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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3839
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Posted: Tue Dec 05, 2006 8:18 am Post subject: L-footings |
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L - Footings
L- Footings is generally required to be provided at the edge of property line. This footing, if not connected to internal column, is subjected to very high moment due to eccentricity of load w r t footing centreline. It is always preferable to connect this footing with some internal column through a strap beam so that moment due to eccentricity is taken care by this beam.For economy in strap beam, it is necessary to take footing dimension in direction of eccentricity,as less as possible,say, half the other.
For edge footing without strap beam,it is difficult to satisfy the no upliftment condition as redistribution is not permitted for footings resting on soils.Even if some redistribution is taken it may give very high value of upliftment of footing and max pressure after redistribution is quite high(it has to be less than SBC) Some designers take advantage of passive soil resistance(with some FOS) offered by the edge wall to relieve eccentric moment but in that case edge wall shall be strong enough to withstand this passive relief moment.
Long back I had seen a paper written by structural expert and author Sh UH Varyani on the analysis of edge footing w/o strap beams. This might be available in some of his books.
with best wishes
vikramjeet
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madsud_shroti General Sponsor
Joined: 26 Jan 2003 Posts: 11
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Posted: Wed Dec 06, 2006 1:27 pm Post subject: L-footings |
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thnx mr. vikramjeet for your reply. yes it is quite difficult to satisfy no uplift condition in L footing. Can you suggest what can be the best remedy/way for the following condition, If by mistake one of the isolated footing(square shaped) is casted as L footing vikram.jeet[AT]jal... wrote: L - Footings
L- Footings is generally required to be provided at the edge of property line. This footing, if not connected to internal column, is subjected to very high moment due to eccentricity of load w r t footing centreline. It is always preferable to connect this footing with some internal column through a strap beam so that moment due to eccentricity is taken care by this beam.For economy in strap beam, it is necessary to take footing dimension in direction of eccentricity,as less as possible,say, half the other.
For edge footing without strap beam,it is difficult to satisfy the no upliftment condition as redistribution is not permitted for footings resting on soils.Even if some redistribution is taken it may give very high value of upliftment of footing and max pressure after redistribution is quite high(it has to be less than SBC) Some designers take advantage of passive soil resistance(with some FOS) offered by the edge wall to relieve eccentric moment but in that case edge wall shall be strong enough to withstand this passive relief moment.
Long back I had seen a paper written by structural expert and author Sh UH Varyani on the analysis of edge footing w/o strap beams. This might be available in some of his books.
with best wishes
vikramjeet
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3839
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Posted: Fri Dec 08, 2006 5:28 am Post subject: L-footings |
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Dear modsud in my opinion ,try to provide strap beam as per situation given below in view of L-footing already cast without strap beam.
i)assuming that columns are not cast above top of footing
In this situation you can provide the strap beam connecting this column to internal column at top of footing level .
ii)assuming that columns are cast above top of footing upto GL/PL In this situation you can provide the strap beam(tie beam) connecting this column to internal column at GL/PL but edge column reinf provided shall be adequate enough to transfer the eccentric moment upto strap / tie beam level. You can take advantage of passive relief from soil(with some FOS) on edge wall on either side of column .Also the orthogonal cross wall (if it exists) will help. ( by way of its triangular load component beyond footing line through dispersion which will provide counter eccentricity)
with best wishes
vikramjeet
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