Building design made simple

May 2004 » Feature Article
The conventional construction provisions contained in the International Building Code (IBC) are entirely prescriptive.
John R. Henry, P.E.

The conventional construction provisions contained in the International Building Code (IBC) are entirely prescriptive and are intended to apply to buildings constructed of repetitive, light, wood framing members consisting of studs, joists, and rafters. Prescription is defined as the action of laying down authoritative rules or directions. Prescriptive means acquired by, founded on, or determined by prescription or by long-standing custom. Together, these two definitions describe clearly the nature of the conventional wood-frame construction provisions in the IBC: they are a set of rules based upon long-standing custom.

History of the provisions

The underlying philosophy of prescriptive, conventional construction originated with the repetitive, right-frame, wood construction provisions of the 1970 Uniform Building Code (UBC). It has remained unchanged through all editions of the UBC—up to and including the 1997 edition—and states, "The requirements contained in this section are intended for light-frame construction. Other methods may be used, provided a satisfactory design is submitted showing compliance with other provisions of this code." In other words, one need not conform to the restrictions, limitations, and requirements of the provisions if a design conforms to the engineering requirements of the code.

In addition, the 1970 UBC included an exception in the engineering chapter that stated, "Buildings or portions thereof that are constructed in accordance with the conventional, light-framing requirements specified in Chapter 25 of this code shall be deemed to meet the requirements of this section." The 1997 UBC has the same exception, with the added phrase, "Unless otherwise required by the building official, buildings or portions thereof that are constructed in accordance with the conventional, light-framing requirements…" The indication of these two statements is simple: a wood-frame building either must conform to all of the restrictions, limitations, and requirements prescribed in the conventional construction provisions or an engineered design must be provided that demonstrates compliance with the other requirements of the code.

There is another important aspect of how the UBC dealt with conventional construction. There is a clear dividing line between the requirements that may be disregarded when engineering is provided and those requirements that still must be followed. The requirements covered in the other sections of the wood chapter are mandatory, not discretionary as are the conventional construction provisions. For example, the requirements for minimum standards of quality, general design and construction requirements, decay and termite protection, wall and floor framing, and shear walls and diaphragms must be followed without exception.

Although there are comparable prescriptive provisions for repetitive, light-frame wood construction in both the National Building Code (NBC) and the Standard Building Code (SBC), they are different from the UBC because they do not state explicitly that wood-frame construction must conform to conventional construction provisions or be designed in accordance with the engineering provisions.

Conventional construction in the IBC

The same philosophy of prescriptive, conventional construction provisions of the UBC described above was intentionally incorporated into the IBC. Like the UBC, the provisions of IBC Section 2308 are prescriptive in nature and permit an individual who is not a registered design professional (architect or engineer) to design a wood-frame building simply by following the rules of the code. This section also permits the American Forest and Paper Association’s (AF&PA) Wood Frame Construction Manual for One- and Two-Family Dwellings (WFCM) to be used as an alternative. In order to use the WFCM, however, a building must meet all the limitations established in Section 2308 as well as the limitations within the WFCM itself.

Conversely, conventional, wood-frame construction provisions of the International Residential Code (IRC) are not necessarily the same as the IBC; there are some cases where the requirements of the IRC differ from the IBC. The main reason for these differences is that the IRC and IBC had somewhat different origins. The primary base document for development of the IRC was the 1998 International One- and Two-Family Dwelling Code (IOTFDC), while the basis for the IBC was the model codes.

This article will discuss briefly the components of conventional construction that are defined in the IBC (all sections listed herein refer to the IBC unless noted otherwise). Because of space considerations, additional details from each sub-section of this article can be found at the end of this article.


Light-frame buildings are easy to design by following
the conventional construction rules.

