Double layer reinforcement on one-way slab and ACI 318-14 Table 7.3.1.1 Minimum Slab thickness

[Torres_Structural]

Hi everyone,

I have a two-part question in regards to a one way span for a slab. I've attached a sketch of the slab, with loading and dimensions.

First part is the slab is supported on steel beams which are 8ft apart, and the slab cantilevers 3 ft on each end. It will have two cantilevers. I'm trying to figure out the reinforcement for positive and negative moment. Assuming that we run top and bottom reinforcement, are you to consider the steel as part of the compression value for the combination of steel and concrete compression (similar analysis to a doubly reinforcement)?

part two, In the ACI 318-14 code, Table 7.3.1.1 the minimum slab thickness is shown for different conditions based on the span. My question lies in regards to the span value for consideration of either a cantilever limitation or the simply supported. Based on the above information, does the cantilever span become 3ft or do I consider the 8ft? Can I part out the spans for two different scenarios a cantilever with a span of 3ft and account the l/10; and use the span 8ft for the l/28 (both ends continuous)? I'm not sure how to treat that part.

Thanks!

 

[Lomarandil]

1. Unless your slab becomes much thicker than I expect (or very small cover), you can consider both top and bottom steel to contribute to your capacity, but it's unlikely that you really get "compression steel". Consider the strain profile and you'll usually find that both sets of bars are in tension (although one will be at yield, and the other usually carries much less stress due to its position near the neutral axis). Many designers will ignore that latter contribution, but I find it valuable.

2. Yes, you can part out the spans. The cantilever is straight forward. The interior span may not behave quite as "both ends continuous". Somewhere between that and the simply supported case -- but exactly where is hard to say. Other parameters in your design will probably drive whether it's worth really trying to figure out precisely, or if you should just use the simply supported case and move on to solving other problems.

 

[hokie66]

The beam is statically determinate, so just do the comps. No need to resort to any assumptions. But you should have several load cases, with or without each of the point loads.

As to counting reinforcement on the compression face, forget about it.