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Using "Fixed But" Support Instead Of Fixed Support in G+3 RC Building with Isolated Footing

 
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ANINDITA
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Joined: 23 Aug 2018
Posts: 2

PostPosted: Mon Mar 28, 2022 11:18 am    Post subject: Using "Fixed But" Support Instead Of Fixed Support in G+3 RC Building with Isolated Footing Reply with quote

Is it better option to provide "Fixed But" Support condition in Staad Pro model while designing a RC building structure (g+3) with soil condition of isolated footing?? Or I should go with fixed Support only?
Also it will be very helpful to be guided in how to use "Fixed But" support? Should I only release My and Mz?
And How to put sub-grade modulus in the value box beside.
Any guidance or advice will be very helpful
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vikram.jeet
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Joined: 26 Jan 2003
Posts: 3462

PostPosted: Tue Mar 29, 2022 9:51 am    Post subject: Reply with quote

For Building columns on isolated footing :
Although column - footing joint is monolithic , the soil strata beneath footing needs to be seen , wether it allows slight rotation due to compatibility moment caused by gravity loading or not.

Rocky strata -  No rotation of footing , Fully fixed support

Hard soil strata -  some partial rotation is expected - Hence condition is in between fixed and hinged . Designer can judicially choose .

Medium and soft soils -  Soil allows rotation of footing and a slight rotation ceases compatibility moment under gravity loading . Hence Hinged baSe can be taken.

However in case of Equilibrium moments caused by horizontal forces due to wind / EQ , the condition of joint is fully fixed.

Irony of input data is that for different load cases viz gravity and EQ separate set of support conditions can not be  given in computer softwares and therefore fixed condition remains the only option left.
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