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Design of Footing for EQ resistant structures
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vijayarathnam
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PostPosted: Fri Sep 22, 2017 6:12 am    Post subject: Design of Footing for EQ resistant structures Reply with quote

Dear Sirs,
Kind Attn:Dr.N.Subramanian

Which is the better way of modelling the shallow foundation systems for Earthquake resistant structures.
1.Providing grade beam of sufficient stiffness at plinth level and treating the Footing as hinged. Allowing the moments to be carried by these beams and axial forces by footing.The axial load can be either gravity alone or envelope of gravity and seismic loads. (or)
2.Keeping the Footing as fixed  allowing all the gravity and seismic load combinations together.There will be a plinth beam too.

3.I read from Dr.Subramanian's book Pg 623 (Design of RCC Structures). that when grade beams are provided only axial loads from gravity and seismic needs to be considered.
4.However, some Engineering Institutions to whom we approach for vetting insist on option at Sl No.1.This leads to unrealistic design especially when building is at Zone V and SBC in the order of 75KN/Sqm.

seeking for Expertise opinion from learned members.

With regards!

Vijayarathnam
Globe Consultancies, Port Blair, Andaman.
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sakumar79
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PostPosted: Sat Sep 23, 2017 3:37 am    Post subject: Reply with quote

Dear Sir,
     If you refer the book "Some Concepts in
Earthquake Behaviour of Buildings" by CVR Murthy et al, Page 36 under "
Effect of Degree of Fixity at Member Ends" discusses that "
when the flexibility of soil underneath the footings of columns allows rotation of the columns; this happens when individual footings are used. Highly flexible soils make column bases as good as hinged, and rocky layers below as good as fixed. The
extent of fixity at column bases controls overall behaviour of buildings
"

    A fixed base reduces the sway and column reinforcement while pinned base makes the foundations more economical. Fixed base may be used only if the foundation is on hard strata (or for structural stability purposes)

   In your case, you have indicated the soil to be basically of very mediocre capacity (loose to medium sand, etc). Hence, I think a pinned base is appropriate.

  Do also note that you should also check for sliding (may not be very critical in RCC buildings) even if you have pinned base.

  I am also confused by your question wording "Providing grade beam of sufficient stiffness at plinth level" - what is SUFFICIENT and who decides it?

Hope this helps
Arunkumar
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vijayarathnam
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PostPosted: Sat Sep 23, 2017 3:47 am    Post subject: Reply with quote

Dear Arunkumar sir,
I meant provision of designed Plinth Level Beams which acts as grade beam.

Regards
Vijayarathnam
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Dr. N. Subramanian
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PostPosted: Mon Sep 25, 2017 6:37 pm    Post subject: Re: Design of Footing for EQ resistant structures Reply with quote

Dear Er. Vijayarathnam,

Modelling of the fixity at the bottom of columns is always a problem for structural engineers. There was a big debate in this forum between me and Prof. ARC. However no conclusions were reached.

As Er. Arun has suggested in Non-earthquake zones or Earthquake zones, we may assume  fixed end conditions for columns if it is founded on Rock, Pile or Raft foundation.

In Non-earthquake zones  we may assume  pinned end conditions for columns if it has individual footings and the soil is compressible and the frames do not extend through floors below grade. You may refer the book by Prof. CVR Murty et al. suggested by Er Arun-It is free to download at: http://www.iitk.ac.in/nicee/IITK-GSDMA/EBB_001_30May2013.pdf

One drawback of the pinned base condition is that the drift of the frame, esp. the inter-storey drift at the lowest storey is difficult to control within code allowable limits. Pinned base may lead to the development of soft or weak stories, which are prohibited in IS 1893 (Part 1).

If drift of the structure exceeds acceptable limits, then rotational restraint can be increased at the foundation by adding grade beams or by considering soil springs to simulate foundation flexibility.

Of course, the statement in page 623 of my book is from the work of Paulay and Priestley 1992. It only says that the footings can be designed for axial force and the grade beams can absorb the bending moments resulting from EQ. I suggest you to provide grade beams(not small beams as in plinth beams, but with a depth of about 450 mm), consider this in analysis also. You will see that the foundation will be subjected to mainly AF.

For moment frame exceeding through floors below grade a complex assumption of vertical and horizontal soil springs are to be taken in the analysis. I  assume that the current software package have facility to handle such soil springs. The stiffness of soil spring to be considered is another big discussion. You may browse through this forum to get the results. You may also refer Bowles book for a simplified treatment.
Best wishes
NS

"
vijayarathnam wrote:
Dear Sirs,
Kind Attn:Dr.N.Subramanian

Which is the better way of modelling the shallow foundation systems for Earthquake resistant structures.
1.Providing grade beam of sufficient stiffness at plinth level and treating the Footing as hinged. Allowing the moments to be carried by these beams and axial forces by footing.The axial load can be either gravity alone or envelope of gravity and seismic loads. (or)
2.Keeping the Footing as fixed  allowing all the gravity and seismic load combinations together.There will be a plinth beam too.

