Bearing Calculations for Spread Footings
Bearing Calculations for Spread Footings
(OP)
I'm looking for information on calculating bearing capacities on spread footings with large eccentricities. ie. e > L/2.
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Bearing Calculations for Spread Footings
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Bearing Calculations for Spread FootingsBearing Calculations for Spread Footings(OP)
I'm looking for information on calculating bearing capacities on spread footings with large eccentricities. ie. e > L/2.
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RE: Bearing Calculations for Spread Footings
RE: Bearing Calculations for Spread Footings
DaveAtkins
RE: Bearing Calculations for Spread Footings
Laura
RE: Bearing Calculations for Spread Footings
It would depend on the situation. I believe that most codes would require your loading condition to produce a conservative result.
For example, when analyzing the external stability of a MSE retaining wall, live loads supported directly on the MSE fill are ignored when calculating the overturning and sliding resistance. Live loads on the retained soil are included in these cases, as they will tend to increase the eccentricity or driving lateral force.
Self-weight of the foundation should be included in the eccentricity calculation, as should the weight of the soil directly above the footing, but not the column/wall stem.
I would explicitly ignore any "passive pressure" other than self-weight provided by soil directly above a foundation. Watch out for assuming soil above the foundation if structural loads will be applied before backfilling.
Good Luck!
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Bearing Calculations for Spread Footings
http://www.usda.gov/rus/electric/bulletins.htm
The weight of the soil is a factor.
RE: Bearing Calculations for Spread Footings
DaveAtkins
RE: Bearing Calculations for Spread Footings
I looked in the RUS Bulletin you mentioned (good reference by the way, I wasn't aware of this one, not being in the electrical industry). If the eccentricity is equal to half the footing width, e=b/2, then the safety factor is equal to 1.0 for overturning (substitute Eq. 8-18 and 8-20 into 8-19). However the bulletin states that a safety factor between 1.5 and 2.0 is adequate. This agrees with what DaveAtkins stated above and with which I concur; you cannot safely use a spread footing where the eccentricity is greater than half the footing width. If you cannot make the footing wider, then you should consider piles or drilled shafts.
RE: Bearing Calculations for Spread Footings