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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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.    

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Once the general contractor has been selected and the contract awarded, the construction work begins, as described in the   contract documents  . The contract documents are virtually the same as the bidding documents, except that the contract documents are part of a signed legal contract between the owner and the contractor. They generally do not contain Division 00 of the MasterFormat.

In preparing the contract documents, the design team’s challenge is to efficiently produce the graphics and text that effectively communicate the design intent to the construction professionals and the related product suppliers and manufacturers so that they can do the following:

  •    Propose accurate and competitive bids
  •    Prepare detailed and descriptive submittals for approval
  •    Construct the building with a minimum number of questions, revisions, and changes
   
SHOP DRAWINGS

The construction drawings and the specifications should provide a fairly detailed delineation of the building. However, they do not describe it to the extent that fabricators can produce building components directly from them. Therefore, the fabricators generate their own drawings, referred to as   shop drawings , to provide the higher level of detail necessary to fabricate and assemble the components.

Shop drawings are not generic, consisting of manufacturers’ or suppliers’ catalogs, but  are specially prepared for the project by the manufacturer, fabricator, erector, or subcontractors. For example, an aluminum window manufacturer must produce shop drawings to show that the required windows conform with the construction drawings and the specifications. Similarly, precast concrete panels, stone cladding, structural steel frame, marble or  granite flooring, air-conditioning ducts, and other components require shop drawings  before they are fabricated and installed.

Before commencing fabrication, the fabricator submits the shop drawings to the general  contractor. The general contractor reviews them, marks them “approved,” if appropriate, and then submits them to the architect for review and approval. Subcontractors or   manufacturers cannot submit shop drawings directly to the architect.

The review of all shop drawings is coordinated through the architect, even though they may actually be reviewed in detail by the appropriate consultant. Thus, the shop drawings pertaining to  structural components are sent to the architect and then to the structural consultant for review and approval. The fabricator generally begins fabrication only after receiving the architect’s review of the shop drawings.

The review of shop drawings by the architect is limited to checking that the work indicated therein conforms with the overall design intent shown in the contract documents. Approval of shop drawings that are later discovered to deviate from the  contract documents does not absolve the general contractor of the responsibility to comply with the contract documents for quality of materials, workmanship, or the dimensions of the fabricated components,  Figure  1.9.


MOCK-UP SAMPLES

In addition to shop drawings, full-size mock-up samples of one or more critical elements of the building may be required in some projects. This is done to establish the quality of materials and workmanship by which the completed work will be judged. For example, it is not unusual for the architect to ask for a mock-up of a  typical area of the curtain wall of a high-rise building before the fabrication of the actual curtain wall is undertaken. Mock-up samples go through the same approval process as the shop drawings.

OTHER SUBMITTALS

In addition to shop drawings and any mock-up samples, some other submittals required from the contractor for the architect’s review are:

•   Product  material  samples 
•   Product  data 
•   Certifications
•   Calculations

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The preconstruction phase generally begins after the construction drawings and specifications have been completed and culminates in the selection of the construction team. The construction of even a small building involves so many specialized skills and trades that the work cannot normally be undertaken by a single construction firm. Instead, the work is generally done by a team consisting of the   general contractor  and a number of specialty subcontractors .

Thus, a project may have roofing; window and curtain wall; plumbing; and heating, ventilation, and air-conditioning (HVAC) subcontractors, among others, in addition to the general contractor. The general contractor’s own work may be limited to certain components of the building (such as the structural components—load-bearing walls, reinforced concrete beams and columns, roof and floor slabs, etc.), with all the remaining work subcontracted.

In contemporary projects, however, the trend is toward the general contractors not performing any actual construction work but subcontracting the work entirely to various sub-contractors. Because the subcontractors are contracted by the general contractor, only the general contractor is responsible and liable to the owner.

In some cases, a subcontractor will, in turn, subcontract a portion of his or her work  to another subcontractor, referred to as a   second-tier subcontractor, Figure 1.6. In  that  case, the general contractor deals only with the subcontractor, not the second-tier subcontractor.

