Part II of this two-part article provides a brief overview of the more significant changes to Chapter 16 of the 2012 IBC, including wind loads, mapped acceleration parameters, and atmospheric ice loads.
By John R. Henry, P.E.
About 100 successful code changes to the structural provisions of the 2009 International Building Code (IBC) were subsequently incorporated into the current 2012 edition of the IBC. The 2012 edition of the IBC references several hundred national standards, which are listed alphabetically in Chapter 35. The main structural standards referenced in the 2012 IBC for loads and materials are shown in Part 1 of this article (See July, 2013 issue).
This article is a continuation of the brief overview of the more significant changes to Chapter 16 of the IBC, which covers design loads for structures. Perhaps the most significant structural standard referenced by the 2012 IBC is the 2010 edition of Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7 (ASCE 7-10). For a complete discussion of the significant changes to the 2012 IBC, including structural and non-structural fire and life safety provisions, refer to Significant Changes to the International Building Code, 2012 Edition, available from ICC.
Section 1609, Wind Loads
The wind design requirements of Section 1609 were extensively revised to update and coordinate them with the latest wind load provisions in ASCE 7-10. A very significant change is that the wind load maps in ASCE 7 and the IBC are now based on ultimate design wind speeds, Vult, which produce a strength level wind load similar to seismic load effects. In developing the new wind speed maps, it was decided to use multiple ultimate event or strength design based maps in conjunction with a wind load factor of 1.0 for strength design. For allowable stress design (ASD), the load factor has been reduced from 1.0 to 0.6, thus the load combinations in Section 1605 had to be modified accordingly. Several important factors related to more accurate wind load analysis were considered that led to the decision to move to strength based ultimate event wind loads.
In developing the new wind speed maps, it was decided to use multiple ultimate event or strength design based maps in conjunction with a wind load factor of 1.0 for strength design. For allowable stress design (ASD), the load factor has been reduced from 1.0 to 0.6, thus the load combinations in Section 1605 had to be modified accordingly.
As a result of the new strength-based wind speed, new terminology had to be introduced into the 2012 IBC. The former term “basic wind speed” has been changed to “ultimate design wind speed” and is designated Vult. The wind speed that is equivalent to the former basic wind speed is now called the “nominal design wind speed,” Vasd, and the conversion between the two is given by Equation 16-33, as Vasd=Vult.√0.6
The conversion from Vasd to Vult is a result of the wind load being proportional to the square of the velocity and the ASD wind load being 0.6 times the strength level ultimate wind load (V2asd = 0.6V2ult). It should also be noted that the term “basic wind speed” in ASCE 7-10 corresponds to the “ultimate design wind speed” in the 2012 IBC.
Because many existing provisions in the code are driven by the wind speed, it was necessary to modify the wind speed conversion section so that the many requirements triggered by wind speed were not changed. The terms “ultimate design wind speed” and “nominal design wind speed” were incorporated in numerous locations to help the code user distinguish between the two. In cases where the code has used wind speed to trigger a specific requirement, the ultimate wind speed, Vult, must be converted to a nominal design wind speed that corresponds to the former basic wind speed. Thus, a new table in the 2012 IBC converts Vult to Vasd so that the mapped wind speed thresholds in various parts of the code can still be used:
For example, in a case where the 2009 IBC imposed requirements where the basic wind speed exceeds 100 mph, the 2012 IBC now imposes the requirements where Vasd exceeds 100 mph. A nominal design wind speed, Vasd, equal to 100 mph corresponds to an ultimate design wind speed, Vult, equal to 129 mph.
|Table 1609.3.1, Wind Speed Conversions.|
Over the past 20 years, wind speed maps have changed from fastest mile to 3-second gust and now to ultimate 3-second gust wind speeds. For a comparison of ASCE 7-93 (fastest mile) wind speeds, ASCE 7-05 (3-second gust) ASD wind speeds, and ASCE 7-10 (3-second gust) ultimate wind speeds, refer to Table C26.5-6 of the ASCE 7-10 commentary. Note that the conversion in ASCE 7-10 is given by Vult=Vasd.√1.6, which produces slightly different values than does IBC Equation 16-33.
