Engineering and Construction

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During the CD stage, the architect and consultants prepare their own sets of drawings, referred to as   construction drawings . Thus, a project has architectural construction drawings, civil and structural construction drawings, MEPF construction drawings, landscape construction drawings, and so on.

Construction drawings are dimensioned drawings (usually computer generated) that  fully delineate the building. They consist of floor plans, elevations, sections, schedules, and  various large-scale details. The details depict a small portion of the building that cannot be  adequately described on smaller-scale plans, elevations, or sections.    

Construction drawings are the drawings that the construction team uses to build the  building. Therefore, they must indicate the geometry, layout, dimensions, types of materials, details of assembling the components, colors and textures, and so on. Construction drawings are generally two-dimensional drawings, but three dimensional isometrics are sometimes used for complex details. Construction drawings are also used by the contractor to prepare a detailed cost estimate of the project at the time of bidding.

Construction drawings are not a sequence of assembly instructions, such as for a bicycle. Instead, they indicate what every component is and where it will be located when the building is completed. In other words, the design team decides the “what” and “where” of the building. The “how” and “when” of the building are entirely in the contractor’s domain.

NOTE

Working Drawings and Construction Drawings The term working drawings was used until the end of the  twentieth century for what are now commonly referred to as construction drawings.

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Once the schematic design is approved by the owner, the process of designing the building in greater detail begins. During this stage, the schematic design is developed further—hence the term   design development  (DD) stage.

While the emphasis in the SD stage is on the creative, conceptual, and innovative aspects of design, the DD stage focuses on developing practical and pragmatic solutions for the  exterior envelope, structure, fenestration, interior systems, MEPF systems, and so forth.

This development involves strategic consultations with all members of the design team. Therefore, the most critical feature of the DD stage is decision making, which may range  from broad design aspects to details. At this stage, the vast majority of decisions about  products, materials, and equipment are made. Efficient execution of the construction documents depends directly on how well the DD is managed. A more detailed version of the specifications and probable cost of the project is also prepared at this stage.  

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The schematic design gives graphic shape to the project program. It is an overall design concept that illustrates the key ideas of the design solution. The major player in this stage is the architect, who develops the design scheme (or several design options) with only limited help from the consultants. Because most projects have strict budgetary limitations, a rough estimate of the project’s probable cost is generally produced at this stage.

The schematic design usually goes through several revisions, because the first design scheme prepared by the architect will rarely be approved by the owner. The architect communicates the design proposal(s) to the owner through various types of drawings—plans, elevations, sections, freehand sketches, and three dimensional graphics (isometrics, axonometrics, and perspectives). For some projects, a three-dimensional scale model of the entire  building or the complex of buildings, showing the context (neighboring buildings) within which the project is sited, may be needed.

With significant developments in electronic media technology, especially building information modeling (BIM), computer-generated imagery has become common in architecture and related engineering disciplines. Computer-generated walk-through and flyover simulations are becoming increasingly popular ways of communicating the architect’s  design intent to the owner and the related organizations at the SD stage.

It is important to note that the schematic design drawings, images, models, and simulations, regardless of how well they are produced, are not adequate to construct the building. Their objective is merely to communicate the design scheme to the owner (and to consultants, who may or may not be on board at this stage), not to the contractor.

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The    design phase   begins after the selection of the architect. Because the architect (usually a firm) may have limited capabilities for handling the broad range of building-design activities, several different, more specialized consultants are usually required, depending on the size and scope of the project.

In most projects, the design team consists of the architect, civil and structural consultants, and mechanical, electrical, plumbing, and fire-protection (MEPF) consultants. In complex projects, the design team may also include an acoustical consultant, roofing and waterproofing consultant, cost consultant, building code consultant, signage consultant, interior designer, landscape architect, and so on.

Some design firms have an entire design team (architects and specialized consultants) on staff, in which case the owner will contract with a single firm. Generally, however, the design team comprises several different design firms. In such cases, the owner typically contracts the architect, who in turn contracts the remaining design team members,  Figure 1.1. Thus, the architect functions as the prime design professional and, to a limited degree, as the owner’s representative. The architect is liable to the owner for his or her own work and that of the consultants. For that reason, most architects ensure that their consultants carry adequate liability insurance.

