Engineering and Construction

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During the   predesign phase  (also called the   planning phase  ), the project is defined in terms of its function, purpose, scope, size, and economics. This is the most crucial of the five phases, and is almost always managed by the owner and the owner’s team. The success or failure of the project may depend on how well this phase is defined, detailed, and managed. Obviously, the clearer the project’s definition, the easier it is to proceed to the subsequent phases.

Some of the important predesign tasks are:

•    Building program definition
•    Economic feasibility assessment,  including the project’s overall budget and financing
•    Site assessment and selection,  including verifying the site’s appropriateness and determining its designated land use
•    Governmental constraints assessment,   for example, building code and zoning constraints and other legal aspects of the project
•    Sustainability rating —whether the owner would like the project to achieve the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) certification at some level
•    Design team selection

BUILDING (PROJECT) PROGRAM 

This includes defining the activities, functions, and spaces required in the building, along with their approximate sizes and their relationships with each other. For a house or another small project, the programis usually simple and can be developed by the owner without external assistance. For a large project, however,where the owner may be an institution (such as a corporation, school board, hospital, religious organization, or governmental entity), developing the program may be a complex exercise. This may be due to the size and complexity of the project or the need to involve several individuals—a corporation’s board  of directors, for example—in decision making. These constituencies may have different views of the project, making it difficult to create a consensus.

Program development may also be complicated by situations in which the owner has a fuzzy idea of the project and is unable to define it clearly. By contrast, experienced owners tend to have a clear understanding of the project and generally provide a detailed, unambiguous program to the architect.

It is not unusual for the owner to involve the architect and a few other consultants of the design team in preparing the program. In this instance, the design team may be hired during the predesign phase. When the economic considerations of the project are paramount, the owner may also consult a construction cost analyst.

Whatever the situation, preparing the program is the first step in the project delivery process. It should be spelled out in writing and in sufficient detail to guide the design, reduce the liability risk for the architect, and avoid its misinterpretation. If a revision is made during the progress of the project, the owner’s written approval is necessary.

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The process by which a building project is delivered to its owner may be divided into the following five phases, referred to as the   project delivery phases . Although there is usually some overlap between adjacent phases, they generally follow this order:

  •     Predesign  phase
  •     Design  phase
  •     Preconstruction  phase
  •     Construction  phase
  •     Postconstruction  phase

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Estimate the depth of wood rafters on the basis of the horizontal (not slope) distance from the outside wall of the building to the ridge board in a gable or hip roof and the horizontal distance between supports in a shed roof. A 2 X 4 rafter spans approximately 7 feet (2.1 m), a 2 X 6 10 feet (3.0 m), a 2 8 14 feet (4.3 m), and a 2 X 10 17 feet (5.2 m).

The depth of wood light roof trusses is usually based on the desired roof pitch. A typical depth is one-quarter of the width of the building, which corresponds to a 6/ 12 pitch in a gable truss. Trusses are generally spaced 24 inches (600 mm) o.c. and can span up to approximately  65 feet (20 m).

Estimate the depth of wood floor joists as follows: 2 x 6 joists span up to 9 feet (2.7 m), 2 x 8 joists 11 feet (3.4 m), 2 x 10 joists 14 feet (4.3 m), and 2 x 12 joists 17 feet (5.2 m).

Estimate the depth of manufactured  wood I-joists as follows: 9½-inch (240-mm) joists span 16 feet (4.9
m), 117 /8 -inch (300-mm) joists 19 feet (5.8 m), 14-inch (360-mm) joists span 23 feet (7.0 m), and 16-inch (400-mm) joists span 25 feet (7.6 m).

Estimate the depth of  wood floor  trusses as 1/ 18 of their span. Typical depths of ß oor trusses range from 12 to 28 inches (305-710 mm) in 2-inch (51-mm) increments.

2 X 4 wood studs 24 inches (600 mm) o.c. can support attic and roof loads only. Either 2 X 4 studs 16 inches (400 mm) o.c. or 2 6 studs 24 inches o.c. can support one floor plus attic and roof. 2 X 6 studs 16 inches o.c. can support two ß oors plus attic and roof.

Framing members in light frame buildings are usually spaced either 16 or 24 inches (400 or 600 mm) o.c. For actual sizes of dimension lumber in both conventional and metric units, see Figure 3.22.

