Allowable deflection of residential steel beam supporting garage prestressed floor system 1

[TroyD]

I assist high-end home builders with various structural engineering issues. A common feature locally in high-end single family dwellings are garages with large basements below. The garage floor system is 8" precast plank with a 4" sloped concrete topping. On particularly large garages, a steel beam is needed to support the prestress plank spans. What is the allowable Dead and Dead + Live deflection criteria for this steel beam? I know for supporting concrete or masonry block, a more conservative L/600 is used. Does that apply here?

Attached is a sketch from the prestressed supplier to illustrate.

 

[KootK]

I'm fine with 360/240 for this. There's nothing much to damage until after the topping hardens at which point most of the dead load will be baked in. Once the topping is on and the keyways are grouted, it'll effectively be a composite beam with a 12" slab over by way of the weld plates. Pretty stiff.

For what it's worth, I used to be a part time hollow core designer and I've seen a lot of these setups go by. There's no way that the EOR's that I worked with were specifying L/600 beams. A lot of W8/W12.

 

[HouseBoy]

I do many high end homes too and I might be more conservative that KK.
Generally I go with L/600 Live load and L/480 total load for something like that.
L/480 live and L/360 total load deflection limit is the absolute least requirement I'd want.
If I need to get the beam size down some, I would figure a way to discount (or set aside) the deflection due to the plank weight alone and then make the topping and/or live load deflection limit a tight number.

 

[OldDawgNewTricks]

L/600 is for very rigid and brittle items like brick walls. But reinforced concrete slabs are not so brittle, especially in the out-of-plane direction. Are the precast planks also designed for L/600? If not, then it is probably overkill for the steel beam.

 

[TroyD]

I ended up using a W12x40 to span 15 ft, supporting ~22 ft trib. width of plank. My L+D total load deflection was ~L/400, and my live load deflection was ~L/1200. Weld plates bracing to plank at 4 ft spacing.

 

[Brad805]

Some of your well to do clients might be interested a system like the Delta Beam from Piekko. Nice clean ceiling for all those fancy cars. Delta Beam No affiliation, just an FYI. We have done that with a plate to the U/S of a wide flange, but I was not keen on the design.

 

[KootK]

Similarly, you could also recess the beam to get more depth. I've never been a huge fan of the detail but it's a fairly common thing to do.

 

[jayrod12]

I've done Koot's detail, but utilized an HSS in lieu of a plated wide flange. Usually this is on those pesky block and hollowcore school projects and the new sizing of ducting required to pass by the corridor load-bearing walls. Is anybody else noticing that the ducting requirements in buildings just seem to be getting worse and worse? With no desire to adjust floor to floor heights to accommodate.

 

[KootK]

I like the HSS version better mechanically. The limitation with it is usually just the limited depths of HSS available for long span / high load applications.

I like this version better too in some ways. Some precasters will tell you that it's all but impossible to yard notch the plank; other precasters just get it done. Accomplishing the notching is usually pretty rough exercise, though, that sometimes leaves me wondering about the remaining shear capacity of the plank webs.