FIGURE 1.1 Members of a typical design team, and their interrelationships with each other and the owner in a traditional contractual setup. A line in this illustration indicates a contractual rela- tionship between parties. (“MEPF consultants” is an acronym for mechanical, electrical, plumbing and fire consultants.)

In some projects, the owner may contract some consultants directly, particularly a civil consultant (for a survey of the site, site grading, slope stabilization, and site drainage), a geotechnical consultant (for investigation of the soil properties), and a landscape architect (for landscape and site design),  Figure 1.2   . These consultants may be engaged before or at the same time as the architect.  

FIGURE 1.2 Members of a typical design team, and their interrelationships with each other and the owner in a project where some consultants are contracted directly by the owner. A solid line in this illustration indicates a contractual   relationship between parties. A dashed line indicates a communication link, not a contract.

Even when a consultant is contracted directly by the owner, the architect retains some liability for the consultant’s work. This liability occurs because the architect, being the prime design professional, coordinates the entire design effort, and the consultants’ work is influenced a great deal by the architectural decisions. Therefore, the working relationship between the architect and an owner-contracted consultant remains essentially the same as if  the consultant were chosen by the architect.

In some cases, an engineer or another professional may coordinate the design process.

This  generally occurs when a building is a minor component of a large-scale project. For example, in a  highly  technical project such as a power plant, an electrical engineer may be  the prime design professional.

In most building projects, the design phase consists of three stages:

  •     Schematic  design   stage
  •     Design  development   stage
  •     Construction  documents   stage

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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.

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The 2 x 4 (38 x 89 mm) has been the standard-size wall stud since light framing was invented. In recent  years, however, pressures for heating fuel conservation have led to energy code requirements for more thermal insulation than can be inserted in the cavities of a wall framed with  members only 3½ inches (89 mm) deep. One solution is to frame walls with 2 x  6 (38 x 140 mm) studs, usually at a spacing of 24 inches (610 mm), creating an insulation cavity 5½ inches (140 mm) deep. Alternatively,  2 x 4-framed walls may be covered either inside or out with insulating plastic foam sheathing, thus reaching an insulation value about the  same as that of a conventionally insulated 2   6-framed wall. For even greater insulation performance, 2   6 studs and insulating sheathing may be used in tandem. Alternatively, in very cold climates, even deeper wall  assemblies that can achieve greater insulation values may be constructed of two separate layers of wall studs  or of vertical truss studs made up of pairs of ordinary studs joined at intervals by plywood plates. Some of these constructions methods are illustrated in Figures 7.17 through 7.21.

Figure 7.17 Insulation levels in walls of light frame buildings can be increased from the R13 to R-15 (RSI-90 to RSI-104) of a 2 x 4 stud wall to R19 (RSI-132) or more by using either 2 x 6 framing and thicker batt insulation (left) or 2 x 4 framing with plastic foam sheathing in combination with batt insulation (right). The foam sheathing insulates the wood framing members as well as the cavities between them but can complicate the process of installing some types of siding.

Figure 7.18 Window and door headers in 2 x 6 framing require special detailing. Two alternative header details are shown here in section view: (a) The header members are installed flush with the interior and exterior surfaces of the studs, with an insulated space between. This detail is thermally efficient but may not provide sufficient nailing for interior finish materials around the window. (b) A 3 x 2 spacer provides full nailing around the opening.

Figure 7.19 Two alternative corner post details for 2 x 6 stud walls, shown in plan view: (a) Each wall frame ends with a full 2 x 6 stud, and a 2 x 4 nailer is added to accept fasteners from the interior wall finish. (b) For maximum thermal efficiency, one wall frame ends with a 2 x 4 stud flush with the interior surface, which eliminates any thermal bridging through studs.

Figure 7.20 With the two framing methods shown here, walls can be insulated to any desired level of thermal resistance.

Figure 7.21 A raised-heel roof truss provides plenty of space for attic insulation at the eave.

