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swe_mani General Sponsor
Joined: 05 Jan 2013 Posts: 6

Posted: Thu Jan 24, 2013 9:05 am Post subject: seismic design of Desilting Tank 


Dear SefIndians
I am in a position to design an underground desilting basin for the new thermal power plant in India. I am interested to design the structure for seismic resistance since the structure in seismic zone III.
I used some literature for the seismic design of liquid retaining structures. The Impulsive & convective hydrodynamic pressure have been calculated as per the reference of IS1893 Part 2 and same is applied in staad pro
Since the tank is underground, i used MononobeOkabe theory for the calculation dynamic active and passive earth pressure and the active dynamic earthp ressure and passive dynamic earth pressure have been calculated and applied as hydrostatic load (load option in staad pro.)
After completing all of these i am not sure whatever i did are right. I have few doubt about my design.....
1. The impulsive and convective hydro dynamic forces calculated as per IS1983 Part2 is only for rectangular and circular tanks, but my desilting basin in not perfect rectangular which is somewhat irregular in shape. How to i calculate the forces and expression for parameters of spring mass model for the irregular tank structure?
2. IS the MO theory alone enough for design the tank or should i used soil spring constant. if i should apply the soil spring constant means how to calculate the soil spring constant from Active and passive dynamic earth pressure co efficient?
3. Is it possible to use response spectrum method?
4. Since the structure is very close to the sea shore, the liquefaction effects should be considered. How the liquefaction effects to be considered for the design? Should i use any factor of safety?
I will be very graceful if some most respected engineers clarify these doubts.
Thanks & Regards
N Manikandan M.Tech., MICI.,
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5430 Location: Gaithersburg, MD, U.S.A.

Posted: Thu Jan 24, 2013 5:03 pm Post subject: Re: seismic design of Desilting Tank 


Dear Er Manikandan,
Please look into the report published by IITKNICEE at
http://www.iitk.ac.in/nicee/IITKGSDMA/EQ08.pdf
This also contains worked examples. It may be useful to look into the review of codal provisions at
http://www.iitk.ac.in/nicee/IITKGSDMA/EQ04.pdf
Best wishes,
NS
swe_mani wrote:  Dear SefIndians
I am in a position to design an underground desilting basin for the new thermal power plant in India. I am interested to design the structure for seismic resistance since the structure in seismic zone III.
I used some literature for the seismic design of liquid retaining structures. The Impulsive & convective hydrodynamic pressure have been calculated as per the reference of IS1893 Part 2 and same is applied in staad pro
Since the tank is underground, i used MononobeOkabe theory for the calculation dynamic active and passive earth pressure and the active dynamic earthp ressure and passive dynamic earth pressure have been calculated and applied as hydrostatic load (load option in staad pro.)
After completing all of these i am not sure whatever i did are right. I have few doubt about my design.....
1. The impulsive and convective hydro dynamic forces calculated as per IS1983 Part2 is only for rectangular and circular tanks, but my desilting basin in not perfect rectangular which is somewhat irregular in shape. How to i calculate the forces and expression for parameters of spring mass model for the irregular tank structure?
2. IS the MO theory alone enough for design the tank or should i used soil spring constant. if i should apply the soil spring constant means how to calculate the soil spring constant from Active and passive dynamic earth pressure co efficient?
3. Is it possible to use response spectrum method?
4. Since the structure is very close to the sea shore, the liquefaction effects should be considered. How the liquefaction effects to be considered for the design? Should i use any factor of safety?
I will be very graceful if some most respected engineers clarify these doubts.
Thanks & Regards
N Manikandan M.Tech., MICI., 


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swe_mani General Sponsor
Joined: 05 Jan 2013 Posts: 6

Posted: Fri Jan 25, 2013 9:11 am Post subject: 


