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Use different R values for foundation and superstructure

 
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shakya
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PostPosted: Tue Sep 23, 2014 1:33 am    Post subject: Use different R values for foundation and superstructure Reply with quote

Foundations are designed for the reaction forces from the superstructure. The reaction forces becomes smaller if the superstructure is designed as SMRF and becomes higher if superstructure is considered as OMRF.
So is it OK to design the superstructure as OMRF however, the foundation of the same structure is designed for the reaction forces obtained for the SMRF. If it is OK, then the foundation sizes can be reduced.
What happens if the situation is reversed i.e. superstructure is designed as SMRF and the foundation is designed for the reaction forces of OMRF.

I hope for some expert advise.
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nimish.khanolkar
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PostPosted: Tue Sep 23, 2014 8:20 am    Post subject: Reply with quote

"So is it OK to design the superstructure as OMRF however, the foundation of the same structure is designed for the reaction forces obtained for the SMRF. If it is OK, then the foundation sizes can be reduced"


Why should this be ok? From where is this coming?
The superstructure would dump the reactions which would be a function of what the superstructure is. A SMRF superstructure would dump SMRF base shear on foundations.
An OMRF superstructure would dump OMRF shears.
An SMRF superstructure would NOT dump OMRF base shear on the foundations .
An OMRF superstructure would NOT dump SMRF base shear on the foundations.
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Manoharbs_eq
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PostPosted: Tue Sep 23, 2014 11:30 am    Post subject: Reply with quote

Is codes does not clearly specify the for this kind of situations.
However, the type of frame has lot of effect on lateral load carrying capacity.So the SMRF is detailed effectively to transfer the forces.The OMRF is simple arrangement.

This R value is specified for structure, so the R value of the conservative one shall be considered for design.


Rgds
Manohar
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mtamil
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PostPosted: Wed Sep 24, 2014 6:43 pm    Post subject: Reply with quote

Hi Shakya,

As long as you are consistent with your approach, it is ok.

If the super structure is designed and detailed as SMRF then the siesmic response will be lower, therefore,  the resulting foundation forces shaĺl be used for foundation design.

If the super structure is OMRF, then corresponding foundation forces shall be used to design the foundations.

Mixing up things appears unfounded.

Regards
Tamilarsan
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shakya
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PostPosted: Sat Sep 27, 2014 7:48 am    Post subject: Use different R values for foundation and superstructure Reply with quote

Thank you very much for the kind responses.

nimish.khanolkar
"An SMRF superstructure would NOT dump OMRF base shear on the foundations .
An OMRF superstructure would NOT dump SMRF base shear on the foundations."

I completely agree with you but my one of the confusions/questions is does really the ratio of 5/3 (R for SMRF/ R for OMRF) for the base shear coefficient occurs when a building is subjected to the same earthquake.

Let us assume that a SMRF and OMRF buildings are designed for earthquake level (Sa/g = 2.5) and also assume that these buildings experience an earthquake which has spectral acceleration (Sa/g) <=2.5 so that both buildings behave elastically. So in this situation, the ratio of base shear coefficients Ah(OMRF)/Ah(SMRF) (Note that not the ratio of base shear) will be 1 or 5/3? I think it will be 1 (I may be wrong though).
R value completely depends on the detailing of structures rather than the force/acceleration input.
Particularly for steel buildings/structures the ductile detailing of joints and the fabrication are time consuming and cumbersome eventually may not be cost effective. Hence, to avoid the tedious procedure and to save the space and cost of piling, I am searching for the possibility of designing superstructure as a OMRF and designing the foundation for lower reaction forces (V(SMRF)).

Manohar
"This R value is specified for structure, so the R value of the conservative one shall be considered for design."

Yes if we strictly follow the code provision, conservative solution is always preferred. But is it always a better solution? Can't we search for a better engineering solution.

One of the reasons why I raised this issue is to have some productive discussion about the current code provisions.

mtamil
"If the super structure is OMRF, then corresponding foundation forces shall be used to design the foundations.
Mixing up things appears unfounded."

Yes IS is silent about it but the Canadian Code provides some conservative solution and for your kind reference I have copied the statement from the code which is as follows:

1) For anchored footings or elements other than foundations supporting the walls or frames, a factored resistance to develop the maximum load effects determined with loads calculated using RdRo = 1.3.

2) Where footings supporting the walls or frames are unanchored, a factored resistance to develop the maximum load effects determined with loads calculated using RdRo = 2.6 for Rd greater than or equal to 2 and RdRo = 1 .3 for Rd less than 2.

Both of these provisions are leading to conservative design of foundations.

RdRo is equivalent to R in IS.
Rd = 1.5 and Ro is 1.3 for conservative construction i.e. equivalent to OMRF in IS. RdRo is 1.95 for the design of superstructure but considered 1.3 for foundation.
Rd is >= 2 for depending on the nature of ductile detailing.


Kind Regards
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