Whether the general contractor performs part of the construction work or subcontracts  the entire work, the key function of the general contractor is the overall management of  construction. This includes coordinating the work of all subcontractors, ensuring that the  work done by them is completed in accordance with the contract documents, and ensuring  the safety of all workers on the site. A general contractor with a good record of site safety  not only demonstrates respect for the workers but also improves the profit margin by lowering the cost of construction insurance.

FIGURE 1.6  Members of the construction team and their interrelationships with each other and the owner. A solid line in this illustration indicates a contractual relationship between parties. A dashed line indicates a communication link, not a contract. The relationships shown here are not absolute and may change with the nature of the project.

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Architectural design typically involves Divisions 2 to 14 of the Facilities Construction Subgroup. Although the basis for sequencing the Divisions in this subgroup is far more complicated, the first few divisions (those that are used in virtually all buildings) may be deduced by visualizing the sequence of postearthwork operations required in constructing the simple building shown in  Figure 1.5. The building consists of load-bearing masonry walls, steel roof joists, and wood roof deck.

FIGURE 1.5  A simple load-bearing masonry wall building with steel roof trusses and wood roof deck used as an aid to recalling the sequence of the first few (architecturally important) divisions of the MasterFormat.

The first operation is the foundations for the walls. Because foundations are typically made of concrete,   Concrete  is Division 03. After the foundations have been completed, masonry work for the walls can begin. Thus,   Masonry  is Division 04. After the walls are completed, steel roof joists can be placed. Thus, Division 05 is   Metals . The installation of the wood roof deck follows that of the steel joists. Hence,   Wood, Plastics, and Composites is Division 06.


After the roof deck is erected, it must be insulated and protected against weather. Therefore,   Thermal and Moisture Protection  is Division 07. Roofing and waterproofing (of basements) are part of this division, as are wall insulation and joint sealants. The next step is to protect the rest of the envelope; hence, Division 08 is   Openings . All doors and windows are part of this division, regardless of whether they are made of steel, aluminum, or wood.

With the envelope protected, finish operations, such as those involving the interior dry- wall, flooring, and ceiling, can begin. Thus, Division 09 is   Finishes . Division 10 is Specialties , which consists of several items that cannot be included in the previous divisions, such as toilet partitions, lockers, storage shelving, and movable partitions.

Obviously, the building must now receive all the necessary office, kitchen, laboratory, or other equipment. Thus, Division 11 is   Equipment . Division 12 is   Furnishings ,  followed  by Special Construction  (Division 13) and   Conveying Equipment   (Division  14).   

Before any construction operation can begin, there must be references to items that apply to all divisions, such as payment procedures, product-substitution procedures, contract-modification procedures, contractor’s temporary facilities, and regulatory requirements imposed by the city or any other authority having jurisdiction. This is Division 01, General Requirements  . Division 00 (  Procurement and Contracting  Requirements  ) refers to the requirements for the procurement of bids from prospective contractors.

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Architectural design typically involves Divisions 2 to 14 of the Facilities Construction Subgroup. Although the basis for sequencing the Divisions in this subgroup is far more complicated, the first few divisions (those that are used in virtually all buildings) may be deduced by visualizing the sequence of postearthwork operations required in constructing the simple building shown in  Figure 1.5   . The building consists of load-bearing masonry walls, steel roof joists, and wood roof deck.

FIGURE 1.5 A simple load-bearing masonry wall building with steel roof trusses and wood roof deck used as an aid to recalling the sequence of the first few (architecturally important) divisions of the MasterFormat.

The first operation is the foundations for the walls. Because foundations are typically  made of concrete,   Concrete  is Division 03. After the foundations have been completed, masonry work for the walls can begin. Thus,   Masonry  is Division 04. After the walls are completed, steel roof joists can be placed. Thus, Division 05 is   Metals . The installation of the wood roof deck follows that of the steel joists. Hence,   Wood, Plastics, and Composites is Division 06.