Scope, limitations, and restrictions

The basic limitations of the conventional construction provisions of Section 2308 are summarized as follows:

  1. Buildings are limited to a maximum of three stories above grade. For buildings in Seismic Design Category (SDC) D or E, cripple stud walls are considered a story. Solid-blocked cripple walls not exceeding 14 inches in height need not be considered a story.
  2. Bearing wall floor-to-floor (and floor-toceiling) heights cannot exceed 10 feet.
  3. Maximum design loads must not be violated. Average dead loads cannot exceed 15 pounds per square foot (psf) for roofs, exterior walls, floors, and partitions. Floor live loads cannot exceed 40 psf, and ground snow loads cannot exceed 50 psf.
  4. The 3-second gust wind speed at a building site cannot exceed 100 mph for buildings in Exposure Category C or D, and cannot exceed 110 mph for buildings in Exposure Category A or B.
  5. Roof trusses and rafters cannot span more than 40 feet between points of vertical support.
  6. The conventional, light-frame construction provisions cannot be used for buildings in Seismic Use Group III buildings that are in SDC B, C, D, E, or F (Seismic Use Group III buildings are Category IV essential facilities per the Table 1604.5).

In addition to the basic limitations described above, there are further, specific seismic-related requirements in Section 2308.11 for buildings in SDC B or C, and in Section 2308.12 for buildings in SDC D or E.

Design of portions

Section 2308.4 specifically allows a building of otherwise conventional construction that contains nonconventional structural elements to have those elements designed to resist the forces specified in Chapter 16. Also, the section states that the design need only demonstrate compliance of non-conventional elements with the engineering provisions of the code, and the design must be compatible with the performance of the conventional framed system. Therefore, only those portions of the building that do not comply with the conventional construction provisions must be engineered to resist the applicable forces. The purpose of this allowance is to permit designs to have portions of the structure deviate from the prescriptive requirements without requiring complete engineering for the entire building. This often is referred to as partial engineering.

A fairly typical example of a condition requiring partial engineering occurs when a building conforms with the provisions and the wall bracing requirements, but has an exterior wall at the entrance—such as with an open stairway to the second floor—with exterior studs that exceed the permissible height of 10 feet (as prescribed in Section 2308.2.1 and Table 2308.9.1). In this case, the exterior wall studs must be designed and connected to the rest of the structure to resist the effects of combined axial compression and flexural bending for out-of-plane wind loads. Another typical case occurs when a building does not meet all of the requirements for location, spacing, or percentage of wall bracing required. In this instance, that portion must be designed to resist the governing lateral load (either seismic or wind), and be designed and detailed properly to transfer all forces from the point of origin to the resisting element in accordance with Section 1604.4. To satisfy the code, engineering analysis and distribution of governing lateral loads is required, and the chords, collectors, shear walls, footings, and all connections must be designed to ensure a complete load path.

Gravity and lateral support systems

In addition to the general construction requirements—such as permissible notching and boring of framing members (studs, joists, and rafters)—the conventional construction provisions also include prescriptive methods for achieving adequate support systems for gravity loads and lateral loads. The gravity loads are roof live load, roof snow load, and floor (occupancy) live load. The lateral loads are from wind pressure and seismic ground motion. And there are additional loads to consider such as those for balcony rails, stairways, and other components.

The vertical system that provides support for gravity loads consists of the roof framing, floor framing, and wall framing—which are comprised of posts, beams, headers, and girders—and the foundation system. This type of framing generally is known as repetitive, light-frame construction because of the repetitive nature of studs, joists, and rafters. The minimum size, maximum spans, and minimum quality of the various framing members are given in tables within Chapter 23. For example, span tables for floor joists are easy to use because they are based upon the species and grade of lumber for a specific loading condition.


Gravity and lateral systems are
accommodated in the prescriptive code.

