Adding 2 stories on top of 4 story 12" thick brick wall 1

[milkshakelake]

I want to add 2 stories on top of a 4 story 12" thick red brick party wall. Foundations are rubble wall. The new stories will be lightweight (steel joists and plywood). Axial force calculations and foundation bearing capacity are all okay with the new loading. I've taken loading from both sides. I conservatively assumed f'm = 1500psi. I'm running new columns to take concentrated loads. Laterally, I have CMU walls in the short direction, and relying on the brick walls in the long direction. I've even done this once 5 years ago, and the building is still standing.

My question: Even if it meets code, is this a smart thing to do? Does anyone have real life "negative" experience doing this? Legally, if everything passes code, I should be okay. But I really don't want to kill anyone due to a trick of nature.

I typically make a new building within the building if I feel that the loads are too high, but here, everything seems within reasonable limits. And I see lots of big, old 6 story brick buildings. Not sure about their wall thickness, but knowing how cheap people are, they're probably 12".

 

[JAE]

I’ve got a few comments
1. You didn’t mention lateral wind and seismic. Those are usually the main challenge vs gravity design.
2. Old brick masonry at f’m of 1500 psi? I wouldn’t extend a building like that without some actual sampling and prisms tested.
3. Not as critical as the lateral but having accurate knowledge of the soil conditions and actual foundation size and condition would be absolutely necessary.
4. Attachment of new structure to the existing brick would be very difficult, especially for any uplift from new members attaching to old 3-wythe brick walls since tension capacity in brick with stress perpendicular to bed joints is small or non-existant.

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[milkshakelake]

JAE: Thanks for your comments.
1,4. I'm using 2 sides of 60' brick in the long direction for lateral. The new stories will have light gauge strap bracing in the long direction. With a 60' moment arm, I feel that the uplift/downward forces will be minimal, and might be negated by 0.6 x dead load. If I find that the uplift is more than 1 kip, I might scrap this whole idea, and build a new building within the building.
In short direction, I'm adding 2 new intermediate reinforced CMU walls. The uplift from them will be transferred to gravity walls with a grade beam.
2. The client is cheap and doesn't want to do lab testing. Maybe if I assume f'm = 1200psi, it might be okay?
3. Agreed, I've done test pits and borings. Some of the foundation is okay, and some is not. I plan to frame out the parts that are not okay.

 

[dhengr]

Milkshakelake:
Old rubble found. walls are notoriously difficult to deal with. They look fine, and in pretty good shape, until you touch them. Then, all hell breaks loose, touch one stone and three more fall out, and you just don’t know where to stop fixin. Try to apply any concentrated loading and you have some trouble being confident about how it is distributed out into the wall, let alone actually making the connection. Sometimes you can break out the old mortar and repoint a good thickness of the wall on one side (or maybe both sides), but you still don’t know how to put numbers on it. Sometimes, you can cast a full width bond beam atop the existing brick and tie it all together below your new construction, and hide this in the new floor construction.

 

[racookpe1978]

Can you install what an interior finish carpenter would call a "reveal" - A deliberate, visible break above the existing wall that will serve to form a self-sufficient beam that holds the new two floors?

Think of the existing rubble-brick-crumbling wall, a horizontal concrete or steel beam covered by an architectural finish or trim or contrasting paint/plaster of some kind, then the next two floors? That would take the upper load off of the lower wall, give you something to tie the top of the crumbling rubble wall to up high for additional lateral strength.

 

[OldBldgGuy]

I have done this and variations of it numerous times. My best advice echoes the last line of dhengr's post: pour a reinforced concrete bond beam on top of the existing brick wall and build on that. Essentially you have a new "footing" for your new building, and you can deal with your loading from there. The new bond beam is a lintel for every opening below it, it's a tie to keep everything together, and it distributes the loads from above into the unreinforced masonry below.

One big big caution: I don't know where you are but I am in an area where seismic loading is insignificant. Because of this I have little experience with seismic design but be sure you know what you have to consider for it before you put a new structure on top of a brittle masonry structure.

 

[milkshakelake]

dhengr: I haven't had a situation yet where rubble walls come loose, but then again, I've only done one job where I added additional load. I always make a building within a building so I don't have to think about this aspect. But it's hard to get clients that way; there's always another structural engineer who doesn't blink at adding 1,000 stories to a 3 story building. Thanks for the experience, I don't want to deal with a situation where a stone breaks out.

racookpe1978: I know when you were born! But anyway, this kind of beam would have to be supported by columns. This is what I'm planning to do at this point. Otherwise, it's too much liability. Thanks for the suggestion.

 

[bookowski]

I assume you’re in nyc area. My 2 cents:
- f’m = 500 to 600 for historic masonry (axial is still rarely a problem)
- have you verified wall thickness, most of those buildings step down to 8” unless it was a heavier commercial building and not residential
- I’ve had a rubble wall come apart once, very scary, but I still do these , just more diligent in the investigation phase
- if possible I’d have a section cut into the brick and rubble in a few spots, you can at least do an observation as to composition and mortar condition
- repoint as much as necessary, which is sometimes almost everything

 

[HouseBoy]

OBG - Do you anticipate any issues with (relatively) hard concrete behaving badly in contact with softer brick?
I'm guessing that since it's 3 course brick at 4 stories and a party wall (assuming it's inside and not as weather resistant etc.) it might be softer brick.
Maybe not such a problem if it is not exposed to weather.
Hardness compatibility is often an issue with older brick.

 

[OldBldgGuy]

You never want to point soft old masonry with hard new mortar, or patch into it with stronger materials, but in this instance the hard concrete is bearing on the softer brick, and I have had no problems yet; I have projects pushing 20 years old. The interface wants to be as level and straight as you can make it, not stepping up and down. The concrete will bond to the top of the masonry, which could create problems with differential temperature and moisture movements between the two materials, but as I said, I haven't had any issues. As far as the bearing itself, concrete footings almost always bear on much softer, weaker materials. That's the point of them.

 

[bob33]

Beware of rubble masonry that was built with lime mortar. This was commonly done until Portland cement became readily available. Water can slowly dissolve the lime mortar and leave only voids and sand behind. I have seen this with exterior walls, basement walls and foundation walls, and with wall and column footings. If mortar is okay, best to use empirical design for rubble masonry compressive strength.

 

[SteelPE]

I'm surprised no one else has commented on this yet, but it's kind of hard to help you out if we don't know what codes you are dealing with.

In my jurisdiction this building would fall under the IEBC, and most certainly would require the entire structure to be in compliance with the current building code. My jurisdiction also prohibits plain reinforced masonry shear walls for all structure heights and Seismic Design Categories. So adding 2 stories would be incredibly difficult, and I don't believe we could add a 2 stories on top of an URM wall. I try not to care what other people would do, but sometimes it's hard.

I recently looked at a single story masonry building with grouted cells at 60" o.c. The client wants to add a second story to the building. As I said above, my jurisdiction prohibits URM buildings and Ordinary Reinforced Masonry buildings. They only allow Intermediate Reinforced Masonry which the structure does not comply with. I told the owner what he needed to do to make the building compliant so they could add the second floor. He wasn't happy, but it was early in the design process so we are not pushed into a corner yet.