The applications of Sections 188.8.131.52 and 184.108.40.206 have caused some confusion, and we hope that this Code Simple will clear things up.
Section 220.127.116.11 states that for seismic design category (SDC) D-F buildings having listed irregularities, the design forces determined from the equivalent lateral force procedure of Section 12.8.1 must be increased 25 percent for connections of diaphragms to vertical elements and to collectors, and for connections of collectors to the vertical elements of the seismic force-resisting system. The listed irregularities are: torsional irregularity, extreme torsional irregularity, re-entrant corners, diaphragm discontinuity, out-of-plane offsets of the vertical lateral force-resisting elements (Horizontal Structural Irregularity Type 1a, 1b, 2, 3, or 4 of ASCE 7-05 Table 12.3-1) and in-plane discontinuity in vertical lateral force-resisting elements (Vertical Structural Irregularity Type 4, ASCE 7-05 Table 12.3-2). with overstrength apply, no further increase is warranted.”
The primary source of confusion has been the last sentence of Section 18.104.22.168, which states that collectors and their connections should also be designed for the increased forces, unless they are designed for the seismic load combinations with overstrength factor in accordance with Section 22.214.171.124. Section 126.96.36.199 requires collector elements and their connections to be designed to resist load combinations with overstrength factor in structures assigned to SDC C-F.
The question that is asked by engineers is: For the design of collectors and their connections, doesn’t Section 188.8.131.52 always govern and therefore Section 184.108.40.206 won’t apply? In order to answer this question, it is helpful to summarize Sections 220.127.116.11 and 18.104.22.168 as follows:
|ASCE 7-05 Section 22.214.171.124
(25 percent increase in design force)
|ASCE 7-05 Section 126.96.36.199
(design using load combinations with overstrength factor)
|Applicable to:||Applicable to:|
In cases where both sections 188.8.131.52 and 184.108.40.206 apply, there is no point in using Section 220.127.116.11 for the design of collectors and their connections because Section 18.104.22.168 is more restrictive. It should be apparent from the table that there is no situation covered by Section 22.214.171.124 that isn’t already covered by Section 126.96.36.199.
Please note that Section 188.8.131.52 has been improved in ASCE 7-10. The new language corrects the incorrect reference to the equivalent lateral force base shear in ASCE 7-05 Section 12.8.1 (and, by implication, the corresponding vertical distribution) and refers to the diaphragm design force in ASCE 7-10 Section 184.108.40.206 instead. Also, a part of the last sentence is deleted and replaced with the following:
EXCEPTION: Forces calculated using the seismic load effects including overstrength factor of Section 12.4.3 need not be increased.
This change in ASCE 7-10 makes the intent of the provision in ASCE 7-05 clearer.
Another question that is asked regarding these two sections is: Section 220.127.116.11 has an exception applying to light-frame construction. Can that exception be carried over when applying Section 18.104.22.168?
No. There really is no exception to Section 22.214.171.124. This section addresses a potentially weak link in SDC D-F structures having the irregularities specifically identified in the section. Section 126.96.36.199 does refer to Section 188.8.131.52, but that doesn’t mean the exception to Section 184.108.40.206 applies to Section 220.127.116.11. The exception to Section 18.104.22.168 would need to be repeated in Section 22.214.171.124 in order for it to be applicable. If your structure is assigned to SDC D-F, is of light-frame construction, and has one of those irregularities listed in Section 126.96.36.199, the collectors and their connections need to be designed for either 1) design forces increased by 25 percent, or 2) forces given by Section 188.8.131.52 load combinations with overstrength factor.
S.K. Ghosh Associates Inc. is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S. K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at email@example.com and firstname.lastname@example.org, respectively, or at www.skghoshassociates.com.