|Beyond the adoption of the new strength design wind speed maps, the wind load calculation provisions that were contained in Chapter 6 of ASCE/SEI 7-05 have been reorganized and divided into six separate chapters (26 through 31) for improved clarity and ease of use. This is similar to the reorganization of the seismic design provisions in ASCE 7-05, which were broken down into several chapters to facilitate their use.|
Beyond the adoption of the new strength design wind speed maps, the wind load calculation provisions that were contained in Chapter 6 of ASCE/SEI 7-05 have been reorganized and divided into six separate chapters (26 through 31) for improved clarity and ease of use. This is similar to the reorganization of the seismic design provisions in ASCE 7-05, which were broken down into several chapters to facilitate their use. This reorganization into multiple chapters required several coordination revisions to the IBC code language. In addition, ASCE/SEI 7-10 also includes a new simplified method for use in the determination of wind loads for buildings up to 160 feet in height. Figure 26.1-1 of ASCE 7 shows a flowchart of the new wind design procedure and the various chapters, which is reproduced on page 38.
The alternate all-heights wind design procedure is maintained in the 2012 IBC and was updated to conform to the new ultimate wind design procedure in ASCE 7-10.
Section 1613.5.1. Mapped Acceleration Parameters
The 2012 IBC seismic ground motion maps have been updated to reflect the 2008 maps developed by the United States Geological Survey (USGS) National Seismic Hazard Mapping Project and the technical changes adopted for the 2009 NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (FEMA P750). The risk-targeted maximum considered earthquake (MCER) ground motion response accelerations are defined as the most severe earthquake effects considered by the code, determined for the orientation that results in the largest maximum response to horizontal ground motions adjusted for targeted risk.
The USGS National Seismic Hazard Mapping Project and the technical changes adopted for the 2009 NEHRP (FEMA P750) are part of the ongoing federal effort to make the most current earthquake hazard information available to users of the IBC. The ground motion maps in the 2012 IBC also incorporate technical changes adopted for the 2009 NEHRP Provisions that include: 1) use of risk-targeted ground motions, 2) the maximum direction ground motions, and 3) near-source 84th percentile ground motions.
The titles of the maps in the IBC were revised from the former “Maximum Considered Earthquake (MCE) Ground Motion” to “Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Response Accelerations” to reflect the changed titles in the 2009 NEHRP and ASCE 7-10. Although the maps in the IBC are generally illustrative of the earthquake hazard, the contours in some regions cannot be read clearly enough to provide exact design values for specific sites. Precise seismic design values can be obtained from the USGS website http://earthquake.usgs.gov/hazards/designmaps/ using the longitude and latitude of the building site. The latitude and longitude of proposed building sites can be obtained from GPS mapping programs or websites such as google.com, topozone.com or trails.com.
Section 1613.4, Alternatives to ASCE 7
Many of the alternatives in Section 1613.6 of the 2009 IBC to ASCE 7-05 have been deleted in the 2012 IBC because they have subsequently been incorporated into ASCE 7-10. For example, Sections 1613.6.1, 1613.6.3 through 1613.6.8, and 1613.7 in the 2009 IBC provide alternative amendments to various requirements in ASCE 7-05 and are no longer necessary because similar provisions were incorporated into the 2010 edition of ASCE 7.
Section 1614, Atmospheric Ice Loads
A new section, definition and notation for atmospheric ice loads and ice-sensitive structures is added to the 2012 IBC for consistency with ASCE 7-10. An ice-sensitive structure is a structure for which the effect of an atmospheric ice load governs the design of the structure or portion thereof. These include lattice structures, guyed masts, overhead lines, light suspension and cable-stayed bridges, aerial cable systems for ski lifts or logging operations, amusement rides, open catwalks and platforms, flagpoles and signs. Section 10.1 of ASCE 7-10 requires atmospheric ice loads to be considered in the design of ice-sensitive structures. The term “ice-sensitive structure” is defined in Section 10.2 of ASCE 7-10 and this definition has been added to the IBC to provide the technical basis for determining which structures must be considered ice-sensitive and therefore required to be designed for ice loads in accordance with the applicable provisions of ASCE 7 Chapter 10. The LRFD load combinations in Section 1605.2.1 and ASD load combinations in Section 1605.3.1.2 (other loads) were modified to include ice loads where applicable. Where atmospheric ice loads must be considered, these sections reference ASCE 7 Section 2.3.4 for LRFD and Section 2.4.3 for ASD, respectively.
For a complete discussion of the more important provisions in the 2012 IBC, both structural and non-structural fire and life safety requirements, refer to the 2012 International Building Code Handbook, available from ICC.
John R. Henry, P.E., is the principal staff engineer, International Code Council. Contact him at email@example.com.