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The generic roof shapes for wood light frame buildings are shown in Figure 5.50. These are often combined to make roofs that are suited to the covering of more complex plan shapes and building volumes.

Figure 5.50 Basic roof shapes for wood light frame buildings.

For structural stability, the rafters in gable and hip roofs must be securely tied together at the top of the supporting walls by well-nailed ceiling joists to make what is, in effect, a series of tri- angular trusses. If the designer wishes to eliminate the ceiling joists to expose  the sloping underside of the roof as  the finished ceiling surface, a beam or bearing wall must be inserted at the  ridge or a system of exposed horizontal  ties must be used in place of the ceiling joists. Sometimes a designer wishes  to raise the ceiling joists or exposed ties to a higher elevation than the tops of the wall plates. This greatly increases  the bending forces in the rafters and should be done only after consultation with a structural engineer.

Although a college graduate  ar  chitect or engineer would find it difficult  to lay out all the cuts of the rafters for a sloping roof using trigonometry, a  skilled carpenter, without resorting to  mathematics, has little problem making the layout if the  pitch (slope) is  specified as a ratio of rise to run. Rise is  the vertical dimension and run is the  horizontal. In the United States, pitch  is usually given on the architectÕs drawings as inches of rise per foot (12 inches) of run. A skilled carpenter uses  these two figures on the two edges of  a framing square to lay out the rafter as  shown in Figures 5.52 and 5.58. The  actual length of the rafter is never figured, nor does it need to be, because all the measurements are made as  horizontal and vertical distances with  the aid of the square. Today, many  carpenters prefer to do rafter layout  with the aid of tables that give actual  rafter lengths for various pitches and  horizontal distances; these tables are  stamped on the framing square itself  or printed in pocket-size booklets. Also available are hand-held calcula- tors that are specially programmed to find dimensions of rafters.

Hips and valleys introduce another level of trigonometric complexity  in rafter layout, but the experienced  carpenter has little difficulty  even  here: Again, he or she can use published tables for hip and valley rafters or do the layout the traditional way, as illustrated in Figure 5.56. The  head carpenter lays out only one rafter of each type by these procedures. This then becomes the  pattern rafter from which other carpenters trace  and cut the remainder of the rafters (Figure 5.59).

In areas that are subject to hurricanes, the building codes may require  that rafters be attached to their sup- porting walls with sheet metal rafter anchors. The type, size, and spacing of the nails that attach the roof sheathing to the rafters are also closely controlled.  The intent of both of these measures is to reduce the likelihood that the roof  will be blown off in high winds. Similar to requirements for bridging in floor joists, the International  Residential Code requires bridging  for ceiling joists and rafters where the  depth of these members exceeds their  thickness by a ratio of 6 to 1 based on   nominal dimensions (for conventional solid wood framing, members deeper  than 2 12 or 38 286 mm). Where  the undersides of the joists or rafters  remain unfinished, wood strapping  nailed perpendicular to the bottoms of the joists or rafters may be used instead of solid blocking or crossbracing installed between framing members.

Figure 5.52 Roof framing details and procedures: The lettered details are keyed to Figure 5.53. The remainder of the page shows how a framing square is used to lay out a pattern rafter, reading from the fi rst step at the lower end of the rafter to the last step at the top.

Figure 5.53 Step Seven: framing the attic fl oor and roof. The outer ends of attic fl oor joists are not usually headed off, but instead are face nailed to the rafter pairs that overlap them.

Figure 5.54 A summary of the major details for the structure shown in Figure 5.55, aligned in relationship to one another. The gable end studs are cut as shown in A and face nailed to the end rafter.

Figure 5.55 Step Eight: The sheathed frame is completed. The letters A, B, and C indicate portions of the framing that are detailed in Figure 5.54.

Figure 5.56 Framing for a hip roof. The diffi  cult geometric problem of laying out the diagonal hip rafter is solved easily by using the framing square in the manner shown. Toward the bottom of the rafter shown in the lower part of this fi  gure is a birdsmouth cut, an angled notch in the rafter that allows the rafter to seat securely on the top plate of the wall.

