Concentrated Load Masonry Wall - Arching Action ACI 530

[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?

 

[RebarRyan]

jgeng,

Good question. Unfortunately I don't have a good answer. But I do have a copy of ACI 530-05.

I'm assuming your interested in a concentrated load on masonry laid in running bond, because your asking about arching action, correct?

ACI 530-05 Commentary on Section 2.1.9 Concentrated loads, assumes that the running bond distributes the axial compressive stress along the length of wall, known as the effective length. There are two scenarios shown on fig. 2.1-14 (a). Each scenarios shows a 45 degree angle between the bearing point and the distribution triangle. See attachment.

As you are probably well aware, in ACI 530-08, Concentrated loads are now listed in Section 1.9.7. After reviewing the commentary on p. CC-17 for more information, it appears that the two listed references, are perhaps clues to help answer your question.

 

[msquared48]

FYI, from page 188 of my Reinforced Masonry Engineering Handbook" by Jim Amrhein, he says in section 12.4c...

"... A conservative arch angle would be 45 degrees; actually an angle of 30 degrees would be more realistic to the true arch action of modern reinforced masonry."

The diagram he shows for the arch shows the 30 degrees measured from the horizontal line of the lintel, not the vertical, that's why he says 45 degrees is conservative. In other words, you do not have to take the ikltra-conservative approach of the Mayan Arch scenario.

He does not seem to differentiate between point loads and uniform loads in the application.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

I'm not a codewriter but I will stick with my first response of 45° or 1:1. I cannot visualize masonry arching in a 1:2 fashion.

thread507-283504

 

[msquared48]

kikflip:

I agree with you.

If appropriate, I commonly use 45 degrees too ... just to be conservative if I do use it. Not using it at all would be the most conservative.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask