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L-footings

 
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vikram.jeet
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PostPosted: Tue Dec 05, 2006 8:18 am    Post subject: L-footings Reply with quote

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
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PostPosted: Wed Dec 06, 2006 1:27 pm    Post subject: L-footings Reply with quote

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
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PostPosted: Fri Dec 08, 2006 5:28 am    Post subject: L-footings Reply with quote

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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