Lateral Deflection 3

[AggieYank]

To simplify the structure I'm designing, lets say it is a steel framed 100' x 40' x 22' tall building. Each wall is CMU. Using CMU walls as shear walls can lead to cracking in the CMU (since to act properly, CMU shear walls crack). Instead, I'm forced to use moment frames. The lateral deflection limit is going to make a large difference in my design. I have several questions on it.

1) In IBC table 1604.3, deflection limit for wind for exterior walls with brittle finishes is L/240. That seems like a low limit for CMU. The allowable deflection would be 1.1". ACI 530 doesn't give a deflection limit. Is L/240 the correct deflection limit for CMU walls?
2) Other engineers in my office use the L/240 limit for lateral deflection. However, they calculate it in an interesting way. They say that the building acts like half of a beam. "The top of the wall is equivalent to the midpoint of a beam." Saying this, you could say that to properly use a deflection limit of L/240, you'd have to double the building height. Or, in other words, use L/120 for the lateral deflection. They have used this same concept for cantilevers. I can understand their reasoning, but have been hesitant to use this method, because I'm not sure what the IBC intends. My gut feeling is to not use this method, and we know what they say about gut feelings.

-I'm thinking of using L/240 (true L/240), and on my CMU wall braces from the steel beams, detailing them so that the steel frames can move without moving the wall or creating a stress in the brace or wall.

Thanks in advance for the feedback.

 

[AggieYank]

It appears I may have answered my own question. I found this guide which is based on "Serviceability Design Considerations for Low-Rise Buildings", which was apparently sponsored by AISI and AISC.

This metal building supplier provides a very nice summary of different deflection limits.

I'll also assume that the "cantilever" analysis where the limit is L/120 as opposed to L/240 due to the idea of the building being half of a beam is incorrect.

http://www.wwbldgsys.com/pdf/service.pdf

I'd still very much appreciate feedback on different limits other engineers use.

 

[JAE]

Hmmm - I don't know about using H/100 for lateral drift. That seems like a very very high allowable drift. For your 22' high building you'd be able to allow 2.6" of lateral drift - with CMU walls you'd have a mess.

There was either an ASCE or an AISC (probably AISC) Journal article - I've got it at my office and I'll dig it up on Monday to see what it says. I do remember that, depending on the external skin, the lateral drift limit was in the range of H/400 to H/500.

 

[14159]

You need to get your hands on the AISC serviceability seminar notes from a few years back. I'd be willing to bet that somebody in your office has a copy of this. It has a very good writeup on drift limits. It also has a copy of the AISC EJ article by Griffis on the subject (probably the one JAE is referring to). You could get a copy of this paper free at AISC's website assuming that you're a member.

I think you're going to end up using H/400, but that's for a 10-year wind. Your code wind load is 50 year wind, so you'll have to convert it. The conversion is 50%+/- or 70%+/- depending on the wind speed and this stuff is detailed in the seminar notes. The conversion is also in ASCE 7, keeping in mind that pressure is proportional to the velocity squared.

BTW, given the proportions of your building, unless your walls are swiss-cheese, I don't buy the proposed problem with using CMU shearwalls. It's difficult to imagine a situation where that wouldn't be better than moment frames with CMU serving more as cladding. If you have CMU walls all around, then why not bear directly on the CMU and not have exterior columns? This is done very frequently.

I also don't understand "since to act properly, CMU shear walls crack." While this is probably correct at the ultimate strength, I think it's too much of a broad brush and has no relevance for this thread. I see no reason to shy away from using CMU shearwalls because they'll crack if used as shearwalls. They're a lot more likely to crack (by about 4 million times) if supported by your relatively flexible moment frames!

14159

 

[AggieYank]

As always, thanks for the responses. I'll take a look at finding that Design Guide 3. I'm not a member of AISC, but it looks like its about time I joined.

JAE (or anybody), if you could post what you find on Monday that would be awesome.

 

[WillisV]

AISC Design Guide 3 is certainly a good resource and as it was co-authored by Griffis it contains much of the recommendations in a previous paper. A couple of important points to note:

1. The drift limits in the seismic section of the IBC are just that...drift limits for seismic action. These drift limits are not serviceability limit states but rather help maintain the overall stability of the system under strength based earthquake loads.

2. H/400 is a reasonable servicability type wind load deflection and as someone pointed out you can use this with a load case of 0.7W to take the 50 year code wind down to a 10-year. I wrote about this previously:

http://www.eng-tips.com/viewthread.cfm?qid=123091