The 2006 International Building Code: Overview of the significant structural changes - Part 1

November 2006 » Feature Article
The 2006 IBC - Significant structural changes - Part 1
John R. Henry, P.E.

The roundtable article titled, "Three issues facing the structural engineering industry" in the April 2006 issue of Structural Engineer stated that many of the recent code changes in the 2006 International Building Code (IBC) were intended to clarify, consolidate, and reorganize the provisions to improve language and make the code more user- friendly. In addition, many of the structural changes were designed to eliminate duplicate provisions that appeared in both Chapter 16 of the code and in the ASCE 7 standard so that the design criteria are in the code and the technical provisions (and criteria) are within the standard. Another reason for structural code changes is to incorporate advances in engineering science and technology and to permit new and innovative materials and methods of construction.

The purpose of this article is to present a summary overview of some of the significant changes to the structural provisions in the 2006 IBC. The article covers changes to the code itself, not the referenced standards. As all code users now realize, the continuing trend in the IBC is to reference national consensus standards rather than transcribe the requirements of the standards into the code. For an in-depth discussion of the changes to the new standards, refer to the commentaries in the various structural standards themselves.

This is the first 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 Chapter 16, and the second article (which will be printed in the December issue of Structural Engineer) will detail the materials chapters of the code.

Referenced standards

As far as structural loads are concerned, the 2006 IBC references the American Society of Civil Engineer’s Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05), including Supplement No.1 (ICC product #9002S05), which has an extensive commentary. Since the seismic provisions of the 2006 IBC and ASCE 7—05 are based on the 2003 National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings, refer to the commentary in that document for seismic-related issues. The 2003 NEHRP is FEMA 450 and is available at www.bssconline.org.

In addition, each new triennial edition of the IBC often includes updated, referenced structural standards. All the standards referenced in the IBC are listed alphabetically in Chapter 35. IBC Section 102.4 states that where differences occur between provisions of the code and referenced codes and standards, the provisions of the code apply. Table 1 shows the referenced structural standards in the 2003 and 2006 IBC.

Note that in the 2006 IBC the standards for design of steel (ASIC 360—05) and wood (AF&PA NDS—05) each combine both ASD and LRFD procedures into a single standard. Further discussion of the materials chapters will be included in Part two of this article.

One of the most significant changes in the 2006 IBC is the elimination of many overlapping and duplicate technical provisions in Chapter 16 that were also included in the ASCE 7 standard. The 2003 IBC was often difficult to use because in many cases the technical provisions were in the code and standard and in some cases the code amended the language in the standard. This problem has been resolved in the 2006 IBC by essentially removing the technical design provisions from Chapter 16 of the code. The design criteria remain in the code (and in the standard) for use by the building official, but the technical design provisions are now only located in the ASCE 7 standard (ICC Product #9002S05).

Changes to Chapter 16

Although there were no major changes to the structural provisions of the 2006 IBC, there were a variety of noteworthy format changes of which code users should be aware. The format used in this article is by the section in the 2006 IBC and the code change number. For example, "Section 1604.5 (S9-04/05)" refers to structural code change number S9 made to Section 1604.5 during the 2004/2005 code development cycle. The following is a brief description of the significant structural changes to the 2006 IBC.

Section 1604.5 (S9-04/05)—This code change simplified the seismic design provisions of the code by eliminating the Seismic Use Group and replaced it with the more general classification of Occupancy Category. Table 2 shows the 2003 IBC Seismic Use Group and 2006 Occupancy Category.

Instead of assigning earthquake, snow, and wind importance factors within Table 1604.5, the specific earthquake, snow, and wind load provisions in the ASCE 7 standard prescribe the corresponding importance factors based on the building’s Occupancy Category given in Table 1604.5. See Tables 3 and 4. It should be noted that Table 1-1 of ASCE 7 and Table 1604.5 are not the same. Since IBC Section 102.4 states that where differences occur between provisions of the code and referenced codes and standards, the provisions of the code apply. Thus, Table 1604.5 should be used to determine Occupancy Category rather than ASCE 7 Table 1-1.

The multiple occupancy requirements have been relocated to Section 1604.5.1, so they now apply regardless of the type of load effect being considered. The code prescribes two design options for mixed-occupancy buildings: The entire structure can be designed as a single building based on the requirements for the most restrictive Occupancy Category in the building, or the registered design professional can structurally separate portions of the structure containing distinct occupancy categories and design each portion according to its specific Occupancy Category classification.

Table 1604.5 (S7- 04/05)—The 2006 IBC criterion for Occupancy Category III buildings has been changed to read "covered structures whose primary occupancy is public assembly with an occupant load greater than 300" to achieve consistency with the legacy model codes and the NEHRP Provisions. The 2003 IBC table stated, "buildings where more than 300 people congregate in one area," which was confusing and led to inconsistent interpretation and enforcement of the provision. It appeared to apply to any occupancy classification, even large commercial buildings where an occupant load greater than 300 in a single room is not unusual. That interpretation was a departure from past practice under the legacy codes; therefore, the language was changed in the 2006 IBC to better reflect the intent.

