Trimmer bars at re-entrant corners of slabs 1

[StructuralDzine]

The Australian code, and I guess most others, requires additional reinforcement at re-entrant corners, openings, etc to control cracks starting from the corners. We usually use trimmer bars at 45 degrees across the corner.
Does anyone have a rational method to determine the quantity and orientation of this reinforcement?

 

[ishvaaag]

For the openings that may be assigned some definite loadings one should be able to use both the strut and tie method and the elastic analyisis to determine stresses, and from there required rebar.

The elastic analyisis is at hand for plates, and then from the stresses we can reinforce, for almost any situation. Maybe some bigger safety factor should be used there.

The more stringent cases, such a cut at the end of the beam to make space for support have some specific procedures stated in the books, but normally I think don't include such bigger safety factor...except in using quite limited tensile stress (and strain) for the rebar.

 

[StructuralDzine]

I was thinking of the case where stresses are caused by shrinkage. In one form or another the problem of reinforcing for shrinkage cracking at re-entrent corners occurs in almost every concrete slab or wall.

Chris Rohde
Editor, Structural D-zine

http://www.topica.com/lists/Structuraldzine

 

[ishvaaag]

I have seen the kind of analysis you refer to in one Concrete International magazine of 2-3 years ago. Have it entrants or not seems not relevant to what that procedure can give, I assume. In any case it looks the rationale of examining if the restrained shrinkage stresses exceed the available tensile strain seems the way to go. The problem I think is that 3D modeling the early concrete and the early tensile strenght on half hardened concrete is not precisely established ground. In reality not even a proper simple 3D criterium of 3D final strength of normal concrete (what would be the equivalent to some Tresca or Von mises for ductile) seems to have gained the general favour, less for non early concrete. Stress concentrations also may make more difficult to model.

 

[Ron]

Chris,
The rationale is for shrinkage control and I look at it in the same manner as direct tension control, though the actual stress distibution is mostly bi-directional.

As an example, consider a block-out box for a column. A majority of shrinkage is perpendicular to each "flat" side of the box. This results in a concentration of shearing occurring at the same time in two different directions, resolving to a 45 degree line off the re-entrant corner.

The result of this bi-directional loading is a bar that is in bending and shear simulaneously, with no good means of predicting the level of stress!

For no good rhyme or reason, when I consider reinforcing across the re-entrant corner, I consider the thickness of the slab (related to mass shrinkage) and use 1 bar perpendicular to the 45 degree line, 3 to 4 inches off corner, assuming a development length of 12 inches on each side of the anticipated crack. Bar is located upper part of thickness, near center.

For thicker slabs, increase to two bars, parallel, spaced about 12 inches apart.

You will not completely stop the cracks, but keep them tight.

 

[Riz]

Institution of Structural Engineers' design manual has a prescription as well:
1. Area of rebars interrupted by openings in slabs ahould be replaced by an equivalent amount half of which should be placed along each edge of the opening. Diagonals bars are thus calculated accordingly.
2. For openings in shear/core walls the rebars that are to be provided diagonally should be able to resist a tensile force equal to twice the shear force in the vertical components of the wall. This should not be less than two 16mm dia. bars across each corner of the opening. {T = 2Q; As = T/(0.87fy)}. In addition to this, vertical and horizontal trimmer bars should also be applied. This can be calculated as in 1.

 

[Riz]

Institution of Structural Engineers' design manual (British)has a prescription as well:
1. Area of rebars interrupted by openings in slabs ahould be replaced by an equivalent amount half of which should be placed along each edge of the opening. Diagonals bars are thus calculated accordingly.
2. For openings in shear/core walls the rebars that are to be provided diagonally should be able to resist a tensile force equal to twice the shear force in the vertical components of the wall. This should not be less than two 16mm dia. bars across each corner of the opening. {T = 2Q; As = T/(0.87fy)}. In addition to this, vertical and horizontal trimmer bars should also be applied. This can be calculated as in 1.