CMU Bearing Area

[structeng2012]

I'm designing a steel lintel in a CMU wall (wide flange beam on a steel bearing plate). Per the MSJC masonry code: Is A1 the area of the plate or the area of the bottom flange on the wall? I ask because I see in section 4.3.4 A_br shall not exceed 2*A_1. So is A_br the area of the bearing plate and I must ensure that this area is less than 2* the area of the bottom flange on the wall? And if it's not I use 2*A_1 in my bearing stress equation?

 

[PittEng88]

A[sub]1[/sub] is the area of the base plate. To calculate the bearing stress on the cmu, you need to calculate A[sub]br[/sub], which is the smaller of:

1. A[sub]1[/sub]*(A[sub]2[/sub]/A[sub]1[/sub])[sup]1/2[/sup]
where A[sub]2[/sub] is the base of the frustum of a pyramid or cone with the loaded area at the top and sides that slope at 45 degrees that is wholly contained within the masonry.

2. 2*A[sub]1[/sub]

Typically, the bearing areas for lintels are grouted solid, so you can use whole area of the frustum that is contained within the cmu, not just the face of the block. However, keep in mind that if a portion of this area falls outside the cmu, due to edge distance, it cannot be included in the A[sub]2[/sub] calculation.

Here is a good example calculation of this from the Concrete Masonry Association of California and Nevada:
Link

Hope this helps!

 

[JLNJ]

Adding to what Pitt says above: In order to count on the base plate area as A1, you need to make the base plate stiff enough to distribute the load out uniformly to whatever bearing area you wish to consider. A 3/8" plate might not be stiff or strong enough.

Bearing often controls in masonry because the allowable values are so low. The stresses don't distribute out all that well since the stresses tend to concentrate at the edge of the opening.

 

[structeng2012]

Thanks for the information both of you!