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FOUNDATION DESIGN
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harish_purohit
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PostPosted: Tue Feb 21, 2017 3:42 pm    Post subject: FOUNDATION DESIGN Reply with quote

Dear all
As per code 1893 while designing the foundations with earthquake loads, the bearing pressure can be increased by 25%  or 50%, depending upon the type of soil and type of foundation . Using this criteria(for the worst load combination)the size of foundation is coming less than the service condition size, (DL+LL). Now, pl let me know which condition we have to take and why ? Now a days, every building is designed considering earthquake force.If (DL+ LL) is correct then why the code is allowing increase in soil pressure. And why we should not design for DL+LL+ EQ .

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HARISH PUROHIT
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Abishek_Siingh
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PostPosted: Wed Feb 22, 2017 6:29 am    Post subject: Reply with quote

This is a very interesting topic and I dint have clarity over this for a long time but following is my understanding of the subject:

From Geotechnical Point of View: For dense sands, rocks etc the bearing pressure increase by 25% (50% for raft) can seem to be logical but for loose sands, clays and other soils having large flow this may not be suitable. This is clearly specified in 6.3.5.2 of IS 1893 where in alternative foundation types like deep piles are advised.

Experiments have suggested (cant remember who did those) that the dense sand theory is actually wrong. Under earthquake local shear failure happens in dense soil etc and the capacity actually reduces by 30% or so. So increase in bearing capacity should be taken with caution and with due consultation of soil engineers.

From ASCE load combination (and natural logic) point of view: The ASCE says that when two transient loads are acting simultaneously, in our case LL and EQ, then both cannot have their full value acting together. So to mitigate that ASCE reduces the EQ and LL under overturning case by 25%. Hence their combination reads as

DL+0.75LL+0.75EQ which is quite logical.

In Indian code, using same common sense, if you see we have DL+0.8LL+0.8EQ .

This is under serviceability condition but you must remember that increase in bearing pressure under earthquake has to be taken for serviceability load combinations only and we only do a pressure check but not provide steel for that. If the pressure check fails then we increase the area of footing.

Now either you reduce the load, that is take 0.8LL and 0.8EQ into account, then no need for earthquake pressure check because 1/0.8 = 1.25 which means increase in 25% pressure. So either you increase the bearing capacity by 25% under full loads or you reduce the loads in the load combinations and no need to do earthquake increased pressure check.

Hope this helps,

Abhishek
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harish_purohit
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PostPosted: Wed Feb 22, 2017 9:57 am    Post subject: Reply with quote

thanks abhishek for your reply ,but i think this subject need some more clarifications from our seniors.

harish purohit
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bijoyav
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PostPosted: Wed Feb 22, 2017 2:14 pm    Post subject: Reply with quote

Design of foundation (bearing area and type- not the structural design of foundation elements) is done NOT on Characteristic Load applied with Partial Safety factors. Instead, FACTOR OF SAFETY(FOS) is applied on Bearing Capacity to arrive at Safe Bearing Capacity. Normally the FOS for foundation is 3, i.e., 300%, because of high uncertainty. Since Earth Quake is less probable in Zone I & II, the IS code, 1893, allows 25% increase in pressure.
Note that this allowance is not allowed in zone III, IV, & V, where SPT is less than 15 and also in Zone II, where SPT is less than 10. Also, the places where liquifaction can happen. (6.3.5 - Increase in permissible Stresses)

Bijoy AV
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pankajshah
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PostPosted: Wed Feb 22, 2017 5:44 pm    Post subject: Reply with quote

1. Adopt size which is safe in all loading combinations. It is not necessary that combinations having EQ or Wind loads will give higher size of footing than DL+LL case.

2. Kumar Abhishek Singh has explained the concept behind this.
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kishorpss
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PostPosted: Fri Feb 24, 2017 8:52 am    Post subject: Reply with quote

Dear Senior Sefians,

I request you all to give your valuable comments in this connection. When we are designing foundations for structures subjected to wind or eartquake by manually with help of excel spreadsheets or by help of softwares, large size foundations are comming due to tension developed at base of footings. To avoid this non-practicable size, how would you be modifying it to accepted sizes ?

During designing of footings ( Isolated/ combined), I am considering the effects of these following outputs of reactions FX, FZ, FY, MX & MZ generated in softwares. Also I am considering the enhancement of net SBC (from 25% to 50%) and reduction of live loads as per guideline of code.

Enhancement of net SBC will have no effect when there is negetive pressure develops in base of foundation (tension develops at base).

Suggestings are most welcome as SEFI is the pool of expert professionals.

Brgds,

Kishore
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sakumar79
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PostPosted: Fri Feb 24, 2017 12:36 pm    Post subject: Reply with quote

Dear Er Kishore,
     You can make use of the weight of soil above to counteract the uplift due to bending. This can further be enhanced considering the angle of internal friction to take not just the weight of soil directly above, but the soil covered by the fructum of cone starting with the footing base and projecting out. This methodology is covered under NPTEL IIT Madras course Design of Steel Structures-II under "Foundation of Towers" subheadings "Soil resistances for designing foundation" and "Design procedure for foundation"

Hope this helps
Arunkumar
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kishorpss
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PostPosted: Sat Feb 25, 2017 2:51 am    Post subject: Reply with quote

Dear Er. Arunkumar,
Thanks a lot. I have also already applied the weight of soil covered by the fructum of cone starting with the footing base and projecting out .Still my problem is unsolved. I had cross checked many tower foundation drawings issued by companies, where foundation sizes are smaller than my calculated sizes. I want to know the theory behind that.

I need your valuable openion for the foundation design for earthquake loads, where negetive pressure develops due to FX, FZ, MX & MZ even FYs are in positive value.

Thanking you.

Brgds,

KISHORE
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harish_purohit
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PostPosted: Sat Feb 25, 2017 5:24 am    Post subject: Reply with quote

Thanks KISHORE u have made my point more Clear . I am also facing the same problem ,designing foundation with excel sheet to avoid negative pressure, we have to increase the foundation size,I am using software for design , it takes maximum value of   P MX AND MZ out of the 13 load combinations ( with increase soil capacity by 25%) When I compare it with simple DL+ LL the size is less. Why it so ??


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HARISH PUROHIT
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kishorpss
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PostPosted: Sat Feb 25, 2017 6:33 am    Post subject: Reply with quote

Dear Er.Harish,

Sorry to intimate you that your problem & my problem are different. We have to design the foundation for the worst load case among all load cases.

Your word "it takes maximum value of   P MX AND MZ out of the 13 load combinations" is not clear to me. Design of foundation by taking maximum values of P, MX & MZ out of all load combinations is not the correct procedure. I have been found some structural engineers are adopting this procedure while cross checking their calculation sheets.

Foundations are to be designed for each load cases. Sizes of foundation such as Length, Breadth & Depths are to be finalised, which are safe in all loading combinations. Softwares have been prepared adopting the same procedure. Cross check your design calculation sheet once again, definitely you will be arrived with a best conclusion.

Thanking you.

Brgds,

Kishore
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