Conventional roof framing

Although many wood-frame buildings use engineered, wood trusses, Section 2308.10 permits roof systems to be framed conventionally with rafters, purlins, purlin braces, and rafter ties. The conventional roof framing provisions of this section only apply to roofs having a slope of 3:12 or greater. Where the roof slope is less than 3:12, members supporting the rafters and ceiling joists, such as ridge boards, hips, and valleys must be designed as beams. The rafters themselves may be determined from the span tables. The allowable spans for rafters are determined from Tables 2308.10.3(1) through 2308.10.3(6) for the particular species, grade, size, spacing, and load given. For other grades and species, the code references AF&PA’s Span Tables for Joists and Rafters.

Gravity framing tables

The IBC contains several tables that existed in standards but were not included in all three of the model codes. Such tables include header and girder span tables for exterior and interior bearing walls, required rating for wind uplift connectors at rafters, and rafter tie connections for snow loads. The WFCM has similar header and girder span tables, as well as helpful figures and illustrations that explain the proper use of the tables.

Wall bracing

The lateral load supporting system consists of braced wall lines and braced wall panels. A braced wall line extends across a building and consists of a series of braced wall panels. Although a shear wall and a braced wall panel serve essentially the same purpose (to provide lateral support for wind and seismic loads), they are not the same thing. The difference is that a braced wall panel is a prescriptive element, constructed in accordance with a set of rules. A shear wall, on the other hand, is an engineered element that is design to resist lateral loads from wind and seismic forces. Therefore, the prescriptive systems have rules instead of loads, but the type and amount of bracing still is driven by the site-related design parameter SDC. Buildings in SDC A, B, and C must have exterior and interior braced wall lines spaced not more than 35 feet on center. And, buildings in SDC D and E must have braced wall lines spaced not more than 25 feet on center.

In addition to the spacing requirements for braced wall lines, there are requirements of the amount, location, and percentage of braced wall panels within each braced wall line as shown in Figure 2308.9.3.

Cripple wall bracing

According to the provisions, cripple walls having a stud height exceeding 14 inches are considered a story and must be braced the same as other braced wall lines in SDC A, B, or C. The requirement is the same in SDC D or E, except that cripple walls must be meet additional bracing requirements. And, in SDC D and E, where interior braced wall lines occur without a continuous foundation below, the length of parallel exterior cripple wall bracing must be increased to 1 1/2 times the length otherwise required for braced wall lines.

Alternate braced panels

To accommodate relatively slender braced wall panels, the code permits the use of alternate braced wall panels for any of the braced wall panels described above. The alternate braced wall panel actually is a shear wall that is incorporated into prescriptive provisions. The intent is to allow alternate braced wall panels to be used where it is difficult to fit the 4-foot braced wall panel.

Conventional foundation systems

Section 2308.6 requires that foundations and footings conform to the requirements of Chapter 18, and Section 1805.4.2 requires the design, materials, and construction of concrete footings to comply with the provisions of Chapter 19. However, there is a specific exception for concrete footings supporting walls of light-frame construction. Where a design is not provided, these concrete footings may be constructed in accordance with Table 1805.4.2, which prescribes the minimum footing width and thickness based upon the number of floors supported. The footings are permitted to support a roof in addition to the specified number of floors listed in the table. Again, note that the requirements of IBC Table 1805.4.2 are not the same as IRC Table R403.1.

Summary

Section 2308 of the IBC contains conventional construction provisions that are intended to permit wood-frame structures to be designed without requiring calculations from a registered design professional. However, the provisions are prescriptive and the restrictions, limitations, and requirements of the section must be adhered to strictly.

In addition to the conventional, wood-frame construction provisions, the IBC also includes prescriptive foundation provisions that can be used in conjunction with Section 2308. This allows an individual who is not a registered design professional to design an entire wood-frame building simply by following the rules of the code.

Also, the code allows partial engineering of buildings constructed conventionally that contain non-conventional structural portions or elements. In these cases, only those specific portions or elements must be designed by a registered design professional.

John R. Henry, P.E., is a senior staff engineer with the architectural and engineering services department of the International Code Council’s and is based out of the Los Angeles regional office. He can be reached at jhenry@iccsafe.org.

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