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Stiffness modifier and cracking in composite Pile

 
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Sunate
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PostPosted: Thu Oct 07, 2021 11:10 am    Post subject: Stiffness modifier and cracking in composite Pile Reply with quote

Hi Sefians

I am designing a pile-supported raft foundation system based in South America https://speedtest.vet/ https://vidmate.bid/.
This pile will be a cast-iron hollow tube (170x7.5), installation is such that pile will be hammered in ground with a conical shoe at bottom(320 dia) at the same time concrete will be poured with pressure inside pile and allowed to go out above shoe around cast iron thus making system 320 dia with concrete outside and inside cast-iron pile.
concrete is without any reinforcement and there are lateral loads. Design code is ACI and it says to use stiffness modifier. I understand there will be no cracking in concrete inside tube.
What will be composite behavior of this pile?
Should I consider outer concrete to be fully cracked?
or use modifier only for outer concrete?
or convert this entire composite section to an equivalent concrete section and apply modifier after that?
what kind of modeling will capture actual behavior of this system?


Last edited by Sunate on Fri Oct 08, 2021 11:49 am; edited 1 time in total
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vikram.jeet
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PostPosted: Fri Oct 08, 2021 4:31 am    Post subject: Reply with quote

I think The structural components resisting loads and moments are steel shell pipe and concrete inside it which is confined in the shell. The outer concrete is not treated to be structural , but protective covering around steel pipe. Stiffness is provided by composite sections of (Pipe Pile + inside concrete).

Bonding between outer concrete shell and steel pipe pile is not appreciable for considering structural strength , but for pile capacity of composite pile , the outer concrete may contribute in increasing perimeter ,thus,  skin friction area. But here also it is to be seen that capacity shall be based on lesser of two:
(I) Friction between steel pipe  perimeter area and concrete
(ii)Friction between soil and outermost concrete perimeter area
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pvgraju
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PostPosted: Sun Oct 10, 2021 1:50 pm    Post subject: Stiffness modifier and cracking in composite Pile Reply with quote

Dear sir,

Pile may behave like circular concrete section with hollow tube as continuous reinforcement.

Regards

P.V.Gavarraju
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vikram.jeet
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PostPosted: Sun Oct 10, 2021 6:01 pm    Post subject: Reply with quote

Pile is definitely a circular section but reservation is on outer concrete which is unreinforced ,  has got no circular ties resisting lateral force as result of axial compression on pile . The outer concrete will try to split out and totally dependent on soil around which is not stiff enough to stop splitting . Also the moments on pile due to EQ may cause high tensile stresses in outermost fiber , may  not be controlled by outer PCC.

Just a personal view , as not dealt such situation .
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pvgraju
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PostPosted: Tue Oct 12, 2021 2:45 am    Post subject: Stiffness modifier and cracking in composite Pile Reply with quote

Dear sir,

Pile capacity is based on skin friction and end bearing.Only casing cannot generate skin friction.But out side concrete cover can generate skin friction.This will help to reduce pile foundation depth. Then if there is any doubt on cover then hollow tube filled with concrete can be taken as structural member.It is to be designed as per the codal guidelines which speaks about members like hollow tube filled with concrete.Then suitable stiffness modifiers to be applied as per that code instead of ACI.

Regard,

P.V.Gavarraju
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vikram.jeet
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PostPosted: Tue Oct 12, 2021 3:43 am    Post subject: Reply with quote

Yes Er Gavarraju saheb , appreciate your to the point response.

Concern is only about outer concrete ring which is unreinforced.
Even in a normal RCC pile , the pile capacity is based on skin friction from perimeter area of concrete  ( +point berg) , but steel reinforcement  in form vertical bars and circular ties is provided as per  str design of pile to take care  of vertical compressive loads as well as EQ moments .

Plain concrete piles (without any reinforcement) are not there in  pile types , may be for reasons of splitting since soil around pile is not stiff enough to prevent such splitting.

In present case steel ring  plus inside concrete is structural member  but for pile capacity outer concrete ring may count as discussed in first response fm my end.

Best regards
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