PRECAST CHANNEL SLABS

[alumpkin]

Looking for some help for diaphragm action using Precast Channel Slabs ......made back in the 1950's....maybe 1960's. Basically, it is a 8'-0" x 4'-0" x 2-3/4" plank with thicker ribs at the edges. Typically, they are connected to the steel below by a single finger clip at every other channel slab. I have a very large project where the owner is considering renovation or a new facility. The Building Code officials will require that the entire facility be evaluated per our latest IBC codes (2012) and deficient parts and pieces be brought up to code. I am trying to validate a roof diaphragm attached to the steel below but....a thin finger clip is not a legitimate connection. Would all of these panels bind up by friction when loaded? Would I have to go back and begin a process to attach these panels together and then attach the connected panels to the steel framing?

 

[KootK]

Would all of these panels bind up by friction when loaded?

To some degree but that would be tough to rely on, especially under seismic which may reduce the effective weight of the panels. There's no topping or anything? Horizontal bracing below?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[alumpkin]

No.....there is no horizontal bracing. I would agree....this is a difficult issue to resolve. As for a topping....I am unclear at this point. No drawings exist and current access to the area is very limited due to production. See the attachment for a typical precast channel slab.

 

[KootK]

According to the literature, the intended diaphragm connection is welds to the support steel. And those welds would be hidden from view. Are you concerned that those welds may be missing?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

I've run into this myself a few years back on a large warehouse renovation/retrofit in Missouri. Same channel slabs; same clips as shown on page 7 of your attachment. You're looking at the metal edge plank KootK.

I didn't see any evidence that the clips were welded. I think they were maybe screwed into the channel flange on one side of the joint, but that's about it. I saw nothing to indicate that the system is capable of performing as a diaphragm, but this building required a diaphragm for stability. It's a mystery to me. Luckily we were not required to bring the building up to code due to limited alterations at the roof. Oh, and by the way, it was untopped.

 

[alumpkin]

Yes.....that's right. I am unclear about our requirements to bring this building into compliance per our building officials. Nevertheless, it is a functioning building for 50-60 years. It may not have seen true design lateral loads but I do not see how to legitimately call this a functioning diaphragm for seismic code analysis. We've kicked it around here in the office and thought about about adding some sort of flat strap bracing across the entire roof. This would entail a complete roof removal, installation of some bracing, re-roofing and so on. This would be an approximate 240,000 sq ft roof re-work....could be millions. That's why this is such an important question. Thanks for commenting.

 

[KootK]

I would think that you could do something from the underside. Either stitch the panels together or install strap bracing below. Stitching seems promising as the pieces would be small to handle and the diaphragm capacity of the planks themselves is obviously not in question.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[alumpkin]

KOOTK.....Sure that would work......but alas no access. In production with these products.