Figure 5.57 Framing for an overhanging rake.

Figure 5.58 A framing square being used to mark rafter cuts. The run of the roof, 12 inches, is aligned with the edge of the rafter on the blade (the wider leg) of a square, and the rise, 7 inches in this case, is aligned on the tongue (the narrower leg) of the square. A pencil line along the tongue will be perfectly vertical (a plumb cut) when the rafter is installed in the roof, and one along the blade will be horizontal (a level cut). True horizontal and vertical distances can be measured on the blade and tongue, respectively. The layout of these types of cuts can also be seen in Figures 5.52 and 5.56.

Figure 5.59 Tracing a pattern rafter to mark cuts for the rest of the rafters. The corner of the building behind the carpenters has let-in corner braces on both fl oors, and most of the rafters are already installed.

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Wall framing, like floor framing, is laid out in such a way that a framing member, in this case a stud, occurs under each vertical joint between sheathing panels (Figures 5.29-5.32). The  lead carpenter initiates wall framing by laying out the stud locations on the  top plate and sole plate of each wall. Other carpenters follow behind to cut the studs and headers and assemble the walls in a horizontal position  on the subfloor. As each wall frame is completed, it is tilted up and nailed into position, with temporary bracing  as needed (Figures 5.34-5.37).

Figure 5.29 Typical ground-floor wall framing details, keyed by letter to Figure 5.30.

Figure 5.30 Step Four in erecting a platform frame building: The ground-floor walls are framed. The letters A, B, and C indicate portions of the framing that are detailed in Figure 5.29.

Figure 5.31 Framing details for nonloadbearing interior partitions.

Figure 5.32 Steps in the framing of a typical wall and details at wall intersections and a window opening. In D, the short studs above the header and below the rough sill are called trimmer or cripple studs.

Figure 5.33 As the fi rst step in constructing a wall frame, the chief carpenter aligns the top plate and sole plate side by side on the floor platform and marks each stud location on both of them simultaneously with a pencil and framing square.

Figure 5.34 Nailing studs to a plate, using a pneu- matic nail gun. The triple studs are for a partition intersection.

Figure 5.35 Tilting an interior partition into position. The gap in the upper top plate will receive the projecting end of the upper top plate from another partition that intersects at this point.

Figure 5.36 Fastening a wall to the floor platform. Horizontal blocking between studs as seen toward the left in this photo is typically installed to provide a solid substrate for the later attachment of bathroom hardware, exterior panels, or any number of possible other items.

Figure 5.37 Lower-level wall framing is held up by temporary diagonal bracing until the floor joist framing is in place above and wall bracing or sheathing is complete, after which the frame becomes completely self-bracing. The outer walls of this building are framed with 2 x 6 (38 x 140 mm) studs to allow for a greater thickness of thermal insulation, whereas the interior partitions are made of 2 x 4 (38 x 89 mm) stock.

Figure 5.38 Prefabricated shear wall panels are especially useful in garage wall framing or other walls with large openings where only narrow, solid sections of wall remain to resist lateral forces. They may be made of all wood components or, where even greater strength is required, of wood and metal, as shown here. The corrugated galvanized steel in this panel is over 1/ 8  inch (3.5 mm) thick, and the panel’s capacity to resist lateral forces is several times greater than that of comparable all-wood prefabricated panels. The bottom of the panel will be anchored with bolts embedded 21 inches (533 mm) or more into the concrete foundation, and the sides and top will be screw-fastened to the surrounding wood framing. Holes in the panel can accommodate wiring runs within the wall.

Long studs for tall walls often must be larger than a 2 X 4 in order  to resist wind forces. Because long  pieces of sawn dimension lumber tend not to be straight and may be less  readily available, studs manufactured  of structural composite lumber may  be used instead. The International Residential Code requires horizontal  wood blocking to be installed between studs at midheight in wall frames that are taller than 10 feet (3 m). The purpose of this blocking is to stop off the cavities between studs to restrict the  spread of fire.

