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COMBINED FOOTING

 
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anitha harwal
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PostPosted: Mon Nov 14, 2016 11:41 am    Post subject: COMBINED FOOTING Reply with quote

Respected seniors,
1.I have combined footing to design with the load details as in attachment.
the problem i am facing is we dont have the staad advance foundation design licence to design the same as raft footing in advance staad foundation .what would be the alternative ,i tried to design it as a mat footing in staad but a fail.
2.And secondly wind and seismic laods are given in both + and - values how to treat these values while designing.
Regards,
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saikiran gone
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PostPosted: Tue Nov 15, 2016 7:14 am    Post subject: Re: COMBINED FOOTING Reply with quote

anitha harwal wrote:
Respected seniors,
1.I have combined footing to design with the load details as in attachment.
the problem i am facing is we dont have the staad advance foundation design licence to design the same as raft footing in advance staad foundation .what would be the alternative ,i tried to design it as a mat footing in staad but a fail.
2.And secondly wind and seismic laods are given in both + and - values how to treat these values while designing.
Regards,



Hi Anitha Harwal.

1. In my view, it is better to design the  combined footing for that structure , using manually or excel.

2.Regarding loads for a design, you consider the load combinations as per the IS 456-2000. If the structure is symmetric on both opposite faces it is better to take forces only in one direction either +ve or -ve.

The forces you attached is for which joint out of four joints.

With Best,
Saikiran.
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anitha harwal
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PostPosted: Wed Nov 16, 2016 4:53 am    Post subject: Reply with quote

Thanks for the reply sir,
the reaction given are on all the individual pedestals,and foundation has to be designed in a confined area.
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sakumar79
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PostPosted: Wed Nov 16, 2016 10:37 am    Post subject: Reply with quote

Dear Er Anitha,
       Since this is symmetric pedestal, I would consider,

1. Best this is done as a combined raft.
2. Below considerations may be impacted by SBC and depth of foundation as per soil report.
3. Create a excel sheet.
4. Add loads and their locations in tabular format.
5. Dead Min can be considered as Dead Load and Dead Max - Dead Min can be taken as Live Load.
6. Due to symmetry, we can consider WLX, WLZ, ELX and ELZ with only positive sign. However, vertical loads will be positive for 2 pedestals and negative for the other two. For example, in WLX and ELX, the two pedestals on the left will have uplift and the two on the right will have downward reaction. Similarly for WLZ and ELZ, upper pedestals will have downward reaction (considering up is +z) and lower two will have uplift.
7. Calculate moments at CG due to vertical loads and horizontal forces. Moments due to vertical components of DL and LL will be 0 due to symmetry and will be a couple for WL and EL - This will give MZ for WLX and ELX and MX for WLZ and ELZ. Moments due to horizontal components of all cases will be the Force x (Depth of Foundation bottom from reaction applied level), with MZ from X component and MX from Z component of the forces.
8. For a given raft footing size and thickness, calculate its area, Zxx, Zzz, Weight, Weight of soil above, etc. Include weight of all pedestals and footing in DL.
9. For each load combination, calculate P/A +/- Mxx/Zxx +/- Mzz/Zzz, check for P max <SBC> 0, sliding check and overturning check.
10. Design the footing for the maximum Pmax case.

Seniors may correct me if I am wrong.
Hope that helps.
Yours sincerely
Arunkumar
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PostPosted: Wed Nov 16, 2016 11:31 am    Post subject: Reply with quote

sakumar79 wrote:
Dear Er Anitha,
       Since this is symmetric pedestal, I would consider,

1. Best this is done as a combined raft.
2. Below considerations may be impacted by SBC and depth of foundation as per soil report.
3. Create a excel sheet.
4. Add loads and their locations in tabular format.
5. Dead Min can be considered as Dead Load and Dead Max - Dead Min can be taken as Live Load.
6. Due to symmetry, we can consider WLX, WLZ, ELX and ELZ with only positive sign. However, vertical loads will be positive for 2 pedestals and negative for the other two. For example, in WLX and ELX, the two pedestals on the left will have uplift and the two on the right will have downward reaction. Similarly for WLZ and ELZ, upper pedestals will have downward reaction (considering up is +z) and lower two will have uplift.
7. Calculate moments at CG due to vertical loads and horizontal forces. Moments due to vertical components of DL and LL will be 0 due to symmetry and will be a couple for WL and EL - This will give MZ for WLX and ELX and MX for WLZ and ELZ. Moments due to horizontal components of all cases will be the Force x (Depth of Foundation bottom from reaction applied level), with MZ from X component and MX from Z component of the forces.
8. For a given raft footing size and thickness, calculate its area, Zxx, Zzz, Weight, Weight of soil above, etc. Include weight of all pedestals and footing in DL.
9. For each load combination, calculate P/A +/- Mxx/Zxx +/- Mzz/Zzz, check for P max <SBC> 0, sliding check and overturning check.
10. Design the footing for the maximum Pmax case.

Seniors may correct me if I am wrong.
Hope that helps.
Yours sincerely
Arunkumar



Hi Arun kumar,

The procedure what you told for raft can be similarly followed for combined footing also (i.e) one footing for 2 joints.

In my view, it's easy to design the combined footing manually or by excel than raft foudation. But the final decision will completely depend on a designer.

With Best,
Saikiran.
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anitha harwal
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PostPosted: Fri Nov 18, 2016 10:09 am    Post subject: COMBINED FOOTING Reply with quote

Thank you so very much for your valuable reply sir.
But in point no -6 of saikiran sir i.e
6. Due to symmetry, we can consider WLX, WLZ, ELX and ELZ with only positive sign. However, vertical loads will be positive for 2 pedestals and negative for the other two. For example, in WLX and ELX, the two pedestals on the left will have uplift and the two on the right will have downward reaction. Similarly for WLZ and ELZ, upper pedestals will have downward reaction (considering up is +z) and lower two will have uplift.
How the symmetry affects the +/- sign of WL/EQ kindly explain.
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PostPosted: Tue Nov 22, 2016 10:37 am    Post subject: Re: COMBINED FOOTING Reply with quote

anitha harwal wrote:
Thank you so very much for your valuable reply sir.
But in point no -6 of saikiran sir i.e
6. Due to symmetry, we can consider WLX, WLZ, ELX and ELZ with only positive sign. However, vertical loads will be positive for 2 pedestals and negative for the other two. For example, in WLX and ELX, the two pedestals on the left will have uplift and the two on the right will have downward reaction. Similarly for WLZ and ELZ, upper pedestals will have downward reaction (considering up is +z) and lower two will have uplift.
How the symmetry affects the +/- sign of WL/EQ kindly explain.


Hi anitha ,

It is not exactly related to effects of lateral force. What exactly above point says is:

If the structure is symmetry in plan & elevation means the (say x direction) length of the structure is same on front and back side i.e 2.6 m on the front and back side it results in symmetry in plan .

Related to elevation the openings and walls (enclosures ) are same on front and backside . vice versa in y direction.

So , if structure satisfies the above criteria then if apply the forces in both directions (EQ+x & EQ-X) will results same as one direction load application.

1 2
3 4

if structure is symmetry in nature you apply loads in one direction only (+ve X or -Ve X). Results you will get same and you need to provide same foundation for (1& 2 ), (1&3) if combined footing (1+3) & (2+4).

With Best,
Saikiran.
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