Limitations on Height of Free Standing Wall 2

[Lion06]

Hi all. I have a situation where a free standing wall was damaged by a tornado and is being rebuilt. It's a large wall (approximately 37' tall) and about 5' way from the exterior wall of the main building. I was shocked to see it was a double wythe brick wall with no interior structure and no reinforcing. With that said - as mentioned, the wall was damaged by a tornado. Because it's a risk category II structure, however, it's not required to be designed for a tornado and neither the contractor nor the insurance company have any desire to design it for anything beyond what is required by code... so no tornado design. I can't find any height limitation on a free standing wall, but a 37' tall wall that is being replaced because it was damaged by a tornado and not being designed for a tornado scares me a little. It scares me from a safety perspective and a liability perspective.
What are your thoughts on this situation? Is there a height limitation for such a wall that I'm missing? Would any of you have similar hesitations or no... just design it as a free standing wall for code required wind forces and call it a day?
I should note there is a possibility to laterally brace the top of the wall, but it will add significantly to cost to open up the existing envelope to tie a bracing structure into the roof diaphragm of the main building, not to mention the watertightness and thermal implications.

 

[WesternJeb]

If it isn't required by code to be designed for a tornado and the owner doesn't want to design it to tornado forces then don't. The codes are based on statistical probabilities and a certain risk is associated with it. If the building isn't designed for a tornado, neither should the wall adjacent to it.

Are you in a seismic zone? That could trigger some height limitations on the wall. The foundations for that wall (overturning) are going to have to be massive though compared to what was originally there (which will be difficult for everyone to understand, as it just withstood large wind forces without falling down). I would imagine the footing would need to extend all the way to the existing building's foundations and maybe then some.

 

[bones206]

What are you planning to replace it with? CMU?

 

[Lion06]

@WesternJeb - Wind controls over seismic for this wall. I hear you and I typically take that approach. I was just thinking about the lack of redundancy in a 37' tall free standing wall and the potential liability if it falls in a high wind event and were to hurt someone. 37' covers a big part of the ground around it! The footing does need to be much larger than what was there, but, surprisingly, not nearly as large as I was expecting (the self weight of the wall and the brick cladding are a big help to held it down).

@bones206 - Yes, it will be a CMU core clad with brick.

 

[phamENG]

the self weight of the wall and the brick cladding are a big help to held it down

Are you considering second order effects? A bit of deflection and that self weight will start working against you real quick.

Are they using an existing building code provision that lets them rebuild it as it was before it was damaged regardless of code requirements? Some places put value limits on this, others don't.

 

[bones206]

Maybe you can incorporate some CMU pilasters with generous proportions, then use bond beams to break the wall into smaller spanning segments. That makes the wall itself much more resilient. The overturning aspect is still non-redundant, so you will want to have a nice comfy OT safety factor.

 

[WesternJeb]

I will say that I did just a very quick check with a wall pressure of 35 psf and am getting a failing wall with 13" deflection at the top.. Design is 12" CMU, f'm = 2.5 ksi, fully grouted, #6 @ 8" E.F., o.c...

How are you getting this wall to work? What is your design wind pressure?

 

[Lion06]

@Pham - It's not getting rebuilt as it was before. It was just a double wythe wall with no core. I am just starting to study this, so I haven't studied the second order effects yet with an accurate reflection of the cracking moment. I have second order effects accounted for without cracking, but I haven't accounted for the cracking yet.

@WesternJeb - I'm using 16" CMU with 3ksi and #8 @ 16" each face. Interestingly, the wind pressure for case B works out to be close to 35 psf. Case A is 22 psf. I'm getting about 1.5" of deflection at the top before accounting for cracking.

 

[Lion06]

Thanks everyone for the input.
One more thought - can you reasonably get composite behavior out of the wall with the brick cladding on either side? It's not needed for strength, but it would add quite a bit of stiffness and help with the second order effects and cracking. I can design reinforcement for the shear flow, but I'm having a hard time finding literature from NCMA that discusses using brick and CMU as a composite wall.

 

[bones206]

I think it would be reasonable to consider a composite moment of inertia for displacement calcs. Full scale blast testing of CMU-brick cavity walls have shown the veneer can contribute a significant stability effect.

I would just use it as part of a bounding analysis though, and still err on the conservative side so I don’t have to deal with cracking complaints after a wind storm. But it’s a judgement call.

 

[Lion06]

Thanks, bones. I think where I landed is that I will design the CMU core and reinforcement for the full wind load for strength and only count on the brick veneer in the deflection analysis to, as you said, not have to deal with cracking complaints with a wind storm. I'm currently around h/670 for out-of-plane wind deflection at the top with the service load wind. The strength level wind is more like h/380, but we can check deflection with the service level wind. I'm sizing the footing to make sure I'm in the kern under the full wind load.

Thanks, everyone!