The document discusses how to analyze and design a raft foundation modeled with plate elements in STAAD Pro. It explains that STAAD provides shear stresses (SQX, SQY) directly from the stress contour output. These stresses can be used to calculate shear forces and check design requirements for bending, one-way shear, and punching shear. The thickness of the plate model needs to be checked and updated if not adequate based on the design checks. Proper node numbering and orientation of local axes for each element is also important for interpreting the stress results correctly.

1. while we
design raft
foundation
by plate
element in
stadd
pro,how we
can see one
way
shear,punchi
ng shear,how
we can
identify that
thickness we
provide to
plate is
correct and
economical?
what are the
sqx, sqy (in
help content
these defined
as shear)?
what should
be the values
for safe
design.
please clairify
these.
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Kooluthum
Sukhesh
Bronze Sponsor
Joined: 06 May
2010
Posts: 12
Posted: Mon Jun 07, 2010 10:21 am Post subject: Raft foundation design
Dear Engr. Guruganesh,
First step is to identify the local axis of the element from nodal connectivity data. As we
know, node numbering of the nods for 4-noded plate shell element is 1-2-3-4 (I-J-K-L in
the attached fig.) from left bottom node to counterclockwise direction. If so our local x-axis
is along the nodes 1-2 and that of y-axis is along the 2-3 nodes and top plane is element
top surface. (It is advisable to follow a consistent patterern of node numbering during
geometry meshing to bring local axis of all elements in one direction)<xml><o></o>
<o> </o>
For shear force calculation we have to plot the stress contour for SQx- the out of plane
shear across x-section (cutting local x-axis) and SQy that across the local y(cutting local
y). Product of average the element stresses SQx along the line parallel to local y to the
thickness will give the shear force Vx. and that of SQy in a line parallel to local- x
multiplied by thickness will give shear force Vy. (The plane of shear shall be taken at a
distance d from the face of column). Since in STAAD PRO out put element stresses are at
the centre of the element we can use direct linear interpolation for accuracy or use higher
value for conservative design.<o></o>
<o> </o>

2. Follow same procedure for finding the column punching shear force. But the shear plane
shall be the punching shear plane (A plane along the perimeter at a distance d/2 from the
face of the column).Product of average shear stress along this line (Vx along local y –
direction and Vy along local x-direction) multiplied by thickness and perimeter will give the
punching shear force.<o></o>
<o> </o>
For identifying the thickness requirement we have to check the depth requirement for
bending, one way shear, and punching and minimum crack width requirement. Thickness
needs to be changed in the plate model if not adequate by design. Repeat the procedure
for final depth. <o></o>
<o> </o>
For economy we have to proportion the length and width of the footing.<o></o>
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sukanta.adhi
kari
General Sponsor
Joined: 26 Jan
2003
Posts: 666
Posted: Mon Jun 07, 2010 10:44 am Post subject: Re: raft foundation by plate element in
stadd pro
SQX, SQY are called Shear stresses (Force/ unit len./ thk.)...The sketch of
STAAD help menu explains this..PLease note STAAD analysis of plate will give
shear stress value and not shear force...for a foundation design we have shear force
value and from that u find the shear stress value by dividing shear force by
area...here the calculation is simplifed..STAAD directly gives the shear stress
value and this is for one way shear...After getting this value refer SP16 table 61
page 178....wherein depending on shear stress u can check the percentage of
reinforcement...
Regards,
S.Adhikari

