Section 18.104.22.168 of The American Society of Civil Engineers— Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) is a section we suspect may at times be missed. This section requires that for seismic force-resisting systems comprised solely of moment frames in structures assigned to Seismic Design Categories D, E, or F, the design story drift cannot exceed Δa/ρ for any story. This requirement is also included as footnote “b” to ASCE 7-05 Table 12.12-1 and translates into the limiting story drift being reduced by a factor of 1.3 when compared to that of a redundant moment frame system as shown in Table 1.
ASCE 7-05 Section 22.214.171.124 indicates that a ρ = 1 may be used for the determination of drift and P-delta effects. Although one can assume ρ = 1 for drift calculation purposes and P-delta effects, the allowable drift cannot exceed Δa/ρ for systems comprised solely of moment frames in structures assigned to Seismic Design Categories D, E or F, which could be argued as essentially being the same as requiring ρ to be used in the determination of drift.
It should be emphasized that this provision applies only in Seismic Design Categories D, E and F and applies to all moment resisting frames —” ordinary, intermediate, and special. Item C in Table 12.2-1 lists moment-resisting frame systems. Ordinary and intermediate reinforced concrete moment frames are not permitted in Seismic Design Categories D, E and F; however, ordinary and intermediate steel moment-resisting frames are permitted in these seismic design categories provided certain limitations are met in accordance with the table footnotes referenced for these systems.
This provision of ASCE 7-05 Section 126.96.36.199 represents an interesting trade-off between redundancy and lateral stiffness. Redundancy is the concept of multiple load paths —” more than one way for loads to be transmitted from the point of application down to the ground. Redundancy helps a structure, especially a moment frame system, to attain a comparatively large deflection without significant strength loss. This is because when a particular beam loses moment connections at both ends, the load that can no longer be carried by that beam can be picked up by the remaining beams and the structure can go on deflecting without appreciable strength loss.
In the absence of such redundancy in moment frame systems, it is wise to cut down on lateral deflections by making the building laterally stiffer because that will keep the secondary (P-delta) effects low and thereby avert instability problems. The penalty imposed by a ρ value of 1.3 is a strength penalty. Stronger usually means stiffer, but the two terms are definitely not synonymous. The provision in question is intended to ensure added stiffness, not just additional strength, in the absence of redundancy.
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 firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com