Dear Sir
Thanks for your fast reply Sir.
I already followed the reference which you specified(IITKGSDMA GUIDELINES
for SEISMIC DESIGN OF LIQUID STORAGE TANKS  Provisions with Commentary and Explanatory Examples).
My question is, some parameters used on the commentary Table C1 which is based on the spring mass model only applicable for rectangular and circular c/s. But my tank is not perfect rectangle. I just assumed the tank is rectangle and calculated the parameters. But I am not sure my assumption is right.
As per Clause 4.13.3 the dynamic earth pressure can be used. When seismic excitation about +X is applied the first wall will subjected by active earth pressure and the second end wall will subjected by passive earth pressure as per the basic theory. But at the same time the effect of wall will be more when the wall move towards the back fill compared to the wall movement away from the back fill. How do i consider this effect in staad pro. It is only possible when i use soil spring support. Is there any method is available to find the soil spring model for dynamic pressure. I already used MO theory to calculate the dynamic active and passive earth pressure, Can i use to calculate the spring constant by (Dynamic earth pressure x influence area of plate) /permissible deflection of wall....?
And the code does not specifies the effect of liquefaction of soil. How to consider the effect of liquefaction in the design. IF this liquefaction effect is only for stability check, what should we do if the structure is in liquefaction prone region.?
Kindly
N Manikandan


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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5430 Location: Gaithersburg, MD, U.S.A.

Posted: Fri Jan 25, 2013 3:47 pm Post subject: 


Dear ER Manikandan,
You may assume an equivalent rectangle for calculating the loads.
I am not having much experience with STADD pro. May be others will help on this.
Liquefaction will be there only on certain types of soils(sandy and silty soils). Three critical factors must be present for sediments to be prone to liquefaction. The sediment must be 1) saturated with ground water, 2)composed of sand or siltsized particles, and 3) compacted fairly loose. For liquefaction to occur, all three factors must be present at the same time.
It is not possible to simply design for liquefaction. Perhaps the simplest method of dealing with liquefaction is to locate
structures in areas that do not have liquefiable soils. Problems with liquefaction may be mitigated by altering the site soil
characteristics. Examples include lowering the ground water table with drains or pumps, densification of the soils by dynamic compaction or vibration, installation of stone columns, and grouting.Structurally, foundation support may be
redistributed through the use of piles or caissons which extend through the liquefiable layersthis may help reduce liquefaction induced damage. Specially designed mat foundations have also been used in some buildings.
Best wishes,
NS
swe_mani wrote:  Dear Sir
Thanks for your fast reply Sir.
I already followed the reference which you specified(IITKGSDMA GUIDELINES
for SEISMIC DESIGN OF LIQUID STORAGE TANKS  Provisions with Commentary and Explanatory Examples).
My question is, some parameters used on the commentary Table C1 which is based on the spring mass model only applicable for rectangular and circular c/s. But my tank is not perfect rectangle. I just assumed the tank is rectangle and calculated the parameters. But I am not sure my assumption is right.
As per Clause 4.13.3 the dynamic earth pressure can be used. When seismic excitation about +X is applied the first wall will subjected by active earth pressure and the second end wall will subjected by passive earth pressure as per the basic theory. But at the same time the effect of wall will be more when the wall move towards the back fill compared to the wall movement away from the back fill. How do i consider this effect in staad pro. It is only possible when i use soil spring support. Is there any method is available to find the soil spring model for dynamic pressure. I already used MO theory to calculate the dynamic active and passive earth pressure, Can i use to calculate the spring constant by (Dynamic earth pressure x influence area of plate) /permissible deflection of wall....?
And the code does not specifies the effect of liquefaction of soil. How to consider the effect of liquefaction in the design. IF this liquefaction effect is only for stability check, what should we do if the structure is in liquefaction prone region.?
Kindly
N Manikandan 


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xuanyanzi SEFI Member
Joined: 29 Apr 2013 Posts: 3


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honey_verma General Sponsor
Joined: 31 May 2017 Posts: 2

Posted: Wed May 31, 2017 9:57 am Post subject: EQ load for underground pit 


Dear N Manikandan,
Did u find the solution for hydrodynamic earthquake pressure for irregular basin. right now i'm also facing the same problem. No proper guidance is available for irregular basin. it will be very kind if u can guide me in right direction.
Thanks in Advance
Regards,
Honey Verma


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