Concentrated Load on Masonry Wall - Lintel Design

[jgeng]

In analyzing the effect of a concentrated load on the top of a masonary wall I have been confused by my resources. One resource says that a concentrated load can be neglected in analyzing a lintel if its distribution taken as 45° down a distance 4x the wall thickness stops before the apex of a 45°zone above the lintel because of the arching action. Other resources say the concentrated load should be distributed at 30° and down a distance of half the wall height. If this is the case most all concentrated loads on top of a wall above a lintel will need to be evaluated for there effects on the linetel? what happened to the arching action? I only have ACI 530-08 not ACI 530-05 but I believe there was a change to section 2.1.9.1?

 

[BAretired]

Forget arching action. Design the lintel for the concentrated load and sleep soundly at night.

BA

 

[AVak]

I agree with BA on this one. That is what I do. You never know when a vertical control joint might end up close to your concentrated load and do away with your arch-action.

 

[rscassar]

I'll agree with BA and AVak regarding the lintel design it assuming no arching action.

For the design of CMU, when considering arching I take the load down at 1:1 (45°).

 

[csd72]

It depends on the bedding, if staggered bedding then I would use the arching (it works - 20000 plus railway bridges cannot be wrong!)unless it is right at the end and there is not enough to resist the horizontal thrust.

If the masonry is not staggered (i.e. the vertical joints go all the way up) then I would definately not use arching action and would use a point load.

I would ignore arching in anything less than 4 bricks high.

 

[sponcyv]

jgeng,

I have been confused by this as well. According to "Six Minute Solutions for Structural I PE Exam Problems", the 45 degree method is chosen. It states:

"The concentrated load is distributed downward at an angle of 45 degrees over an effective length equal to four times the wall thickness plus the width of the bearing."

It then shows a diagram which is different that what you would expect....it shows the 45 degree triangle area drawn directly above the lintel, not below the load. It states that since the concentrated load is above the apex of the triangle that it does not load the lintel.

 

[concretemasonry]

There is no such thing as "stack bond" today. It is now really defined as "other than running bond" (ACI 530). Running bond is defined as where successive courses overlap by one-fourth of the unit length. This provides the continuity for the masonry of the wall to act as a structural component, as opposed to the vertical strips in "other than running bond".

It is based on the historic testing of masonry and the load distribution without an effects of reinforcement, which can open the doors to other design concepts and load distributions.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[rscassar]

[qutoe]It then shows a diagram which is different that what you would expect....it shows the 45 degree triangle area drawn directly above the lintel, not below the load.[/quote]

That's what I would expect to see, anything above that 45deg line will have the ability to arch over the opening as long as the opening is less than 8 times the thickness of the CMU.

 

[csd72]

The brick above that 45 degree line is an arch, the brick below that 45 degree line relies on the lintel for support.

 

[a2mfk]

Every time this subject comes up, you get answers from 30-45 degrees. I don't cut it too close on this, but also take into account your loading- if from a roof, how much of that dead and live load are really there? Makes it easier for me to use arching when there is little chance of the full design load every being applied. I think I use 45 degrees usually...

But everyone had good advice, but I am confused. You guys don't show masonry control joints on your drawings??? I always have, I want control of this aspect. The architect then shows his stucco joints at the same locations and can add additional as necessary... This also may be critical in your lateral design, it has for me lots of times..

AVak (Structural)
" You never know when a vertical control joint might end up close to your concentrated load and do away with your arch-action."