Building Allow. Drift (Deflection) vs. C&C Allow. Drift (Deflection)

[Sam78053]

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

 

[JAE]

Are you looking at wind deflection or seismic? 0.02H is for seismic and your window wall would be damaged (but shouldn't fall off the building) in a seismic event.

Most general maximum wind deflections are in the order of L/400 to L/500.

 

[DST148]

@Sam78053: Drifts under seismic loading are computed using the strength level seismic forces whereas under wind loading it is computed at reduced service level forces.
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.

 

[Sam78053]

Thank you to both.

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

 

[DST148]

@Sam78053: The limits given in the code are upper bound. You need to coordinate with SEOR for the actual drifts of the structure.
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.

 

[JAE]

Sam78053,

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).

 

[wannabeSE]

I have seen different details to accommodate drift at curtain walls and storefronts. For instance, the Steel Network manufactures connectors that will accommodate floor deflection and drift (

http://www.steelnetwork.com/Site/ProductLines#Drift

). I have also seen clips designed and detailed so they bend to accommodate drift. I don't know if there is a sure-fire solution at corners. In high seismic areas, it is not uncommon to see horizontal joints for drift in the cladding on multistory moment frame buildings.

 

[mijowe]

@DST148 Those limits are what the curtain wall manufacturer is going to limit his system too (ie the mullion deflection at midspan under wind load), they are not the same as the drift limits of the building.

 

[DST148]

@mijowe: In my second post I have clearly said that the curtain wall manufacturer restricted drifts to ........ I never said those limits related to the building structure.