Moments induced in a masonry wall

[whyme]

Greetings masonry designers:
Any hints on how to calc permissible locally applied bending loads on an unreinforced masonry wall (assume no openings, and vertical height known).
Eg, I want to bolt an awning onto a brick wall subject to uplift. Bolts spread 'x' cm (in) apart. Wall subject to applied bending due to loaded cantilever awning, and tension on bolts.
so brick mortar shear (to prevent pull-out of brick from anchor) and localised wall bending needs to be checked.

 

[BAretired]

Does your code permit the use of an unreinforced masonry wall to carry bending moments which result in net tensile stress in the mortar? It doesn't sound like a good idea to me.

BA

 

[whyme]

LOL - you're right - no it doesn't. But provided there is sufficient wall height to give compression that can resist any tension, at least I can crunch the numbers.
Obviously something like a small domestic shelf bracket is ok - it's just stepping up in size till something says "NO MORE!!"
The way I see it, looking at all potential modes of failure, I'd need..
1) shear strength of mortar to prevent the brick with the anchor from being pulled out or pushed in from direct tension or comp.
2) resistance of the brick or effective group of bricks from rotating or tilting within the wall, resisted by compression from wall ht above
3) direct vertical lifting

I'd think it might be similar to a mass block of concrete pad resisting a cantilevered post where overturning resistance and compression at the toe is considered.
It's complex - I agree

why an I asking?
Someone has installed a bracketed awning that has been up for several years. I want to do one similar - but justify that it can work.

Cheers

 

[hokie66]

This type structure is inherently dangerous, thus the reason most building codes don't allow them. Periodically, there are reports of injury/death resulting from unreinforced masonry walls falling due to cantilevered awnings, basketball backboards, etc. being attached. They also fall over due to wind loading and soil loading. Reinforcing resists tension, mortar doesn't.

 

[whyme]

Thanks, I'm very much aware of all that, but regardless of codes, there must be a reasonable engineering method.
What if it's solid 2 or 3 skins, still unreinforced, but solid masonry. Hence weight of wall above.
Surely there must be something!

 

[XR250]

I thought you were allowed to design for flexural tension - just not direct tension. Has something changed in the code?
Millions of basement walls would disagree :>

 

[SteelPE]

This is covered in ACI 530 which is the only "comic book" sized code we have left in the states (that I know about) and is not that hard to find. I would be surprised if your jurisdiction doesn't have similar information contained within their code. What code are you designing to?

 

[EngineeringEric]

I agree with all the posters above on not doing this... but I will say that NMCA does publish that Type M and S mortar can have some tensile capacity; i believe it is 45 psi but cannot confirm (see TEKs). Of course this is only until the masonry is cracked at which point capacity is zero for non-reinforced. So using this tensile value one can calculate a bending stress allowable from the cross section using an axial self weight.

Again, this is not the best thing to do. I would recommend tieing your strucutre back into the studs/wall and check those for the additional load. Brick is a veneer, and a poor veneer in most cases, not structural in the typical structure.

 

[TDIengineer]

What about treating the wall similar to plain concrete and use f'c = 1500 psi.

Apply the same mechanics as plain concrete.

.. just a thought.

 

[whyme]

Thanks for the responses.
Code is AS 3700 Masonry Code (Australia) 204 pages.
Structure is an OLD hotel, 2 stories, external walls are about 6m (20') high.
Walls are rendered, so I'd need to do some destructive testing to ascertain mortar condition and type.
I'm expecting to assume zero tensile strength. I do however expect to find it is SOLID masonry, not brick veneer against timber frame like domestic BV.
Possibly 2 or 3 veneers, hopefully at least 2 veneers as a solid construction on the outside.
So my assumption, as above, is to assume stability based on net vector reaction of lateral and vertical load.
Like a mass pad footing where e <= D/6 so that the pad doesn't tip. No tension assumed.
Problem is also ensuring compressive capacity of the mortar isn't exceeded by this. (Like checking soil pressure at the toe of an eccentrically loaded or moment pad footing).
Get where I'm going with this?