After the roof deck is erected, it must be insulated and protected against weather. Therefore,   Thermal and Moisture Protection  is Division 07. Roofing and waterproofing (of basements) are part of this division, as are wall insulation and joint sealants. The next step is to protect the rest of the envelope; hence, Division 08 is   Openings . All doors and windows are part of this division, regardless of whether they are made of steel, aluminum, or wood.

With the envelope protected, finish operations, such as those involving the interior dry-wall, flooring, and ceiling, can begin. Thus, Division 09 is   Finishes . Division 10 is   Special-
ties , which consists of several items that cannot be included in the previous divisions, such
as toilet partitions, lockers, storage shelving, and movable partitions.

Obviously, the building must now receive all the necessary office, kitchen, laboratory, or other equipment. Thus, Division 11 is   Equipment . Division 12 is   Furnishings ,  followed  by  Special Construction  (Division 13) and   Conveying Equipment   (Division  14).   

Before any construction operation can begin, there must be references to items that  apply to all divisions, such as payment procedures, product-substitution procedures, contract-modification procedures, contractor’s temporary facilities, and regulatory requirements imposed by the city or any other authority having jurisdiction. This is Division 01, General Requirements  . Division 00 (  Procurement and Contracting  Requirements  ) refers to the requirements for the procurement of bids from prospective contractors.

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Just as the construction drawings are prepared separately by the architect and each consultant for their respective portions of the work, so are the specifications. The specifications from various design team members are assembled by the architect in a single document called the   project manual . Because the specifications consist of printed (typed) pages (not graphic images), a project manual is a bound document—like a book.

The major component of a project manual is the specifications. However, the project manual also contains other items, as explained later in this chapter.

The set of construction drawings (from various design team members) and the project manual together constitute what is known as the   construction document set, Figure 1.3. The construction document set is the document that the owner and architect use to invite bids from prospective contractors.

FIGURE 1.3 A construction document set consists of a set of architectural and consultants’ construction drawings plus the project manual. The project manual is bound in a book format.

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Buildings cannot be constructed from drawings alone, because there is a great deal of information that cannot be included in the drawings. For instance, the drawings will give the locations of columns, their dimensions, and the material used (such as reinforced concrete), but the quality of materials, their properties (the strength of concrete, for example), and the test methods required to confirm compliance cannot be furnished on the drawings. This information is included in the document called   specifications. 

Specifications are written technical descriptions of the design intent, whereas the drawings provide the graphic description. The two components of the construction documents—the specifications and the construction drawings—complement each other and generally deal with different aspects of the project. Because they are complementary, they  are supposed to be used in conjunction with each other. There is no order of precedence between the construction drawings and the specifications. Thus, if an item is described in  only one place—either the specification or the drawings—it is part of the project, as if  described in the other.

For instance, if the construction drawings do not show the door hardware (hinges, locks, handles, and other components) but the hardware is described in the specifications, the owner will get the doors with the stated hardware. If the drawings had precedence over the specifications, the owner would receive doors without hinges and handles.

Generally, there is little overlap between the drawings and the specifications. More importantly, there should be no conflict between them. If a conflict between the two documents is identified, the contractor must bring it to the attention of the architect promptly. In fact, construction contracts generally require that before starting any portion of the project, the contractor must carefully study and compare the drawings and the specifications and report inconsistencies to the architect.

If the conflict between the specifications and the construction drawings goes unnoticed initially but later results in a dispute, the courts have in most cases resolved it in favor of the specifications—implying that the specifications, not the drawings, govern the project. However, if the owner or the design team wishes to reverse the order, it may be so stated in the owner-contractor agreement.

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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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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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Wood construction has evolved into two major systems of on-site construction: The hand-hewn frames of centuries past have become the heavy timber frames of today, used both for single-family houses and for larger buildings. And from the heavy timber frame has sprung wood light frame construction, which is the dominant  system for houses, small commercial  structures, and apartment buildings  even as tall as four or five  stories.

