arching of masonry walls

[bjb]

I have masonry openings that are close to the corners of the building, and other openings that have control joints at one end of the opening. Is it considered acceptable to draw a 45 degree line from the end away from the corner/joint to the other end of the lintel, and assume that the lintel only has to support the masonry under this line? This results in a masonry "right" triangle. Can the support mechanism for the masonry above this line be assumed to be similar to corbelling?

 

[JAE]

We assume exactly what you described - a right triangle distributed load with the vertical leg adjacent to the corner/control joint and sloping down toward the other end.

 

[jheidt2543]

Just a couple of clarifying questions:

1. In the case where an opening is in the middle of a wall (not near a corner or control joint) and you are using the 45 deg. loading on the lintel, how high do you choose the masonry load? Do you use the width of the opening or take it all the way up the wall?

2. Same question for the case of an opening near a corner or control joint.

I would just like to know what others are doing for these situations.

 

[bjb]

This reply is to Jheidt2543:
For a masonry opening away from a corner or control joint, the gravity load on the lintel takes the shape of an isoscoles triangle, defined by drawing 45 degree lines from the lintel supports. The lintel must support this weight of masonry, in addition to any other vertical loads such as beam reactions, etc. Some engineers I know conservatively use a uniform load, where the magnitude is defined by the intersection of the two 45 degree lines described above. The Masonry Designers Guide, and other masonry textbooks I have seen deal well with masonry arching for this type of opening, but don't seem to consider the case where you have a control joint at one end or are near a corner. These are 2 very important and common situations. The control joint interupts the running bond and eliminates vertical load transfer, and at the corner you don't have sufficient mass of masonry to resist the horizontal thrust from arching. See JAE's response to my first postiing.

 

[dougantholz]

bjb,
If you have a control joint, or the end of a wall near an openning, you CANNOT count on arching action, for the beam design. In order for a 45 degree right triangle to be formed over an openning, a jamb must be present in order to resist the horizontal thrust. Also, there must be sufficient height above the theoretical top of the triangle in order to bridge over the masonry under the triangle needing support.
The Masonry Designer's Guide has some guidence in chapter 8. What I have used is there must be half the openning on either jamb to resist thrust, and the masonry above the openning must be as tall as the openning (half of that is in the triangle, half above it). Probably conservative, but I don't have a clear guideline

 

[bjb]

dougantholz:

I realize that you cannot count on arching action. For the openings I am talking about I do have an adequate jamb on one side of the opening. If the lintel were to be removed from this opening, I don't believe that you would have a vertical failure surface in the masonry if you have running bond. The failure surface would be a diagonal line starting at the end away from the joint/corner. I have seen this in walls that have been partially collapsed. Instead of arching, you would get some support from corbelling. In new construction with proper detailing, you are supposed to have control joints in the masonry at the openings. Of coarse, this depends on the number and size of the openings. In the majority of openings in new construction therefore, you probably can't count on arching action.