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bending of plates and soil-structure interaction

 
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tosifsayyed
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PostPosted: Sat Oct 05, 2013 7:35 pm    Post subject: bending of plates and soil-structure interaction Reply with quote

Respected Sefians,
I am thinking to work on bending of plates for my M. Tech. project. At this stage what I want to do is, I am going to analyse bending of plates by conventional method. After that replacing a plate by foundation I will try to study "bending of foundation and soil-structure interaction". right now this is the only idea I have. Looking forward for help from experts. Please also suggest me some tool for analysis. Any kind of query and help will definitely improve my knowledge.
Waiting for your help!
Thanks and Regards-
Sayyed Tosif
tosifsayyed@gmail.com
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VPandya
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PostPosted: Sun Oct 06, 2013 7:04 am    Post subject: To account for effects of Foundation Rotation, Soil-Struct. Reply with quote

Dear Er.  Sayyed Tosif ,
SOIL STRUCTURE INTERACTION:  We engineers normally do two separate analysis:
a) Super   Structure , Steel Frames or Concrete Frames with Support Conditions as Fixed Base or  Pinned Base.
b) Sub Structure or Foundations Model, A Raft or Footings on Soil Springs  and  Pile Foundation on  Pile Springs.       

Now in this approach of two separate analysis model, we are ignoring the effect on Super Structure of Sub structure or  Foundation Rotation due to Overturning Moments . Member Forces and Lateral Deflection of Super Structure will change if you do a combined model of Super Structure and Sub Structure. That is all SOIL-STRUCTURE  Interaction means.

We were doing two separate analysis few years back because of Limited Computer Power In those days. Now with the new Computer Power we  can  do a Combined Model.

Here is a simple STAAD/PRO EXample-3 (American Examples) illustrating Soil-Structure interaction. In this Example Footings are modeled as Concrete Beam Elements but you can Model them with Concrete  Plate and Shell Elements as in a Raft Foundation Model.  

-----------------------------------------------------------------------------------------------------  
Example Problems.
3. Soil springs for portal frame

A portal frame type steel structure is sitting on concrete footing. The soil is to be considered as an elastic foundation. The value of soil subgrade reaction is known from which spring constants are calculated by multiplying the subgrade reaction by the tributary area of each modeled spring.

This problem is installed with the program as
/SProV8i/STAAD/Examp/US/EXAMP03.STD
.

Figure 1-3:

Example Problem No. 3: A) dimensions and B) analytical model


Where:

B = 20 ft, H = 10 ft, F1 = 4 ft, F2 = 8 ft
P = 5 kips, w = 3 kips/ft
Soil Subgrade Reaction = 250 kips/ft3


Table 1-1:

Spring constant calculationSprings of JointsSpring Constant

1, 5, 10, & 14 (Edges)

8 x 1 x 250 =
2,000 kips/ft

2, 3, 4, 11, 12, & 13 (Interior)

8 x 2 x 250 =
4,000 kips/ft



Actual input is shown in bold lettering followed by explanation.

STAAD PLANE PORTAL ON FOOTING FOUNDATION

-------------------------------------------------------------------------------------------------

I will send you a STAAD/PRO input file of this example through your E-Mail.

Regards.

Vasudeo Pandya P.E. ; S.E.
Structural Engineer
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tosifsayyed
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PostPosted: Tue Oct 08, 2013 2:14 pm    Post subject: Re: To account for effects of Foundation Rotation, Soil-Stru Reply with quote

Respected Sir,
Thank you fo your help.
Thanks and Regards-
Sayyed Tosif
tosifsayyed@gmail.com
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Dr. N. Subramanian
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PostPosted: Wed Oct 09, 2013 2:29 pm    Post subject: Re: To account for effects of Foundation Rotation, Soil-Stru Reply with quote

Dear Er Pandyaji,

We all know that the support condition for footing will be between hinged and fixed, unless the soil is rock, in which case, fixed condition can be safely assumed. Also if the foundation is on piles, fixed condition can be assumed (but during earthquake piles can also have deformations). Considering a soil with SBC = 150 kN/m2 (clay soil), what will be the effect of soil-structure interaction? Have you done any parametric study using the software?.


