Engineering and Construction

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Careful planning in scheduling the construction operations for a building and in providing the forms can assure the maximum economy in formwork and also the highest efficiency by labor, both of  which will reduce the cost of formwork.

Consider the six-story building in Figure 2-1, to be constructed  with concrete columns, girders, beams, and slabs. The floor area is  large enough to justify dividing the floor into two equal or approx-imately equal areas for forms and concreting. A construction joint through the building is specified or will be permitted. If the structure is symmetrical about the construction joint, the builder will be fortunate. If the building is not symmetrical about the construction joint, some modifications will have to be made in the form procedures presented hereafter.

Each floor will be divided into equal units for construction purposes. Thus, there will be 12 units in the building. One unit will be completely constructed each week, weather permitting, which will include making and erecting the forms; placing the reinforcing steel, electrical conduit, plumbing items, etc., and pouring the concrete. The carpenters should complete the formwork for unit 1 by the end of the third day, after which time some of them will begin the form-work for unit 2 while others install braces on the shores and other braces, if they are required, and check; if necessary, the carpenters will adjust the elevations of girder, beam, and deck forms. One or two carpenters should remain on unit 1 while the concrete is being placed. This will consume one week.

FIGURE 2-1  Construction schedule for concrete frame of building.

During the second week, and each week thereafter, a unit will be completed. Delays owing to weather may alter the timing but not the schedule or sequence of operations. Figure 2-1 shows a simplified section through this building with the units and elapsed time indicated but with no provision for lost time owing to weather.

Forms for columns and beam and girder sides must be left in place for at least 48 hours, whereas forms for the beam and girder bottoms, floor slab, and vertical shores must be left in place for at
least 18 days. However, concrete test cylinders may be broken to determine the possibility of a shorter removal time of shores. Form-work will be transferred from one unit to another as quickly as time requirements and similarity of structural members will permit.

Table 2-2 will assist in determining the number of reuses of form units and total form materials required to construct the building illustrated in Figure 2-1. Although the extent to which given form sections can be reused will vary for different buildings, the method of analyzing reusage presented in this table can be applied to any building and to many concrete structures.

If the schedule shown in Table 2-2 will apply, it will be necessary to provide the following numbers of sets of forms: for columns and beam and girder sides, two sets and for beam bottoms, slab decking,
and shores, three sets.

TABLE 2-2  Schedule of Use and Reuse of Formwork for a Building (Continued  )

If structural sections such as columns, girders, beams, and floor panels in odd-numbered units 1 through 11 are similar, and those in the even-numbered units 2 through 12 are similar, but those in units 1 through 11 are not similar to those in units 2 through 12, it will be necessary to move form sections to higher floors above given units. For example, forms for unit 1 cannot be used in unit 2, or those from unit 3 in unit 4, and so on. Under this condition, it will be necessary to provide one set of columns and beam and girder sides for unit 1 and another set for unit 2, which will be sufficient for the entire building.

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As discussed in Section 3.4, many of the structural strategies used for wind load resistance and earthquake resistance are essentially similar. Buildings are, therefore, generally designed to resist either earthquake loads or wind loads, whichever causes the worst effect (greater stresses). This is based on the assumption that there is a negligible probability of maximum wind speeds occurring at the same time as an intense earthquake. In regions of low seismic activity, wind loads govern the design of the lateral resistance of buildings; in regions of intense seismic activity, the reverse is the case.

The design of lightweight envelope components, such as glass curtain walls and roof membranes, is governed by wind loads even in highly seismic regions. (Remember that earthquake loads are influenced by the weight of the component.)  Despite the similarities between the provisions for wind load and earthquake load resistance, there are several differences in the details. An important difference is that as an earth-quake shakes the ground on which the building rests, it affects all components of the building—exterior as well as interior components. Damage to the building’s contents and injury to occupants may occur by falling interior contents,  Figure 3.24   . Thus, all compo-nents, equipment, and fixtures in a building located in a seismically active zone must be adequately anchored to remain intact during an earthquake.

FIGURE 3.24 Effect of earthquake shaking (1989 Loma Prieta earth-quake) on the contents of a building’s interior. Note that the building envelope is intact.

Wind, on the other hand, acts on the building envelope. It is through the envelope that wind loads are transmitted to the building structure. If the envelope remains intact, the building will generally retain its overall structural integrity in a storm. Thus, the wind damage in a building usually begins with the damage to envelope components, such as the roof, exterior walls, exterior doors, windows, and other cladding elements. Once the building changes from fully enclosed to partially enclosed (see  Figure 3.15), the wind loads on the building increase due to the ballooning effect, which may result in additional damage or collapse of the structure, damage to interior contents, and injuries to occupants, Figure 3.25.

FIGURE 3.25 Damage to the glass curtain wall of the 37-story Bank One Building, Fort Worth, Texas, by a forceful tornado on March 28, 2000. As soon as the glass in the curtain wall shattered, substantial damage to the building’s interior (ceiling, drywall, electrical fixtures, etc.)

