Brick Bearing Pressure 2

[Stress02]

What would be an appropriate allowable bearing pressure for brick masonry? Not finding anything in the ACI 530, but I don't think it's really applicable as this not a recent building.
Looking at a bearing plate on a 12" thick wall of 3 wythes of 4' brick.

 

[FalsePrecision]

Not having any data on the strength designation of the brick, only about 100 psi allowable bearing pressure. I looked at my 1976 UBC, unreinforced brick masonry tables.

 

[Stress02]

I have always used 100 psi for all masonry types; I was hoping for more.
Thanks

 

[UcfSE]

Why don't you use 0.25f'm? That's in ACI 530 2.1.9.3. If it's veneer only I could see using a much lower value for bearing but then if it's veneer it shouldn't have much load on it anyway.

 

[Stress02]

Not a veneer; Solid brick bearing wall. Lintel beam load runs about 5.6 kips per foot so the reactions are about 31 kips for the beam.

A 1982 Navy manual uses quite high bearing values based on .25 f'm but they assume that the bricks have a unit compressive strength of 8000 psi. Although the building looks good, that sounds pretty high based on concrete test results that we get nowadays, so I am not going there.

Other much earlier references show a bearing ranging from 100 psi to 400 psi depending on brick compressive strength and mortar type.

At 100 psi, the bearing plates will be unmanagable.

 

[UcfSE]

The minimum required f'm is 1500 psi. 0.25 of that is 375 psi. That might seem pretty high but clay masonry brick are very very hard. If 375 psi makes you uncomfortable how about say half of that? That's still better than 100 psi.

Check out the Brick Industry Association

http://www.bia.org

They have many technical reports available online at the following link:

http://www.bia.org/html/frmset_thnt.htm

You can scroll through them on the left side of the page. This is taken from Tech. note 17b:

"The designer should check the stress condition at bearing points for RBM beams. This applies to loads on the beam and to the beam's reaction on the wall. The MSJC Code limits the bearing stress to 0.25 f'm, where f'm is the specified compressive strength of masonry. A rule-of-thumb recommended for many years is to provide a minimum of 4 in. (100 mm) of bearing length for masonry beams. The masonry directly beneath a bearing point should be constructed with solid brick or with solidly grouted hollow brick. Concentrated loads should not bear directly on ungrouted hollow brick masonry because of the potential for localized cracking or crushing of the face shells."

If you feel better using 100 or 200 psi or something like that by all means do so. This site along with the ACI suggests that 0.25f'm is sufficient. In other technical notes on that page - such as the material properties one - the compressive strength of some of the brick them sleves exceeds 10000 psi. To me, that means assuming 1500 psi should be sufficient, giving you a bearing stress of 375 psi. 200 or 300 would be good too, and definitely better than 100 psi.

 

[FalsePrecision]

OK, I am thinking 300 psi would be "ballpark" - but not defendable in a lawsuit. Even wood has a similar bearing pressure.

 

[DaveAtkins]

It's 0.25 f'm, so you will have at least 375 psi. Fired brick will have a greater compressive strength than CMU, which has f'm = 1500 psi.

DaveAtkins

 

[DTGT2002]

The fact that it is older brick concerns me.

Any chance of testing some of the brick?

Bricks expand with age as they draw in water. Yesterdays bricks were not necessarily made like today's bricks.

I am assuming no design data is available on the building or similar ones in the area?

Good luck

daniel

 

[Stress02]

No documentation.
It's an oldie, but it really doesn't look bad.