There is a difference of opinion between the consultants on the condition to be assumed, in the case of ordinary soils. There was a big debate on this- Prof. ARC says the footing should be fixed, whereas we were assuming hinged- assuming hinged may be better for the safety of columns-but may not be economical for the GF columns- Also it may not be good for foundations as they will be subjected only to AF. If fixity is assumed, there may be economy in columns ( if the fixity is relieved by soil rotation, columns will be subjected to greater BM and may even result in the failure of columns in extreme cases) but may result in heavy foundations. Additionally, detailing may also be slightly different in both cases.  

Some are of the opinion that the column bases can be assumed as hinged when there are grade beams just above the footing level.

Please comment.

Regards,
NS

VPandya wrote:
Dear Er.  Sayyed Tosif ,
SOIL STRUCTURE INTERACTION:  We engineers normally do two separate analysis:
a) Super   Structure , Steel Frames or Concrete Frames with Support Conditions as Fixed Base or  Pinned Base.
b) Sub Structure or Foundations Model, A Raft or Footings on Soil Springs  and  Pile Foundation on  Pile Springs.       

Now in this approach of two separate analysis model, we are ignoring the effect on Super Structure of Sub structure or  Foundation Rotation due to Overturning Moments . Member Forces and Lateral Deflection of Super Structure will change if you do a combined model of Super Structure and Sub Structure. That is all SOIL-STRUCTURE  Interaction means.

We were doing two separate analysis few years back because of Limited Computer Power In those days. Now with the new Computer Power we  can  do a Combined Model.

Here is a simple STAAD/PRO EXample-3 (American Examples) illustrating Soil-Structure interaction. In this Example Footings are modeled as Concrete Beam Elements but you can Model them with Concrete  Plate and Shell Elements as in a Raft Foundation Model.  

-----------------------------------------------------------------------------------------------------  
Example Problems.
3. Soil springs for portal frame

A portal frame type steel structure is sitting on concrete footing. The soil is to be considered as an elastic foundation. The value of soil subgrade reaction is known from which spring constants are calculated by multiplying the subgrade reaction by the tributary area of each modeled spring.

This problem is installed with the program as
/SProV8i/STAAD/Examp/US/EXAMP03.STD
.

Figure 1-3:

Example Problem No. 3: A) dimensions and B) analytical model


Where:

B = 20 ft, H = 10 ft, F1 = 4 ft, F2 = 8 ft
P = 5 kips, w = 3 kips/ft
Soil Subgrade Reaction = 250 kips/ft3


Table 1-1:

Spring constant calculationSprings of JointsSpring Constant

1, 5, 10, & 14 (Edges)

8 x 1 x 250 =
2,000 kips/ft

2, 3, 4, 11, 12, & 13 (Interior)

8 x 2 x 250 =
4,000 kips/ft



Actual input is shown in bold lettering followed by explanation.

STAAD PLANE PORTAL ON FOOTING FOUNDATION

-------------------------------------------------------------------------------------------------

I will send you a STAAD/PRO input file of this example through your E-Mail.

Regards.

Vasudeo Pandya P.E. ; S.E.
Structural Engineer
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VPandya
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PostPosted: Thu Oct 10, 2013 12:15 pm    Post subject: Combined Model of Super Structure and Foundation is best. Reply with quote

Dear Dr.  Subramanian,

I recently did on Tall Steel Structure (45m) on Pile foundation, a combined model , Super Steel Structure with Pile Cap and Piles.  This gave 38% more lateral deflection at top of Structure against a Super Structure only Model with hinge base. Steel Column Base detail with Anchor Bolts in Web only.That is a Hinge Base Detail. So I recommend a Combined Model of Foundation  and Super Structure.

With current Computer Power this is not hard to do but post processing of such large
model is more work.  38% difference is big when we account for Soil Structure Interaction.  Pile springs are easily calculated from Geotech report  Load Test Values of Piles for Axial Compression and Lateral Load.  
I will do same method for Super Structure on Raft Foundation with Soil Springs (Modulus of Subgrade Reaction Values).

Your second question:

"Considering a soil with SBC = 150 kN/m2 (clay soil), what will be the effect of soil-structure interaction? Have you done any parametric study using the software?. "  


I will do the study and get back to you.


Regards.

