and reports. Various levels of membership exist to meet particular needs. Student memberships are available.

Publications
Concrete International. Published monthly. C overs institute, chapter, and industry news. Several technical articles following a specific theme appear in each issue.

ACI Materials Journal. Published bimonthly . Describes research in materials and concrete, related ACI
International standards, and committee reports.

ACI Structural Journal. Published bimonthly. Includes technical papers on structural design and analysis,
state-of-the-art reviews on reinforced and structural elements, and the use and handling of concrete.

Other publications: ACI International makes available over 300 technical publication on concrete.

Information is also av ailable in computer software and compact disc formats. A free 72-page publications catalog describing what ACI International has to offer is available.

Other Activities
ACI International provides technical information in the form of high-quality conventions, seminars, and symposia.

COMPRESSIVE STRENGTH OF HARDENED CONCRETE BASIC AND TUTORIALS


It is well recognized that silica fume can contribute significantly to the compressive strength development of concrete. This is because of the filler effect and the excellent pozzolanic properties of the material, which translate into a stronger transition zone at the paste–aggregate interface.

The extent to which silica fume contributes to the development of compressive strength depends on various factors, such as the percentage of silica fume, the water/cement + silica fume ratio, cementitious materials content, cement composition, type and dosage of superplasticizer, temperature, curing conditions, and age.

Superplasticizing admixtures play an important role in ensuring optimum strength development of silica-fume concrete. The water demand of silica-fume concrete is directly proportional to the amount of silica fume (used as a percentage replacement for Portland cement) if the slump of concrete is to be kept constant by increasing the water content rather than by using a superplasticizer.

In such instances, the increase in the strength of silica-fume concrete over that of control concrete is largely offset by the higher water demand, especially for high silica-fume content at early ages. In general, the use of superplasticizer is a prerequisite to achieving proper dispersion of the silica fume in concrete and fully utilizing the strength potential of the fume.

In fact, many important applications of silica fume in concrete depend strictly upon its utilization in conjunction with superplasticizing admixtures. Silica-fume concretes have compressive strength development patterns that are generally different from those of Portland cement concretes.

The strength development characteristics of these concretes are somewhat similar to those of fly-ash concrete, except that the results of the pozzolanic reactions of the former are evident at earlier ages. This is due to the fact that silica fume is a very fine material with a very high amorphous silica content.

The main contribution of silica fume to concrete strength development at normal temperatures takes place between the ages of about 3 and 28 days. The overall strength development patterns can vary according to concrete proportions and composition and are also affected by the curing conditions.

Carette and Malhotra (1992) reported investigations dealing with the short- and long-term strength development of silica-fume concrete under conditions of both continuous water curing and dry curing after an initial moist-curing period of 7 days. Their investigations covered superplasticized concretes incorporating 0 and 10% silica fume as a replacement by weight for Portland cement and water/cement + silica fume ratios ranging between 0.25 and 0.40.

As expected, the major contributions of silica fume to the strength took place prior to 28 days; the largest gains in strength of the silica-fume concrete over the control concrete were recorded at the ages of 28 and 91 days, although this gain progressively diminished with age. For concretes with water/cement + silica fume ratios of 0.30 and 0.40, the gain largely disappeared at later ages.

Under air-drying conditions, the strength development pattern was found to be significantly different from that of water-cured concretes up to the age of about 91 days; thereafter, however, air drying clearly had some adverse effect on the strength development of both types of concrete.

The effect was generally more severe for silica-fume concrete, where some reduction in strength was recorded between the ages of 91 days and 3.5 years, especially for concretes with water/cement + silica fume ratios of 0.30 and 0.40.

These trends of strength reduction have not yet been clearly explained, but they appear to stabilize at later ages and therefore are probably of little practical significance.

Curing temperatures have also been shown to affect significantly the strength development of silicafume concrete. This aspect has been examined in some detail by several investigators in Scandinavia. In general, these investigations have indicated that the pozzolanic reaction of silica fume is very sensitive to temperature, and elevated-temperature curing has a greater strength-accelerating effect on silica-fume concrete than on comparable Portland cement concrete.

