Footing Ties in Seismic Areas

[tclat]

Hi,

IBC 1805.4.2.2 requires footing ties for structures in Seismic Design Category D, E and F for site class E and F. This can be omitted if the equivalent restraint is provided by beams in the slab or the slab itself. I interpret this to mean that the tie does not have to be directly at the footing level.

I have a situation where the footing is 4-6 feet below the top slab. I have grade beams at the top of the pedestal to minimize the moments in the pedestal by distributing the shear force of a braced frame over several pedestals. Is there a limit, in terms of distance between the grade beam and footing, for footing ties as required by IBC?

Thanks

 

[msquared48]

I am in seismic zone "D". For residences, if you bury the footing in the soil 7" or deeper, then the footing will not "walk" laterally in an event, which is what you are trying to prevent. For larger structure wioth larger footings, you should bury deeper, or use interconnecting grade beams, particularly with a pile system.

I think there would be a limit as to just restraint at the floor level. The longer the column connecting to the footing, the more the tendency for the footing to walk.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[tclat]

Mike

The structure is a single storey steel structure with 20'x20' bays. The wind uplift is high which is why I have the footings with a minimum of 5 feet ground/fill cover. The top of my grade beam is 1 foot below the slab on grade and the beam is 18" deep. Therefore between the base of the footing and the soffit of the grade beam is between 2 and 4 feet. Do you think I have satisfied IBC requirements?

 

[msquared48]

Looks like it to me from what you describe.

I guess you have to ask yourself is "Where would the footings go in a selemic event?" With the soil in place, ane with the slab on top, unless this is in a soil liquefaction zone where all bets would be off, I would be comfortable with your scenario.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

@tclat: In response to your OP - A lot depends where the lateral loads due to earthquake are resisted (level of restraint); whether at the level of the footings or at the level of the tie beams / grade beams / slab on grade.
If lateral loads are resisted at grade level then the difference in height between the top of the footings and the underside of the tie beams will not affect the forces and hence the design of the stub columns / pedestals.
A difference in height of about 3 to 4 feet would not affect the design of the stub columns either way.

In response to your second post - Steel structure supported on concrete pedestals - Providing ties at grade level is a better option since a combination of stub columns + tie beams make a better framework in resisting lateral loads whereas stub columns would behave as cantilevers if seismic ties were located at footing level.
In my opinion, codal requirements are met so long as ties are provided in two directions at grade level and the framework of ties + stub columns can transmit the lateral loads to the founding level.

 

[RareBugTX]

Looks like there isn't too much in IBC regarding footings in seismic zones.

 

[tclat]

Thanks for the comments to all. I agree that the grade beam is better at the top of the pedestal in my case. My gut feeling is that it is ok. Thanks again for the input.