Headers over window and door openings must be sized in accordance  with building code criteria. Typically,  a wall opening header consists of two nominal 2-inch members standing on  edge, separated by a plywood spacer  that serves to make the header as  thick as the depth of the wall studs.  Headers for long spans and/or heavy loads are often made of laminated  veneer lumber or parallel strand lumber, either of which is stronger and stiffer than dimension lumber. Several manufacturers market prefabri- cated headers that include thermal insulation as a way of reducing the heat loss that occurs through these difficult-to-insulate assemblies. At either end, headers rest on shortened studs called trimmer or jack studs, which  themselves are nailed to full-height king studs. At the bottom of a window  opening, the  rough sill is supported  on cripple studs (Figure 5.32).

Each corner and partition intersection must furnish nailing surfaces for the edge of each plane of exterior  and interior finish materials. This requires a minimum of three studs  at each intersection, unless special  metal clips are used to reduce the number to two (Figure 5.32).

Sheathing was originally made of solid boards, usually 6 to 10 inches  (150 to 250 mm) wide. If applied  horizontally, these boards did little  to brace the building against wracking caused by the forces of wind or
earthquake. If applied on a diagonal, however, they produced a rigid frame. Today, walls are sheathed with  either plywood or OSB, which provide permanent, very stiff bracing, or with let-in bracing  as described below. Panels are applied as soon as possible after the wall  is framed, often while it is still lying on the floor platform.

In regions of strong winds or earthquakes, wall sheathing plays  a very important part in the lateral stability the structure. A properly sheathed wall acts as a  shear wall to  resist lateral forces. Both interior and  exterior walls can act as shear walls,  which must be provided in both east west and north-south orientations  and must be distributed more or less symmetrically in the floor plan. Stresses within shear walls are proportional  to their length, with shorter walls being subjected to higher stresses than those that are longer. Where walls have large openings for windows or doors, the remaining solid portions  that can contribute to lateral force resistance may become relatively short and therefore exposed to very  high stresses. In such cases, sheathing may have to be attached with  larger nails at very close spacings, the  horizontal edges of sheathing panels  may have to be supported by wood  blocking to keep them from buckling, and studs over which sheathing panels join may need to be larger in  size, such as 3 x or 4 x (64 mm or 89 mm) members, in order to hold  the required nails without splitting.

Where the needed strength cannot  be reliably achieved using site-con- struction methods, factory-fabricated  panels made of wood or steel com- ponents may be used (Figure 5.38).  Shear walls subject to high forces may also require hold-downs to prevent the  walls from pulling up off the foundation or floor platform (Figures 5.39-5.41). Consultation with a structural engineer is recommended (and often  legally required) when building in areas that are earthquake-prone or subject to very high wind forces.

Sheathing panels made from wood or paper fiber, plastic foam, and glass fiber are intended principally as  thermal insulation and to provide a base for water-resistant building paper or house wrap. Most panels of these types are nonstructural, so walls sheathed with such panels rely on  letin diagonal bracing or strategically located structural panels for lateral force  resistance. Let-in bracing may be made  of wood members, such as 1 x 4 (19 x 89 mm) boards, or light steel members that are recessed into the outer  face of the studs of the wall before it is sheathed (Figures 5.32 and 5.42).

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each floor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.

Figure 5.42 Applying a panel of insulating foam sheathing. Because this type of sheathing is too soft and weak to brace the frame, diagonal bracing is inserted into the outside faces of the studs at the corners of the building. Steel bracing, nailed at each stud, is used in this frame and is visible just to the right of the carpenter’s leg.

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Floor framing (Figure 5.19) is laid out in such a way that the ends of un-cut 4-foot by 8-foot (1.2-m by 2.4-m) subflooring panels will fall directly over joists; otherwise, many panels will have to be cut, wasting both materials and time. Standard joist spacings are 16 or 24 inches o.c. (406 or 610 mm o.c.; o.c. stands for on  center,  meaning that the spacing is measured from center to center of the joists). Though less common, a joist spacing of 19.2 inches (486 mm) can also be used. Any of these spacings automatically provides a joist to support each end of every 8-foot (2.4-m) panel (Figure 5.23).