Vasudeo Pandya P.E. ; S.E.
Structural  Engineer.
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Dr. N. Subramanian
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Joined: 21 Feb 2008
Posts: 5430
Location: Gaithersburg, MD, U.S.A.

PostPosted: Thu Oct 10, 2013 4:36 pm    Post subject: Re: Combined Model of Super Structure and Foundation is best Reply with quote

Dear Er Pandyaji,

Thank you very much for your feedback. Due to 38% more lateral deflection, what was the difference in column moments compared to hinged case?

I take this opportunity inform you all that I will be in India from 20th to 26th Oct. 2013.

Regards,
NS
VPandya wrote:
Dear Dr.  Subramanian,

I recently did on Tall Steel Structure (45m) on Pile foundation, a combined model , Super Steel Structure with Pile Cap and Piles.  This gave 38% more lateral deflection at top of Structure against a Super Structure only Model with hinge base. Steel Column Base detail with Anchor Bolts in Web only.That is a Hinge Base Detail. So I recommend a Combined Model of Foundation  and Super Structure.

With current Computer Power this is not hard to do but post processing of such large
model is more work.  38% difference is big when we account for Soil Structure Interaction.  Pile springs are easily calculated from Geotech report  Load Test Values of Piles for Axial Compression and Lateral Load.  
I will do same method for Super Structure on Raft Foundation with Soil Springs (Modulus of Subgrade Reaction Values).

Your second question:

"Considering a soil with SBC = 150 kN/m2 (clay soil), what will be the effect of soil-structure interaction? Have you done any parametric study using the software?. "  


I will do the study and get back to you.


Regards.

Vasudeo Pandya P.E. ; S.E.
Structural  Engineer.
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mahantesh_83
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Joined: 20 Dec 2011
Posts: 11
Location: Mumbai, India

PostPosted: Thu Oct 10, 2013 5:51 pm    Post subject: Reply with quote

Dear Pandya Sir,

I agree with you sir. It is better to use springs in analysis, since given SBC is for 25mm vertical settlement.

NS sir, If we consider vertical settlement only, then there will not be any difference in column moments due to lateral loads as well as vertical loads, but column moments will change for vertical loads if we consider lateral displacement. Sir correct me if I am wrong.

Pandya Sir I want to know how much lateral displacement is to be considered?

Regards,

Mahantesh Nagannavar
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Meera P
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Joined: 11 May 2009
Posts: 21

PostPosted: Fri Oct 11, 2013 8:45 am    Post subject: Re: Combined Model of Super Structure and Foundation is best Reply with quote

Dr. N. Subramanian wrote:
Dear Er Pandyaji,

Thank you very much for your feedback. Due to 38% more lateral deflection, what was the difference in column moments compared to hinged case?

I take this opportunity inform you all that I will be in India from 20th to 26th Oct. 2013.

Regards,
NS
VPandya wrote:
Dear Dr.  Subramanian,

I recently did on Tall Steel Structure (45m) on Pile foundation, a combined model , Super Steel Structure with Pile Cap and Piles.  This gave 38% more lateral deflection at top of Structure against a Super Structure only Model with hinge base. Steel Column Base detail with Anchor Bolts in Web only.That is a Hinge Base Detail. So I recommend a Combined Model of Foundation  and Super Structure.

With current Computer Power this is not hard to do but post processing of such large
model is more work.  38% difference is big when we account for Soil Structure Interaction.  Pile springs are easily calculated from Geotech report  Load Test Values of Piles for Axial Compression and Lateral Load.  
I will do same method for Super Structure on Raft Foundation with Soil Springs (Modulus of Subgrade Reaction Values).

Your second question:

"Considering a soil with SBC = 150 kN/m2 (clay soil), what will be the effect of soil-structure interaction? Have you done any parametric study using the software?. "  


I will do the study and get back to you.


Regards.

Vasudeo Pandya P.E. ; S.E.
Structural  Engineer.
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tosifsayyed
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Joined: 10 Aug 2013
Posts: 44

PostPosted: Fri Oct 11, 2013 6:27 pm    Post subject: Reply with quote

Respected Er. Pandya and Dr. Subramanian,
Can I get some links or name of books to get literature to study the basics of soil structure interaction?
Thanks and Regards-
Sayyed Tosif
tosifsayyed@gmail.com
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