Point Load on Concrete Wall

[ToadJones]

I have an Extremely thick reinforced concrete wall, around 6 feet thick and pretty tall as well, at least 15 feet.
I have to put a temporary concentrated load on the wall.
Owner's engineer wants the wall checked and suggested checking punching shear.
This made no sense to me, but the wall will be loaded near the edge, maybe 12"-16" in from the edge.

Other than bearing, I can't think of what else to check.
Intuitively I can see some sort of break out of the edge of the wall, similar to side face blowout.

Thoughts?

 

[normm]

ToadJones

The problem can occur if there is not enough horizontal steel near the point of application of applied load. Explained in attached pdf.

The formula for bursting force can be found in end block design in prestressed concrete Codes in any country or any text book.

 

[Lion06]

I've always been curious about that type of failure mechanism, but have never come across an example.

What seems counterintuitive is that the failure plane that makes sense (what is drawn in the sketch) is a principal compression plane, not a principal tensile plane.

 

[ToadJones]

can someone point me toward bursting in the ACI code?

 

[a2mfk]

Toad- you subbing out your sketches now? Where's BA or Hokie when you need them...

In all seriousness I just did some design calcs on something similar, what kind of load are we talking about? The failure mode is as Norm's sketch shows, and I have a method from a textbook for the design of the rebar for such a failure. Its not too bad at all.

 

[ToadJones]

a2mfk

I am estimating the load at 45 kips, for now (WAG).

Care to share your textbook method. I cant find anything in my concrete texts.

 

[ToadJones]

my situation is similar to a highway bridge with steel beams/ rockers bearing on the concrete abutment.

Any takers now?

 

[a2mfk]

Toad- I got you covered, can it wait until Sunday?

 

[slickdeals]

Toad:
Look at ACI 318-05 Section 18.13

 

[ToadJones]

a2mfk-
Now would be great~!

 

[ToadJones]

slick-
do you think that directly applies here?

 

[slickdeals]

I am not sure how directly it applies here. You might want to look at Figure R18.13.5.

 

[BAretired]

Here's a suggestion. Assume weight W is centered 'a' from edge of wall. Bearing plate is b x L where L is parallel to the wall. Say that failure plane occurs at an angle [θ] to the vertical.

Force W can be resolved into component parallel and perpendicular to the failure plane. Parallel component is Wcos[θ]. Length of failure surface is 12/sin[θ].

The shear stress on the failure plane is Wcos[θ]sin[θ]/(a + b/2)L. This will be maximum at 45 degrees. Compare that value with allowable shear for plain concrete.

BA

 

[BAretired]

If W = 45000#, a = 12", b = 8", L = 12" then v = 45000(0.5)(12 + 4)12 = 117 psi (too high, so increase b and L until acceptable).

BA

 

[BAretired]

correction...v = 45000(0.5)/(12 + 4)12 = 117 psi

BA

 

[ToadJones]

BA-
Thank you kindly for the input.

I will run some numbers.

 

[BAretired]

correction 2... Length of failure surface is (a + b/2)/sin?.

BA

 

[slickdeals]

@BA:
Should'nt the shear stress be {W*sin?}/{cos?*(a+b/2)L}

 

[BAretired]

@slick,

I don't think so. See attached.

BA

 

[ToadJones]

Do you think it is reasonable to estimate this failure plane at 45º?