Concrete beam load transfer to vertical supports

[StrEng007]

When a concrete beam is placed over a vertical support, such as a concrete wall, concrete column, or masonry wall, what is the load distribution from the beam to the vertical element?

For instance, say you have a beam that is placed over a 4'-0" CMU wall section. Is there a concentrated force that develops at the dimension "X" shown below?

The same question goes for a beam placed over something like a 3ft wide column. Is the axial load transfer uniform or non uniform over the support?

 

[KootK]

For instance, say you have a beam that is placed over a 4'-0" CMU wall section. Is there a concentrated force that develops at the dimension "X" shown below?

Absolutely. And the more flexible the beams is, the worse the issue. It's a good reason to have a stiff beam.

 

[StrEng007]

How do you determine the relationship between beam stiffness and the distance "x" that the load is distributed?

 

[KootK]

I was hoping you wouldn't ask.

 

[Greenalleycat]

When a concrete beam is placed over a vertical support, such as a concrete wall, concrete column, or masonry wall, what is the load distribution from the beam to the vertical element?

For instance, say you have a beam that is placed over a 4'-0" CMU wall section. Is there a concentrated force that develops at the dimension "X" shown below?

The same question goes for a beam placed over something like a 3ft wide column. Is the axial load transfer uniform or non uniform over the support?

View attachment 7250

A very interesting question and I think it comes down to "it's very complex and many things happen"
I'm assuming here that the wall is poured and is solid first and has a perfectly flat and level top - otherwise, a few mm of variation could be enough to change the whole loading distribution

I think the wall will behave closer to a cantilever 2 support beam than a uniform support condition so it will want to hog over the middle of the wall
However, the loading will be very high at the outside edge so you probably get some localised crushing at the masonry/concrete shell that distributes the load further along the wall
How far this goes is a complete guess depending on relative stiffnesses, loading, etc

By the time you get to the base of the wall I think it will be engaging most/all of the wall width for bearing checks

 

[StrEng007]

Greenalleycat: I'm assuming here that the wall is poured and is solid first and has a perfectly flat and level top - otherwise, a few mm of variation could be enough to change the whole loading distribution

Would it really, though? If the tie-beam is cast on top of the wall, wouldn't any variation of the CMU be "absorbed" by the fact that the concrete hasn't cured yet?

As far as the hogging, it seems like the most conservative thing to do is consider the beam supported at the center of the wall. This increases the beam length for checking the location of an internal support over the continuous beam.

For the x-dimension, if you were going to conservatively make the system behave as mentioned, I'd take the x-dimension to be (1) 8" fully grouted cell. This seems really conservative and TBH I haven't had any discussion with another engineer on this.

Some might say to take (2) cells each side so the beam rests on (2) theoretical 8"x16" supports at each side of the wall, but I don't know how to prove this.

 

[KootK]

Some might say to take (2) cells each side so the beam rests on (2) theoretical 8"x16" supports at each side of the wall, but I don't know how to prove this.

That is exactly what I would do and I wouldn't feel overly concerned with proving it. Assuming the beam to only be supported at the center of the pier is only conservative for the beam, not the pier.

This is a situation that comes up in concrete buildings often and at a much larger scale than this. In addition to wall situations, slabs deliver their load to columns disproportionately through the corners. And nobody checks those corners for increased bearing etc.

I feel that beam and pier materials that are compatible with respect to internal stress capabilities goes a very long way in these situations. Concrete combined with masonry is not as compatible as concrete combined with concrete. But it's a lot better than, say, concrete combined with steel.

Trying to model the situation accurately in production work is greatly complicated by two things:

1) Much depends on the shear strain flexibility between the ends of the pier and the interior. Assumption-a-palooza.

2) Interior wall rebar that runs up through the beam will attempt to hold the beam down over the interior of the pier and thus, set up a prying mechanism that will tend to amplify the axial load delivered to the ends of the pier.

 

[Greenalleycat]

Would it really, though? If the tie-beam is cast on top of the wall, wouldn't any variation of the CMU be "absorbed" by the fact that the concrete hasn't cured yet?

As far as the hogging, it seems like the most conservative thing to do is consider the beam supported at the center of the wall. This increases the beam length for checking the location of an internal support over the continuous beam.

For the x-dimension, if you were going to conservatively make the system behave as mentioned, I'd take the x-dimension to be (1) 8" fully grouted cell. This seems really conservative and TBH I haven't had any discussion with another engineer on this.

Some might say to take (2) cells each side so the beam rests on (2) theoretical 8"x16" supports at each side of the wall, but I don't know how to prove this.

I agree, if it was cast in situ then this does not apply - my comment would apply to a precast beam