plate notes_aak.doc

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.

Back to top

Kooluthum SukheshBronze Sponsor

Joined: 06 May 2010Posts: 12Posted: 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) 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. 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. 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. For economy we have to proportion the length and width of the footing.

Warning:Make sure you scan the downloaded attachment with updated antivirus tools before opening them. They may contain viruses.Use online scannershereandhereto upload downloaded attachment to check for safety.Element.pdfDescription:

DownloadFilename:Element.pdfFilesize:41.77 KBDownloaded:1413 Time(s)

Back to top

sukanta.adhikariGeneral Sponsor

Joined: 26 Jan 2003Posts: 666Posted: Mon Jun 07, 2010 10:44 am Post subject: Re: raft foundation by plate element in stadd pro

SQX, SQYare 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

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.

Back to top

ganesh_gaudGeneral Sponsor

Joined: 05 Jun 2010Posts: 57Posted: Tue Jun 08, 2010 12:55 am Post subject:

dear siri 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 whenwe see stress counter should max value not exceed to 0.39 )please give me your valuable suggetion

thanks and regardsganesh gaur

Back to top

sukanta.adhikariGeneral Sponsor

Joined: 26 Jan 2003Posts: 666Posted: Tue Jun 08, 2010 10:10 am Post subject:

Dear friend,

It isnot 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 siri 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 whenwe see stress counter should max value not exceed to 0.39 )please give me your valuable suggetion

thanks and regardsganesh gaur

Back to top

abhik_shar...

Joined: 06 May 2008Posts: 62Posted: 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

Back to top

ganesh_gaudGeneral Sponsor

Joined: 05 Jun 2010Posts: 57Posted: 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

Back to top

Radha krishna...

Joined: 01 Nov 2008Posts: 70Posted: 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

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_gaudGeneral Sponsor

Joined: 05 Jun 2010Posts: 57Posted: 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.