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approach for handling two different foundation support conditions of building model in staad-pro
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MBR05
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PostPosted: Tue May 17, 2022 3:46 pm    Post subject: approach for handling two different foundation support conditions of building model in staad-pro Reply with quote

Hello All,
I have a RCC building structure with isolated footing (considered fixed support) and combined footing (with 2 or More columns) model with plate mesh with sub-grade.
What shall be the correct approach for handling two different foundation support conditions of building model in staad-pro.
Option -1 Provide fixed-but support to isolated footing column ends with spring value calculated by footing area (after first trial run), SBC and allowable settlement and for plate mesh support shall be elastic mat with SBC/ allow settlement.
Option -2 Provide fixed support to all columns, prepare separate plate mesh model for combined footing/ mat and transfer support reactions and analyse it ( I use to work with options but time consuming exercise)

Thanks,
Manoj
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vikram.jeet
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PostPosted: Wed May 18, 2022 2:34 am    Post subject: Reply with quote

Just a generic  view :

Fix footing areas for footings based on net SBC for both isolated and combined footings as per manual working  , adopt single approach for both .
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MBR05
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PostPosted: Wed May 18, 2022 5:50 am    Post subject: Reply with quote

vikram.jeet wrote:
Just a generic  view :

Fix footing areas for footings based on net SBC for both isolated and combined footings as per manual working  , adopt single approach for both .



Thank you very much Sir.
From this, I understood one single approach can be providing fixed support to all column Staad model and design foundations ( isolated & Raft) manually.
What if we have structure where in water tank connected to few outer columns (tank raft is at different level ), a retaining wall on three sides along with columns and building structure.
Can you please guide in simplifying the analysis.  
Pls. refer image.



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vikram.jeet
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PostPosted: Wed May 18, 2022 7:12 am    Post subject: Reply with quote

Unless full free hand sketch of Water tank with its raft and other cols footing is given , difficult to appreciate the arrangement. If possible pl give.
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MBR05
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PostPosted: Wed May 18, 2022 8:02 am    Post subject: Reply with quote

Dear Sir,
Pls find attached Typ. section. through building and tank.



Thanks,
Manoj



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vikram.jeet
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PostPosted: Wed May 18, 2022 11:55 am    Post subject: Reply with quote

Firm soil is available at 1.5m below as per isolated footings foundation shown in sketch.

It will be wise to either keep retaining wall base at 1.5m below , keeping tank base at same level as shown   0r in present arrangement provide 1.5m thick Lean Pcc 1:4:8  of width about 0.6H or so ,  (H being ht of rt wall)  beneath ret wall cum tank base  - periphery. Idea is to transfer loadings at firm soil level.

Since it is Underground tank (and not OH) , structure  can be jointly analysed.
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MBR05
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PostPosted: Wed May 18, 2022 1:06 pm    Post subject: Reply with quote

[quote="vikram.jeet"]

Thank you Sir,
Structure is being jointly analysed and tank raft support is plate mat with spring based on net SBC
(NSBC/Allow settlement ) and columns will have fixed-but support with spring value calculated by footing area
(after first trial run), NSBC and allowable settlement. With this approach my observation is isolated footings
designed with fixed support required more size than that with spring support.


Can you pls suggest the support type for tank raft and columns while analysing tank and structure jointly.
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vikram.jeet
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PostPosted: Wed May 18, 2022 4:54 pm    Post subject: Reply with quote

Just to say further that :
Since structure house Water tank with outer wall also subject to earth pressure , the structure need to be checked for tank full condition as well as tank empty condition with one horz force acting at a time I.e in tank full water pr without outside earth pressure  and under tank empty full earth pressure +any Live load surcharge  with due consideration to rise of subsoil water. Uplift force on base of tank also act.
Water tank portion need to be designed as uncracked sec in accordance with IS 3370.

Also it needs to be seen that soil at 1.5 m below has different spring value than soil at water tank base. The upper soil strata will be having smaller SBC.
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MBR05
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PostPosted: Wed May 18, 2022 6:39 pm    Post subject: Reply with quote

Yes sir, basic load cases and combinations are considered accordingly.
My concern is regarding support type to be used for tank raft and columns while analysing the structure jointly.
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vikram.jeet
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PostPosted: Thu May 19, 2022 1:33 am    Post subject: Reply with quote

Soil taken as springs , then it need to be modelled for raft as also for col footing as springs.

The retaining wall need to be designed for EP rest coeff  in working condition .
But site may like to do filling behind wall first , prior to casting tank  roof slab/beams and such a situation must be addressed , in in such case Retaining wall is a cantilever retaining wall with active earth pressure coeff.

Encountered such situation while designing a CW Reservoir for WTP wherein Site In charge wanted filling behind wall prior to casting of roof.
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