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Unreinforced masonry with steel straps?

Unreinforced masonry with steel straps?

Unreinforced masonry with steel straps?

(OP)
I have a project where the client is putting on an addition. The existing structure is believe to be unreinforced masonry infill (masonry cut in-between existing steel columns). Where the new building meets the existing building the client wants to remove 75% of the existing wall to provide access into the new structure. I have already briefed the client in regards to the possible implications of this (possibly requiring reinforcing of the remaining 25% of the wall). The architect however has suggested that we use steel straps attached to the face of the wall to help us solve our problem (I guess he has done this before). I said I would look into it to see if it was feasible. Has anyone done such a system before and if you have, what structural system would this fall under in ASCE 7-05?

RE: Unreinforced masonry with steel straps?

I've seen vertical rebar in grouted cells, horizontal steel truss Wythe's in the mortar joints between vertical blocks, vertical plates with through bolts to hold in the blocks. No straps however. What spacing do they want to use the straps on? Do they plan on a strap for every CMU block?

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet

RE: Unreinforced masonry with steel straps?

I believe there are surface applied mixes that can provide marginal tensile capacity. Or make use of carbon fiber straps. but strapping the exterior on both sides, and over the top should act rather similar to rebar assuming you do not have stress concentrations, so ASCE7 reinforced masonry?

RE: Unreinforced masonry with steel straps?

I have never seen this before. Would the straps be stainless steel/galvanized? Also with this systems how would you transmit the forces into the slab (assuming you are spanning vertically)?

RE: Unreinforced masonry with steel straps?

(OP)
I'm not sure I have seen it either. The wall would be inside the addition, they just don't need access through this portion of the building, so the straps would just be regular painted steel.

I'm not sure if the other engineer who worked on this architects project considered the straps reinforcing or not. I can't seem to find a system in ASCE 7-05 that would fit this description. Anything with straps mention is a light framed wall (metal studs).

RE: Unreinforced masonry with steel straps?

Are you suggesting diagonal straps to serve as X-braces against portions of the wall? In other words, would these straps take the place of unreinforced masonry shear walls?

Or are you suggesting vertical straps to create a sort of reinforced wall - replacing what should be vertical embedded reinforcement?

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RE: Unreinforced masonry with steel straps?

(OP)
JAE

I'm not suggesting anything, the architect is suggesting based upon experience with a similar project he completed about 8 years ago. He was referencing horizontal and vertical members to take the place of reinforcing.

However,in my previous experience working for a steel fabricator, I seem to remember structures where the EOR used diagonal straps to reinforce the building.

RE: Unreinforced masonry with steel straps?

The problem with diagonal straps is the attachment to the existing masonry. It would be better to develop a trussed system which can act on its own without reliance on the existing masonry other than perhaps to provide lateral support to compression members.

BA

RE: Unreinforced masonry with steel straps?

Is this a lateral system, or are you talking about straps for out of plane loads?

If a lateral system it doesn't sound worth it. Developing forces through masonry and also at the top and bottom will be tough and do you have to grout the ends or does it work as 1/4 the length unreinforced? Would it make more sense to remove 100% of masonry, add in a post at that 25% extent line, and use traditional bracing?

The other project that the architect is referencing could be completely different application. Maybe they saw someone add external plate reinforcing for out of plane loads and are trying to apply it here.

RE: Unreinforced masonry with steel straps?

If he is going to remove 75% of the CMU, just remove the rest and put the 25% back correctly as reinforced CMU.

Mike McCann
MMC Engineering

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