Overview of the significant structural changes—Part 2

This is the second of a series of two articles that will present an overview of the significant structural changes to the 2006 IBC. This article will highlight the changes in the materials chapters of the code, and the first article (which was printed last month in November) detailed the changes to Chapter 16.

Although there were no major changes to the structural provisions of the 2006 IBC, there were a variety of noteworthy changes that code users should be aware of. The format used in this article is by the section in the 2006 IBC and the code change number. For example, "Section 2304.12 (S66-03/04)" refers to structural code change number S66 made to Section 2304.12 during the 2003/2004 code development cycle. The following is a brief description of the significant structural changes to the 2006 IBC.

For a more in-depth discussion of the changes covered in this article, refer to Significant Changes to the International Building Code, 2006 Edition, by Douglas W. Thornburg, AIA, and John R. Henry, P.E. (ICC product #: 9281S06), which provides an overview of the changes in the 2006 IBC for both non-structural and structural provisions.

Significant structural changes

Sections 1704.7 Soils, 1704.8 Pile foundations, 1704.9 Pier foundations (S73-04/05)—This code change provides specific requirements for special inspection of existing site soils, fill placement, installation and testing of pier and pile foundations, and load-bearing capacity of soils. Three new tables (1704.7, 1704.8, and 1704.9) identify the specific inspection tasks and indicate continuous and periodic special inspection requirements.

Section 1705 Statement of Special Inspections (S76-04/05)—This code change completely reorganized and clarified the special inspection and quality assurance provisions in Chapter 17. The term "statement of special inspections" is consistently used throughout the 2006 IBC instead of the terms "special inspection program" and "quality assurance plan." The statement of contractor responsibility requirements were consolidated and put in a separate Section 1706, eliminating redundant language where it appeared in both the wind and seismic quality assurance plans in the 2003 IBC.

Sections 1808.1, 1810.8 Micropiles (S121-04/05)—The IBC had no provisions for micropiles; however, use was permitted if approved by the building official under the provisions for an alternative materials and methods of construction. New provisions have been added to 2006 IBC Sections 1808.1 and 1810.8 that include provisions for micropiles. Micropiles are defined as bored or grouted in-place piles no more than 12 inches in diameter and incorporating steel pipe casing and/or steel reinforcement.

Sections 1901-1907 Concrete Construction Provisions (S122-04/05)—Sections 1902 through 1907 contain concrete construction provisions that are patterned after Section 2 through 7 of the American Concrete Institute’s Building Code Requirements for Structural Concrete and Commentary (ACI 318). These code sections, which contain general requirements, definitions, specifications for testing and materials, concrete durability requirements, concrete quality, mixing and placing, formwork, embedded pipes, construction joints and details of reinforcement, were revised in the 2006 IBC. The majority of changes to Sections 1902 through 1907 essentially deleted the language in the code and replaced it with references to applicable sections in the 2005 edition of the ACI 318 code (ICC Product #9021S05).

Section 1908 Modifications to ACI 318 (S127-04/05)—Section 1910 of the 2003 IBC contains design provisions for design and construction of concrete structures resisting seismic forces except as modified by Section 1908. This code change deleted Section 1910 and integrated all the seismic design provisions into Section 1908 along with other modifications to the ACI 318-05 standard.

Chapter 21 Masonry (S131-04/05)—Autoclaved Aerated Concrete (AAC) masonry has been used as a building material for many years but is a relatively new construction material in the United States. In the 2000 and 2003 editions of the IBC, its use required approval by the building official under the provisions for alternative materials and methods of construction. This code change incorporated material requirements, construction requirements, and appropriate references to new Appendix A of the 2005 edition Masonry Standards Joint Committee’s (MSJC) Building Code Requirements for Masonry Structures (ACI 530/ASCE 5/TMS 402) for the structural design of AAC masonry. AAC masonry is designed by the strength design procedure in accordance with IBC Section 2101.2.2. AAC masonry is not permitted in the seismic-force-resisting system of structures classified as Seismic Design Category B, C, D, E, or F. (ACI 530/ASCE 5/TMS 402 is ICC Product #9026S05).

Chapter 22 Steel (S149-04/05)—This code change updated the referenced standards for steel construction to the most current editions from the American Institute of Steel Construction (AISC). The updated steel standards are AISC 360-05 Specification for Structural Steel Buildings and AISC 341—05 Seismic Provisions for Structural Steel Buildings, including Supplement No. 1. AISC 360-05 replaces both the AISC 335 ASD and AISC LRFD standards as a unified, dual format standard that permits either the LRFD or ASD methods. The AISC 360-05 and ASIC 341 standards are available from AISC at www.aisc.org. The Steel Construction Manual, Thirteenth Edition (AISC 325-05), and the AISC Seismic Design Manual (AISC 327-05) are available from ICC (ICC Product # 9206S05, # 9055S05).

Section 2210.5 Lateral design (S148-04/05)—This code change deleted all the technical provisions in 2003 IBC Section 2211 related to the design of cold-formed, light-framed shear walls. New Section 2210.5 was added that references the American Iron and Steel Institute (AISI) Standard for Cold-Formed Steel Framing-Lateral Design (AISI-Lateral).

