Celt83
Structural
- Sep 4, 2007
- 2,083
Diving deeper into running strain-compatibility analysis on some beams to verify some computer results I stumbled on the condition of relatively deep beams as defined in ACI 318-14 9.7.2.3 having a depth greater than or equal to 36" and the consideration or lack thereof of the provided skin reinforcement to the capacity of the beam and more generally beams with layers of steel in tension much higher in the cross section than anticipated when looking at shear strength of the cross section.
I looked at a test case of a 12 x 36 cross section with #4 stirrups and (3)#8 main tension face bars and (2)#7 compression face bars which we would typically provide to give the stirrups something to tie too both with and without #4 skin reinforcement spaced per ACI 318-14 24.3.2 up to H/2.
Considering the skin reinforcement nets an increase of about 110 ft-kips to the flexural capacity but a decrease to the shear capacity of the beam of about 4 kips. 4 kips isn't all that large of a decrease but if you run the skin reinforcement for the full height of the beam looking at strain compatibility a lot of those bar layers end up in tension and raise the centroid even further.
So the question I am getting at is does anyone have any resources or any insight into the standard ACI shear equations and in conditions like this is the intent really to take d as the distance to the centroid of all of the steel undergoing tension or was the intent of d in those equations, 22.5.5.1, meant for a typical condition up to like 2 layers of tension steel?
Another condition where we have seen this lower shear capacity in computer analysis and verified by a hand strain-compatibility analysis is two-way slabs with drop panels proportioned to help reduce the amount of flexural reinforcement, ACI 8.2.4, and the existence of a bottom reinforcing mat. Per ACI 318-14 8.7.4.2.1 the bottom steel in the column strip should be continuous or have a class B splice, the problem being in a lot of conditions based on strain compatibility the "bottom" mat at the deeper drop sections is actually in tension thus reducing the tension steel centroid location and the one-way shear capacity.
I have seen this rationalized away as designing the section only considering the steel you need for flexure but my hesitation with this is if the bars are present the response of the cross section will not match the analysis and potentially have a significantly lower shear capacity based on the standard equations.
I looked at a test case of a 12 x 36 cross section with #4 stirrups and (3)#8 main tension face bars and (2)#7 compression face bars which we would typically provide to give the stirrups something to tie too both with and without #4 skin reinforcement spaced per ACI 318-14 24.3.2 up to H/2.
Considering the skin reinforcement nets an increase of about 110 ft-kips to the flexural capacity but a decrease to the shear capacity of the beam of about 4 kips. 4 kips isn't all that large of a decrease but if you run the skin reinforcement for the full height of the beam looking at strain compatibility a lot of those bar layers end up in tension and raise the centroid even further.
So the question I am getting at is does anyone have any resources or any insight into the standard ACI shear equations and in conditions like this is the intent really to take d as the distance to the centroid of all of the steel undergoing tension or was the intent of d in those equations, 22.5.5.1, meant for a typical condition up to like 2 layers of tension steel?
Another condition where we have seen this lower shear capacity in computer analysis and verified by a hand strain-compatibility analysis is two-way slabs with drop panels proportioned to help reduce the amount of flexural reinforcement, ACI 8.2.4, and the existence of a bottom reinforcing mat. Per ACI 318-14 8.7.4.2.1 the bottom steel in the column strip should be continuous or have a class B splice, the problem being in a lot of conditions based on strain compatibility the "bottom" mat at the deeper drop sections is actually in tension thus reducing the tension steel centroid location and the one-way shear capacity.
I have seen this rationalized away as designing the section only considering the steel you need for flexure but my hesitation with this is if the bars are present the response of the cross section will not match the analysis and potentially have a significantly lower shear capacity based on the standard equations.
