Shear in one way slab

[RHB51]

Concentrated load on a one way slab near a support
such as a concrete wall: Is there any good method of determining how much distribution along the wall (width of slab "b" for shear calcs?
If the load is within distance "d" then some engineers say that you can ignore the load. However, ACI 318 specifically indicates that you cannot ignore it when there is a large change in shear in a short distance.
I have the British standard and also have found a paper written by Spangler in the 1930's; the problem is the British standard would provide about 12" distribution and the Spangler research would give an answer of 36"+.

 

[steve1]

RHB51,

Without knowing all the specifics my guess is that punching shear will control over beam shear. Have you checked that? What effective width did you use for that calculation? In the punching shear check the load is assumed to spread out at a 45 degree angle through the slab thickness, why not use the same assumption and analyze a "strip" of your one way slab whose width equals the width of the loading plus two times the width of slab determined by projecting, in plan, a line at 45 degress from the edge of the load to the support. Also, since this is a "beam" analysis don't forget that not all of the concentrated load goes to one end of the beam.

 

[RHB51]

Steve,
Thank you for your suggestion. If I may go into further
detail; yes, we have considered both of the items you have mentioned. A 45 degree plane is reasonable but may be too conservative; the British standard cooresponds fairly well with a 45 degree angle.
Punching shear actually never controls in what we are doing, even though we check it every time.

Punching shear equations recognize the effect of a long face and how the shear actually is more of a one way shear; this is the reason of the length/width ratio in punching shear equations.

What we are doing is concrete shoring leg loads that typically average 14k factored with the specific system
that is used. a 12" wide strip x 5" depth just doesn't cut it. However, this system has been used many years on
many projects with success. My personal belief is that the construction live load does not occur as suggested by
ACI 347.

Anyway, I appreciate your response. If you would like to discuss this more I would like to also.
Thanks

 

[steve1]

RHB51

A couple of further comments:

If I understand correctly you have a shoring tower (typically 4 legged, made of steel, look similiar to scaffolding) that is being used to shore a floor above during a concrete placement. The legs sit on baseplates that are 12" x 5" in plan.

Do you have the baseplates turned so that the 12" is normal to the beam span?

Have you considered setting a bigger plate under the leg baseplate to further distribute the load?

Concerning the floor itself, how is it constructed?

What is the level of stress you are calculating compared to the allowable?

You say this system has been used for many years with success. Who was checking the supporting floor then?

I would not assume that code prescribed loads are wrong. I think you should consider that you are doing the same analysis that any other engineer would and that is the level of care expected.

Finally, are you sure that there are no numerical errors in your calculations?