During fabrication and erection processes for wood construction, wood products should be handled and covered to prevent marring of the surfaces and moisture absorption. Overstressing of members and joints during handling and erection should be avoided. Competent inspectors should check materials and workmanship.
Fabrication of Structural Timber
Fabrication consists of boring, cutting, sawing, trimming, dapping, routing, planing, and otherwise shaping, framing, and finishing wood units, sawn or laminated, including panels, to fit them for particular places in a final structure. Whether fabrication is performed in shop or field, the product must exhibit a high quality of workmanship.
Jigs, patterns, templates, stops, or other suitable means should be used for all complicated and multiple assemblies to ensure accuracy, uniformity, and control of all dimensions. All tolerances in cutting, drilling, and framing must comply with good practice and applicable specifications and controls. At the time of fabrication, tolerances must not exceed those listed below, unless they are not critical and not required for proper performance. Specific jobs, however, may require closer tolerances.
Location of Fastenings. Spacing and location of all fastenings within a joint should be in accordance with the shop drawings and specifications, with a maximum permissible tolerance of +-1⁄16 in. The fabrication of members assembled at any joint should be such that the fastenings are properly fitted.
Bolt-Hole Sizes. Bolt holes in all fabricated structural timber, when loaded as a structural joint, should be 1⁄16 in larger in diameter than bolt diameter for 1⁄2-in and larger-diameter bolts, and 1⁄32 in larger for smaller-diameter bolts. Larger clearances may be required for other bolts, such as anchor bolts and tension rods.
Holes and Grooves. Holes for stress-carrying bolts, connector grooves, and connector daps must be smooth and true within 1⁄16 in per 12 in of depth. The width of a split-ring connector groove should be within 0.02 in of and not less than the thickness of the corresponding cross section of the ring. The shape of ring grooves must conform generally to the cross-sectional shape of the ring. Departure from these requirements may be allowed when supported by test data. Drills and other cutting tools should be set to conform to the size, shape, and depth of holes, grooves, daps, etc., specified in the ‘‘National Design Specification for Wood Construction,’’ American Forest & Paper Association.
Lengths. Members should be cut within 1⁄16 in of the indicated dimension when they are up to 20 ft long, and 1⁄16 in per 20 ft of specified length when they are over 20 ft long. Where length dimensions are not specified or critical, these tolerances may be waived.
End Cuts. Unless otherwise specified, all trimmed square ends should be square within 1⁄16 in per foot of depth and width. Square or sloped ends to be loaded in compression should be cut to provide contact over substantially the complete surface.
Erection of timber framing requires experienced crews and adequate lifting equipment to protect life and property and to assure that the framing is properly assembled and not damaged during handling.
Each shipment of timber should be checked for tally and evidence of damage.
Before erection starts, plan dimensions should be verified in the field. The accuracy and adequacy of abutments, foundations, piers, and anchor bolts should be determined.
The erector must see that all supports and anchors are complete, accessible, and free from obstructions.
Job-Site Storage. If wood members must be stored at the site, they should be placed where they do not create a hazard to other trades or to the members themselves.
All framing stored at the site should be set above the ground on appropriate blocking. Where practical, the members or bundles of material should be separated with strips, so that air may circulate around all sides. The top and all sides of each storage pile should be covered with a moisture-resistant covering that provides protection from the elements, dirt, and job-site debris. The use of clear polyethylene films is not recommended, since wood members may be bleached by sunlight.
Individual wrappings should be slit or punctured on the lower side to permit drainage of water that accumulates inside the wrapping. Particular care should be taken with members such as glued-laminated timber that may be exposed to view in the completed structure.
Glued-laminated members of Premium and Architectural Appearance (and Industrial Appearance in some cases) are usually shipped with a protective wrapping of water-resistant paper. While this paper does not provide complete freedom from contact with water, experience has shown that protective wrapping is necessary to ensure proper appearance after erection. Though used specifically for protection in transit, the paper should remain intact during job-site storage. Removal of the paper during erection is a designer or contractor option. For example, if the paper is removed from isolated areas to make connections from one member to another, either the paper should be replaced and should remain in position until all the wrapping is removed or all the paper should be removed to minimize uneven discoloration due to sun bleaching.
At the site, to prevent surface marring and damage to wood members, the following precautions should be taken:
Lift members or roll them on dollies or rollers out of railroad cars. Unload trucks by hand, forklift, or crane. Do not dump, drag, or drop members.
During unloading with lifting equipment, use fabric belts, or other slings that will not mar the wood. Provide additional protective blocking or padding at edges of members.
Guard against soiling, dirt, footprints, abrasions, or injury to shaped edges or sharp corners.
Equipment. Adequate equipment of proper load-handling capacity, with control for movement and placing of members, should be used for all operations. It should be of such nature as to ensure safe and expedient placement of the material. Cranes and other mechanical-devices must have sufficient controls that beams, columns, arches, or other elements can be eased into position with precision. Slings, ropes, and other securing devices must not damage the materials being placed.
