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Large lateral loads in PEMB without frame ties in Seismic Category D

Large lateral loads in PEMB without frame ties in Seismic Category D

Large lateral loads in PEMB without frame ties in Seismic Category D

(OP)
Throwing this out there to see if anyone has any other ideas or suggestions. We have a pre-engineered rigid frame metal building that has a 200' clear span, which results in some pretty high thrust forces at the base of the columns (90k). The building will be on large (11'x11') spread footings with grade beams on the perimeter. Column spacings are roughly 30' and building length is over 500'. Typically I would do frame ties to withstand thrust forces this large, but in this situation we have deep trenches and pits over 12' below the slab through most of the building. The building will be constructed in two phases, the first being the building and its foundations (footing/grade beams), and after the building is constructed the slab and pits will be built and would occur most likely over the winter here in Illinois. I feel initial restraint is needed beyond the slab due to this lag. The loads are too high for hair pins and due to the presence of pits, frame ties are not an option on some of the columns. We are also in Soil Site Class E, which puts us in Seismic Category D. This adds the IBC 1809.13 requirement for footing ties.

Our initial thoughts were to deepen and strengthen the perimeter grade beams to act as wings for the footing, as well as utilizing a concrete beam parallel with the perimeter grade beams on the interior side of the building, connected with frame ties. This additional grade beam would act as a deadman anchor. Sheetpile deadman could also be used, but adds more equipment to drive them to refusal. The concerns are relying solely on passive pressure for loads this high. I'm leaning towards this option in conjunction with hair pins to meet the IBC seismic requirement.

The other thought was to use battered micropiles under the footing, but battered piles are typically frowned upon in seismic zones.

Drilled shafts would likely get large to obtain a relatively shallow fixity, plus the Contractor is hoping to avoid shafts.

Any suggestions or other thoughts?

RE: Large lateral loads in PEMB without frame ties in Seismic Category D

See if your PEMB engineer is being overly conservative and dumping all of the lateral load into exterior columns and assuming any interior columns are "leaning" columns that just go along for the ride. I ran into a similar situation and that was the case. The PEMB engineer revised his frame analysis such that the interior columns also resisted some of the lateral load. Made a big difference.

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