Reinforced Concrete Beam - Horizontal Joint Shear Reinforcement 1

[Baffled Engineer]

Hello,

I am designing a reinforced concrete beam composite with a slab poured separately as shown below.
I understand this will require shear interface reinforcement for shear friction to resist horizontal slip. My questions are as follows:

1. Are the horizontal shear reinforcements (in the form of stirrups) additive to vertical shear reinforcements? I.e. If I am utilizing stirrups for additional vertical shear resistance (Vs), can these stirrups be utilized for horizontal shear resistance as well via shear friction reinforcement, or do I add more stirrups for this component separately?

2. Will the vertical concrete shear resistance (Vc) be affected by the horizontal joint?

Thank you.

 

[KootK]

1. Are the horizontal shear reinforcements (in the form of stirrups) additive to vertical shear reinforcements?

No, they are not. You can use the same stirrups for both purposes and "double" dip so to speak.

2. Will the vertical concrete shear resistance (Vc) be affected by the horizontal joint?

No, at least not with respect to how you calculate capacity.

I feel that the key to understanding this is to recognize that, at its heart, what you are doing at such a joint is reconstituting the shear compression struts such that they don't fail at the horizonal plane of the construction joint as shown below. Similar to the compression webs of a real truss, each strut/web serves the purpose of completing the load path for both horizontal and vertical shear. That, as it must be since those two aspects of shear are always and forever complementary.

This is a very common question in the minds of designers. Our code provisions generally separate the calculations for vertical and horizontal shear without really making it clear that they are complementary aspects of a single phenomenon that must, eventually, be stitched back together for a complete design.