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Adhesives are widely employed in the factory production of plywood and panel products, laminated wood, wood structural components, and cabinetwork. Some such adhesives, most notably urea-formaldehyde, can release formaldehyde or other unhealthful gases long after the manu-facturing process is complete and the products have been placed into service in buildings. When used for interior carpentry, such products can be the source of indoor air quality problems.

Where products manufactured with adhesives or binders will be used indoors, those made with a more chemically stable exterior adhesive called phenol-formaldehyde, or with alternative formaldehyde-free adhesives, should be considered. Finishes or veneers that encapsulate composites also reduce emissions from the glues and binders used in their manufacture.

Adhesives are used less on the construction site, where it is more difficult to clamp and hold glued  joints and to maintain controlled environmental conditions until the adhesive has cured. In rough carpen-try work, adhesive is most commonly used for securing subflooring panels to wood framing (Figure 5.27). In this application, mastic sealant is applied using a sealant gun. The joint between the subß oor panel and the joist framing is then clamped by nailing or screwing the panels to the framing, the fasteners serving as the primary structural connection and the adhesive acting primarily to reduce squeaks in the floor. Adhesives of various types are also used in finish carpentry, usually in combination with nails or screws, to improve the strength and stability of joints in finish trim, site-built cabinets, and other architectural woodwork.

Figure 5.27 For a stiffer, quieter fl oor, subflooring  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.

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Dozens of ingenious sheet metal and metal plate devices are manufactured for strengthening common connections in wood framing. The most frequently used is the joist hanger, but all  of the devices shown in Figures 3.45 and 3.46 find extensive use. There are two parallel series of this type of  device, one made of sheet metal for  use in light framing and one made of  thicker metal plate for heavy timber  and laminated wood framing. The  devices for light framing are attached  with nails and the heavier devices  with bolts or lag screws. Where such framing devices will be exposed to  the weather or to lumber treated  with corrosive chemical treatments,  they should be made from corrosion- resistant metal such as heavily galvanized steel or stainless steel. To  prevent galvanic corrosion between  dissimilar metals, fasteners for metal  framing devices should always be  made of the same material as the  framing device: Galvanized framing  devices should be fastened with galvanized fasteners, and stainless steel  devices should be fastened with stain-less steel fasteners.

Figure 3.45 Joist hangers are used to make strong connections in fl oor framing wherever wood joists bear on one another at right angles. They are attached with special short nails driven through the holes punched in the hangers. The heavier steel beam hangers are used primarily in laminated wood construction. Post bases serve the twofold function of preventing water from entering the end of the post and anchoring the post to the foundation. The bolts and lag screws used to connect the wood members to the heavier connectors are omitted from this drawing.

Figure 3.46 The sheet metal connectors shown in this diagram are less commonly used than those in Figure 3.45, but are invaluable in solving special framing problems and in reinforcing frames against wind uplift and earthquake forces.

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Fasteners have always been the weak link in wood construction. The interlocking timber connections of the past, laboriously mortised and pegged, were weak because much of the wood in a joint had to be removed to make the connection. In todays wood connections, which are generally based on metal devices, it is usually impossible to insert enough nails, screws, or bolts in a connection to develop the full strength of the members being joined.

Adhesives and toothed plates are often capable of achieving this strength but are largely limited to factory installation. Fortunately, most connections in wood structures depend primarily ondirect bearing of one member on an-other for their strength, and a varietyof simple fasteners are adequate forthe majority of purposes.

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Hardboard is a thin, dense panel made of highly compressed wood fibers.

It is available in various thicknesses  and surface finishes, and in some  formulations is durable against weather  exposure. Hardboard is produced  in configurations for residential siding and roofing as well as in general-purpose and pegboard panels of standard dimension.

Insulating fi  berboard  sheathing is a low-density panel, usually ½ or ¾ inch (13 or 19 mm) thick, made of  wood or vegetable fibers and binders, and coated with asphalt for water  resistance. It has some thermal insulating value and is used in wood construction chieß  y as nonstructural wall  sheathing, although some panels of  this type are strong enough to also  contribute lateral force resistance to  a building frame. Other  cellulosic fiberboard panels are made from finely  processed recycled paper waste and  are used for wall sheathing, acoustical isolation, carpet underlayment, and  even structural roof decking. Panels  of these types are low in cost, make  productive use of waste or recycled  materials, and help to conserve forest  resources.

