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Point Load on Concrete Wall

Point Load on Concrete Wall

Point Load on Concrete Wall

(OP)
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?

RE: Point Load on Concrete Wall

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.

 

RE: Point Load on Concrete Wall

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.

RE: Point Load on Concrete Wall

(OP)
can someone point me toward bursting in the ACI code?

RE: Point Load on Concrete Wall

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.

RE: Point Load on Concrete Wall

(OP)
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.  

RE: Point Load on Concrete Wall

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

Any takers now?  

RE: Point Load on Concrete Wall

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

RE: Point Load on Concrete Wall

Toad:
Look at ACI 318-05 Section 18.13

RE: Point Load on Concrete Wall

(OP)
a2mfk-
Now would be great~!
 

RE: Point Load on Concrete Wall

(OP)
slick-
do you think that directly applies here?  

RE: Point Load on Concrete Wall

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

RE: Point Load on Concrete Wall

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 Wcosθ.  Length of failure surface is 12/sinθ.

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

BA

RE: Point Load on Concrete Wall

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

RE: Point Load on Concrete Wall

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

BA

RE: Point Load on Concrete Wall

(OP)
BA-
Thank you kindly for the input.

I will run some numbers.  

RE: Point Load on Concrete Wall

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

BA

RE: Point Load on Concrete Wall

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

RE: Point Load on Concrete Wall

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

RE: Point Load on Concrete Wall

(OP)
This is also ignoring a shear plane perpendicular to the one show in the sketch, so it is conservative, right?

and the question becomes, what shear allowable to use?

RE: Point Load on Concrete Wall

Toad,

I did not expect the angle to be 45 degrees but the expression cosθsinθ is maximum at 45 degrees.  

The suggested formula does not take into account the shearing resistance at the two ends of the failure zone, so the failure angle could change as a result.

slick,

Okay.

BA

RE: Point Load on Concrete Wall

I think this is clearly a case of over analysing. If a 6 feet thick concrete wall is not capable of supporting this load then we all need help.
Engineering Judgement??

Kieran
 

RE: Point Load on Concrete Wall

Toad,

Yes, I think it is conservative.  The two end areas combined are (a + b/2)^2/tanθ so a revised angle could be calculated based on this additional shear resistance.

Vc is defined in the Canadian code.  Fifty percent of Vc is permitted when shear reinforcement is omitted.  I would think similar provisions can be found in ACI.

BA

RE: Point Load on Concrete Wall

Kieran,

You may be right.  My first thoughts were to simply check the bearing stress and forget about it, but in this case the client wants the issue specifically addressed, so we have been attempting to come up with a way of doing that.

BA

RE: Point Load on Concrete Wall

@Kieran,
I think the location of the load is more important here compared to the thickness of the wall. If the load was smack on the center, I agree with you.

RE: Point Load on Concrete Wall

(OP)
Kieran-
My thoughts were originally the same,...however, I can't simply plop a 45 kip load right near the edge of the wall. The wall could be 5 miles wide at that point but I still can't load the edge without running some numbers.

slick-
Think  phi x 2 x fc'^0.5 is the correct shear formula?  

RE: Point Load on Concrete Wall

I'm not sure if this meets all of your design criteria but I would take a look at PCA Notes on ACI 318-05 Example 14.2 on Page 14-9.  Good luck!  Let me know if you need a copy of the page

RE: Point Load on Concrete Wall

(OP)
a copy would be nice

RE: Point Load on Concrete Wall

(OP)
that you slick.  

RE: Point Load on Concrete Wall

Sorry Toad, my weekend got away from me...

Slick's attached method is the same I use from an older concrete text. I think a few bars in the right spot are cheap insurance. I do this even on embed connections in tilt walls that rely on bearing.

RE: Point Load on Concrete Wall

(OP)
thanks guys.
I never thought to look at PCA until Saturday....I still can't find my notes from '02.
Guess I should get the new ones.
 

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