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- floor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the  floor joists over the foundation.

Figure 5.23 Installing floor joists. Blocking will be inserted between the joists over the two interior beams to prevent overturning of the joists.

Wood composite I-joists or manufactured trusses are frequently used in place of solid wood joists. Their advantages include the ability to span greater distances, straightness  and uniformity, and reduced drying  shrinkage of the floor  platform. These joist types are most frequently  spaced at 19.2 or 24 inches o.c. (488  or 610 mm o.c.) I-joists, with their thin webs, often require reinforcing where concentrated loads occur,  such as at joist ends, at cantilevers, or where interior loadbearing partitions bear on the joists (Figures 5.44 and 5.45).

Figure 5.44 Alternative details to those shown in Figure 5.43 for a fl oor framed with I-joists rather than solid lumber. For clarity, the web of the I-joist is rendered light gray and blocking with a darker shade. The web stiffener shown in A is cut slightly shorter than the height of the web of the joist, and is installed so that a small space remains between the top of the blocking and the underside of the top fl ange. This prevents the blocking from prying the fl anges apart if the I-joist itself shrinks or is compressed slightly (see also Figure 5.45). The I-joist blocking panel in B-1 is exactly analogous to the solid blocking shown in Figure 5.45 B. A squash block is shown in B-2. This is a short section of 2x (38-mm) framing, installed vertically like a very short stud on either side of the I-joist. Squash blocks are used under points of concentrated load, such as under loadbearing posts or studs on either side of a large opening in a wall above. They are cut slightly longer than the full height of the joist to ensure that the loads are transmitted through the blocks and not the joist

Figure 5.45 Blocking where an I-joist frames into a glue-laminated wood beam, as illustrated in Figure 5.44A. This blocking acts as a web stiffener and also prevents the I-joist from rotating within the metal hanger, which is not tall enough to restrain the upper fl ange of the joist. The need for blocking depends on the size of the I-joist, the magnitude of the loads, and the type of hanger used.

After floor framing is complete, subflooring is fastened in place. To reduce squeaking in the finished floor and to increase floor  stiffness, adhesive may be used in conjunction with mechanical fasteners (Figures 5.26 and 5.27). Deformed shank nails or screws, either of which hold subflooring more securely than common nails, also contribute to the prevention of squeaks. Plywood and OSB panels must be laid with  the grain of their face layers perpendicular to the direction of the joists because these panels are considerably stiffer in this orientation (that is, a standard 4-foot by 8-foot (1.2-m by 2.4-m) panel should be laid with its  longer dimension perpendicular to the joists). Sheathing and subflooring panels are normally manufactured  1 /8  inch (3 mm) short in each face dimension so that they may be spaced slightly apart at all their edges  to prevent floor buckling during construction from the expansion of storm-wetted panels. Bridging, consisting of solid blocking or wood or metal crossbracing inserted between joists at  midspan or at intervals not exceeding  8 feet (2.4 m), is a traditional feature of floor framing (Figures 5.19, 5.28). (The term blocking refers, in general, to short lengths of lumber inserted between framing members for any  of a variety of purposes, such as bracing, reinforcing, or providing solid backing where fasteners are required for attachment of finish materials or equipment.) Its function is to hold the joists straight and to help them  share concentrated loads. The International Residential Code requires bridging only for joists deeper than 2 x 12 feet (38 x 286 mm). In better-quality construction, however, bridging is frequently used in all floors, regardless of depth, to improve stiffness and reduce vibration. Solid blocking is also required wherever floor joists span continuously across supporting beams to resist overturning of the joists (Figure 5.18B, D).

Where the ends of joists butt into supporting headers, as around stair openings and at changes of joist di- rection for projecting bays, end nails and toe nails cannot transfer the full  load between members, and sheet  metal joist hangers must be used. Each hanger provides a secure pocket  for the end of the joist and punched holes into which a number of special short nails are driven to make a safe connection.

Figure 5.26 Applying OSB subfl ooring. Note that the longer dimension of the panel runs perpendicular to the joist framing.

