Use of Unreinforced brick masonry in seismic design

[nycstructural]

I have a project in where we are proposing to add one story to an existing three story building. We are also enlarging the building laterally. The existing structure predominently consists of steel framing but there is a rebust brick masonry bearing wall that runs through the middle of the building. Our lateral design includes the addition of steel moment and braced frames for the new structure and retrofitting braced frames within the existing structure. My question is, can we consider the existing brick wall to contribute to resisting the lateral forces (wind and seismic)?

 

[jdgengineer]

Section 3403.4 of IBC says that as long as your addition is independent of your existing structure than the existing lateral load-carrying structural elements shall be permitted to remain unaltered. However, your case will likely not fall within this and therefore I believe Section 3404.4 applies which states that that the lateral load-carrying elements shall be shown to meet the requirements of new code.

So I'd say unless you can show that the wall meets the requirements of new code I wouldn't use it as part of the lateral system. You may want to isolate it from the diaphragm in-plane so that it does not take seismic loads as well.

 

[msquared48]

If you do have to isolate the brick wall, then it will be doing nothing structurally other than fire protection. It will be adding significantly to any lateral due to seismic forces too, depending on the level you have to deal with.

Could you remove it?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[nycstructural]

Thanks for your responses. Unfortunately it would be too invasive and costly to remove it. It's a significant bearing wall and will have to stay that way. The problem is that its dead load adds significant seismic shear to the building which in turn pushes the new steel dunnage up.

 

[msquared48]

OK. Then...

Had to mix concrete shear walls and moment frames in the lateral system of a hospital for an expansion many years ago. You could always perform the lateral analysis based on relative rigidities, but this would put you back where you were in the beginning, having to upgrade the wall.

Frankly, if this is a multi-story brick bearing wall, possibly with let-in wood joists to either side, I have no idea how you could effectively and economically isolate the wall seismically from the rest of the structure.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[nycstructural]

Typo. Meant tonnage, not dunnage

 

[jdgengineer]

I'll be the first to admit I haven't done something like this before and not sure what the as-built detail looks like. Is there a wood ledger the joists are attached to Are they let in? With a ledger couldnt you used a slotted hole to allow the wall to slip in plane? If they are let-in I'm assuming the diaphragm shear transfer is already very insufficient. Seems like there must be a way to keep it bearing but cut it free for in-plane shear

 

[nycstructural]

There is no wood framing. The existing structure consists of reinforced concrete slabs supported by steel beams and columns. There is a single interior unreinforced brick masonry bearing wall which also supports the beams and slabs.

 

[msquared48]

Is it possible that there are steel beams and columns encased within the brick wall, and that the wall is only for fire - making it non-bearing?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[nycstructural]

Indeed it is possible that there are beams and columns embedded in the wall. We have instructed a contractor to do some probes to verify. In any case, it sounds like we have reached the same conclusion: The wall weight must be included in the establishing the seismic design shear and may not be considered as part of the lateral force resisting system without reinforcing it. Correct?

 

[SAIL3]

If the wall is not isolated from the lateral load resisting system of the structure it will attract it's share of seismic loads based on it's relative stiffeness. These loads will not redistribute to the steel members until significant damage has occured to the wall and it's relative stiffeness has been reduced.
If the wall is isolated then I would treat it as a stand-alone element which should have the integrity of resisting it's own mass in a seismic event.
Either way, I would assume the wall would have to be reinforced unless damage to the wall is of no consequence to personnel safety, structural integrity of the system or anticipated cost of repair.

 

[SteelPE]

Could you add new bracing along the bearing wall? Granted, you will probably have problems with the foundations below but that may help solve your problem with stiffness.