1. The concept of netallowable bearinghasbeendiscussedonanumberof threadsin the past insome
detail. WHena loadisplacedon the footing,there are twoaspectsof concern –
1) the shear capacityof the soil towithstandthe grossloadingof the footingand
2) the settlementof the footing.
In almostall casesin granularsoils,settlementof the groundbelow the footinggovernsthe allowable
bearingpressureswhichthe geotechnical engineerwillprovidetoyou - hence it isthe netallowable
bearingpressure - onlythatloadingatthe footinglevel inexcessof the existingoverburdenpressure at
that level will 'cause'settlement(neglectingforpractical purposesthe slightexpansionandthen
recompressionasthe overburdenpressure isremovedandthenreappliedprincipally,assome have
indicated,bythe weightof the concrete.
For clays,however,itmaybe the shear strengthof the clay - or of settlement,too, that will
govern. Generally,onlyforfirmtosoftsoilswouldthe shearstrengthlikelygovern. Inthiscase,the
allowable bearingpressure isthe grosspressure placedbythe loadingonthe footing(the overburden
pressure istakenintoaccountin the computation of the bearingcapacityof the footingbythe
(gamma)*Dterm. SEveral textshave explainedthisinsome detail.
Whena soilsengineergivesyouthe allowable bearingpressure - you have the needto know if it is
the gross bearingpressure (basedon shear) or the net allowable bearingpressure (basedon
settlement). Iusuallyget around this by saying net allowable bearingpressure for settlement
controllingsituationsand allowable bearingcapacity b for those situationsgovernedby shear.