Section 2210.6 Prescriptive framing (S155-04/05)—This code change adds a new Section 2210.6 that references the AISI Standard for Cold-Formed Steel Framing-Prescriptive Method for One- and Two-Family Dwellings (AISI-PM). The standard provides prescriptive construction provisions for detached one- and two-family dwellings and townhouses up to two stories in height constructed of cold-formed, light-gage steel framing. Note that the term "light-frame construction" is defined in IBC Section 202 as "A type of construction whose vertical and horizontal structural elements are primarily formed by a system of repetitive wood or light-gage steel framing members."

Section 2206 Steel joists (S151-04/05)—This code change created four new code sections under Section 2206 for steel joists that clarify the submittal document requirements and the responsibilities of the registered design professional and the joist engineer/specialty structural engineer. The new sections pertain to construction drawings, calculations, and certification. These new provisions require the steel joist industry to meet requirements similar to what has been required for the pre-engineered wood truss industry.

Chapter 23 Wood (S62-03/04)—Improper nail sizes have been used in wood-frame building construction because the pennyweight system of specifying nail sizes is not universally understood and applied. Code users sometimes focus on pennyweight (8d - 8 penny, 16d - 16 penny, etc.) and do not pay sufficient attention to the specific type of nail such as common, box, cooler, sinker, finish, etc. A typical example is substitution of box nails for common nails of the same pennyweight. This code change added the shank length and diameter dimensions of nails to the various fastening tables in Chapter 23 of the IBC.

Section 2303.1.10 Structural log members (S157-04/05)—In the past, the design of log structures was challenging because the building code had no specific provisions that addressed structural capacity and grading requirements for logs used as structural members. New Section 2303.1.10 in the 2006 IBC provides structural capacity and grading requirements for logs used as structural members. The section references ASTM’s Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings (ASTM D3957) for acceptable methods of establishing structural capacities of logs and specifies the requirement for grade stamping and certification of structural logs. ICC is currently developing a standard on the design, construction, and performance of log structures to provide technical design and performance criteria that will facilitate the design, construction, and installation of structures constructed of log timbers.

Section 2303.4 Trusses (S165-04/05)—This code change revised Section 2303.4 to clarify the requirements pertaining to metal-plate-connected wood trusses in order to achieve better consistency with the current practice and eliminate confusion regarding trusses submittals. The provisions include general requirements for the design, manufacture, and quality assurance of metal-plate-connected wood trusses; specific and detailed requirements for truss design drawings; provisions for truss placement diagrams; requirements for the truss submittal package; requirements truss member permanent bracing; and truss anchorage requirements.

Section 2304.12 Long-term deflection (S66-03/04)—This code change deleted the provision that imposed restrictions on wood members supporting concrete and masonry. The 2006 IBC section references the design method for limiting long-term deflections in Section 3.5.2 and Appendix F of the American Wood Council’s National Design Specification for Wood Construction (NDS). The NDS contains provisions to account for time-dependent deformations known as creep. The tabulated modulus of elasticity values, E, in the NDS are used to calculate immediate deformation under load. Under sustained loading, additional time dependent deflection (creep) develops at a slow rate over long periods of time. The total deflection under long-term loading can be estimated by increasing the initial deflection by a factor depending on the type or end use conditions of the wood member.

Section 2305.1 Lateral-force resisting systems (S176-04/05)—Section 2305 contains requirements for wood shear walls and diaphragms resisting wind, seismic, and other lateral loads. This code change added a national consensus standard for lateral design as an alternative to the general design requirements for lateral-force resisting systems contained in Section 2305. This new code provision references The American Forest & Paper Association (AF&PA) Supplement, Special Design Provisions for Wind and Seismic (SDPWS) as an alternate to the general design requirements for lateral-force resisting systems contained in IBC Section 2305.

Section 2305.2.2, 2305.3.2 Deflection (S71-03/04)—Equations 23-1 and 23-2 of the IBC contained the parameters en, G, and t but the values were not included in the 2000 or 2003 IBC or in standards referenced by the IBC. The values that originated in Volume 3 of the 1997 Uniform Building Code (UBC), and this code change incorporated updated tables of the values into 2006 IBC Section 2305.2.2.

Section 2305.3 Design of wood shear walls (S59-03/04)—Section 2305.3 prescribes requirements for design and construction of wood-framed shear walls. The provisions include traditional segmental shear walls, shear walls with force transfer around openings, and perforated shear walls. This code change made many clarifications to the provisions pertaining to shear wall aspect ratio, as well as the definitions of shear wall segment and pier height and shear wall segment and pier width.

