Strap Footing Vs. Isolated Footing ?

[Dren Gashi]

I am designing a group of isolated foundations under columns where the central columns have a Ultimate Load of 1600 kN where the soil bearing pressure is 150 kN/m2. My isolated footing size is 3.2 m x 3.2 m but I heard from another engineer that if you connect isolated footings with strap beams the dimensions of the footings decrease because the moments are distributed to other less loaded footings. Is that true ?

 

[hokie66]

No. For gravity loads, the application for a strap footing is where loads are eccentric to one or both of the connected footings. The moment caused by the eccentricity is then resisted by the strap beam.

For just isolated pad footings, a square footing under each column is appropriate. Your size sounds right for the 1600 kN load. For the lighter loads, use smaller footings.

 

[dik]

Why straps? Seismic area?

Dik

 

[BridgeSmith]

It depends on how much of the soil bearing pressure is uniform and how much is due to moment. The strap beams will spread out the moments applied to the footing, and will therefore decrease the pressure (and uplift) due to moment. If most of the bearing pressure is due to gravity load that is not eccentric to the footing, the effect of strap beams will be small.

 

[dik]

HR: anything over 2 or 3 stories, column moments usually give way to axial loads... Other than for seismic, I don't think I've ever used 'straps'.

Dik

 

[BridgeSmith]

Dik, that makes sense. I tried write my response fairly generalized, since I don't know how the loading breaks down.