Sections 2304, 2305, and 2306 contain the requirements for allowable stress design.This chapter contains provisions for materials, design and construction, and the quality of wood and wood fasteners.
There has been a change in the code regarding the general design requirements of structural elements or systems constructed in part or in whole of wood or wood-based products. These must be in accordance with the following:
Sections 2304, 2305, and 2306 contain the requirements for allowable stress design.
Sections 2304, 2305, and 2307 contain the requirements for load and resistance factor design.
Sections 2304 and 2308 contain the requirements for conventional light-frame construction.
Note this exception: Buildings that have been designed in accordance with the provisions of the AF&PA WFCM are considered to have met the requirements of Section 2308.
Dimensions of lumber are reasoned to be nominal dimensions unless selected as actual dimensions for the purpose of this chapter.
This section contains the minimum standards and quality for the different types of wood and other materials used in construction and include, but are not limited to: preservative-treated wood; structural log members; prefabricated wood I-joists; staples; and nails.
Sawn lumber used for load-supporting purposes must be identified by the grade mark of a lumber grading agency. You can use an inspection agency too, but keep in mind that the agency you use must be approved by an accreditation body that complies with DOC PS 20 or equal. All grading practices and identification has to comply with the procedures of DOC PS 20, as well. A certificate of inspection may be accepted instead of the grade mark, but only if all provisions have been met. You are also permitted to use approved end-jointed lumber instead of solid-sawn members as long as they are of the same species and grade.
There are many items in this chapter that also must follow the guidelines as set forth in other referenced standards. These include the following:
Prefabricated wood I-joists—ASTM D 5055
Structural glued-laminated timber—AITC A 190.1 and ASTM D 3737
Wood structural panels—DOC PS 1 or PS 2
Hardboard—AHA A 135.6
Pre-finished hardboard—AHA A 135.5
Particleboard—ANSI A208.1
Preservative-treated wood—AWPA U1 and M4.
CODE UPDATE |
Conventional light-frame construction is a type of construction where the primary structural elements are formed by a system of repetitive wood-framing members. |
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When applied to the outside of foundation walls, fiberboard wall insulation must be protected below the ground level with a bituminous coating. |
Fiberboard and its various uses must conform to ASTM C 208, as well as fiberboard sheathing. Edges must be tight-fitting with square or U-shaped joints. When fiberboard is used as insulation, regardless of construction, you must ensure its protection with an approved roof covering.
Fiberboards are allowed as wall insulation in all types of construction as long as they are installed and fireblocked. If used in fire walls and barriers, but not treated to comply with Section 803.1 for Class A materials, you must be sure to cement the boards directly to the concrete or other non-combustible base. They must be protected with an approved noncombustible veneer anchored to the base without intervening airspaces.
Wood used as trusses must be preservative treated and marked with a quality mark of an inspection agency that upholds continuing supervision, testing, and inspection over the quality of the preservative-treated wood. All inspection agencies for such wood must be listed by the accreditation body that complies with the requirements of the American Lumber Standards Treated Wood Program. The label that is affixed on the wood must contain the following:
Identification of the manufacturer that treated the wood
Type of preservative used
Minimum preservative retention
End use for which the product is treated
AWPA standard to which the product was treated
Identification of the inspection agency.
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Preservative-treated wood that is used in enclosed locations without a reasonable means of drying must have a moisture content of 19 percent or less before it can be covered with materials such as insulation, interior wall finish, or other materials. |
All fire-retardant treated lumber and wood structural panels also must be labeled with the following:
An I.D. mark from an approved agency
Identification of the treating manufacturer
Name of the treatment
Species of wood
Flame spread and smoke index
Method of drying that was used
Conformance with appropriate standards
For wood exposed to weather, include these words: “No increase in the listed classification when subjected to the Standard Rain Test” (ASTM D 2898).
Wherever fire-retardant treated wood is exposed to damp or wet locations, it must be identified as exterior. This indicates that there is no increase in the listed flame spread index. For interior uses, fire-retardant treated wood cannot have a moisture content over 28 percent when tested in accordance with ASTM D 3201. This type of wood has to be dried so that the moisture content is 19 percent or lower for lumber and 15 percent or lower for wood structural panels.
When working with general construction requirements, you will be working with lay-up patterns. The five standard patterns that can be used are as follows:
Simple span pattern: All pieces are supported by two supports
Two-span continuous pattern: All pieces are supported by three supports with all end joints occurring in line on every other support.
Combination simple and two-span continuous pattern: End spans alternate with simple and two-span continuously. Stagger end joints in adjacent courses only over supports.
Cantilevered pieces intermixed pattern: For this pattern, the decking covers a minimum of three spans. Pieces in the starter course and every third course must be simple span. Cantilever the pieces in the other course over the supports with end joints alternating.
