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Modifications of Elastic Drift for Inter-storey Drift Check in IS 1893 Part 1: 2016

 
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aditya
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Joined: 05 Apr 2008
Posts: 170

PostPosted: Tue Jun 30, 2020 3:21 am    Post subject: Modifications of Elastic Drift for Inter-storey Drift Check in IS 1893 Part 1: 2016 Reply with quote

Dear Sefians,
The Indian seismic design code IS 1893 Part 1: 2016 stipulates in Clause 7.11.1.1 that storey drift in any storey shall not exceed 0.004 times the storey height under the design base shear VB with partial safety factors for all loads taken as 1.0. Thus the code does not consider any modification of elastic drift calculated under seismic load whereas most of the international codes require modifications of this elastic drift for drift check.
However, the same code in Clause 7.11.3 states that the combined deflections of two adjacent buildings shall be multiplied by the Response reduction Factor "R" for checking separation between two adjacent buildings for seismic gap calculation. Even the proposed modifications by IITGN on the code contains the same provisions for the clause 7.11.1.1.
Why is it that drift ratio is to be determined without multiplication by "R" but seismic gap is to be determined after multiplying by "R"? Please explain.
with best regards,
Aditya
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Dr. N. Subramanian
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Wed Jul 01, 2020 3:20 am    Post subject: Re: Modifications of Elastic Drift for Inter-storey Drift Check in IS 1893 Part 1: 2016 Reply with quote

Dear Er Aditya,

As shown in pp. 726 of my book on Design Of Steel Structures, In ASCE 7-16 there is an amplification factor Cd for deflection, whch accounts for the increase in displacement due to the inelastic response of a structure that is not determined by elastic analysis and corrects for the reduction of forces introduced by response modification factor R. Hence deflection is calculated as D = Cd Dxe/I, where I is the importance factor, Dxe is the deflection determined from elastic analysis. For SMF Cd = 5.5 and for OMF Cd=3, Generally Cd is 1/2 to 4/5 the value of R. Generally the more ductile a system is, the greater will be the difference between R and Cd.

But it is difficult to compare the values of different codes. For example the R values are different in ASCE and IS 1893, The load factor for EQ is 1.0 in ASCE whereas it is taken as 1.2 to 1.5 in IS. Moreover in ASCE, the Design basis earthquake is 2/3rd of Maximum considered EQ and that factor is 0.5 in IS code.  Note that R values are considered both in ASCE and IS, though their actual values may differ.

The combined deflections of two adjacent buildings is multiplied by the Response reduction Factor "R" for checking separation between two adjacent buildings for seismic gap calculation. It is because to consider actual defection such that two adjacent buildings will not hit against one another during earthquakes.

Best wishes,
NS

aditya wrote:
Dear Sefians,
The Indian seismic design code IS 1893 Part 1: 2016 stipulates in Clause 7.11.1.1 that storey drift in any storey shall not exceed 0.004 times the storey height under the design base shear VB with partial safety factors for all loads taken as 1.0. Thus the code does not consider any modification of elastic drift calculated under seismic load whereas most of the international codes require modifications of this elastic drift for drift check.
However, the same code in Clause 7.11.3 states that the combined deflections of two adjacent buildings shall be multiplied by the Response reduction Factor "R" for checking separation between two adjacent buildings for seismic gap calculation. Even the proposed modifications by IITGN on the code contains the same provisions for the clause 7.11.1.1.
Why is it that drift ratio is to be determined without multiplication by "R" but seismic gap is to be determined after multiplying by "R"? Please explain.
with best regards,
Aditya
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