Hardwood plywood panels, made  from birch, maple, poplar, or alder veneers, are popular for use in  cabinetry and other interior finish carpentry.

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For structural uses such as sub-flooring  and sheathing, wood panels may be specified either by thickness or by  span  rating. The span rating is determined  by laboratory load testing and is given  on the gradestamp on the back of the  panel, as shown in Figures 3.33 and  3.34. The purpose of the span rating  system is to permit the use of many  different species of woods and types  of panels while achieving the same  structural objectives. Under normal  loading conditions, any panel with  a span rating of 32/16 may be used  as roof sheathing over rafters spaced  32 inches (813 mm) apart or as sub-flooring over joists spaced 16 inches  (406 mm) apart. The long dimension of the sheet must be placed perpendicular to the length of the supporting members. A 32/16 panel may be  plywood, composite, or OSB, may  be composed of any accepted wood  species, and may be any of several thicknesses, so long as it passes the  structural tests for a 32/16 rating.

Figure 3.33 Typical gradestamps for structural wood  panels. Gradestamps are found on the back of each panel. (Courtesy of APA–The Engineered Wood Association)

Figure 3.34 A guide to specifying structural panels. Plywood panels for use in wall paneling, furniture, and other applications where  appearance is important are graded by the visual quality of their face veneers rather than their structural properties.

The designer must also select from  three  exposure durability classifi  cations  for structural wood panels: Exterior,  Exposure 1, and Exposure 2. Panels marked Exterior are suitable for  use as siding or in other applications  permanently exposed to the weather. Exposure 1 panels have fully water- proof glue but do not have veneers of  as high a quality as those of Exterior panels; they are suitable for structural  sheathing and sub-flooring, which will  be protected from the weather once  the building is finished, but must often endure long periods of wetting during  construction. Exposure 2 is suitable  for panels that will be fully protected  from weather and will be subjected  to only a minimum of wetting during  construction. About 95 percent of  structural panel products are classified as Exposure 1.


For plywood panels intended as finish surfaces, the quality of the face  veneers is of obvious concern and  should be specified by the designer  (Figure 3.32). For example, A-B  plywood (with an A grade veneer on  one face and a B grade veneer on the  other) might be specified for cabinet construction, while less expensive  C-D plywood is adequate for exterior  sheathing that will be concealed in  the finished construction. For higher-quality architectural woodwork,  fine  flitch-sliced hardwood face ve- neers may be selected rather than  rotary-sliced softwood veneers and  the matching pattern of the veneers specified.

Figure 3.32 Veneer grades for softwood plywood. Ply- wood with A grade face veneer is costly and limited in availability. It is used only in applications demanding the highest quality appear- ance. Plywood with B face veneer is used for  concrete formwork construction and for less demanding appearance applications. Plywood  with C-plugged face veneer is used for under- layment and combination subfl  ooring/under- layment, where an especially smooth substrate is required to avoid telegraphing irregularities  through the fi nish fl ooring material. Plywood for sheathing and subfl ooring is most com- monly specifi ed as “CDX,” consisting of a C grade face, a D grade face, and constructed  with exterior glue (the panel is installed with the C face oriented toward the weather). C  grade veneer is also the lowest grade permitted in any ply of an Exterior rated panel, that is, a  siding panel intended for permanent exposure to the weather.

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Wood in panel form is advantageous for many building applications (Figure 3.28). The panel dimensions are usually 4 by 8 feet (1220 by 2440 mm).

Panels require less labor for installation than individual boards because  fewer pieces must be handled, and  wood panel products are fabricated  in such a way that they minimize  many of the limitations of boards and
dimension lumber: Panels are more nearly equal in strength in their  two principal directions than solid  wood; shrinking, swelling, checking,  and splitting are greatly reduced.