The dosage of silica fume is obviously an important parameter influencing the compressive strength of silica fume concrete. For general construction, the optimum dosage generally varies between 7 and 10%; however, in specialized situations, up to 15% silica fume has been incorporated successfully in concrete.

FORMS OF SILICA FUMES BASIC AND TUTORIALS

Silica fume is available commercially in several forms in both North America and Europe:

• As-produced silica fume is silica fume collected in dedusting systems known as bag houses. In this form, the material is very fine and has a bulk density of about 200 to 300 kg/m3, compared with 1500 kg/m3 for Portland cement (Malhotra et al., 1987).

As-produced silica fume is available in bags or in bulk. Because of its extreme fineness, this form poses handling problems; in spite of this, the material can be and has been transported and handled like Portland cement.

• Compacted silica fume has a bulk density ranging from 500 to 700 kg/m3 and is considerably easier to handle than as-produced silica fume. To produce the compacted form, the as-produced silica fume is placed in a silo, and compressed air is blown in from the bottom of the silo.

This causes the particles to tumble, and in doing so they agglomerate. The heavier agglomerates fall to the bottom of the silo and are removed at intervals.

The air compaction of the asproduced silica fume is designed so the agglomerates produced are rather weak and quickly break down during concrete mixing. Mechanical means have also been used to produce compacted silica fume.

• Water-based silica fume slurry overcomes the handling and transporting problems associated with as-produced silica fume; the slurry contains about 40 to 60% solid particles. Typically, these slurries have a density of about 1300 kg/m3.

Some slurries may contain chemical admixtures such as superplasticizers, water reducers, and retarders. One such product (known as Force 10,000®) has been successfully marketed in North America.

TRAVELING CRANES BASIC AND TUTORIALS


A traveling tower crane is erected freestanding on a base frame and ballasted by the user to accommodate in service loads. When out of service, it may need to be parked and anchored down to prearranged storm ballasting blocks or guyed to resist storm winds. An installation might be on the ground or mounted on another structure such as a building roof.

The base travels on railroad-type rails set to a very wide gauge. At each corner of the base one or more wheels is provided; when more than one wheel is used, they are mounted in a bogie that will equalize the load on all wheels at any one corner (Figure 6.39).



Some crane manufacturers offer options on the number of wheels to be placed at each corner of any one crane model. As the number of wheels increases, the weight of track and the number of track supports need to decrease. This can have significant ramifications for installation cost, particularly if soil conditions are poor.

Crane rails can be supported in a number of ways, including wooden ties on stone ballast (in this case the term ballast is used to refer to the bed of material placed between the tie and the native soil or sand base), a continuous steel beam on wooden ties and stone ballast or on concrete footings, or a continuous concrete footing or concrete sleepers on stone ballast.

The best system is that which will support the crane properly at least cost; this will be a function of crane wheel loads, soil conditions, and availability and cost of the materials at the jobsite. The crane manufacturer provides the wheel-load data, but the installation designer must make the decisions from that point on.

The spacing of sleepers or ties can be determined from rail strength and the wheel loads. For multiple-wheel arrangements, some continuity can be taken into account, but we suggest that supports outside of the bogie should be taken as simple. Deflection should also be checked to avoid lifting the ties off their beds.

Track splices are designed to carry only shear loads, so that splices must be centered between close-spaced supports or placed directly over a support. The spacing must be set so that the two rail ends do not differ in elevation (as a result of deflection or any other cause) as the wheel passes over the splice. This will prevent horizontal impact forces from occurring, forces that can be quite significant given the inertia of the tall crane above.

Rails must be laid to comply with the tolerances given by the manufacturer or specified by code. There are strict limits to variations permitted in gauge, in elevation along the tracks and between the tracks, in straightness, and in slope.

Crane rails can be laid to curves but only if the bogies are designed to permit it. Centrifugal forces which develop as the crane travels a curve can have an important effect on stability. The manufacturer must supply data for minimum radius of curvature that will permit safe travel at the speed the crane is capable of attaining.

