Live Load Reduction on a Double Tee 1

[dcarr82775]

Live load reduction per ASCE 7 uses the Kll factor. The item is question is a double tee used in an office building, with a 3.5" (average) thickness topping slab . I would use Kll = 1.0 for this double tee. The original calcs used Kll=2.0. This seems optimistic to me, but is it common in the precast industry to use Kll=2.0?

I don't consider it an Interior Beam, but I could see some making that argument.

Thanks

 

[KootK]

I've been doing some double tee work lately and, in particular, studying their ability to load share between units. I'd love to be wrong about this but, as far as I can tell, double tees don't really share load among themselves owing to their low torsional stiffness to flexural stiffness ratio. Certainly, load/response sharing is not as aggressive as it's purported to be with hollow core.

If load sharing is non-existent, then I would argue that [Kll x At] for each unit is equal to the projected area of the individual unit.

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.

 

[dcarr82775]

I can see an argument that the topping slab acts as a transfer mechanism, but it feels wrong to me to assume that load sharing. The slab is always cracked at the joints between DT's so you have shear friction at the joint and some minimal flexural transfer.

 

[EngineeringEric]

Maybe i am wrong, but Kll for an interior beam is 2.0; Kll for a one way slab is 1.0

I would consider a double tee to be a one way slab not a beam and a hollowcore as a slab and not a beam in this scenario. Now if the precast member was being loaded by transverse framing.... i would say it is a beam and therefore kll=2.

Now i know this means you doing get to reduce live loads most likely, and these loads will stack over floors which can be frustrating for load bearing walls (CMU/Concrete) becuse they don't really get to reduce much until multiple floors have added up.

 

[briancpotter]

After the previous discussion about this, I did a bit of digging. There's a report from the Texas DOT (will see if I can find it) that did tests on tee load sharing. As I recall, load sharing only takes place if something like a shear key or notched grout joint is in place on the edges of the flange.

Personally, I've never done any sort of double tee load sharing, for the reasons KootK mentions.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[dcarr82775]

I am firmly in the Kll = 1.0 boat. I has a discussion with 2 other engineers who both routinely use 2.0. I just don't see it unless you have a good topping slab, and even then I don't really like the idea.

 

[KootK]

I can see an argument that the topping slab acts as a transfer mechanism, but it feels wrong to me to assume that load sharing.

There's usually some load transfer mechanism even without a topping. See the related article attached. There pretty much has to be some load transfer ability otherwise you form lips from one plank to the next under differential load. I think that it comes down to an incompatibility between how much transfer can keep adjacent flanges together and how much can truly engage the flexural capacity of the neighboring tee. The former is relatively easy and, one could argue, mandatory. The latter is pretty tough.

I has a discussion with 2 other engineers who both routinely use 2.0.

Any chance those would be precast engineers?

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.