Additionally, panel products make  more efficient use of forest resources  than solid wood products through  less wasteful ways of reducing logs  to building products and through  utilization in some types of panels  of material that would otherwise be  thrown awayÑbranches, undersized trees, and mill wastes. Many wood  panel products are made largely  from recovered or recycled wood  waste, and panels made from rapidly  renewable vegetable fibers are also  available.

Figure 3.28 Plywood is made of veneers selected to give the optimum combination of economy and performance for each application. This sheet of roof sheathing plywood is faced with a D veneer on the underside and a C veneer on the top side. These veneers, though unattractive, perform well structurally and are much less costly than the higher grades incorporated into plywood made for uses where appearance is important.

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Wood-plastic composite (WPCs) products are made from wood lumbers and  plastics of various types, mixed with  other ingredients, such as ultraviolet stabilizers, pigments, lubricants,  and biocides, which are then heated  and pressed, extruded, or injection-molded into final form. In comparison to their solid wood counterparts,

WPC materials offer more consistent  material quality, freedom from defects and distortion, and, depending  on their formulation, superior resistance to moisture. They are used  most notably for exterior decking,  as well as for exterior railing systems  and finish trim, both interior and  exterior. Like structural composites,  WPC products make productive use  of rapidly renewable or waste materials. Some WPCs also have high recycled materials content.

WPC decking, most commonly  made from blends of polyethylene  or polypropylene and wood lumber,  is  available in sizes matching conven-tional solid wood decking in lengths  up to 20 feet (6.1 m). It may be fastened with corrosion-resistant nails or  screws, or with concealed hardware  that engages the edges of the boards.

A variety of maintenance-free colors  and textured finishes are available,  some remarkably similar in appearance to genuine hardwoods.

Ingredients and manufacturing processes used in the making of  composite wood trim vary widely, as do  the workability, surface qualities, and  durability of the finished  products.

Blends of plastic and wood similar  to those used for the manufacture  of composite decking may be used.

Alternatively, formulations with a  higher proportion of wood lumberÑ more similar in composition to traditional wood panel products, such as plywood, OSB, and fiberboard,  discussed later in this part may be  used. Products may be preÞ  nished, factory primed, encapsulated within a dense plastic outer shell, or coated with a resin-impregnated paper that  improves the quality of field-applied finishes.

In comparison to solid lumber, nonstructural composite lumber expands and contracts more with changes in temperature, so greater allowance for thermal movement must be made during installation.

And in the case of spanning members such as decking, the lesser stiffness of composites necessitates closer spacing of joists or other members on which the members are supported.

Wood trim made up from shorter lengths of finger-jointed  and  glued solid wood material is also available as an alternative to conventional finish lumber. In comparison to solid pieces, finger-joint stock is more stable and more consistently free of defects. It also makes use of short-length scraps that might otherwise be treated as waste.

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Lumber sizes in the United States are given as nominal dimensions in inches,  such as 1 x 2 (one by two), 2 x 10, and so on. At one time, sawn lumber may have approached these actual dimensions. Today however, subsequent to sawing, seasoning, and surfacing,  true sizes are less. By the time a kilndried 2 10 reaches the lumberyard, its actual dimensions are close to 1½ by 9¼ inches (38 by 235 mm). The relationship between nominal lumber  dimensions (which are always written without inch marks) and actual dimensions (which are written with inch marks) is given in simplified form in Figure 3.22 and in more complete  form in Figure 3.23. Anyone who designs or constructs wooden buildings soon commits the simpler of these  relationships to memory. Because of  changing moisture content and manufacturing tolerances, however, it is  never wise to assume that a piece of  lumber will conform precisely to its  expected dimensions. Wood members vary in size seasonally with changes in  humidity and temperature. In hot, dry  locations such as attics, wood framing  may shrink to dimensions substantially below its original measurements.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplifi ed chart, which is extracted from the complete chart in Figure 3.23.

Figure 3.23 A complete chart of nominal and actual dimensions for both framing lumber and fi  nish lumber.