Figure 5.27 For a stiffer, quieter fl oor, subfl  ooring should be glued to the joists. The adhesive is a thick mastic that is squeezed from a sealant gun on the tops of the joists just before the subfl  ooring is laid in place.

Figure 5.28 Bridging between joists may be solid blocks of joist lumber, wood crossbridging, or, as seen here, steel crossbridging. During manufacture, the thick steel strip is folded across its width into a V- shape to increase its stiffness. Steel crossbridging with one end toothed requires only one nail per piece and no cutting to size, so it is the fastest to install.

Figure 5.18 Ground-fl oor framing details, keyed to the lettered circles in Figure 5.19. A compressible or resilient sill sealer strip should be installed between the sill and the top of the foundation to reduce air leakage but is not shown on these diagrams. Using accepted architectural drafting conventions, continuous pieces of lumber are drawn with an X inside and intermittent blocking with a single diagonal. The metal termite shield in detail C is used in areas where the risk of termite infestation is high. It prevents subterranean termites from traveling undetected from cracks in the concrete into the wood framing above.

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The foundation sill plate, usually made of preservative-treated wood for added resistance to insects and moisture, is bolted to the foundation as a base for the wood framing (Figure 5.18,  A, B, and C). A single sill, as shown in the details here, is all that is required by code, but in better-quality  work, the sill may be doubled or made from thicker stock for greater stiffness. A thicker sill plate may also  be required in areas of high seismic risk, to provide a stronger connection between the wood framing and  the foundation anchor bolts. Because the top of a foundation wall is usually somewhat uneven, the sill may  also be shimmed up at low spots with wood shingle wedges or plastic shims to provide a more level, even base  for subsequent framing. A  sill sealer, made of compressible or resilient  material, should be inserted between  the sill and the foundation to reduce air infiltration through the gap. Some  sill sealer materials also discourage  moisture wicking up from the foundation into the wood framing (Figure 5.22). Conventional anchor bolts  are sufficient to hold most buildings on their foundations, but tall frames or those subject to high winds  or strong earthquakes may require more elaborate attachments (Figures 5.39-5.41).

Figure 5.22 Carpenters apply a preservative-treated wood sill to a sitecast concrete foundation. A strip of compressible glass fiber sill sealer has been placed on the top of the concrete wall, and the sill has been drilled to fit over the projecting anchor bolts. Before each section of sill is bolted tightly, it is leveled as necessary with wood shingle or plastic shims between the concrete and the wood. As the anchor bolts are tightened, the sill sealer squeezes down to a negligible thickness. A length of bolted-down sill is visible at the upper right. The basement windows were clamped into reusable steel form inserts and placed in the formwork before the concrete was cast. After the concrete was cast and the formwork was stripped, the steel inserts were removed, leaving a neatly formed concrete frame around each window. A pocket in the top of the wall for a steel beam can be seen at the upper left.

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each fl oor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting.

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.

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The erection of a typical platform  frame (referred to, imprecisely, as  rough carpentry in the architectÕs specifications) can best be understood by following this post s sequential  isometric diagrams, beginning with  Figure 5.19. Notice the basic simplici- ty of the building process: A platform  is built on top of the foundation.  Walls are assembled horizontally on the platform and tilted up into place.

Another platform or a roof is built on top of the walls. Most of the work is accomplished without the use of ladders or scaffolding, and temporary  bracing is needed only to support the walls until the next level of framing is  installed and sheathed.

The details of a platform frame  are not left to chance. While there  are countless regional and personal  variations in framing details and techniques, the sizes, spacings, and  connections of the members in a  platform frame are standardized  and closely regulated by building  codes, even down to the size and  number of nails for each connection (Figure 5.21) and the thicknesses and nailing patterns of the sheathing panels.

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- fl oor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the floor joists over the foundation.

Figure 5.20 Alternative ways of constructing an interior line of support for ground-fl oor joists. Space is left over the tops of the beams in B and D to allow for drying shrinkage in the joists.

Figure 5.21 Platform framing members are fastened according to this nailing schedule, which framing carpenters know by memory and which  is incorporated into most building codes and construction standards.