Section 1605.3.1.1 (S9-03/04)—This code change eliminated the restriction on the use of the 0.7 factor on E in combination with the 0.75 load reduction factor where two or more variables loads are combined with dead load in the basic ASD load combinations. The purpose of the 0.7 factor on E in the basic ASD load combinations is to scale the strength level earthquake load effect, E, down to service load level. (In the alternative basic load combinations this is done by dividing E by 1.4). The restriction on the use of the 0.7 factor on E in combination with the 0.75 load reduction factor in the basic load combinations produced an inconsistency with the alternative basic load combinations, as well as ASCE 7. The 2006 IBC load combinations have been expanded and reformatted and are now consistent with ASCE 7—05.

Section 1607.5 (S20-04/05) — The minimum partition load for office floors has historically been 20 pounds per square foot (psf) in the legacy model codes, as well as the IBC. In the 2006 IBC, the partition live load has been reduced from 20 psf to 15 psf for office floors to make the code consistent with the partition load in the ASCE 7-05 standard. Note that the Standard Building Code and Uniform Building Code (UBC) considered the partition load a dead load whereas the National Building Code and IBC treat the partition load as a live load.

Table 1607.1 (S14-03/04)—Residential attics have three possible live load conditions: attics without storage (10 psf), attics with limited storage (20 psf), and habitable attics (30 psf). The 2003 IBC was not clear on how to determine which condition applied. Three new footnotes were added to the 2006 IBC Table 1607.1 that clarify the requirements. Footnote i covers attics without storage and clarifies that the attic live load need not act concurrently with any other live load requirements. Footnote j covers attics with limited storage and footnote k covers habitable attics.

Section 1607.9.2 (S25-04/05)—This code change revised the reduction of live loads on garage floor members so that both the general live-load reduction provisions in Section 1607.9.1 and the alternate floor live-load reduction provisions in Section 1607.9.2 are consistent. In both 2000 IBC and 1997 UBC, the uniform live load for passenger vehicle garages was 50 psf and the maximum live load reduction produced a 30 psf live load. A code change to the 2000 IBC reduced the floor live load for passenger vehicle garages from 50 psf to 40 psf. (Note that the concentrated load was also changed from 2,000 pounds to 3,000 pounds). Using the alternative live-load reduction provisions of Section 1607.9.2, it was possible to reduce the parking garage live load down to 24 psf, which was not the intent and not prudent.

A subsequent code change corrected this condition by imposing the previous minimum live load of 30 psf as it was in the 2000 IBC and 1997 UBC. However, the general provisions in Section 1607.9.1.2 did not permit live load reduction in passenger vehicle garages, yet the alternate provisions in Section 1607.9.2 permitted a 10 psf (25 percent) live load reduction. This code change essentially made the general live-load reduction provisions and the alternate floor live-load reduction provisions in the 2006 IBC consistent in regards to passenger vehicle garages.

Section 1608 (S30-04/05) — The 2003 IBC and ASCE 7-02 standard both contained snow load provisions. This duplication was confusing for designers and building officials to determine which provisions applied. This code change removed the technical requirements relating to the determination of snow loads and simply refers the code user to Section 7 of the ASCE 7-05 standard for snow load design. The snow load design criteria remains in the building code so that the building official can determine the appropriate ground snow load design criteria.

Section 1609 (S32-04/05) — The 2000 IBC referenced ASCE 7-98 and the 2003 IBC referenced ASCE 7-02 for wind load provisions. The 2003 IBC contains simplified wind load provisions that were transcribed from ASCE 7, but also allows the designer to determine wind loads directly from the ASCE 7 standard. In fact, the simplified procedure in 2003 IBC Section 1609.6 is identical to Method 1 in ASCE 7 - 02. Since both the code and the standard contained wind load provisions, it was sometimes confusing for designers and building officials. This code change removed all technical requirements relating to the determination of wind loads on structures and simply refers the code user to Section 6 of the ASCE 7-05 standard. The wind design criteria remain within the building code so the building official can determine the appropriate wind load design criteria.

There were also two technical changes to the wind speed conversion provisions in IBC Section 1609.3.1 and Table 1609.3.1. The values in 2006 IBC Table 1609.3.1 were revised to more accurately reflect the revised curve and data in ASCE 7-02; and a new conversion formula is given in the code (Equation 16-31) that permits the designer to convert from 3-second-gust basic wind speed to fastest-mile basic wind speeds. The new Equation 16-31 is as follows: Vfm = (V3s - 10.5)/1.05  (Equation 16-31), where V3S = 3-second-gust basic wind speed from IBC Figure 1609 (ASCE Figure 6-1).

This code change also expanded the requirements given in Table 1609.1.4. Section 1609.1.4 of the 2003 IBC requires protection of openings for exterior glazing in the lower 60 feet of buildings located in the wind-borne debris region. The section includes an exception for one- and two-family buildings where precut wood structural panels may be used to cover glazed openings and the fastening system must be designed to resist component and cladding loads. For buildings with a mean roof height less than or equal to 33 feet where wind speeds do not exceed 130 mph, Table 1609.1.4 gives prescriptive fastening requirements in lieu of calculating fastener requirements to resist component and cladding loads. The 2006 IBC includes a variety of clarifications and improvements to Section 1609.1.4 and Table 1609.1.2.