Curved track as well as slewing forces, wind, and rail misalignment induce lateral forces on the rails. Rail strength and anchorage must be sufficient to restrain these forces. Magnitudes, however, are not easily determined; the crane manufacturer’s recommendations should be sought and followed.

On poor soils, track differential settlements can be a problem as they may cause track elevations to deviate from permitted tolerances and endanger operations. It would be wise to monitor elevations at marked points. This will show whether settlement is ongoing or stabilizing.

With wooden ties, settlements can be corrected by jacking the rail and tie and resetting the stone ballast. For concrete supports it may be necessary to install steel shim plates with sufficient contact area to prevent the concrete from being crushed.

The parking area must be designed and constructed in advance of crane erection. It will consist, for most cranes, of an area with close support spacing for the rails that will be capable of resisting the storm-wind compressive wheel loads. In addition, there must be four buried ballast blocks to which the crane can be tied down by means of cast-in fittings.

In U.S. practice, the buried ballast together with the traveling ballast must be capable of counterbalancing 1.5 times the maximum overturning moment. At the ends of the tracks, trippers are set that will automatically cause the crane travel brakes to engage. At a distance somewhat beyond the crane stopping distance, end stops, or bumpers, are installed as a last means to prevent the crane from running off the rails.

BUCKLING OF COLUMNS BASIC AND TUTORIALS


Columns are compression members whose cross-sectional dimensions are relatively small compared with their length in the direction of the compressive force. Failure of such members occurs because of instability when a certain axial load Pc (called critical or Euler load) is equated or exceeded. The member may bend, or buckle, suddenly and collapse.

Hence the strength P of a column is not determined by the unit stress (P = AĆ’) but by the maximum load it can carry without becoming unstable. The condition of instability is characterized by disproportionately large increases in lateral deformation with slight increase in axial load. Instability may occur in slender columns before the unit stress reaches the elastic limit.



Stable Equilibrium
Consider, for example, an axially loaded column with ends unrestrained against rotation, shown in Fig. 5.43. If the member is initially perfectly straight, it will remain straight as long as the load P is less than the critical load Pc.

If a small transverse force is applied, the column will deflect, but it will return to the straight position when this force is removed. Thus, when P is less than Pc, internal and external forces are in stable equilibrium.


Unstable Equilibrium
If P = Pc and a small transverse force is applied, the column again will deflect, but this time, when the force is removed, the column will remain in the bent position (dashed line in Fig. 5.43).

The equation of this elastic curve can be obtained from Eq. (5.62):
EI d2y/dx2 = -pCY


in which E modulus of elasticity
I = least moment of inertia
y = deflection of the bent member from the straight position at a distance
x = from one end



TYPE OF BEAMS BASIC AND TUTORIALS

Beams are the horizontal members used to support vertically applied loads across an opening. In a more general sense, they are structural members that external loads tend to bend, or curve. Usually, the term beam is applied to members with top continuously connected to bottom throughout their length, and those with top and bottom connected at intervals are called trusses.



There are many ways in which beams may be supported. Some of the more common methods are shown in Figs. 5.11 to 5.16.


The beam in Fig. 5.11 is called a simply supported, or simple beam. It has supports near its ends, which restrain it only against vertical movement. The ends of the beam are free to rotate.

When the loads have a horizontal component, or when change in length of the beam due to temperature may be important, the supports may also have to prevent horizontal motion. In that case, horizontal restraint at one support is generally sufficient.

The distance between the supports is called the span. The load carried by each support is called a reaction. The beam in Fig. 5.12 is a cantilever. It has only one support, which restrains it from rotating or moving horizontally or vertically at that end. Such a support is called a fixed end.

If a simple support is placed under the free end of the cantilever, the propped beam in Fig. 5.13 results. It has one end fixed, one end simply supported.

The beam in Fig. 5.14 has both ends fixed. No rotation or vertical movement can occur at either end. In actual practice, a fully fixed end can seldom be obtained.

Some rotation of the beam ends generally is permitted. Most support conditions are intermediate between those for a simple beam and those for a fixed-end beam.

