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Design of RCC post's foundation for lateral load

 
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ramlakhan_kumar
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PostPosted: Tue Jul 16, 2019 12:03 pm    Post subject: Design of RCC post's foundation for lateral load Reply with quote

I have to install sprinkler system for near Coal Mines area. For this it has been instructed to provide 4.00 meter height RCC post above ground level which would be circular in shape with día 0.35m.
I would like to ask my dear SEFIans that how would I determine the depth of foundation and also the size of foundation?
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mallesh_ng
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PostPosted: Wed Jul 17, 2019 11:28 am    Post subject: FOOTING DESIGN Reply with quote

Hi Ram, You have to decide based on soil SBC, Since its for sprinkler system footing size will be very less, but keep bottom of footing minimum 1m below as its not framed structure (Pedestal)it may rotate.

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Mallesh
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ramlakhan_kumar
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PostPosted: Wed Jul 17, 2019 1:10 pm    Post subject: Design of RCC post's foundation for lateral load Reply with quote

Is there any design criteria for such kind of structures in which vertical load is very nominal but there could be horizontal loads?
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rojanmathewtsy
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PostPosted: Thu Jul 18, 2019 6:43 am    Post subject: Re: Design of RCC post's foundation for lateral load Reply with quote

ramlakhan_kumar wrote:
Is there any design criteria for such kind of structures in which vertical load is very nominal but there could be horizontal loads?


Dear Ram,

The calculation is similar to the isolated footing having combined action of axial load and bending moments. Since you have less axial load, tension may arise in the foundation. So you have to go to a depth D such that the tension will be removed due to the selfwt of soil above the foundation.
That is you have to consider the load of soil in the axial load and so the sbc should be gross SBC. Not the NET SBC.

Rojan Mathew
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es_jayakumar
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PostPosted: Fri Jul 19, 2019 2:29 am    Post subject: Re: Design of RCC post's foundation for lateral load Reply with quote

ramlakhan_kumar wrote:
Is there any design criteria for such kind of structures in which vertical load is very nominal but there could be horizontal loads?



If you can assess the probable maximum horizontal force on the pole, we can design the foundation, as suggested by SEFIans above.

E S Jayakumar
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mallan
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PostPosted: Fri Jul 19, 2019 7:58 am    Post subject: Reply with quote

My two cents on the topic. I think you can use the weight of soil enclosed in a frustum of a pyramid given by a 45° line upward from the edge of your footing pad to resist the tension generated by overturning moments.
If I remember correctly, this is allowed for foundations of transmission line towers by a technical manual published Central board for irrigation.
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es_jayakumar
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PostPosted: Sat Jul 20, 2019 1:46 am    Post subject: Reply with quote

The frustum of soil resisting the uplift force or overturning shall have 300 inclination with the vertical. The half of the weight of the frustum is assumed to resist the overturning, acting through the heel point. In your case, there cannot be any axial tension on the foundation. The stability check can be done similar to that of a transmission line / communication tower foundation. The details can be read from :IS : 11233-1985, CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF RADAR ANTENNA, MICROWAVE AND TV TOWER FOUNDATIONS. Worked out examples for such structures (tower foundations) are available in the books of Varghese, Swami Saran etc. You can do stability check for your post foundation similar to that of a tower foundation, the exception being that there is no axial pull in your case.

E S Jayakumar
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mjnasar
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PostPosted: Wed Jul 24, 2019 12:58 pm    Post subject: Reply with quote

Eng.Kumar

I suppose basically check stability
by assuming same as  block foundation for Poles  

Considering active / passive earth pressure on the block

see snap shot  attached

regards



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es_jayakumar
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PostPosted: Wed Jul 24, 2019 3:05 pm    Post subject: Reply with quote

Your concept seems OK. What you have named passive earth pressure is actually active earth pressure and vice versa. For pole structures with only wind load as major lateral load, block foundation is appropriate. But, for structures with considerable lateral load like transmission line structures, it will be economical to use pad footing cast in depth, which derives lateral stability & pull-out resistance mainly from the back-filled soil above it.

E S Jayakumar
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