spieng89
Structural
- Jun 30, 2015
- 172
I was having a discussion with a coworker about the load requirement for seismic ties and we came to a disagreement. Wanted to see if anyone here could shine some light on their interpretation.
IBC section 1809.13 states effectively:
Where a structure is assigned to Seismic Design Category D, E or F, individual spread footings founded on soil defined in Chapter 20 of ASCE 7 as Site Class E or F shall be interconnected by ties. Unless it is demonstrated that equivalent restraint is provided by reinforced concrete beams within slabs on grade or reinforced concrete slabs on grade, ties shall be capable of carrying, in tension or compression, a force equal to the lesser of the product of the larger footing design gravity load times the seismic coefficient, SDS, divided by 10 and 25 percent of the smaller footing design gravity load.
I take that to mean 0.1 x Sds x (1.2D + 1.6L)
My coworker takes that to mean, after applying factored loads and determining the larger footing design (1.2D + 1.6L comparing each cap between ties)... then apply following equation; 0.1 x Sds x 1.2D. Considering only gravity loads for the seismic tie. She pulled an example from a SE exam practice question from NCEES.
Her reasoning is live loads are only considered in seismic design for certain situations if LL is large enough. In scenario where LL isn't being considered for base shear or any diaphragm loads, it shouldn't be included for ties. I see where she is coming from and it makes sense, but want to get a consensus. For small loads, doesn't matter much. For larger column loads, the tie design can get beefy depending on the route taken and interpretation of IBC.
IBC section 1809.13 states effectively:
Where a structure is assigned to Seismic Design Category D, E or F, individual spread footings founded on soil defined in Chapter 20 of ASCE 7 as Site Class E or F shall be interconnected by ties. Unless it is demonstrated that equivalent restraint is provided by reinforced concrete beams within slabs on grade or reinforced concrete slabs on grade, ties shall be capable of carrying, in tension or compression, a force equal to the lesser of the product of the larger footing design gravity load times the seismic coefficient, SDS, divided by 10 and 25 percent of the smaller footing design gravity load.
I take that to mean 0.1 x Sds x (1.2D + 1.6L)
My coworker takes that to mean, after applying factored loads and determining the larger footing design (1.2D + 1.6L comparing each cap between ties)... then apply following equation; 0.1 x Sds x 1.2D. Considering only gravity loads for the seismic tie. She pulled an example from a SE exam practice question from NCEES.
Her reasoning is live loads are only considered in seismic design for certain situations if LL is large enough. In scenario where LL isn't being considered for base shear or any diaphragm loads, it shouldn't be included for ties. I see where she is coming from and it makes sense, but want to get a consensus. For small loads, doesn't matter much. For larger column loads, the tie design can get beefy depending on the route taken and interpretation of IBC.