	FIGURE 3.15 Ballooning of a building with a relatively large opening in one wall. Such a building is referred to as a partially enclosed building. A building that is not partially enclosed is called an enclosed building. Most buildings belong to the enclosed building category.

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From Newton’s second law of motion, the other factor that affects the magnitude of the earthquake load is the weight of the building. Lighter buildings attract a smaller earthquake load than buildings constructed of heavy materials. Thus, concrete frame and masonry structures attract a greater earthquake load than wood frame or steel frame structures. Residential structures with wood siding or stucco finishes attract a smaller earthquake load than those with brick or stone veneer.

Indeed, because of its light weight, the earthquake load on a tent is negligible even in a severe earthquake, Figure 3.22. A tent structure has the additional advantage of its built-in resistance toabsorb structural deformations (ductility)—an important structural property for earthquake regions.
Buildings constructed of heavy materials with little or no capacity to absorb deformations are among the most hazardous buildings in a seismically active zone. Such buildings include those whose structural system consists of unreinforced masonry structures (i.e., bricks, stone or concrete block walls with no vertical rein-forcement), Figure 3.23.

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Although roofs are designed to have adequate drainage so that no accumulation of water occurs, loads resulting from accidental accumulation of melted snow or rainwater must be considered as a possibility. Drains may be blocked by windblown debris or hail aggregating on the roof or the formation office dams near the drains.

Long-span, relatively flat roofs are particularly vulnerable to rainwater accumulation because, being flexible, they deflect under the weight of water. This deflection leads to yet more accumulated water, causing additional deflection, which increases accumulation. If adequate stiffness is not provided in the roof, the progressive increase of deflection can cause excessive load on the roof. Water accumulation has been the cause of complete collapse of several long-span roofs.

Generally, roofs with slope greater than 1/4 in. to 1 ft (1:48 or, say, 1:50 slope) are not subjected to accumulated rainwater unless roof drains are blocked. Building codes mandate 1/4 in. to 1 ft as the minimum slope required for roofs, which, apart from providing positive drainage, also helps to increase the life and improve the performance of roof (waterproofing) membranes.

Building codes also require that in addition to primary drains, roofs must be provided with secondary (overflow) drains. The secondary drains must be at least 2 in. above the primary drains so that if the primary drainage system gets blocked, the secondary system will be able to drain the water off the roof,   Figure 3.5.

Water accumulation generally occurs on roofs that are provided with a parapet. In the absence of a parapet, water accumulation will generally not occur. Therefore, secondary drains are not required on unparapeted roofs or on steep roofs.

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The live load on a roof takes into account the weight of repair personnel and temporary storage of construction or repair materials and equipment on the roof. Roof live load is generally given as 20 psf acting on the horizontal projected area of the roof, Figure 3.4.


This applies to a roof that will not be used in the future as a floor. If the building is expected to be extended vertically in the future, the roof live load is the floor live load for the anticipated future occupancy. Additionally, if the roof is to be landscaped, the load due to the growth medium and landscaping elements must be considered as dead load on the roof.

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While the dead load is a permanent load on the structure,   live load  is defined as the load whose magnitude and placement change with time. Such loads are due to the weights of people (animals, if the building houses animals), furniture, movable equipment, and stored materials. As shown in  Figure 3.1   , live loads are further divided into

• Floor live load
• Roof live load


Floor live load depends on the occupancy and the use of the building. Therefore, it is also called the   occupancy load,  and it is different for different occupancies. For instance, the floor live load for a library stack room is higher than the floor live load for a library reading room, which in turn is higher than the floor live load for an apartment building.

Floor live loads are determined by aggregating the loads of all people, furniture, and movable equipment that may result from the particular occupancy. Safety considerations require that the worst expected situation be considered so that the structure is designed for the maximum possible live load that may be placed on it.

Based on a large number of surveys, floor live loads for various commonly encountered occupancies, such as individual dwellings, hotels, apartment buildings, libraries, office buildings, and industrial structures, have been determined and are contained in building code tables. Table 3.1 gives floor live loads for a few representative occupancies. For instance, the floor live load for a library stack room is 150 psf, for a library reading room is 60 psf, and for an apartment building is 40 psf. Note that these are the minimum floor live loads for which the building must be designed. Even if the actual live load on a floor is smaller, the building must be designed for the minimum live load specified by the building code.

The floor live load values in Table 3.1 are conservative. In most situations, the actual loads are smaller than those given. However, the architect or the engineer must recognize unusual situations that may lead to a greater actual load than the one specified in the code. In such situations, the higher anticipated load should be used.  

Additionally, if the live load for an occupancy not included in building code tables is to be obtained, the architect or the structural engineer must determine it from first principles, taking into account all the loads that may be expected on the structure. Most building codes require that such live load values be approved by the building official.

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Dead loads are always present in a building; that is, they do not vary with time. They include the weights of the materials and components that comprise the structure. The dead load of a component is computed by multiplying its volume by the density of the material. Because both the densities and the dimensions of the components are known with reasonable accuracy, dead loads in a building can be estimated with greater certainty than other load types. Densities of some of the commonly used materials are given in  Table 2.


