strut and tie model and punching shear

[oranda]

I have to design a beam to support large point load (column over) which is about 0.5m from the support (column under).

If I use strut and tie model to design this area, am I still required to check for punching shear because the reaction is quite large. (I'm not too familiar with STM method).

Thanks

 

[ishvaaag]

I would say that in this case you have to design and check mainly a corbel. There are provisions for designing corbels in the Structural Journal, and many texts like the one on RC by Nawy also has. If there is more concrete than the corbel, you need design for the shear -and any forces- standing in whatever the kind it comes. But you can reinforce the whole thing (with care) as the superposition ot the corbel thing and the rest. If feasible, better to use a deeper thing than a corbel, but more likely is not feasible, from your exposition.

Search in the Mathsoft Mathcad 2000 Collaboratory, Civil Engineering Folder

Reinforced Concrete Corbel

 

[StephenEd]

I believe the reason for the development of the strut and tie model was to provide a load path which is more accurate than a deep beam model. To this end you have tension and compression forces but no shears.

 

[oranda]

Thanks for both replies. My problem is not really a corbel situation. The beam is just support another column above which slightly offsets from the column below.

Will I still have to check for punching shear with STM?

Thanks

 

[ishvaaag]

As StephenEd signals you need to ensure a load path to the ground for any loads. Then, for example, and out of memory since the column has such small offset I think to remember the Spain’s code precisely indicates one would have to employ strut and tie model to ensure that the load is passed. However this doesn't mean that you forfeit the output of say a FEM model, simply combine both analysis tools for the better. Obviously for a cantilever-like situation the tie thing is very easy to dimension, and it is contrarily the other things that are contemplated during corbel design what may become more problematic (except one is imprudent with the tension tie). Hence looking it as if corbel is not as bad to complement any other thing you make.

But basically you will have to ensure that all main rebar, stirrups and concrete remain within the surmised limits of the code.

 

[StephenEd]

Do you have the new ACI code? There is a quite extensive appendix having to do with the strut and tie models. No "punching shear" to worry about. It doesn't sound like you have a cantilever or corbel (right?).

There are some special ACI provisions for a "regular" beam if the concentrated load is located less than a distance d from the support, but this will not apply in the S and T model.