Controlled random pattern: This decking covers a minimum of three spans with end joints within 6 inches of being in line in either direction. There are no end joints in this pattern.
Be aware of changes for mechanically laminated decking if that is what you choose. This decking consists of square-edged dimension lumber laminations that are set on edge and nailed to the adjacent piece and to the support. Also, you cannot use nails that are less than 2.5 times the net thickness of each lamination either.
When using sheathing nails, make sure that the head or crown is just flush with the surface of the sheathing and not into the sheathing. The code does allow use of clips, glues, or any other methods as long as they are approved. When using fasteners in preservative-treated wood, you must use hot dipped zinc-coated galvanized steel. You can also use stainless steel or copper, but the coating weights must be in accordance with ASTM A 153.
CODE UPDATE |
The following types of wood are considered termite resistant: redwood, Alaska yellow-cedar, eastern red cedar, and both heartwood and all sapwood of western red cedar. |
The exception to this allows fasteners other than nails, timber rivets, or wood screws which are mechanically deposited zinc coated steel, but these must have a coating weight in accordance with ASTM B 695, Class 55 minimum.
As a reminder, fastenings for wood foundations must meet the requirements of AF&PA Technical Report No. 7.
Termite resistant wood must be used for wood floor framing, especially in areas heavily populated with termites. Water-borne preservatives, in accordance with AWPA U1, are used to preserve such wood for use above the ground. There are many woods used above the ground, such as wood joists, sleepers and sills, wood siding, and girder ends, which are connected in one way or another to the foundation.
All buildings and structures need to have lateral-force-resisting systems to resist seismic or wind loads. Then again, compliance with the AF&PA is allowed subject to the limitations of this code. To calculate shear resistance of diaphragms and shear walls, you can use principles of mechanics by using the values of fastener strength and sheathing shear resistance. You must have positive connections and anchorages between the shear panel and the attached components that are capable of resisting the design forces. The capacity of toenail connections cannot be used in Seismic Design Category D, E, or F when calculating lateral load resistance to transfer lateral earthquake forces in excess of 150 pounds per feet.
For structures of Seismic Design Category F, you must follow certain requirements. Wood structural panel sheathing that is used for diaphragms and shear walls that are part of the seismic-force-resisting systems must be applied directly to the framing members, except for wood structural panel sheathing in a diaphragm which is allowed to be fastened over solid lumber planking only if the panel joints and lumber planking joints do not coincide.
CODE UPDATE |
Wood and wood-based materials that use vacuum-pressure impregnation processes to enhance fire retardant or preservative properties are known as treated wood. |
When hardboard siding is used structurally, it must be identified as a conforming material by an approved agency. |
When designing structures with rigid diaphragms, refer to the requirements of Section 12.3.2 of ASCE 7 and the horizontal shear distribution requirements of Section 12.8.4 of ASCE 7, as rigid diaphragms must comply with these sections. Refer to your local code book to find the diaphragm length and width for plan view of open-front buildings.
The following list of applicable standards contains the provisions in which the allowable stress design of wood elements must comply.
American Forest & Paper Association
American Institute of Timber Construction
American Society of Agricultural Engineers
APA—The Engineered Wood Association
Truss Plate Institute, Inc.
There has been a change in the code for some parts of this section, such as wood diaphragms. Wood structural panel diaphragms are allowed to resist horizontal forces by using the allowable shear capacities. The allowable capacities that are found in the code must be increased by 40 percent for wind design, and sheathed lumber (diagonally) diaphragms must be nailed in accordance with nailing requirements.
This brings us to the application of gypsum board, or lath, and plaster to wood framing. There are several steps to take to ensure proper adherence. When joint staggering, make sure the end joints of adjacent courses of gypsum board do not occur over the same studs. You must be familiar with the maximum allowable shear values for blocking, fasteners, gypsum lath, and sheathing for gypsum boards.
As the title suggests, this section is intended for conventional light-frame construction. Other methods are allowed, but only if you submit a satisfactory design that is in compliance in every way necessary with this code. One- and two-family homes and multiple single-family buildings, such as townhomes, no more than three stories above grade plane with separate means of exit, must comply with the International Residential Code.
The spans for floor joists must comply with tables found in your local code book.
For other spans and species that you cannot locate in the building code, please refer to AF&PA Span Tables for Joists and Rafters. The ends of each joist, other than where supported on a 1 inch by 4 inch ribbon strip and nailed, cannot have less than 1½ inches of bearing on wood or metal or less than 3 inches. Make sure that all joists are supported laterally at the ends and at each support by solid blocking, except where the ends of the joists are nailed to the header.
The ends of header joints that are longer than 6 feet must be supported by framing anchors or joist hangers unless the header joints are bearing on a beam or wall.
A braced wall line is a series of braced wall panels in a single story. All braced wall lines must meet all the requirements of this code. Such requirements include location, type and amount of bracing.