In Fig. 5.15 is shown a beam that overhangs both is simple supports. The overhangs have a free end, like cantilever, but the supports permit rotation. When a beam extends over several supports, it is called a continuous beam (Fig. 5.16).

Reactions for the beams in Figs. 5.11, 5.12, and 5.15 may be found from the equations of equilibrium. They are classified as statically determinate beams for that reason.

The equations of equilibrium, however, are not sufficient to determine the reactions of the beams in Figs. 5.13, 5.14, and 5.16. For those beams, there are more unknowns than equations. Additional equations must be obtained on the basis of deformations permitted; on the knowledge, for example, that a fixed end permits no rotation. Such beams are classified as statically indeterminate. Methods for finding the stresses in that type of beam are given in Arts. 5.10.4, 5.10.5, 5.11, and 5.13.



FIBERS FOR CONCRETE MIXES BASIC AND TUTORIALS


As used in concrete, fibers are discontinuous, discrete units. They may be described by their aspect ratio, the ratio of length to equivalent diameter. Fibers find their greatest use in crack control of concrete flatwork, especially slabs on grade.

The most commonly used types of fibers in concrete are synthetics, which include polypropylene, nylon, polyester, and polyethylene materials. Specialty synthetics include aramid, carbon, and acrylic fibers. Glass fiber-reinforced concrete is made using E-glass and alkali-resistant (AR) glass fibers. Steel fibers are chopped high-tensile or stainless steel.

Fibers should be dispersed uniformly throughout a mix. Orientation of the fibers in concrete generally is random. Conventional reinforcement, in contrast, typically is oriented in one or two directions, generally in planes parallel to the surface.

Further, welded-wire fabric or reinforcing steel bars must be held in position as concrete is placed. Regardless of the type, fibers are effective in crack control because they provide omnidirectional reinforcement to the concrete matrix. With steel fibers, impact strength and toughness of concrete may be greatly improved and flexural and fatigue strengths enhanced.

Synthetic fibers are typically used to replace welded-wire fabric as secondary reinforcing for crack control in concrete flatwork. Depending on the fiber length, the fiber can limit the size and spread of plastic shrinkage cracks or both plastic and drying shrinkage cracks. Although synthetic fibers are not designed to provide structural properties, slabs tested in accordance with ASTM E72, ‘‘Standard Methods of Conducting Strength Tests of Panels for Building Construction,’’ showed that test slabs reinforced with synthetic fibers carried greater uniform loads than slabs containing welded wire fabric.

While much of the research for synthetic fibers has used reinforcement ratios greater than 2%, the common\ field practice is to use 0.1% (1.5 lb /yd3). This dosage provides more cross-sectional area than 10-gage welded wire fabric. The empirical results indicate that cracking is significantly reduced and is controlled. A further benefit of fibers is that after the initial cracking, the fibers tend to hold the concrete together.

Aramid, carbon, and acrylic fibers have been studied for structural applications, such as wrapping concrete columns to provide additional strength. Other possible uses are for corrosion-resistance structures. The higher costs of the specialty synthetics limit their use in general construction.

Glass-fiber-reinforced concrete (GFRC) is used to construct many types of building elements, including architectural wall panels, roofing tiles, and water tanks. The full potential of GFRC has not been attained because the E-glass fibers are alkali reactive and the AR-glass fibers are subject to embrittlement, possibly from infiltration of calcium-hydroxide particles.

Steel fibers can be used as a structural material and replace conventional reinforcing steel. The volume of steel fiber in a mix ranges from 0.5 to 2%. Much work has been done to develop rapid repair methods using thin panels of densely packed steel fibers and a cement paste squeegeed into the steel matrix.

American Concrete Institute Committee 544 states in ‘‘Guide for Specifying, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete,’’ ACI 544.3R, that, in structural members such as beams, columns, and floors not on grade, reinforcing steel should be provided to support the total tensile load. In other cases, fibers can be used to reduce section thickness or improve performance. See also ACI 344.1R and 344.2R.