Shear Strengthening - Concrete Joist

[slickdeals]

thread507-221830

There is an existing concrete floor system that is comprised of 12" deep precast joists with a 3" composite concrete topping. There is a change is usage of the floor and subsequent increase in loading.

What are the options for strengthening the beam without doing fiber reinforcing, adding channels etc.?

See attached sketch. The rib above the joist has ties and flexural reinforcing. The rib is in turn connected to the joist using single doweled bars.

My question is:
1. How far should the dowel bar be embedded such that both the sections can act composite for shear.

Does the detail even work?

The bar will be checked for horizontal shear transfer. The surface will be roughened to 1/4" amplitude.

 

[hokie66]

I wouldn't count on the sections acting compositely for shear. The dowels cannot be adequately developed for that to happen. Both sections will resist shear force, in proportion to their bending stiffness.

 

[BAretired]

I would have to disagree with hokie66. If there are enough dowels and they are adequately anchored in the lower joist, composite action will be attained provided the surfaces are roughened to 1/4" amplitude.

BA

 

[hokie66]

BA, maybe we are talking at cross purposes. I agree about the horizontal shear, but I thought the OP was trying to use the dowels as stirrups in resisting beam shear.

 

[Lion06]

I think the dowels will help to achieve composite action for flexure, but not for shear. The dowels and a roughened surface will likely develop the required horizontal shear to along that plane, but there is no continuity between the upper and lower stirrups. A consequence of that is that they will act independently for shear. They will deflect together, so, as hokie notes, they will share shear forces in proportion to their bending stiffness. You will probably only get the full capacity out of either the top or the bottom, but not both.

 

[BAretired]

hokie,
Indeed we are talking at cross purposes. The dowels have no value as stirrups, but they do have value in transmitting horizontal shear betweeen the two joists.

BA