Odd offset masonry pier design in existing structure 1

[mfstructural]

I've been called out to a building where the architectural drawings were not very good. I attached a picture. There are a bunch of offset piers with cantilevers. I'm not worried about the actual sizes of the beams. I've sized those. What I'm worried about is the size of the piers. The first floor piers are made of 4" CMU and limestone stackbond. I should also mention that this is under construction and they're at the top of the first floor. They have all these problems because they never hired a structural engineer. Now they need repair details. I'm concerned about the lateral resistance of the pier. The pier doesn't have to be designed for seismic, building is in Chicago (code states this). Drawings show the channels bearing on the piers, I'm going to splice all steel together to give some continuity. I think this will help the lateral capacity at the front. I am trying to figure out a solution to avoid telling the owner to rebuilding the first floor piers with reinforced 8" block. Thoughts?

 

[jayrod12]

My experience is they cannot realistically install reinforcing in a 4" CMU and expect the grout to completely fill the core. Therefore no lateral capacity.

But that's just what I've seen. The last time we had to come in after the fact and check 4" CMU we ended up attaching channels along the full height of the CMU to provide any out of plane resistance.

 

[BAretired]

Your link does not correspond with your description of the problem. It looks like a three story building which has been completed and the piers appear to be made with 8" concrete blocks, not 4" as you stated.

BA

 

[KootK]

The photo suggests a concrete slab building to me. Is that the case? If so, maybe the piers are just ride along elements and you'll be okay. If the piers are meant to be serious bearing / shear walls, the owner may have a serious problem on his hands.

Regardless of the code outs available to you, I wouldn't disregard seismic, especially for your connections. You've got the New Madrid fault in the vicinity. I'd hate to see one of those piers fly off and crush one of our fellow humans.

Can you share any more detail about the building?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[mfstructural]

I got a copy of the architectural plans and attached first floor. You can't tell from image, but all piers are 4" CMU with stackbond limestone wrapped all the way around. So even with an 8" flange you have some bearing on limestone. The rendering is just to get an idea of the load path and beam orientations.

 

[mfstructural]

The majority of the walls along sides and front are 6" CMU with veneer brick. Pre engineered wood trusses are bearing on the CMU. The entrances to the balconies from the inside are full glass, thus not providing a lot of lateral capacity. Then you you have the piers, even though the balconies are open they will see some lateral load. The piers are definitely meant for bearing. At the each floor the pier nearest the middle has two channels bearing on it. These channels are carrying self weight and balcony loading.

 

[dhengr]

Mfstructural:
In the ‘windy city’ you don’t have to design for wind loads on a bldg. either? “(code states this)”? You’ve got a soft story at the second level, two piers in their weak direction. And, damn little in the way of lateral restraint structure ( two piers?) at the first and third levels. Two 4' screening piers which are not continuous, at levels 1 & 3, just won’t cut it. That you’ve asked this question, the way you have, suggests that you should find a local mentor to help you with this problem. You and the Arch. are in way over your heads on this one, and I don’t mean in Lake Michigan either.

 

[mfstructural]

Like I said, I was just called into this and trying to see what can be done to solve this problem. I have talked to the owner letting him know I'm going to recommend rebuilding the piers with 8" reinforced CMU. There is not much for lateral, but I was not involved in the design. I was literally called up about this 3 days ago first time. Plus I think it's a good lesson learned. No structural engineer was consulted and believe it or not they got a permit to build this building based on only the architect's signature.

 

[BAretired]

Be careful about making any recommendations. If you do that, you are essentially buying the existing design. I think your recommendation to your client ought to be to review the entire structural design before deciding how to proceed. You will probably find other areas of concern during the review.

BA

 

[hokie66]

I agree with BA. If you can't satisfy yourself about the whole structure, I wouldn't get involved with just one part.

 

[mfstructural]

Right. That's the first thing I started thinking about. They want to redesign the front portion but the fact that I"m looking into drawings and doing redesign is tricky. I am essentially liable for the entire structure at that point. That's what I realized when I started looking at the steel and that's how I got into questioning the size of the piers.

 

[mfstructural]

So the client agreed to a redesign of the front of the building. I'm going to have the piers go all the way up with tube steel as well and tie back to the structure.

 

[BAretired]

Are you satisfied that the structure is adequate for wind forces parallel to the front wall?

BA

 

[mfstructural]

Not as it is right now. I basically told them there is not a lot there right now and I need to rework the framing and walls. I have some ideas and am working on this with someone so we'll have to sit down and figure out a solution. Increasing the pier lengths and adding steel is a good start.

 

[mfstructural]

Hey everyone, just giving an update. I suggested taking the piers on the left and middle (looking at the building) all the way up with 8" CMU. I also wanted it on the right but was ok with posts with moment connections. He didn't like it. He doesn't want to redesign because they are selling the building units already (of course). Then he said instead they prefer columns (still eliminating cantilevers). So it's basically a moment frame. I came up with W12x50 columns and W16x67 beams but there's still the issue of having a moment footing as there is just a wall footing. They don't want to do that either so I going to tell them I'm not interested in continuing further.