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The latest edition of the ASCE's Substation Structure Design Guide (ASCE 113-23), references ACI 318-19.  Per section 18.13.4.1 of ACI 318-19 foundation seismic ties are required for piers and caissons in Seismic Design Category C, D, E and F.  This provision makes sense for building design, as the columns and associated foundations need to act as a unit.  Is this provision applicable for flexible substation equipment supports that largely act independently of one another ? 

Unfortunately there is no verbiage in ASCE 113 that addresses the applicability of this provision as it relates to substations.  If applicable, this would be an extreme departure from what has been done in the past and likely be a significant financial burden to the industry. 

Any insights are greatly appreciated !

Thank You

I am the Editor of the 2nd Edition of 113 but don't know a lot about ACI and the foundation design issues. I have sent this question to a couple of 113 committee members that  know about ACI.  My initial thought is the ACI deals mainly with buildings and their foundations as does ASCE 7 for seismic loads. Our substation structures are very different than buildings in that we do not need to consider loss of life and the general public is not allowed inside the substation fence. IMHO, it would be up to the substation owner to decide if the extra cost to tie the individual piers all together in a slab on grade would be necessary. I am thinking of a dead end structure 60' wide and 30' across on 4 drilled piers which might require a slab 80' by 50' where switch stands are under the dead end in the middle of the slab.  You might as well pave the entire substation area with a 20' thick slab with #18 rebar spaced 6" on center in all 3 directions, and try to convince the state PUC every substation in the state should be built this way.    :)   

The latest edition of the ASCE's Substation Structure Design Guide (ASCE 113-23), references ACI 318-19.  Per section 18.13.4.1 of ACI 318-19 foundation seismic ties are required for piers and caissons in Seismic Design Category C, D, E and F.  This provision makes sense for building design, as the columns and associated foundations need to act as a unit.  Is this provision applicable for flexible substation equipment supports that largely act independently of one another ? 

Unfortunately there is no verbiage in ASCE 113 that addresses the applicability of this provision as it relates to substations.  If applicable, this would be an extreme departure from what has been done in the past and likely be a significant financial burden to the industry. 

Any insights are greatly appreciated !

Thank You

I contacted a few Committee members on 113 and the consensus was it is up to the EOR or substation owner to determine if a particular structure in the Sub needed the individual foundations tied together as 318 wants to do. Structures that may be sensitive to movement of individua foundations or differential settlement or liquefaction in a seismic event may need the ties so the structure survives the seismic event. If the owner is not willing to take the small risk of structure failure during a seismic event, then the extra cost of tying a large area of the Sub together like 318 seems to require would be the result. 

Small structures like a 2-leg switch stand supporting 3 phases could have a grade beam tying the legs together for a small relative cost increase. The structure may survive but the porcelain insulators likely would not, so you would replace the switch. A large A-Frame dead end could be very costly to have a large slab tying the foundations together. If the fault line ran between the 4 legs, I doubt any amount of reinforced concrete would be enough to prevent the structure collapse. ASCE 113 is a Manual and not a Standard, so it would be up to the EOR or Owner to decide if the 318 requirement is really necessary or cost effective. The T-Line that serves the Sub can have a failure to an individual tower and the utility moves over a few feet and installs a new tower and puts the wire back up. A failure of the dead end in the substation is more difficult to repair.     

I am the Editor of the 2nd Edition of 113 but don't know a lot about ACI and the foundation design issues. I have sent this question to a couple of 113 committee members that  know about ACI.  My initial thought is the ACI deals mainly with buildings and their foundations as does ASCE 7 for seismic loads. Our substation structures are very different than buildings in that we do not need to consider loss of life and the general public is not allowed inside the substation fence. IMHO, it would be up to the substation owner to decide if the extra cost to tie the individual piers all together in a slab on grade would be necessary. I am thinking of a dead end structure 60' wide and 30' across on 4 drilled piers which might require a slab 80' by 50' where switch stands are under the dead end in the middle of the slab.  You might as well pave the entire substation area with a 20' thick slab with #18 rebar spaced 6" on center in all 3 directions, and try to convince the state PUC every substation in the state should be built this way.    :)   

The latest edition of the ASCE's Substation Structure Design Guide (ASCE 113-23), references ACI 318-19.  Per section 18.13.4.1 of ACI 318-19 foundation seismic ties are required for piers and caissons in Seismic Design Category C, D, E and F.  This provision makes sense for building design, as the columns and associated foundations need to act as a unit.  Is this provision applicable for flexible substation equipment supports that largely act independently of one another ? 

Unfortunately there is no verbiage in ASCE 113 that addresses the applicability of this provision as it relates to substations.  If applicable, this would be an extreme departure from what has been done in the past and likely be a significant financial burden to the industry. 

Any insights are greatly appreciated !

Thank You