Follow-up to the “Is a 1/3 Stress Increase Allowed Anymore?” Blog
After posting the “Is a 1/3 Stress Increase Allowed Anymore?” blog, we had a follow-up question about ASCE 7-10 Section 12.4.3.3 which includes an allowable stress increase that we did not mention in our blog. This allowable stress increase by a factor of 1.2 only comes into play when the “load combinations with overstrength” are invoked.
ASCE 7-10 Section 12.4.3.3 reads as follows:
12.4.3.3 Allowable Stress Increase for Load Combinations with Overstrength
Where allowable stress design methodologies are used with the seismic load effect defined in Section 12.4.3 applied in load combinations 5, 6, or 8 of Section 2.4.1, allowable stresses are permitted to be determined using an allowable stress increase of 1.2. This increase shall not be combined with increases in allowable stresses or load combination reductions otherwise permitted by this standard or the material reference document except for increases due to adjustment factors in accordance with AF&PA NDS.
The allowable stress increase by a factor of 1.2 for load combinations with overstrength is unique in that it provides an increase that applies across the board for all materials when allowable stress design is used. The commentary to ASCE 7-10 simply says: “The allowable stress increase for load combinations with overstrength is to provide compatibility with past practice.”
We understand that some have questioned if this 1.2 increase can apply to allowable vertical bearing and lateral bearing stresses. And the answer is yes, there is no limitation to the application of the 1.2 factor; it also applies to allowable stress-based soil values.
The table below traces the history of the load combinations with overstrength, which originated in the 1997 Uniform Building Code (UBC) and, with it, the history of the 1.2 factor.
Up until ASCE 7-05 introduced ASD-level load combinations with overstrength, there were only strength-level load combinations with overstrength, then called special seismic load combinations. In basic strength design as well as basic ASD load combinations, Eh, the effect of horizontal earthquake ground motion, is amplified by the redundancy factor, ρ. That factor in the 1997 UBC, the 2000 IBC, and the 2003 IBC was 1.0 for SDC C and lower and between 1.0 and 1.5 for SDC D and higher. Starting with ASCE 7-05 and the 2006 IBC, ρ has been 1.0 for SDC B, C and 1.0 or 1.3 for SDC D, E, F. In the special seismic load combinations or load combinations with overstrength, Eh is amplified by the overstrength factor, Ωo. In the 1997 UBC, Ωo was 2, 2.2, or 2.8. In all the editions of the IBC, ASCE 7-05, and ASCE 7-10, Ωo is 2, 2.5, or 3. Thus, in load combinations with overstrength, the horizontal earthquake effect is magnified by a significantly larger factor than in the basic strength design or basic ASD load combinations. The design loads obtained using the special seismic load combinations of the 1997 UBC, the 2000, and the 2003 IBC were judged to be, on an average, 1.7 times those obtained using the basic ASD load combinations. Thus, to preserve the same margin of safety in design, allowable stresses were allowed to be increased by a factor of 1.7 when one did ASD in conjunction with the special seismic load combinations.
When ASCE 7-05 introduced ASD load combinations with overstrength, it was assumed that anyone doing ASD would use the ASD-level, rather than the strength-level, load combinations with overstrength. The allowable stress increase factor of 1.2 in ASCE 7-10 Section 12.4.3.3 is approximately equal to 1.7/1.4 where 1.7 is the old allowable stress increase factor to go with strength level load combinations with overstrength, and 1.4 is the traditional equivalency factor between strength design (SD) and ASD (e.g. SD = 1.4ASD).
The fact that the 2006 IBC did not refer to the allowable stress increase of ASCE 7-05 Section 12.4.3.3 was, we believe, inadvertent.
We’d also like to take this opportunity to address some revised wording in this section between ASCE 7-05 and ASCE 7-10 as reflected by the strikeout and underline below:
12.4.3.3 Allowable Stress Increase for Load Combinations with Overstrength
Where allowable stress design methodologies are used with the seismic load effect defined in Section 12.4.3 applied in load combinations 5, 6, or 8 of Section 2.4.1, allowable stresses are permitted to be determined using an allowable stress increase of 1.2. This increase shall not be combined with increases in allowable stresses or load combination reductions otherwise permitted by this standard or the material reference document except for that combination with the duration of load increases due to adjustment factors permitted in accordance with AF&PA NDS is permitted.
Note that while, previously, an allowable stress increase in addition to the increase by a factor of 1.2 was permitted only for the duration of load as permitted in AF&PA NDS, ASCE 7-10 permits such increase due to “adjustment factors in accordance with AF&PA NDS.” This is a subtle change that may not be noticed by code users. Adjustment factors other than the load duration factor that can result in increases to the allowable stress design values include the flat use factor, repetitive member factor, buckling stiffness factor, and bearing area factor. These factors are material-dependent (i.e. not related to design loads) in much the same manner as the load duration factor.
