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SUBGRADE REACTION ON RAFT

 
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apoorv0289
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Joined: 02 Mar 2012
Posts: 4

PostPosted: Fri May 24, 2019 9:43 am    Post subject: SUBGRADE REACTION ON RAFT Reply with quote

Hello Every One,
I am doing a raft having load of 1345906 Kn (Gravity) and SBC of soil is 180 Kn/sqm
For the above load i need 7477 sqm but i'll provide 8953 sqm
for analyzing and design i'll use safe  
Now in my model soil pressure in center of raft is greater then SBC but outer layer of my raft is in safe zone and deflection of raft is also less then 75 mm

So is their any way to over come from this situation or this can be ignored...



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Anunay_Jain
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Joined: 04 May 2019
Posts: 5

PostPosted: Sun May 26, 2019 5:05 am    Post subject: Reply with quote

Hello Apoorv,

I went through your SAFE file. Here are some of the things that I can conclude:

1) For COMBINATION - 2 that is the factored load combination, the Soil Reaction exceeds the SBC quite drastically. (Even reaches a value close to 370 kN/m2). Hence the design is extremely unsafe.

2) There is differential settlement in some parts of the Raft foundation because the settlement varies from 70 mm to 40 mm in quite a less distance.

3) You might want to check the point loads that you have defined in the SAFE file because as I went through the point loads and some values seemed unrealistic unless a very heavy lateral load is acting on the structure.

Please share the ETABS file for load calculation. I might be able to see that and confirm if the loads have been applied correctly.

Moreover, if a lateral load is acting then you can increase the SBC as per codal provisions.

4) In case the load application is OK, then in order to bring down the soil reactions and take care of the differential settlement you may go for the following:

a) Increase the depth of the RAFT (I wouldn't personally recommend that because the depth is already 950 mm)

b) Provide a grade beam connecting columns with heavy moments in X and Y direction (provide grade beam in both directions). This would take care of the moments.

c) You might want to explore providing deep foundation for some of the heavy columns in case either of the above two options don't work.

In case of any other discussion, feel free to email me at anunay.jain.civ15@iitbhu.ac.in

P.S. - If you wish, I can work with you in designing this foundation. You can check out my bio on https://www.linkedin.com/in/anunay-jain-a2ba40104/

Thanks and regards,
Anunay Jain
Senior Undergraduate Student
IIT (BHU) Varanasi
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apoorv0289
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Joined: 02 Mar 2012
Posts: 4

PostPosted: Tue May 28, 2019 6:56 am    Post subject: SUBGRADE REACTION ON RAFT Reply with quote

i use dl+ll load combination
and my question is all about up thrust reaction of raft base
as per book Soil Mechanics & Foundation engg. By Dr. K.R.Arora sir
in Chapter 23
topic 23.3 Gross & Net footing Pressures
(2) foundation not back filled

Upthrust must be less or equal to gross SBC

So in my case SBC is 182.3
& my raft top is usable no back fill

according to book i'll calculate Gross SBC which is about 260
so what should i doo check with Gross sbc or SBC
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Anunay_Jain
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Joined: 04 May 2019
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PostPosted: Tue May 28, 2019 7:09 am    Post subject: Reply with quote

Approv,
If you have considered only gravity load in the analysis then how come the columns are subjected to such heavy moments (1000 - 2000 kN.m and even more) ?
Such heavy moments in both directions on interior columns can be there only when lateral load is acting on the structure. So unless you share the load analysis files, I cannot help with anything.

Regards,
Anunay Jain
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rojanmathewtsy
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Joined: 21 Apr 2013
Posts: 29

PostPosted: Wed May 29, 2019 5:47 am    Post subject: Re: SUBGRADE REACTION ON RAFT Reply with quote

apoorv0289 wrote:
Hello Every One,
I am doing a raft having load of 1345906 Kn (Gravity) and SBC of soil is 180 Kn/sqm
For the above load i need 7477 sqm but i'll provide 8953 sqm
for analyzing and design i'll use safe  
Now in my model soil pressure in center of raft is greater then SBC but outer layer of my raft is in safe zone and deflection of raft is also less then 75 mm

So is their any way to over come from this situation or this can be ignored...



Dear Apoorv,

The base pressure is resultant of both axial loads and bending moments.You have considered axial loads only in your area calculation. This can be done to get an approximate size of the raft. So, You need to change the raft layout to accommodate the high base pressure which is a result of combined action of both gravity loads and moments.
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apoorv0289
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Joined: 02 Mar 2012
Posts: 4

PostPosted: Mon Jun 03, 2019 2:31 pm    Post subject: Re: SUBGRADE REACTION ON RAFT Reply with quote

rojanmathewtsy wrote:
apoorv0289 wrote:
Hello Every One,
I am doing a raft having load of 1345906 Kn (Gravity) and SBC of soil is 180 Kn/sqm
For the above load i need 7477 sqm but i'll provide 8953 sqm
for analyzing and design i'll use safe  
Now in my model soil pressure in center of raft is greater then SBC but outer layer of my raft is in safe zone and deflection of raft is also less then 75 mm

So is their any way to over come from this situation or this can be ignored...



Dear Apoorv,

The base pressure is resultant of both axial loads and bending moments.You have considered axial loads only in your area calculation. This can be done to get an approximate size of the raft. So, You need to change the raft layout to accommodate the high base pressure which is a result of combined action of both gravity loads and moments.






Dear Sir,

Yes sir i totally agree with you
and i'll check area requirment for
P/A  +- M/Z
BUT  
In my case if i'll increase area soil pressure is not change in center area & i'll try with only Gravitational loads nature of the raft is same...
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rojanmathewtsy
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Joined: 21 Apr 2013
Posts: 29

PostPosted: Tue Jun 04, 2019 9:34 am    Post subject: Re: SUBGRADE REACTION ON RAFT Reply with quote

apoorv0289 wrote:
rojanmathewtsy wrote:
apoorv0289 wrote:
Hello Every One,
I am doing a raft having load of 1345906 Kn (Gravity) and SBC of soil is 180 Kn/sqm
For the above load i need 7477 sqm but i'll provide 8953 sqm
for analyzing and design i'll use safe  
Now in my model soil pressure in center of raft is greater then SBC but outer layer of my raft is in safe zone and deflection of raft is also less then 75 mm

So is their any way to over come from this situation or this can be ignored...



Dear Apoorv,

The base pressure is resultant of both axial loads and bending moments.You have considered axial loads only in your area calculation. This can be done to get an approximate size of the raft. So, You need to change the raft layout to accommodate the high base pressure which is a result of combined action of both gravity loads and moments.






Dear Sir,

Yes sir i totally agree with you
and i'll check area requirment for
P/A  +- M/Z
BUT  
In my case if i'll increase area soil pressure is not change in center area & i'll try with only Gravitational loads nature of the raft is same...


Raft foundation design is based on flexible theory. It is not based on rigid theory where the base pressure is assumed to be uniform.So in raft , the base pressure may vary depend on the loading condition. You can increase the thickness in the locations where soil pressure is more, that will reduce the base pressure. You can provide grade beams connecting columns or u can go for pedestals.

Rojan Mathew
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