Story Drift - Horizontal Combination of Lateral Systems

[RMalaska]

In a large CMU Shearwall Building (100,000 sq ft, 350’x285’), in Seismic Design Category D, if at the center of the building, in one direction, a line of special steel moment frames is added, such that the moment frames will be taking one-half of the building seismic shear (flexible roof diaphragm); what is the drift limit for that moment frame line?

The Building Dept is arguing that the roof as a whole is assigned a single story drift value from Table 12.12-1 of ASCE 7. Since the exterior of the building is masonry cantilever shearwalls, then the story drift for the roof is 0.010*hsx. Since the moment frame is drift controlled, and taking 1/2 of the building shear, I have to do all sorts of difficult gyrations to make the moment frame work at the 0.010 limit.

Is there any published white paper that talks about the combination of these two lateral systems and, similar to section 12.2.3.2, since we can use independent R values for the masonry shearwalls and moment frames and since we have a flexible diaphragm, shouldn’t we also be able to use independent drift limits (as long as the diaphragm deflection will allow it, which it does in this case)?

 

[JAE]

Does footnote c of table 12.12-1 help? If all your interior partitions can take the 0.02h drift, then perhaps the city would buy off on the increased limit.

Also, section 12.12.2 suggests the same thing.

 

[RMalaska]

footnote c in Table 12.12-1 only applies to the 0.025*hsx limit, not to masonry buildings.

12.12.2 only addresses diaphragm deflection, and doesn't seem to address vertical LRFS story drift.

 

[JAE]

I guess I was thinking that 12.12.2 might be a way to convince your building dept. that as long as you contemplate, and check, that all attached elements can withstand the higher deflection in the center, and that the average deflection for the tributary diaphragm near the shear walls is less than the 0.01 limit they might concede.

 

[wannabeSE]

I can’t think of any exceptions in the code for the combination of different lateral systems on separate lines. However, the commentary to Chapter 4 of FEMA 450 discusses the purpose of the drift limits (limiting inelastic strain, stability, etcetera) which may be useful.

On another note, have you considered increasing the diaphragm stiffness and analyzing the building with a semi-rigid diaphragm? Provided there is enough shear wall to handle the increased load, the moment frame deflection may decrease to an acceptable level. Since the frame is drift controlled, using the masonry wall R and Cd factors for the moment frame may not be a big deal.

I am curious what kind of diaphragm is being used. I only have a little experience with bare metal deck and composite deck diaphragms, so I am surprised to hear about a steel moment frame with a flexible diaphragm (diaphragm deflection > 2x frame drift per section 12.3.1.3).

 

[JAE]

wannabeSE may be on to something - if you check your deck deflection against your shear wall and moment frame deflections, you may include diaphragm stiffness in the analysis and this may redistribute loads away from the center moment frame.