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[sathyace]

Hi All,

Is it efficient by designing the combined footing using staad,making plate mesh and assigning the support as elastic mat by entering the value of subgrade modulus?

Herewith i have attached the staad file for reference.

If this is not efficient which is the best method to design combined footing?

Regards,
Sathya.

[es_jayakumar]

Dear Sathya,
Your design is theoretically robust. Are the column loads 626kN and 574kN respectively ? You can better use "Plate Mat" opton for foundation.

Regards,
E S Jayakumar

[sathyace]

Thank you very much sir.

Yes,column loads are 626KN and 576KN.

What is the difference between elastic mat and plate mat sir? and will that give better result?

Regards,
Sathya

[es_jayakumar]

Dear Sathya,

The differencebetween Elastic Mat and Plate Mat is explained as below in the STAAD TechnicalReference Manual :
The ELASTIC MAT option: If you want to have STAADcalculate the influence area for the joint (instead of you specifying an areayourself) and use that area along with the sub-grade modulus to determine thespring stiffness value, use the MAT option. Situations where this may beappropriate are such as when a slab is on soil and carries the weight of thestructure above. You may have modelled the entire slab as finite elements andwish to generate spring supports at the nodes of the elements.
The PLATE MAToption: Similar to the Elastic Mat except for the method used to computethe influence area for the joints. If your mat consists of plate elements andall of the influence areas are incorporated in the plate areas, then thisoption is preferable. Enter a list of plates or YRANGE f1 f2 at the beginningof the command, the joint influence areas are then calculated using the sameprinciples as joint forces would be from uniform pressure on these plates. Thismethod overcomes a major limitation of the Delaunay triangle method used in theELASTIC MAT option, which is that the contour formedby the nodes of the mat must form a convex hull.
Discussion on this in SEFI can be found at :
//www.buonovino.com/forum/viewtopic.php?p=25014
Further, you can check the pressure exerted on the soil, by referringto the �Base Pressure� table and see the maximum pressure is within the SBC ofthe soil and the minimum pressure >0. In the present case, they are 105 kN/m²and 63 kN/m²respectively.

Regards,
E S Jayakumar

[sathyace]

Thank you very much sir.

How do we get the base pressure table?

Regards,
Sathya

[es_jayakumar]

Right click on working area -> Tables -> Check "Base Pressure" Box -> OK -> Summary -> Pick values of "Max Py" and "Min Py"

E S Jayakumar

[sathyace]

Thank you sir for your reply.It was very useful to me.

Regards,
Sathya.

[sathyace]

Dear Sir,


After the analysis of combined footing,taking Mx(local) & My(local) values from plate stress contour and design for the maximum moment,is it the efficient way to provide reinforcement details?

Thanks&Regards,
Sathya.

[es_jayakumar]

You can find Mx and My either from the "Tables" option or by double clicking the elements in the region where you expect maximum BM Mx and My and checking those values in "Centre Stresses" & "Corner Stresses" options (trial and error). You can find maximum Shear Force also, by checking S_QXand S_QYvalues at appropriate sections.

Best wishes,
E S Jayakumar

[es_jayakumar]

The attached Excel sheet can be used for designing the footing, once the design BM, punching shear and one-way shear values are known.

E S Jayakumar