3. gaurganesh wrote:
while we design raft foundation by plate element in stadd pro,how we can see one
way shear,punching shear,how we can identify that thickness we provide to plate
is correct and economical? what are the sqx, sqy (in help content these defined as
shear)? what should be the values for safe design. please clairify these.
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ganesh_gaud
General Sponsor
Joined: 05 Jun
2010
Posts: 57
Posted: Tue Jun 08, 2010 12:55 am Post subject:
dear sir
i have another quary. as u said stadd gives stresses directly. when we see stress
counter how can we determine stress are not exceding permissible limit.
(i.e. we use M25 ,fe 415 % of steel 0.3, than according to sp-16 value comes out
0.39,than when we see stress counter should max value not exceed to 0.39 )
please give me your valuable suggetion
thanks and regards
ganesh gaur
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sukanta.adhi
kari
General Sponsor
Joined: 26 Jan
2003
Posts: 666
Posted: Tue Jun 08, 2010 10:10 am Post subject:
Dear friend,
It is not like this...u have done a analysis...after analysis u get the value of moment
and shear ..next while designing we need to see that ur section is adequate enough to
withstand this moment and shear..and accordingly u provide the section and steel
required..
now for u case u got the shear stress say 0.33N/sq mm...for this the percentage of
steel is 0.2% as per SP16...provide this steel..in that way ur section will be be
adequate to take this value of shear stress...but yes there are some limit to the
maximum value of shear strength of a section...that u dont exceed...
Regards,
S.Adhikari
gaurganesh wrote:
dear sir
i have another quary. as u said stadd gives stresses directly. when we see stress
counter how can we determine stress are not exceding permissible limit.
(i.e. we use M25 ,fe 415 % of steel 0.3, than according to sp-16 value comes out
0.39,than when we see stress counter should max value not exceed to 0.39 )
please give me your valuable suggetion
thanks and regards
ganesh gaur
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4. abhik_shar
...
Joined: 06 May
2008
Posts: 62
Posted: Tue Jun 08, 2010 2:21 pm Post subject:
Hello Sefians
For elastic mat foundation( soil supported) , soil is modelled as spring
I have noticed that the deflection profile of the mat as concave surface,
Foundation detal : 11m x 11m . pedestal 5m x 5m.
My question :
1.For bending one-way shear stress , I have to take maximum shear stress form
STAAD file
(as in my case maximum shear is at the pedestal face)
or
2. I have to take at a distance effective distane from face of pedestal.
with regards
Abhishek Sharma
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ganesh_gaud
General Sponsor
Joined: 05 Jun
2010
Posts: 57
Posted: Tue Jun 08, 2010 3:38 pm Post subject:
dear sefians,
i modelled raft in stadd as plate element give thickness 700mm i got max stress
1.43(sqx). is it permissible or i have to increase thickness.moments are not too high
(mx 300kn-m/m). according to is code two way shear is permissible is
1.25(.25*sqrt(25)).
please guide me.how can i check output stresses that comes from stress.
thanks in advance
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Radha
krishna
...
Joined: 01 Nov
2008
Posts: 70
Posted: Wed Jun 23, 2010 11:59 pm Post subject:
Dear abhik_shar,
How you have modeled pedestal?
if it is beam element then the load will be applied at a single node instead of 25 m2
area ( increases the mat concave surface), preferable to apply distributed loads over
the area.
Regarding the shear check you have to take effective depth from face of the pedestal.
abhik_shar wrote:
Hello Sefians

5. For elastic mat foundation( soil supported) , soil is modelled as spring
I have noticed that the deflection profile of the mat as concave surface,
Foundation detal : 11m x 11m . pedestal 5m x 5m.
My question :
1.For bending one-way shear stress , I have to take maximum shear stress form
STAAD file
(as in my case maximum shear is at the pedestal face)
or
2. I have to take at a distance effective distane from face of pedestal.
with regards
Abhishek Sharma
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ganesh_gaud
General Sponsor
Joined: 05 Jun
2010
Posts: 57
Posted: Wed Jul 14, 2010 11:35 pm Post subject:
hi sefine
i request to experts please give their valuable suggestion on following
1. while designing raft or shear wall by plate elemets, any stresses should not exceed
in any case tau c max as give in IS456 table no 20.
2. if stresses are with in limit i.e. tau c given in table 19 than shear reinforcement
could be nominal otherwise should provided accordingly tau V.
are above obsevations correct?
what should be the correct approach to check shear wall or raft that is done by plate
element.

6. For elastic mat foundation( soil supported) , soil is modelled as spring
I have noticed that the deflection profile of the mat as concave surface,
Foundation detal : 11m x 11m . pedestal 5m x 5m.
My question :
1.For bending one-way shear stress , I have to take maximum shear stress form
STAAD file
(as in my case maximum shear is at the pedestal face)
or
2. I have to take at a distance effective distane from face of pedestal.
with regards
Abhishek Sharma
Back to top
ganesh_gaud
General Sponsor
Joined: 05 Jun
2010
Posts: 57
Posted: Wed Jul 14, 2010 11:35 pm Post subject:
hi sefine
i request to experts please give their valuable suggestion on following
1. while designing raft or shear wall by plate elemets, any stresses should not exceed
in any case tau c max as give in IS456 table no 20.
2. if stresses are with in limit i.e. tau c given in table 19 than shear reinforcement
could be nominal otherwise should provided accordingly tau V.
are above obsevations correct?
what should be the correct approach to check shear wall or raft that is done by plate
element.