The erector should determine the weights and balance points of the framing members before lifting begins, so that proper equipment and lifting methods may be employed. When long-span timber trusses are raised from a flat to a vertical position preparatory to lifting, stresses entirely different from design stresses may be introduced. The magnitude and distribution of these stresses depend on such factors as weight, dimensions, and type of truss. A competent rigger will consider these factors in determining how much suspension and stiffening, if any, is required and where it should be located.
Accessibility. Adequate space should be available at the site for temporary storage of materials from time of delivery to the site to time of erection. Material-handling equipment should have an unobstructed path from job-site storage to point of erection.
Whether erection must proceed from inside the building area or can be done from outside will determine the location of the area required for operation of the equipment. Other trades should leave the erection area clear until all members are in place and are either properly braced by temporary bracing or are permanently braced in the building system.
Assembly and Subassembly. Whether done in a shop or on the ground or in the air in the field, assembly and subassembly are dependent on the structural system and the various connections involved.
Care should be taken with match-marking on custom materials. Assembly must be in accordance with the approved shop drawings. Any additional drilling or dapping, as well as the installation of all field connections, must be done in a workmanlike manner.
Heavy-timber trusses are often shipped partly or completely disassembled. They are assembled on the ground at the site before erection. Arches, which are generally shipped in half sections, may be assembled on the ground or connections may be made after the half arches are in position. When trusses and arches are assembled on the ground at the site, assembly should be on level blocking to permit connections to be properly fitted and securely tightened without damage. End compression joints should be brought into full bearing and compression plates installed where intended.
Before erection, the assembly should be checked for prescribed overall dimensions, prescribed camber, and accuracy of anchorage connections. Erection should be planned and executed in such a way that the close fit and neat appearance of joints and the structure as a whole will not be impaired.
Field Welding. Where field welding is required, the work should be done by a qualified welder in accordance with job plans and specifications, approved shop drawings, and specifications of the American Institute of Steel Construction and the American Welding Society.
Cutting and Fitting. All connections should fit snugly in accordance with job plans and specifications and approved shop drawings. All cutting, framing, and boring should be done in accordance with good shop practices. Any field cutting, dapping, or drilling should be done in a workmanlike manner, with due consideration given to final use and appearance.
Bracing. Structural elements should be placed to provide lateral restraint and vertical support, to ensure that the complete assembly will form a stable structure.
This bracing may extend longitudinally and transversely. It may comprise sway, cross, vertical, diagonal, and like members that resist wind, earthquake, erection, acceleration, braking, and other forces. And it may consist of knee braces, cables, rods, struts, ties, shores, diaphragms, rigid frames, and other similar components in combinations.
Bracing may be temporary or permanent. Permanent bracing, required as an integral part of the completed structure, is shown on the architectural or engineering plans and usually is also referred to in the job specifications. Temporary construction bracing is required to stabilize or hold in place permanent structural elements during erection until other permanent members that will serve the purpose are fastened in place. This bracing is the responsibility of the erector, who normally furnishes and erects it. Protective corners and other protective devices should be installed to prevent members from damaged by the bracing.
In wood truss construction, temporary bracing can be used to plumb trusses during erection and hold them in place until other secondary framing and roof sheathing are installed. The major portion of temporary bracing for trusses is left in place, because it is designed to brace the complete structure against lateral forces.
Failures during erection occur occasionally and regardless of construction material used. The blame can usually be placed on insufficient or improperly located temporary erection guys or braces, overloading with construction materials, or an externally applied force sufficient to render temporary erection bracing ineffective.
(See ‘‘Handling, Installing and Bracing Metal Plate Connected Wood Trusses,’’ Truss Plate Institute, Madison, Wis., for guidance in erection of lightweight metal plate connected, wood trusses.)
Structural members of wood must be stiff, as well as strong. They must also be properly guyed or laterally braced, both during erection and permanently in the completed structure. Large rectangular cross sections of glued-laminated timber have relatively high lateral strength and resistance to torsional stresses during erection.
However, the erector must never assume that a wood arch, beam, or column cannot buckle during handling or erection.
Specifications often require that:
1. Temporary bracing shall be provided to hold members in position until the structure is complete.
2. Temporary bracing shall be provided to maintain alignment and prevent displacement of all structural members until completion of all walls and decks.
3. The erector should provide adequate temporary bracing and take care not to overload any part of the structure during erection.
While the magnitude of the restraining force that should be provided by a cable guy or brace cannot be precisely determined, general experience indicates that a brace is adequate if it supplies a restraining force equal to 2% of the applied load on a column or of the force in the compression flange of a beam. It does not take much force to hold a member in line; but once it gets out of alignment, the force then necessary to hold it is substantial.