Deep Beam Cranking Moment into Thin Wall
Deep Beam Cranking Moment into Thin Wall
(OP)
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.
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.






RE: Deep Beam Cranking Moment into Thin Wall
...unless of course you are talking about some other more conventional construction material?
;)
RE: Deep Beam Cranking Moment into Thin Wall
RE: Deep Beam Cranking Moment into Thin Wall
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.
RE: Deep Beam Cranking Moment into Thin Wall
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.
RE: Deep Beam Cranking Moment into Thin Wall
RE: Deep Beam Cranking Moment into Thin Wall
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.
RE: Deep Beam Cranking Moment into Thin Wall
RE: Deep Beam Cranking Moment into Thin Wall
RE: Deep Beam Cranking Moment into Thin Wall
RE: Deep Beam Cranking Moment into Thin Wall
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.
RE: Deep Beam Cranking Moment into Thin Wall