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Eccentric Footing Design
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ramu_se
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PostPosted: Wed Dec 14, 2022 4:10 pm    Post subject: Eccentric Footing Design Reply with quote

Dear All,

Can we design the eccentric footing for the attached plan as per the attached calculation?

Kindly guide me on this.

Thanks & Regards,
Ramu.



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vikram.jeet
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PostPosted: Thu Dec 15, 2022 4:31 am    Post subject: Reply with quote

Eccentric footing on boundary edge :

Dear Er -

First and foremost task in sbove footing is working out base pressure correctly.

You have taken uniform base pr,, You can do so in case you tie this column to some opposite interior column by a beam called Strap beam .

You may see for yourself that bade pressre Max will exceed many times, if Strap beam is not provided

But in this case : Pl see Max base pr as under -
P  = Column vertical load
B = Footing size in ecc direction
L = footing size in ortho dir
d  = col dim in ecc dir

Since yr col load is at edge of footing,  there is eccentricity of load wrt to footing centrline.
e = (B/2  - d/2)
Moment on footing = P * e

Footing will lift and max base pr will be very very high

Please refer sny text book for design calcs

Some relief in moment on footing due to above ecc is provided by passive soil resistance thru edge wall,, but is not much compared to ecc moment.
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vikram.jeet
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PostPosted: Thu Dec 15, 2022 4:39 am    Post subject: Reply with quote

You can refer earlier detailed discussions on Edge footings in topic " Some general view '
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ramu_se
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PostPosted: Thu Dec 15, 2022 9:36 am    Post subject: Reply with quote

Thank you Sir.

Kindly share if you have any references to designing the eccentric footing with a strap beam.

For example,

Edge Footing

Load on column - 300 kN

SBC - 200 kN/Sqm

Column size - 200 x 450 mm

Attached is the image for your reference.

Thanks & Regards,
Ramu.



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PostPosted: Thu Dec 15, 2022 10:59 am    Post subject: Reply with quote

Et Ramu,,

By connecting edge column to inner column at footing level itself thru strap beam,, the base pressure could be approx P/A  ,,  but strap beam has to resist eccentric moment P*e as discussed in previous post.

Reiforcement is provided at top.,, bottom bars could be nominal ones.
Approx Moment in strap beam can be taken as M=P*e  with 10% reduction for soil passive relief,, but better workout passive relief from side wall in footing zone.

Shesr force - entire shear is balanced within the footing  being resisted by footing concrete ,, hence stps can be nominal but pl see some text book also for better understand ing of subject.
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PostPosted: Fri Dec 16, 2022 2:36 pm    Post subject: Reply with quote

Very old posting from respected  BV Harsoda saheb  wherein discussion on edge footing was there :-


B.V.Harsoda wrote:
In previous posting Er. Vikramjeet has been Explained nicety as Below:-


"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
."



In Second Post said that

"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)"






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PostPosted: Fri Dec 16, 2022 2:43 pm    Post subject: Reply with quote

Earlier posting from my end :

Plinth beam at plinth level can also help , but in that case there is need to  
for additional forces to be taken into consideration .

There are two alternatives - - - - -

1 ] Alternative A - - Additional forces in Column , Plinth beam as well as check
of other columns on plinth beam grid

Provide additional reinf in column for eccentric moment M. This reinf needs to  
be provided from footing to plinth beam (with Ld embedment in both ), and
plinth beam need s to be designed for additional tensile force T= M/Df .  
The columns on inner side with which this plinth beam is tied also to be  
checked for a force T/N

Df is the foundation depth
( More precisely vertical distance from from CL of footing to CL of Plinth B)

N - - is the nos of other columns in that grid tied by plinth beam.

2 ] Alternative B - - Plinth beam designed as strap beam with filler 345thk
brick wall for light loadings/and pcc wall 1:3:6 for high loading between
footing -top and plinth -beam -underside.

This wall will be sandwiched between footing and plinth beam. This will be
subject to upward force =column load which in turn act on plinth beam
to be designed as strap beam.


best regds

vikramjeet



Dear Er vikram.jeet Sir If we provide Plinth beam instead of strap beam in case of eccentric footing so that it connects to inner column to take care of eccentric moment Please comment on this Regards
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PostPosted: Fri Dec 16, 2022 2:57 pm    Post subject: Reply with quote

Earlier posting of 31.05.2010

vikram.jeet wrote:
Dear Er Venkatesh ji ,

14" brick walls are definitely supportive and bearing capacity of 30 tonne /sqm is  
very good strata.But since the structure is old, 14" walls may need re-plastering.
Also please check the footing offset from wall by excavating(exposing) from outside.

For extension , if the walls are planned exactly over the GF walls, extension may
be carried out but it must be seen that walls must have cross walls at about  
10' to 12' interval (which generally exists in a residential building).Footing
width must be checked for (new loading+existing loading).

As regards L - type footing -- These are the one sided footings with column
located at edge .
The corner footings of this type are eccentric in both the directions with column  
located at corner of the footing.These will require strap beams in both directions
The intermediate L type footings are eccentric in one direction with column located
,though at edge ,but at centre in length direction.Thse will require strap beam in
one direction only.
These footings are provided out of compulsion when
(i)column happens to be on property line and it is not possible to provide  
a concentric footing.
(ii) When additions /alterations in existing structure are required -- as the case
under present discussion.

For design ,must refer any standard text book on structures design.
Unless concept is clear ,don't move ahead in design.

However as a quick design, please calculate the moment for which strap beam
to be designed as under:
Moment = P * e
P --- Column Load
e - Eccentricity ( Distance between centre line column to centre line footing in direction
of eccentricity)
Some reduction in this moment is possible due to( passive less active) relief of soil on footing  
back face(and wall face if wall at back exists).

If L -footing happen to be at corner, it has to be planned as square shape
However at intermediate location, try to keep Length of footing along the property line
and L/B shall be planned >2.0 to 2.5 .This would keep the eccenricity minimal and  
result in economical design of strap beam.

For such footings the column pedestal dimension on eccentric side shall be increased
to about 22.5 to 30 cm as against 15cm for generally given for normal footings

Direction of tensile reinf in strap beam-- As already discussed two situations may be posiible
(i) Edge columns on property line will have only one posiibility -- L- footings with footings
inside the plot connected to opposite column(Concentric/eccentric).Strap beam runs
in direction of footing Overhang and passes through it

(ii) Additions /Alterations cases: Possibility of providing revese L type footings is also  
possible in addition to (i). Strap beam runs in reverse direction of footing Overhang  

For (i) Tensile reinf in strap beam is needed at TOP
For (ii) Tensile reinf in strap beam is needed at BOTTOM  

Shear reinf in strap beams is nominal as entire shear is generally balanced by footing
over hang and it is a case of more or less uniform moment (approx no shear)


regards

vikramjeet

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PostPosted: Sat Dec 17, 2022 3:50 pm    Post subject: Reply with quote

Situation now a days reg L footings :

In big projects with proper supervision , whatever detais a str engineer  give in drg ,, are followed without if / but.

But in projects where client is ignorant ,, no / poor supervision  ,, that too by non Civil er  ,, there are possibilities that Strap beams are not provided ,  and unless supervised ,, no body knows ,  Missing is due to fact that additional excavaton for strap beam right upto found lvl. Even many site ers have tendency to ignore here.
However in present times with what's app facility photos must be demanded by Str Consultant when reinf is placed and strap beam ,provided/ not can be easily seen distantly .
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PostPosted: Tue Dec 20, 2022 6:10 am    Post subject: Reply with quote

Thank you, Sir.

I will resolve one problem and update you.
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