These approximations are valid only for purposes of preliminary building layout and must not be used  to select final member sizes. They apply to the normal range of building occupancies such as residential, office, commercial, and institutional buildings. For manufacturing and storage buildings, use somewhat larger  members.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplified chart

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Roof trusses, and to a lesser extent floor trusses, are used in platform frame buildings because of their speed of erection, economy of material usage, and long spans. Though many floor  trusses are light enough to be lifted and installed by two carpenters, most truss assemblies are erected with the  aid of a small crane that often is attached to the truck on which the trusses are delivered (Figure 5.65).  Roof trusses are particularly slender in proportion, usually only 1½ inches (38 mm) thick and capable of spanning 24 to 32 feet (7.5 to 10 m). They must be temporarily braced during construction to prevent buckling or  the dominolike collapse of all the trusses until they are adequately secured permanently by the application of roof sheathing panels and interior finishes (Figure 5.66).

Figure 5.65 Roof trusses are typically lifted to the roof by a boom mounted on the delivery truck. This is one of a series of identical attic trusses, which will frame a habitable space under the roof.

Manufactured wall panels have been adopted more slowly than roof and floor trusses, and are used mostly  by large builders who build hundreds or thousands of houses per year. For  the smaller builder, wall framing can be done on site with the same amount of material as with panels and with  little or no additional overall expenditure of labor, especially when the building requires walls of varying heights and shapes.

Figure 5.66 A roof framed with prefabricated trusses. Approximately midway up the upper chords of the trusses, temporary strapping ties the trusses to one another for bracing. Other diagonal bracing, not visible in this photograph, ties the trusses to floor or ceiling framing to prevent the entire row from collectively tipping sideways.

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In wood light frame structures constructed with  advanced framing techniques (also called optimum value engineering), special attention is given to minimizing the use of redundant or structurally unnecessary wood members, thereby reducing the amount of lumber required to construct the frame and, once the frame is insulated, increasing its thermal efficiency (Figure 5.64). A variety of techniques may be used, including:

Spacing framing members at 24 inches (610 mm) rather than 16 inches (406 mm) o.c.: Wider spacing of framing members reduces the amount of lumber required. In exterior walls,  thermal efficiency is also improved by the reduction in thermal bridging area in comparison to walls framed with more closely spaced members.

Designing to a 24-inch (610-mm) module: When the outside dimensions of a framed structure conform  to a 24-inch module, sheathing panel waste is minimized. Planning rough opening sizes and locations in floors,  walls, and roofs to conform, where possible, to modular dimensions can  reduce waste even further.

Designing to modular dimensions also reduces wastage of interior wallboard.

Using single top plates in all walls, both bearing and nonbearing: In the  case of bearing walls, this requires  floor or roof framing members to align directly over studs in the walls  that support them.

Figure 5.64 Comparison of walls framed with  conventional and advanced techniques. Wall A is framed . Studs are spaced at 16 inches (406 mm) o.c., the layout  of the wall and its openings is not coordinated with the framing module,  and standard details are used for corners, openings, and other features.  In wall B, studs are spaced at 24 inches (610 mm) o.c., the length of the wall and the location and size of its openings have  been coordinated to the greatest extent possible with the 24-inch module, and redundant framing members have been eliminated. Note that with the single top plate, fl>oor joists or roof rafters (shown dashed in these figures) bearing on wall B must fall directly over studs below. The total length of framing lumber used in  wall B is half of that required for wall A. Though only a foot and a half shorter in  length, wall B also can be sheathed with five standard-sized sheathing panels, whereas wall A requires six. Even if  wall A is constructed of 2 X 4 studs and wall B is constructed of 2 X 6 studs, the  overall savings in materials and reduction in waste in wall B are substantial.

Minimizing  other  unneeded framing members: Don't use headers over openings in nonbearing walls,  since they are not needed; in bearing walls, use headers only as deep as  required for the loads and span.  Where corner studs serve only to provide nailing surfaces or support  for wallboard, use other, less waste- ful blocking techniques or metal clips  designed for this purpose instead. Replace jack studs supporting headers at window and door openings with metal hangers; eliminate unneeded cripple studs under rough  sills. All of these techniques save lumber and, in exterior walls, increase energy efficiency by reducing thermal bridging through solid framing members.

Eliminating unneeded plywood and OSB wall sheathing: Where let-in bracing is structurally adequate  for lateral force resistance, eliminate structural panel wall sheathing entirely and cover walls with insulating  sheathing for better thermal efficiency. Where structural panels are required, use the minimum extent of  panels necessary.

Advanced framing techniques rely on unconventional framing methods and significantly  reduce  redundancy in the building frame. For these reasons, they should not  be used without guidance from a  structural engineer or other qualified design professional, and special review and approval from local building authorities may be required. Nevertheless, where these techniques  are used, significant benefits can be realized. According to the National Association of Home Builders Partnership for Advancing Technology in Housing, advanced framing techniques can reduce the amount of  lumber used in a wood light frame  structure by up to 19 percent and  improve the energy efficiency of the insulated structure by as much as 30 percent.