Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
(OP)
Hello Fellow Structural Engineers,
I have been recently hired and I am surprised by the design of window curtain walls and story fronts.
The allowable story-drift given in ASCE is .025, .015, or 0.020h. for a 10 feet story the allowable drift is between 1.8 and 3 inches.
The maximum allowable story drift for a curtain wall system is 0.75 in.
The difference in the allowable is pretty large. Do structural engineers consider this difference in their design? What happens at the corner conditions?
I would greatly appreciate your help, ideas, and input.
Sam
I have been recently hired and I am surprised by the design of window curtain walls and story fronts.
The allowable story-drift given in ASCE is .025, .015, or 0.020h. for a 10 feet story the allowable drift is between 1.8 and 3 inches.
The maximum allowable story drift for a curtain wall system is 0.75 in.
The difference in the allowable is pretty large. Do structural engineers consider this difference in their design? What happens at the corner conditions?
I would greatly appreciate your help, ideas, and input.
Sam






RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
Most general maximum wind deflections are in the order of L/400 to L/500.
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
For guidance on serviceability considerations, you may refer to commentary to Appendix C given in the ASCE 7-05 or the version you are following.(Appendix C is not a mandatory part of the standard)
Per commentary to Appendix C, the limits given are 1/600 to 1/400 of the building or story height. Smaller drifts are appropriate if the cladding is brittle. An absolute limit on interstory drift of 3/8" may be imposed in case of glazing unless detailing practices to tolerate movement are adopted. See CC.1.2 for more details.
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
As you can see my problem is that the building is governed by seismic forces and the curtain window wall are goverened by wind loads. How are they compared? Is the manufacturer responsible to provide a connection that the curtain wall can deflect 3 inches? I understand the service load deflection but the seismic allowable deflection is still greater.
Thank you
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
For one of our projects we had curtain wall. Since it was a striking feature of the project from the architect and the developer's perspective, the curtain wall manufacturer was on board during the design development phase and the related issues were sorted out earlier.
The curtain wall manufacturer restricted drifts to the following limits: L / 175 for span < 13' - 6", and L / 240 + 1/4" for spans > 13' - 6".
You may also refer to discussion in thread507-168653: Curtain Wall / Storefront Wall Design.
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
You say: "the curtain window wall are goverened by wind loads".
That may be true in terms of the structural competence of the window wall itself - i.e. wind loads are more direct and significant on the window since the window doesn't have much mass so wind controls.
However, the window would be subjected to the seismic drifts of the building - the building response would expose the window system to distortions which might actually "control" parts of the window-to-structure connections as well as affect the window-to-structure joint details (widths).
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)
RE: Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)