Brick Spandrel Deflections and Applied Loads 2

[woody1235]

I searched out a thread here that came very close to answering this question, but not quite. I hope you can provide a little more comment to this question.

We often design composite steel floor systems (also sometimes concrete on bar-joist floors)that have brick veneer clading on spandrel panels as the exterior skin. These spandrel panels are supported by the perimeter beams/girders, thus subject to the deflection of these members.

When limiting the deflection to L/600 or better, we typically consider the deflection caused by superimposed loads that would be felt by the spandrel. This would be the weight of the spandrel and any supported wall elements above (like windows), floor tributary live load, and miscellaneous superimposed floor tributary dead loads.

We typically do not consider the weight of the floor slab and beams since these are in place before the spandrel construction begins, and do not effect the deflection of the spandrel.

However, the Code simply says l/600 total load, which we have had others interpret to mean ALL loads need to be considered, not just superimposed. This can result in the need for very big beams, especially along the load bearing sides.

What are your interpretations and practices for this condition?

Thanks!

 

[JAE]

We only consider the TOTAL load that the brick veneer would experience. Thus, the self-weight of the beam and slab system wouldn't be included. However, it could be that the veneer would feel the weight of some "later" loads like flooring, ceilings, ductwork, etc.

The other question is whether to include the full weight of the veneer itself in the total load. As the mason builds the veneer, the supporting structure deflects downward. So technically, some of the stress and movement in the brick is partial as the wall is built (i.e. the lower courses will experience the deflections from the upper courses, but the upper courses will not feel the deflections from the lower ones.

 

[msquared48]

Considering the additional deflection from the long term creep of the concrete, definitely use the total load scenario...always with brick veneer.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[DaveAtkins]

Mike--these are steel beams. Or is there creep with composite steel beams? That is something I have never thought about.

Woody--I agree with your method. Normally I check deflection assuming total load, but in a pinch, I have used your method.

DaveAtkins

 

[jike]

We also use the method that has been put forward by Woody and JAE. Studies of office loadings seem to indicate that floors are rarely if ever fully loaded, however, we use the full load in limiting the deflection.

 

[woody1235]

Thank you all for your responses.

I'm glad that our approach seems to generally follow other's practices.

Although we also tend to be conservative in the loading we use for deflection control, sometimes the results of using ALL loads in this spandrel condition just don't seem realistic.

 

[vandede427]

I believe that the designing the deflection to L/600 for the TOTAL load is overly conservative.

I would design the beam to have L/600 for only the loads imposed AFTER the brick is laid (live load and superimposed dead load).

Whatever deflection is in the beam after the concrete hardens will still most likely be small enough (if you've properly sized the beam) for the masons to adjust to. Masons are typically very good at what they do.

 

[JAE]

vandede427 - the L/600 for "total" loads is a requirement in Section 1 of ACI 530. That's where this post is coming from. The question is really the definition of what "total" means. To me, obviously, any load that causes deflection before the masonry even exists shouldn't count - any load that causes deflection that the masonry will experience should.

 

[vandede427]

My ACI 530 says "dead plus live" in Section 1.10. I don't see the word "total" in this section.

 

[JAE]

I'll check my copy - you are probably right.

 

[TXStructural]

Also mind the 0.3 inch maximum deflection, not just L/600. We also consider that the deflection is D + L, for loads applied after masonry installation begins. Deflection prior to applying the veneer is irrelevant.

Also, I have worked with architects who assured me that L/600 will result in cracks in the veneer.

 

[WillisV]

0.3" maximum removed in -08 edition.

 

[TXStructural]

Thanks. I didn't have an -08 handy, and haven't designed using it. Obviously.

 

[vandede427]

.3" controlled for beams greater than 15' span, which is usually the case for steel. If that is removed in the '08 code, that is a huge change in design. That is major.

 

[DHKpeWI]

I agree with using the super imposed loads only. I have done this in buildings in Wisconsin without any problems. I also recommend Design Guide 3 From AISC. It can be downloaded for free from the AISC web site if you are a member.

If your beams are getting large you might also consider the arching effect of the masonry around the opening. The following resource has some guidelines:

http://www.amazon.com/Masonry-Struc...=sr_1_6?ie=UTF8&s=books&qid=1266862107&sr=1-6

If the openings are wide arching may not work in your application.