Section 2306, 2307 ASD/LRFD (S189-04/05)—This code change deleted the reference to AF&PA/ASCE 16-95 Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction and replaced it with a reference to the 2005 NDS. The 2005 NDS is a dual-format specification that contains provisions for both allowable stress design (ASD) and load and resistance factor design (LRFD) procedures. (NDS—05 is ICC Product # 9542S)

Section 2305.3.11, 2308.12 Sill plate size and anchorage in Seismic Design Category D, E, or F (S83-03/04, S179-04/05)—Section 2305.3.10 contains special provisions for anchorage of foundation plates in buildings assigned to Seismic Design Category D, E, and F. Section 2308.12 contains special provisions for buildings of conventional, light-frame construction in Seismic Design Category D and E. The 2000 and 2003 IBC required 2 inches by 2 inch by 3/16-inch steel plate washers between the foundation sill plate and the nut at shear walls and braced wall lines. A code change in the 2004 Supplement changed the requirement for steel plate washers to be a minimum of 3 inch by 3 inch by 1/4-inch-thick, which is consistent with the NEHRP. The subsequent code change to the 2006 IBC changed the minimum thickness of the plate washers from 1/4 inch to 0.229 inches to allow the plate washers to be manufactured from cold-rolled sheet steel instead of 1/4-inch hot-rolled steel. Cold-rolled steel of this thickness is much more readily available and more economical.

In addition, cold-rolled steel is available with a galvanized finish, while hot-rolled steel is hot dipped galvanized after fabrication. Some steel hardware manufacturers recommend hot-dipped galvanized plate washers where in contact with preservative-treated wood. The hole in the plate washer is permitted to be diagonally slotted with a width up to 3/16-inch wider than the bolt diameter with a length up to 1-3/4 inches, provided a standard cut washer is used between the square plate washer and the nut. The slotted holes originated with the 2003 National Earthquake Hazards Reduction Program (NEHRP) provisions.

Table 2306.4.1 Shear walls (S77-03/04); Section 2307.1.1 LRFD (S2-04/05)—Footnote (i) of Table 2306.4.1 in the 2000 and 2003 IBC requires single, 3-inch nominal framing members at abutting panel joints of shear walls in Seismic Design Category D, E, or F, where the design shear exceeds 490 pounds per lineal foot (plf) for LRFD or 350 plf for ASD. Since Section 2036 is for ASD and Table 2306.4.1 contains allowable shear values for ASD, it is not appropriate to refer to 490 plf for LRFD in footnote (i). In the 2006 IBC, footnote (i) of Table 2306.4.1 was revised to no longer refer to the 490 plf LRFD value. The requirement was appropriately relocated to Section 2307 which covers requirements for LRFD.

New Section 2307.1.1 requires wood structural panel shear walls in Seismic Design Category D, E, or F where shear design values exceed 490 plf to have a single, 3-inch nominal member at framing members receiving edge nailing from abutting panels. A more significant change in the 2006 IBC is that footnote (i) of Table 2306.4.1 and Section 2307.1.1 of the 2006 IBC both permit the use of two, 2-inch nominal members nailed together as an alternate to the single, 3-inch nominal member. Cyclic shear wall testing done by APA-The Engineered Wood Association demonstrated that double 2x members properly nailed together to resist the design shear in accordance with the NDS provide equivalent performance as single, 3-inch nominal members. However, it should be noted that 2006 IBC Section 2305.3.11 still requires a single, 3-inch nominal foundation sill plate in shear walls resisting a design load greater than 490 plf LRFD or 350 plf ASD.

Sections 2308.1.1, 2308.4 Design of portions and elements (S190-04/05, S195-04/05)—The IBC allows portions of otherwise conventional wood-frame buildings to be engineered. These code changes clarified the "design of portions" requirements under the conventional wood-frame construction provisions and distinguishes between "portions" and "elements." The code change clarifies that the code permits elements and members as well as rooms or a series of rooms to be engineered in an otherwise conventionally constructed building. In addition, the code change added the phrase "and supporting load path" to emphasize the importance of providing a continuous load path for the engineered portions or elements.

Section 2308.9.3.2 Alternate bracing adjacent to a door or window opening (S80-03/04)—A new Section 2308.9.3.2 was added in the 2006 IBC, which provides an additional alternative bracing method that can be used adjacent to window or door openings. This new alternative bracing method allows a reduction of the width of the full-height segment of alternate braced wall panels from 32 inches to 16 inches wide for a one-story building, and from 32 inches to 24 inches wide for the first story of a two-story building. Figure 2308.9.3.2 illustrates these new provisions.

Section 2308.9.3 Wall bracing (S197-04/05)—This code change revised the distance that braced wall panels must start from each end of a braced wall line from 8 feet to 12.5 feet, which made Section 2308.9.3 consistent with Section R602.10.1 of the IRC. In addition, the requirement that a designed collector be provided if the braced wall panel begins more than 12.5 feet from the end of a braced wall line was deleted. To conform to the prescriptive requirements, the code requires braced wall panels to be located not more than 12.5 feet from the ends of braced wall lines. If not, then the bracing system does not conform to prescriptive requirements and engineering is required. The design of the collector is only one essential part of the engineering for the lateral-force resisting system.

Conclusion

The code changes to the 2006 IBC are made to make the provisions more clear and user-friendly. In addition, many reference documents have been written to assist practitioners in the transition.

John R. Henry, P.E., is the principal staff engineer at the International Code Council’s Los Angeles office. He can be reached at jhenry@iccsafe.org.