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Design of Footing for EQ resistant structures
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hemal
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Joined: 01 Apr 2008
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PostPosted: Sun Oct 01, 2017 7:36 am    Post subject: Reply with quote

Dear sir,

Recent IS 1893_2016-1 requirement for modeling foundation fixity is as below:

"6.1.5 The soil-structure interaction refers to effects of the flexibility of supporting soil-foundation system on the response of structure. Soil-structure interaction may not be considered in the seismic analysis of structures supported on rock or rock-like material at shallow depth."

While requirement of older code IS 1893_2002-1 for the same is as below:

"6.1.4 Soil-Structure Interaction
The soil-structure interaction refers to the effects of the supporting foundation medium on the motion of structure. The soil-structure interaction may not be considered in the seismic analysis for structures supported on rock or rock-like material."

If we compare both codal provisions critically. We can conclude that base can be fixed only if we have  foundation at shallow depth on rock or rock like material as per new code. We even need to consider soil structure interaction if we have deep (say pile) foundation in rock.

As per old code base can be fixed for shallow or deep foundation in rock.

Regards
Hemal Mistry
Surat
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hemal
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PostPosted: Sun Oct 01, 2017 10:16 pm    Post subject: Design of Footing for EQ resistant structures Reply with quote

Dear sir,

Recent IS 1893_2016-1 requirement for modeling foundation fixity is as below:


"6.1.5 The soil-structure interaction refers to effects of the flexibility of supporting soil-foundation system on the response of structure. Soil-structure interaction may not be considered in the seismic analysis of structures supported on rock or rock-like material at shallow depth."


While requirement of older code IS 1893_2002-1 for the same is as below:


"6.1.4 Soil-Structure Interaction
The soil-structure interaction refers to the effects of the supporting foundation medium on the motion of structure. The soil-structure interaction may not be considered in the seismic analysis for structures supported on rock or rock-like material."


If we compare both codal provisions critically. We can conclude that base can be fixed only if we have foundation at shallow depth on rock or rock like material as per new code. We even need to consider soil structure interaction if we have deep (say pile) foundation in rock.


As per old code base can be fixed for shallow or deep foundation in rock.


Regards
Hemal Mistry
Surat


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hemalmistry
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Joined: 01 Jul 2009
Posts: 16

PostPosted: Sun Oct 01, 2017 10:32 pm    Post subject: Design of Footing for EQ resistant structures Reply with quote

Dear sir,

Recent IS 1893_2016-1 requirement for modeling foundation fixity is as below:


"6.1.5 The soil-structure interaction refers to effects of the flexibility of supporting soil-foundation system on the response of structure. Soil-structure interaction may not be considered in the seismic analysis of structures supported on rock or rock-like material at shallow depth."


While requirement of older code IS 1893_2002-1 for the same is as below:


"6.1.4 Soil-Structure Interaction
The soil-structure interaction refers to the effects of the supporting foundation medium on the motion of structure. The soil-structure interaction may not be considered in the seismic analysis for structures supported on rock or rock-like material."


If we compare both codal provisions critically. We can conclude that base can be fixed only if we have  foundation at shallow depth on rock or rock like material as per new code. We even need to consider soil structure interaction if we have deep (say pile) foundation in rock.


As per old code base can be fixed for shallow or deep foundation in rock.


Regards
Hemal Mistry
Surat

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vijayarathnam
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Joined: 16 Jul 2011
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PostPosted: Tue Oct 03, 2017 1:44 am    Post subject: Reply with quote

The suggestion is for Soft soils we can have pinned support and for rock or hard soil we may adopt fixed condition. Is that what is inferred?
But we see different people have different views on the same issue.Its very difficult to draw a conclusion.

Vijayarathnam
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Yogesh.Pisal
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Joined: 18 May 2008
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PostPosted: Tue Oct 03, 2017 3:45 am    Post subject: Reply with quote

Dear Er Vijayarathnam,

Cl 6.1.4, IS1893 Part 1 : 2016
Soil structure interaction may not be considered in seismic analysis of structures supported on rock or rock like material at shallow depth.

Cl 7.2.5, IS1893 Part 1 : 2016
Sa/g shall be estimated corresponding to 'T' of structure considering soil structure interaction.

My views
1. Cl6.1.4 and Cl7.2.5 are having contradiction. Cl6.1.4 gives exemption for rock or rock like material whereas Cl7.2.5 do not propose such exemption.
2. As per Cl6.1.4 exemption is allowed for shall depth only. This is again qualitative term, hence difficult to interpret.
3. Hence, I think it is better to consider spring supports to all structures (excluding minor structures).

Best Regards,
Yogesh Pisal
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hemal
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PostPosted: Tue Oct 03, 2017 5:37 pm    Post subject: Reply with quote

For equivalent static method generally approximate time period from imperical equations is used, this time period is not affed by soil structure interaction. Time period calculated from modal analysis which is used in dynamic method is affected by soil structure interaction.

According to cl. 6.4.3 (page 14) static method can only be used for regular structures having appx. time period less than 0.4 sec.

According to cl. 7.7.1, dynamic analysis is required for all building except regular building having height less than 15 m in zone 2.

Combining cl. 6.4.3 and 7.7.1, dynamic analysis is required for all buildings except regular building with height less than 15 m in zone 2 with appx time period less than 0.4 sec.

Now, combining 6.1.5, 7.2.5, 6.4.3 & 7.7.1, soil structure interaction is not required for regular buildings founded on rock or rock like material at shallow depth having height less than 15 m in zone 2 with approximate time period less than 0.4 sec.

Regards
Hemal Mistry
Surat
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Yogesh.Pisal
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Joined: 18 May 2008
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PostPosted: Wed Oct 04, 2017 2:55 am    Post subject: Reply with quote

Dear Hemal,

I agree with the fact that SSI will not affect the response of buildings founded on rock or rock like material.

But, as there is scope for difference in opinion in codal provisions itself, we need to be more careful while choosing the method when the structure going to be reviewed by somebody else.

Best Regards,
Yogesh Pisal
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sankar
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Joined: 26 Jan 2003
Posts: 25

PostPosted: Thu Oct 05, 2017 12:15 pm    Post subject: Reply with quote

Yogesh.Pisal wrote:
Dear Hemal,

I agree with the fact that SSI will not affect the response of buildings founded on rock or rock like material.

But, as there is scope for difference in opinion in codal provisions itself, we need to be more careful while choosing the method when the structure going to be reviewed by somebody else.

Best Regards,
Yogesh Pisal




Guideline is based on  velocity of seismic shear wave through the founding media. Generally, SSI is not required to be considered/has negligible effect if Vs through the supporting subgrade  is >= 760 m/s (hard rock).
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vijayarathnam
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Joined: 16 Jul 2011
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PostPosted: Tue Oct 10, 2017 12:49 pm    Post subject: Reply with quote

I would appreciate if somebody can clarify:
1. can we conclude that  from the above discussion,  for rock like base soil-structure interaction is required for soft soils it is not necessary
2. If we want to establish soil-structure interaction how do we ensure?
3. Is it  by providing Fixed support conditions?


Vijayarathnam
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