4" Unreinforced Masonry Block Pop Top Issue

[FranCStructures]

I have been reading threads for the better part of a year and is time I present my first question. I'm designing reinforcement options for an unreinforced 4" masonry block wall. Using the allowable flexural tension equation for hollow block, I see that adding another floor, thus higher dead load, helps for both in-plane and out-of-plane loading conditions. That being said, with the increased dead load, I get very little over 4 psf allowable wind load. This leads me to think that I can provide vertical reinforcement at a certain interval along the outside of the wall (grouting cells is not an option) to allow the wall to obtain full compressive capabilities. How can you determine the horizontal span in which the block will transfer wind load to vertical supports anchored to the foundation and attached at the second floor level? Would I use the flexural tension equation to find this horizontal span? How does this all relate to TMS 402 (Equation 8-11) for combined axial compression and flexure? Does my allowable flexural tension Fb replace the Fb in the equation?
Any advice on how to approach this would be much appreciated.
Thanks!

 

[hokie66]

I don't consider 4" block to be a structural material if it has to resist wind loading.

 

[FranCStructures]

hokie: I'm in agreement with you, but these are the cards I've been dealt on this one.

 

[JLNJ]

If you want the block to take just the vertical load and some other element to suck up the wind load, you will need to look at their relative stiffnesses. The block will be pretty stiff and will likely not want to divest itself of all wind load.

I don't have any experience combining the horizontal-spanning bending stress with axial stress in the other (normal) direction. I suspect masonry design doesn't pretend to be that precise.

I'm not sure that adding a second floor really makes things better for the block. If your load is even the least bit eccentric it will be trying to kill the wall. Isn't the empirical limit for CMU h/t 18? if so, the code is telling you that 4" block is good for just 6' tall bearing walls. You can stretch this, of course, especially with reinforcement; however, with no reinforcement and no grouting of cells, it seems a bit iffy. You might think that 150 psi is a lot of capacity until you get to a door or window opening and realize there is just no way to accommodate the concentrated local combined loading.

 

[wannabeSE]

FranC
I don't know what equation you are trying to use. Isn't 8-11 for AAC (Autoclaves Aerated Concrete)? I wouldn't combine flexural tension with axial compression in the unreinforced masonry.

 

[FranCStructures]

Wannabe: Equation 8-11 in TMS 402. Combined axial compression and flexure. Flexural tension exists in all unreinforced masonry and is directly related to the compressive stresses in the block. More commonly this block would be used as an interior wall and even then the flexural tension calc will govern and suggest that 4" block can't even withstand interior partition loads.

Thanks for the response JLNJ, good call on the relative stiffness. Agreed, the 4" block is not adequate for exterior wall construction as hokie and I discussed. You live and you learn, next time a client approaches me about 4" unreinforced masonry, I will take the correct actions, whether that be letting the job go or recommending demolition.

 

[wannabeSE]

I misunderstood the question. I thought you were asking about combining stresses from horizontal bending (between new reinforcement/supports) and vertical axial force. You must be looking at a newer edition of the TMS. Is equation 8-11 ASD or strength design?

 

[dhengr]

FranCStructures:
I’m not sure I really understand your situation with this 4" conc. blk. wall. Is it existing or to be built? Why is wind loading just coming to light? Can you use a thicker block, and/or grout and rebars? I agree with Hokie, I don’t consider those much of a structural wall, and agree with you that they will hardly check with interior partition loads if not adequately supported. If it is a good strong existing wall and/or a strong conc. block, you could possible post tension it vertically to add significant compression to it, so as to counteract/overcome any bending tension. This is a fair amount of work, effort and detailing, but it can be used to make a conc. blk. wall work. It involves anchors in the lower slab, very carefully located to match conc. blk. cores; threaded couplers on the anchors; tensioning rods to the top of the wall; top bond beam, pl. washers and tensioning nuts; then pressure grouting the cores with the steel rods.