Section 1613 (S42-04/05)—The 2006 IBC has 14 maximum considered earthquake (MCE) spectral response acceleration maps that are based on the 2002 U.S. Geological Survey (USGS) probabilistic maps. See IBC Figures 1613.5(1) through 1613.5(14). There are two enlarged maps (0.2 second and 1.0 second) for the New Madrid and Charleston, S.C., fault regions. The updated USGS MCE ground motion maps replaced the 10 maps in the 2000 and 2003 editions of the IBC. At the time this article was written, the seismic design parameters DVD program was not yet available from USGS. However, the ground motion parameters for new buildings can be determined from the USGS website.

Section 1613 (S39-04/05)—This code change removed the technical requirements relating to the determination of seismic loads on structures and simply refers the code user to the seismic provisions in the ASCE 7-05 standard. The seismic design criteria and parameters remain in the building code so that the building official can determine the appropriate seismic design criteria. The provisions that remain within the building code relate to local geologic, terrain, or other environmental conditions such as seismic ground motion maps, site classification, design spectral response acceleration, and seismic design category. In addition, the seismic design provisions in the ASCE 7-05 standard were totally reformatted and reorganized to more logically follow the typical seismic design process. The goal of the reorganization of ASCE 7-05 seismic design provisions was to create a user-friendly set of provisions that can be readily understood and correctly interpreted by the average engineer when designing the average building.

For a more complete discussion of the changes in the 2006 IBC seismic design provisions, refer to the "2006 IBC seismic design provisions: Significant improvement come from reorganization," by S.K. Ghosh, Ph.D., and Susan Dowty, S.E., that appeared in the January 2006 issue of Structural Engineer.

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.


References

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 structural and non-structural provisions.

For a detailed analysis of the changes to the structural provisions from the 2003 to the 2006 IBC, refer to 2006 Analysis of Revisions to the IBC - Structural Provisions, by S.K. Ghosh, Ph.D., Susan Dowty, S.E., and P. Dasgupta (ICC product #: 9281S06).

Code users who want the complete history of a particular code change are referred to the ICC publication, 2006 IBC Code Changes Resource Collection, which is a compilation of all published information for each successful code change from the 2003 to 2006 editions of International Building Code (ICC product #4016S06). It includes the submitted code changes, committee actions and modifications, assembly actions, successful public comments, and the final action on each successful code change.


Table 1: Referenced structural standards in the IBC.

Standard Type 2003 IBC Referenced
Standard
2006 IBC Referenced Standard
Loads ASCE 7-02 ASCE 7-05
Concrete ACI 318-02 ACI 318-05
Aluminum AA ADM 1-00 (2000) AA ADM 1-00 (2000)
Masonry ACI 530-02/ASCE 5-02/TMS 402-02 ACI 530-05/ASCE 5-05/
TMS 402-05
Steel (ASD) AISC ASD 1989 (9th)
Supplement No. 1, 2001
AISC 360-05
Steel (LRFD) AISC LRFD 1999 (3rd)
AISC HSS 2000
AISC 360-05
Steel (Seismic) AISC Seismic 2002 AISC 341-05
Steel (Cold-formed light gage) NASPEC 2001 (AISI) NAS 01 Including 2004 Supplement
 + General AISI General—01  AISI General—04
 + Header AISI Header—01 AISI Header—04
 + Truss AISI Truss—01 AISI Truss—04
 + Wall Studs None AISI WSD—04
 + Lateral None AISI Lateral—04
 + Prescriptive Framing
    OTFD
None AISI PM—04
Wood (ASD) AF&PA NDS—01 AF&PA NDS—05
Wood (LRFD) AF&PA/ASCE 16-95 AF&PA NDS—05

Table 2: 2003 IBC Seismic Use Group versus 2006 IBC Occupancy Category

2003 IBC
Seismic Use Group
2006 IBC
Occupancy Category
Description (See IBC Table 1604.5 for complete description of building use)
I I Buildings and other structures that represent a low hazard to human life in the event of failure
I II Buildings and other structures except those listed in Occupancy Categories I, III, and IV
II III Buildings and other structures that represent a substantial hazard to human life in the event of failure
III IV Buildings and other structures designated as essential facilities

Table 3: Importance Factors in ASCE/SEI 7-05

Load Condition ASCE 7-05 Section / Table
Wind Section 6.5.5 / Table 6-1
Snow Section 7.3.3 / Table 7-4
Seismic Section 11.5.1 / Table 11.5-1

Table 4: Importance Factors in ASCE/SEI 7-05

  Occupancy Category
Load Condition I II III IV
Wind—Non Hurricane 0.87 1.00 1.15 1.15
Wind—Hurricane 0.77 1.00 1.15 1.15
Snow 0.8 1.0 1.1 1.2
Seismic 1.0 1.0 1.25 1.5

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