Deep Beam Cranking Moment into Thin Wall

[ATSE]

I have a relatively strong beam 350mm wide x 700mm tall [14"x28"] that is bearing on a 250mm [10"] thick wall.
See the attached sketch.
The beam is going to overpower the wall at the joint (negative moment). While there will be some compatibility rotation of the wall, a horiz crack near the top of the wall (bottom of beam elevation) will pop from Day 1, and grow for many years due to resulting loading and creep.
In addition to the rotation and cracking problem, seems like localized crushing (due to rotation) at the inside edge may be a problem.
Beam reaction (vertical shear) is about 265kN [60 kips]. End moment is difficult to determine, but it is obviously bounded between "pinned" (0 moment) and "fixed" (about 300 kip-feet).
This is not a unique situation, but I usually don't have such a stiffness ratio problem.
Question: What is the criteria for adding a pilaster below the beam. I realize that adding a column pilaster (thickened wall section) at the beam centerline will alleviate my concern for this case, but I'm looking for guidance (published info from PCA / ACI would be nice) for this problem specifically, and also for the general condition.

 

[charliealphabravo]

just reinforce the gingerbread wall under the candy bar beam with some horizontal licorice wire and you should be fine

...unless of course you are talking about some other more conventional construction material?

 

[ron9876]

Shouldn't crack--at least not visable--if designed for the forces at the joint. Either the wall will carry it or not.

 

[ATSE]

CAB,
I was hoping for a slightly more technical approach.
To be clear, the beams and walls are reinforced concrete, hence the reference to ACI and PCA.
28-day strength = 28MPa (4 ksi) with typical reinforcing, but the reinforcing specifics probably won't change the conclusion.
The cracking moment of the beam per ACI 318-05 eq 9-9 is larger than design strength (phi-Mn) for the effective wall dimensions (10"x54"), so I am thinking that I have no choice but to use a pilaster.
Looking for some technical backing more substantial than my initial hunch.

 

[charliealphabravo]

hehe, sorry, i couldn't resist the troll. it should be perfectly clear to anyone in the profession i'm sure.

my thinking is that, as the connection begins to behave more like a perfect pinned connection due to cracking and rotation over time, that the eccentricity of the connection approaches 5 inches. if the effective wall section will not take the applied moment then a pilaster would be needed.

 

[hokie66]

If your beam is stiff enough, no need to worry, but if crushing on the inside face is a concern, place some soft material there, say 20 mm thick. I wouldn't be concerned about the crack on the outside. Assume a pinned connection for the beam design.

 

[ATSE]

Hokie,
Cracking at the outside would be the better condition. A more unfortunate condition is if the beam uncracked moment capacity is larger than anticipated, and the beam end cranks moment into the relatively weak wall.
There are dowels that tie the beam and wall together, and the beam will be cast into a formed pocket in the wall. So the dowels provide moment capacity, even if placed in the middle of the wall. This means the weakest capacity is closer to the mid height of the wall. The moment diagram is relatively constant from base to top. If you have a good detail using "soft material" I'd be interested, but this seems to lend itself to precast much better than cast-in-place.
Work is being completed in Vietnam, so I avoid anything fancy.

 

[ron9876]

I still say that if you design the joint to handle the stresses then there should be no problem. If you can't design it for the stresses then something needs to change.

 

[hokie66]

For limiting crushing at the inside edge, you can just use a 20x20 rubber rod, or something like Abelflex (a closed cell polyethylene foam). With normal dimensions of the beam and wall, I have never experienced the type distress in the wall which you are concerned about.

 

[shobroco]

ATSE can't you make the beam simply supported by using smooth dowels & greasing them? With no moment your wall shouldn't have any issues.

 

[ishvaaag]

You may even pierce the wall entirely and through a seat center the load. Tack work may do for waterproofing issues if any.

You could also design a tip of beam and facing plate connection able to allow for the required rotation for a less stringent or even null moment.

You can as well use a metallic seat (corbel) to relieve great part of the moment.

And of course you can use any kind of direct support (steel or concrete column, pilaster) to solve the issue ... analysis will determine the controlling aspects of the selected design. If a pilaster added to an existing wall, you may decide if false and a column, or pinned to the wall. Some FEM or simplified model can give light on the worse stresses appearing to direct reinforcement.

 

[DST148]

@ATSE: Your OP says - End moment is difficult to determine. Why do you think so? You also mentioned about stiffness ratio problem. The distribution of moments will depend on the ratio of stiffnesses(I/L) of beam to wall and not just Ibeam / Iwall. Moreover, you can not develop a standard hook for a bar larger than #4 into a 10" wide wall. If for some reason you have to develop a certain amount of negative moment in the beam, then like you said, adding a pilaster would help.