Hi,
I am designing a two-way PT slab with ACI 318-08. "CS Code Min. Reinforcement Location" is set to "tension face."
However, when I look at the "Code Minimum Design Top/Bottom Reinforcement" plans, I see the same code minimum bars detailed at the top and bottom in many locations (whereas I would expect to see minimum bars only pver the top of columns). An example is shown below. All code minimum bars in this region are controlled by ACI 318-08 18.9.3.3 (shown below) which only specifies top bars.
I took a look at top/bottom stresses for service level design (shown below), and it does not seem these bars are applied to the tension face only.
20191014 L2-L5_grids 5-9_class u_flat slab_EWD.cpt
Top/Bottom Reinforcement
Top/Bottom Stresses (Service Design)
Why is the Minimum Reinforcing required placed on the wrong slab face?This sometimes happens for an ACI318 or BS8110 / TR43 design.RAM Concept locates the minimum reinforcing required by certain design criteria on the tension face of the slab (or the face with the least amount of compression); this normally works well for both elevated slabs and mat foundations. The programs uses the envelope of bending moments for the Code Minimum rule set for this determination (not the service design stresses). However, in certain cases the moment at a design strip cross section is of the opposite sign of what would be expected given the location. For an elevated slab this can lead to reinforcing at columns being at the bottom of the slab and reinforcing at mid-span being at the top of the slab.For example, for ACI318 or TR43 if there is no tension at a slab location under service conditions, then RAM Concept places the minimum support rebar on the face with the least amount of compression. This could be the bottom face at a column.You can overrule this by choosing Elevated Slab for the design strip property CS Min. Reinforcement Location.
Answer Verified By: Eric Diels
Thank you! Does this explain why there are identical bars detailed at the top and bottom of (for example) the rightmost column in the screenshot above? Why is RAM putting bars on both faces instead of the tension face (or face with the least compression)?
Ah, so the envelope has positive and negative moment at this column, so code minimum bars are placed at top and bottom because they are both considered tension face? Is that right?Also, thank you for pointing out where to find this. That was very helpful.
Correct, and the moments are very small because it's a corner condition. Certainly a case where one set of bars (top) should suffice.