NinerStruct
Structural
- Nov 5, 2012
- 36
I’m working on a renovation of a PEMB which is having the slab removed and replaced and an interior independent floor put in. We didn’t have any information or drawings on the size or reinforcing in foundations or slab other than an e-mail from the original contractor saying what he thought the size/depth of the exterior and interior footings were. I went with the assumption that the slab had hairpins to withstand the lateral forces. The building is 3 bays wide, with two rows of interior columns through the middle of the building. On the drawings, I noted that the slab should be removed in one outer bay and tie footings be placed from the exterior to the interior footings to take the lateral forces. Then once those are in place, the rest of the slab can be removed and the same interior footings be placed on the opposite side of the building to tie interior to exterior footings.
Now that construction has begun, the owner has been pressing the contractor to find any savings that they can. The contractor has been having these conversations with the owner without anyone on the design team.
We received a vague and confusing RFI from the contractor yesterday asking about whether we had the reactions building design reactions. Then we received an e-mail from the Owner this morning saying that they “approve of the Cost savings”. After a call to the contractor, he mentioned that he was the contractor for the original construction of the PEMB and informed me that the original building has no hairpins into the slab or any other connection of the slab to the foundations. He said it sat for 3 years without a slab when it was first built. He thought the lateral loads were so small that the soil outside could withstand the forces because metal buildings with interior columns don’t have the large kick-out forces. I explained to him that until he hears otherwise from me, that those footings are to be installed as shown on the drawings but that I would review the design and respond accordingly in the RFI now that I understand what his question is.
The only metal building I had worked on had larger kickout forces, these lateral reactions are 5 kip or less. Has anyone done a PEMB using only passive pressure to resist the lateral? I assumed the use of hairpins was typical regardless of the forces. I also typically neglect using passive pressure within frost zones. Although, when searching yesterday I found this thread “ and it’s got me second guessing whether I overdesigned in the first place. At the same time, just because a building stayed up-right for a few years without a slab doesn’t mean that it’s right, it just means that it may not have seen its design loads.
Anyway, we still don’t have a confirmed depth of footing, but I ran a quick calc using the depth of the trench (given by the contractor) as 42” minus 8” for lower grade, and the passive pressure of drained cohesive soil (260 pcf), neglecting the top 12” of soil, but included it as surcharge. With that, it looks l and it would take roughly 5 feet of the perimeter trench to resist the lateral force. So should I be considering this, or should I simply stick to my original design? I appreciate all opinions, but understand the final decision is my own.
Thanks.
Now that construction has begun, the owner has been pressing the contractor to find any savings that they can. The contractor has been having these conversations with the owner without anyone on the design team.
We received a vague and confusing RFI from the contractor yesterday asking about whether we had the reactions building design reactions. Then we received an e-mail from the Owner this morning saying that they “approve of the Cost savings”. After a call to the contractor, he mentioned that he was the contractor for the original construction of the PEMB and informed me that the original building has no hairpins into the slab or any other connection of the slab to the foundations. He said it sat for 3 years without a slab when it was first built. He thought the lateral loads were so small that the soil outside could withstand the forces because metal buildings with interior columns don’t have the large kick-out forces. I explained to him that until he hears otherwise from me, that those footings are to be installed as shown on the drawings but that I would review the design and respond accordingly in the RFI now that I understand what his question is.
The only metal building I had worked on had larger kickout forces, these lateral reactions are 5 kip or less. Has anyone done a PEMB using only passive pressure to resist the lateral? I assumed the use of hairpins was typical regardless of the forces. I also typically neglect using passive pressure within frost zones. Although, when searching yesterday I found this thread “ and it’s got me second guessing whether I overdesigned in the first place. At the same time, just because a building stayed up-right for a few years without a slab doesn’t mean that it’s right, it just means that it may not have seen its design loads.
Anyway, we still don’t have a confirmed depth of footing, but I ran a quick calc using the depth of the trench (given by the contractor) as 42” minus 8” for lower grade, and the passive pressure of drained cohesive soil (260 pcf), neglecting the top 12” of soil, but included it as surcharge. With that, it looks l and it would take roughly 5 feet of the perimeter trench to resist the lateral force. So should I be considering this, or should I simply stick to my original design? I appreciate all opinions, but understand the final decision is my own.
Thanks.