!Definition alert |
Floor joists: The main sub-floor framing members that support the floor span. |
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When joist framing is on opposite sides of a beam or girder, make sure it is lapped at least 3 inches. If not lapped, then tie the opposite ends together in the manner which has been approved. A joist must be cut away to give way for floor openings. Specifications usually require that headers be double and framed between the full-length joists, also known as trimmers, on either side of the floor opening. |
There are several methods to construct braced wall panels. Use one of the following:
Wood boards of 
-inch minimum thickness applied diagonally on studs with spacing of no more than 24 inches
Fiberboard sheathing panels of no less than ½-inch thickness applied vertically or horizontally on studs with spacing of no more than 16 inches
Portland cement plaster on studs spaced 16 inches
Remember that ceiling joists must have a minimum allowable span.
Purlins that are installed for roof load support can be used to reduce the span of rafters within the allowable limits. These must be supported by struts to bearing walls and under no circumstance can a purlin be smaller than the rafter that it is supporting. This horizontal structural member must adhere to the allowable limits. For example, a purlin that is 2 inches by 4 inches, has a maximum length of 4 feet, and if the purlin is 2 inches by 6 inches, the maximum length is 6 feet.
As mentioned several times previously, there are many changes to the International Building Code. One of these changes states that engineered wood products, which can be composite lumber, prefabricated I-Joists, and structural glue-laminated timber, cannot be notched or drilled unless the manufacturer has given permission to do so or unless the effects of doing so are specifically stated in the design by a registered design professional.
Prulins: Framing members that support a roof panel assembly. |
I make it a point to mention that there are additional requirements for conventional construction in Seismic Design Category B or C. In Seismic Design Category C, structures of conventional light-frame construction cannot be more than two stories in height, and concrete or masonry walls cannot extend above the basement. Be aware of the following exceptions:
You may use masonry veneer in the first two stories above grade plane or in the first three stories where the lowest story has concrete or masonry for Category B, only if panel wall bracing is used and is 1½ times the required length.
You may use masonry veneer in the first story above grade plane or the first two stories above grade plane where the lowest story has concrete or masonry walls in Category B or C.
You may use masonry veneer in the first two stories above grade plane in Category B or C if the type of brace used in Section 2308.9.3 is Method 3 and the bracing of the top story is located at each end and at least every 25 feet. You must also provide hold-down connectors at the ends of braced walls for the second floor to the first floor assembly with a design of 2000 pounds.
CODE UPDATE |
The written, graphic, and pictorial depiction of each individual truss shall be provided to the building official for approval prior to installation. |
You may not use cripple walls.
In the remainder of this chapter I will cover irregular structures. You may not use conventional light-frame construction in irregular parts of structures in Seismic Design Category D or E.
There are six conditions in which a portion of a building or structure will be deemed irregular. If your building or structure contains one or more of these conditions you have an irregular structure. Such conditions are as follows:
Exterior braced wall panels are required to be in one plane vertically from the foundation to the uppermost story. If this is not true of your structure, it will be considered to be irregular. As with many sections of this book, this too has exceptions. Floors with cantilevers that are not more than four times the depth of the floor joists are permitted provided that the following is true:
– Floor joists are 2 inches by 10 inches or larger.
– The back span to the cantilever has a ratio of 2:1.
– Floor joists at the ends of the braced walls are doubled.
– A continuous rim joint is connected to the ends of cantilevered joists.
The ends of cantilevered joists cannot carry gravity loads from more than a single story.
Any section of a floor or roof that is not laterally supported by braced wall lines on all edges will be considered irregular; however, portions of roofs or floors that do not support braced wall panels are allowed to extend up to 6 feet.
CODE UPDATE |
Wood products that are impregnated with chemicals by a pressure process shall be processed in a closed vessel under pressure not less than 50 pounds per square inch gauge. |
The design for the transfer of loads and anchorage of each truss to the supporting structure is the responsibility of the registered design professional. |
If the end of a required braced wall panel is more than 1 foot over an opening in the wall below, your structure is irregular. Braced wall panels are allowed to extend more than 8 feet in width if the header is 4 inches by 12 inches or larger.
If portions of a floor level are vertically offset so the framing members on either side cannot be lapped or tied, you will qualify for an irregular structure. Framing supported directly by foundations do not need to be lapped or tied.
If braced wall lines are not perpendicular to each other, you have an irregular structure.
Openings in floor and roof diaphragms that have a maximum dimension more than 50 percent of the distance between lines of bracing will put your structure in the irregular structure category.
CODE UPDATE |
Where wood joists or the bottom of a wood structural floor without joist are closer than 18 inches, or wood girder are closer than 12 inches to the exposed ground in crawl spaces or unexcavated areas located within the perimeter of the building foundation, the floor construction shall be of naturally durable or preservative-treated wood. |