The dead load for which a building component is designed includes the self-load of the component plus the dead loads of all other components that it supports. For example, the dead load on a column includes the weight of the column itself plus all the dead load imposed on it. In Figure 3.3, the dead load on a column is the weight of the column plus the dead load from the beams and slab resting on it. Similarly, the dead load on a beam is the weight of the beam itself plus the dead load from the slab that it supports. The dead  load on the slab is only the self-weight of the slab. However, if the slab supports a floor finish, ceiling, light fixtures, or plumbing and electrical pipes, their weights must be included in the dead load acting on the slab.

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The general contractor will normally have an inspection process to ensure that the work of all subcontractors is progressing as indicated in the contract documents and that the work meets the standards of quality and workmanship. On smaller projects, this may be done by the project superintendent  . On large projects, a team of quality-assurance inspectors generally assists the contractor’s project superintendent. These inspectors are individuals who, by training and experience, are specialized in their own areas of construction—for example, concrete, steel, or masonry.


Additional quality control is required by the contract through the use of independent testing laboratories. For instance, structural concrete to be used on the site must be verified for strength and other properties by independent concrete testing laboratories.

Leaving quality control of materials and performance entirely in the hands of the contractor is considered inappropriate. It can render the owner vulnerable to omissions and errors in the work, and it places an additional legal burden on the contractor. Therefore, the owner usually retains the services of the project architect to provide a third-party level of scrutiny to administer the construction contract. If not, the owner will retain another independent architect, engineer, or inspector to provide construction contract administration services. The contracting community favors this third-party oversight of its work.

The architect’s role during the construction phase has evolved over the years. There was a time when architects provided regular supervision of their projects during construction, but the liability exposure resulting from the supervisory role became so adverse for the archi-tects that they have been forced to relinquish this responsibility. Instead, the operative term for the architect’s role during construction is   field observation  of the work.

The observational role still allows the architects to verify that their drawings and specifications are transformed into reality just as they had conceived. It also provides a sufficient safeguard against the errors caused by the contractors’ misinterpretation of contract documents in the absence of the architects’ clarification and interpretation. The shift in the architect’s role to observer of construction also recognizes the important and entirely independent role that the contractor must play during construction. This rec-ognition provides full authority to the general contractor to proceed with the work in the manner that the contractor deems appropriate. This reinforces the earlier statements that:

•    The architect determines the   what  and   where . 
•    The contractor determines the   how  (means and methods) and   when  (sequence) of construction.

In other words, daily supervision or superintendence of construction is the function of the contractor—the most competent person to fulfill this role. The architect provides periodic observation and evaluation of the contractor’s work and notifies the owner if the work is not in compliance with the intent of the contract documents. This under-scores the  division between the responsibilities of the architect and the contractor during construction.

Note that by providing observation, the architect does not certify the contractor’s work. Nor does the observation relieve the contractor of its responsibilities under the contract. The contractor remains fully liable for any error that has not been discovered through the architect’s observation. However, the architect may be held liable for all or part of the work observed, should the architect fail to detect or provide timely notification of work not conforming with the contract documents. This omission is known as failure to detect.
 
Because many components can be covered up by other items over days or hours, the architect should visit the construction site at regular intervals, as appropriate to the progress of construction. For example, earthwork covers foundations and underground plumbing, and gypsum board covers ceiling and wall framing. Observing the work after the components are hidden defeats the purpose of observation.

On some projects, a resident project architect or engineer may be engaged by the architect, at an additional cost to the owner, to observe the work of the contractor. Under the conditions of the contract, the contractor is generally required to provide this person with an on-site office, water, electricity, a telephone, and other necessary facilities.

INSPECTION OF WORK 

There are only two times during the construction of a project that the architect makes an exception to being an observer of construction. At these times, the architect inspects the work. These inspections are meant to verify the general contractor’s claim that the work is (a) substantially complete and (b) has been completed and hence is ready for final payment. These inspections, explained in Section 1.10, are referred to as:

•     Substantial  completion   inspection
•    Final  completion  inspection

PAYMENT CERTIFICATIONS 

In addition to construction observation and inspection, there are several other duties the architect must discharge in administering the contract between the owner and the contractor. These are outlined in the box “Summary of Architect’s Functions as Construction Contract Administrator.” Certifying (validating) the contractor’s periodic requests for payment against the work done and the materials stored at the construction site is perhaps the most critical of these functions. An application for payment (typically made once a month unless stated differently in the contract) is followed by the architect’s evaluation of the work and necessary documentation to verify the contractor’s claim. Because the architect is not involved in day-to-day supervision, the issuance of the certificate of payment by the architect does not imply acceptance of the quality or quantity of the contractor’s work. However, the architect has to be judicious and impartial to both the owner and the contractor and perform within the bounds of the contract. 

CHANGE ORDERS 

There is hardly a construction project that does not require changes after construction has begun. The contract between the owner and the contractor recognizes this fact and includes provisions for the owner’s right to order a change and the contractor’s obligation to accept the   change order  in return for an equitable price adjustment. Here again, the architect performs a quasi-judicial role to arrive at a suitable agreement and price between the owner and the contractor.