Members in older buildings may have been manufactured to full nominal  dimensions or to earlier standards of  actual dimensions such as 15/ 8 inches or 1¾ inches (41 or 44 mm) for a  nominal 2-inch member.

Pieces of lumber less than 2 inches in nominal thickness (38 mm  actual thickness) are called  boards.
Pieces ranging from 2 to 4 inches  in nominal thickness (38 to 89 mm  actual thickness) are referred to collectively as  dimension lumber. Pieces  nominally 5 inches (actual size 114  mm) and more in thickness are  termed timbers.  Dimension lumber is usually supplied in 2-foot (610-mm) increments  of length. The most commonly used  lengths are 8, 10, 12, 14, and 16 feet  (2.44, 3.05, 3.66, 4.27, and 4.88 m),  but retailers frequently stock rafter  material in lengths to 24 feet (7.32 m).

Actual lengths are usually a fraction of  an inch longer than nominal lengths.

Lumber in the United States is  priced by the  board foot. Board foot measurement is based on nominal  dimensions, not actual dimensions.

A board foot of lumber is deÞ  ned as  a solid volume 12 square inches in  nominal cross-sectional area and 1  foot long. A 1 x 12 or 2 x 6 10 feet long contains 10 board feet. A 2 x  4 10 feet long contains [(2 x 4)/12] 10 6.67 board feet, and so on. Prices of dimension lumber and timbers  in the United States are usually quoted on the basis of dollars per thou- sand board feet. In other parts of the world, lumber is sold by the cubic meter. Because the board foot is based on lumber nominal dimensions and  metric sizes use actual dimensions,  there is no direct conversion between  board feet and cubic meters.

The architect and engineer specify lumber for a particular construction use by designating its species, grade,
seasoning, surfacing, nominal size, and chemical treatment, if any. When ordering lumber, the contractor must additionally give the required lengths of pieces and the required number of pieces of each length.

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The strength of a piece of wood depends chießy on its species, its grade, and the direction in which the load acts with respect to its grain.

For example, in both tension and compression, wood is typically several times stronger parallel to grain  than perpendicular to grain. With its usual assortment of defects, it is  stronger in compression than in tension. Allowable strengths (structural  stresses that include factors of safety) vary tremendously with species and  grade. For example, allowable com- pressive strength parallel to grain  for commercially available grades  and species of framing lumber varies  from 325 to 1700 psi (2.24 to 11.71 MPa), a difference of more than five times. Figure 3.21 compares the average structural properties of framing lumber to those of some other  common structural materialsÑbrick  masonry, steel, and concrete. Of the  four materials, only wood and steel  have useful tensile strength. Defect- free wood is comparable to steel on  a strength-per-unit weight basis, but  with the ordinary run of defects, an  average piece of lumber is somewhat  inferior to steel by this yardstick.

Figure 3.21 Comparative physical properties of four common structural materials: wood (shaded row), brick masonry, steel, and concrete. Wood has signifi cant strength in both tension and compression. The ranges of values of wood strength and stiffness refl ect differences among species and grades of lumber.

When designing a wooden struc- ture, the architect or engineer determines the maximum stresses that are  likely to occur in each of the structural  members and selects an appropriate  species and grade of lumber for each.

In a given locale, a limited number of  species and grades are usually avail- able in retail lumberyards, and it is  from these that the selection is made.

It is common practice to use a stronger  but more expensive species (Douglas  for or Southern pine, for example) for  highly stressed major members and  to use a weaker, less expensive species  (such as Eastern hemlock) or species  group for the remainder of the structure.  Within each species, the designer se- lects grades based on published tables of allowable stresses. The higher the  structural grade, the higher the allowable stress. But the lower the structural  grade, the less costly is the lumber.

There are many factors other  than species and grade that inß  uence  the useful strength of wood. These include the length of time the wood will  be subjected to its maximum load, the  temperature and moisture conditions  under which it serves, and the size  and shape of the piece. Certain fire retardant treatments also reduce the  strength of wood slightly. All these factors are taken into account when engineering a building structure of wood.