3.I read from Dr.Subramanian's book Pg 623 (Design of RCC Structures). that when grade beams are provided only axial loads from gravity and seismic needs to be considered.
4.However, some Engineering Institutions to whom we approach for vetting insist on option at Sl No.1.This leads to unrealistic design especially when building is at Zone V and SBC in the order of 75KN/Sqm.

seeking for Expertise opinion from learned members.

With regards!

Vijayarathnam
Globe Consultancies, Port Blair, Andaman.
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N. Prabhakar
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PostPosted: Tue Sep 26, 2017 5:34 am    Post subject: Reply with quote

Dear Er, Vijayarathnam, Er. Arunkumar and Dr. NS,

I would like to add, in order to consider lateral resistance of foundation with horizontal soil springs, the foundation is to be cast against the virgin soil, i.e. not in over-sized excavation or back-filled soil.  A note to this effect is to be added on the foundation drawing for proper execution at site.  

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208
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vijayarathnam
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PostPosted: Tue Sep 26, 2017 5:41 am    Post subject: Reply with quote

Thanks Er.Prabhkar sir.

Vijayarathnam
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knsheth123
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PostPosted: Tue Sep 26, 2017 1:13 pm    Post subject: Foundations : Fixed or Pinned Reply with quote

Respected Senior SEFIANs,

A generalised guideline is to use Pinned base for low SBC soil support and use Fixed base for High SBC Soil or Rock.

The rotation at base depends on stiffness of Soil, Foundation and Columns meeting at the base.

Nowadays software have provision to idealise soil as springs. All raft foundations are designed using this concept. Same concept can be extended to isolated footings by modelling their size and thickness with soil support as springs. To carry out this modelling one need size of Footings. Hence a preliminary analysis with Fixed /pinned support is required, footings are proportioned and then iterate to final design.
(Pl. note : Winkler's Spring model is a crude representation of soil strata. Hence one need to solve for range of values learning some basic Geotechnical guidelines-IS 2950 / Bowles etc., Minimum 3 values to be used.)

To overcome the strict Audit requirements, provide Beams connecting all the columns at the foundation level as we do for strap beams or Beam & Slab type Combined Footings and keep the support at column points Pinned. Thus EQ moments are transferred to column till foundation level and Beam moments satisfy equilibrium. Relative Stiffness of the Beams and Column defines rotation of the support. This solution is easy to adopt when the complete plan area is excavated. This is also a best solution for all soil with low SBC <10 T/m2. It accommodates differential settlement also.

This is my perception having of a blend of Structural and Geotechnical background.  

Based on illustrative problems worked out for M.Tech. Students, fixed base solution remains conservatively close to spring solution for SBC > 20 T/m2.

K. N. Sheth
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Yogesh.Pisal
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PostPosted: Wed Sep 27, 2017 3:50 am    Post subject: Reply with quote

Dear Er Vijayarathnam,

I prefer following philosophy for the considering foundation stiffness.

1. Analysis 1: Assign fixed support irrespective of tie / Grade beams.

2. Work out foundation sizes based on the forces from Analysis 1

3. Analysis 2: Assign spring supports as per Tb 13, IS1893 Part 4: 2015

4. Compare support reactions from Analysis 2 with Analysis 1

5. If there is substantial difference in support reactions, redesign
the foundations.

Further, Cl7.2.5, IS1893 Part 1: 2016 makes it mandatory to use the time period evaluated by considering soil structure interaction.

Best Regards,
Yogesh Pisal
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aditya
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PostPosted: Wed Sep 27, 2017 5:45 am    Post subject: Reply with quote

Dear Respected K. N. Sheth Sir,
Thanks a lot for your suggestion on this important topic. Would you please attach a diagram for elaborating your following suggestion regarding modeling of foundation beam and column in a software-" provide Beams connecting all the columns at the foundation level as we do for strap beams or Beam & Slab type Combined Footings and keep the support at column points Pinned. Thus EQ moments are transferred to column till foundation level and Beam moments satisfy equilibrium".
I am confused as to whether at the foundation level, the foundation beam and column will be rigidly connected at below this level, foundation top will be pinned to this arrangement in computer analysis model?
Please kindly explain.
Thanks and regards,
Aditya
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vijayarathnam
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PostPosted: Wed Sep 27, 2017 7:54 am    Post subject: Reply with quote

Yes that will be a great idea. Bcos again we see different people model the structure in many different ways using the software. Understanding the software of its full capability itself is a herculean task. Sketches with illustration will make easy for understanding.

Vijayarathnam
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