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Binding reinforcements and bursting reinforcements of pile cap

 
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ghoshuttamkumar
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Joined: 25 Jan 2013
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PostPosted: Wed Jun 20, 2018 6:18 am    Post subject: Binding reinforcements and bursting reinforcements of pile cap Reply with quote

Hello SEFIANS,
I need your kind help to solve an issue of pile cap reinforcements. As per SP 34 , horizontal ties are required to resist bursting. In clause no. 6.9.4.2 it is mentioned that the reinforcements should be provided around outer piles. In the reinforcement details sketches (fig. 6.10) these rebars are provided like side face rebars. In fig 6.12, binding steel around projected pile bars are shown separately for a 3-piled cap. My question is whether both the binding steel(around pile bars) and bursting steel(as side face)are to be provided-if so there will be congestion of rebars. For your ready reference two pages from SP 34 are enclosed. Kindly help.

With best regards,

Uttam K Ghosh



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N. Prabhakar
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Joined: 25 Apr 2009
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PostPosted: Fri Jun 22, 2018 2:27 pm    Post subject: Reply with quote

Dear Er. Uttam K  Ghosh,

The details of pile reinforcement given in SP-34 is bit vague and not fully correct.  It is well known that for small sized pile caps, up to 5-6 caps, the load transfer to piles is by tie and strut action where the bursting force is resisted by bottom tension reinforcement and horizontal links provided around in the pile cap.  The bottom tie reinforcement is to be fully anchored at the ends by adequate bond length.  The area of horizontal links is to be 1/4 of the main tie reinforcement area (Ref.: Report of the Shear Study Group by A.L.L. Baker, C.W. Yu and P.E. Regan, IStructE Journal: The Structural Engineer, July 1969).

I am attaching herewith two sketch sheets showing typical reinforcement in a 3 pile cap which is included in the RCDesign software I have developed (Ref.: www.rcdesignspreadsheets.com).

I trust  you will find the above information useful.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208



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ghoshuttamkumar
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PostPosted: Tue Jun 26, 2018 10:58 am    Post subject: Reply with quote

Thank you very much Sir. It's a big help for me. Thanks again.

Regards,
UK Ghosh


N. Prabhakar wrote:
Dear Er. Uttam K  Ghosh,

The details of pile reinforcement given in SP-34 is bit vague and not fully correct.  It is well known that for small sized pile caps, up to 5-6 caps, the load transfer to piles is by tie and strut action where the bursting force is resisted by bottom tension reinforcement and horizontal links provided around in the pile cap.  The bottom tie reinforcement is to be fully anchored at the ends by adequate bond length.  The area of horizontal links is to be 1/4 of the main tie reinforcement area (Ref.: Report of the Shear Study Group by A.L.L. Baker, C.W. Yu and P.E. Regan, IStructE Journal: The Structural Engineer, July 1969).

I am attaching herewith two sketch sheets showing typical reinforcement in a 3 pile cap which is included in the RCDesign software I have developed (Ref.: www.rcdesignspreadsheets.com).

I trust  you will find the above information useful.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208
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Dr. N. Subramanian
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PostPosted: Wed Jun 27, 2018 2:31 am    Post subject: Re: Binding reinforcements and bursting reinforcements of pile cap Reply with quote

Dear Er Ghosh


The Indian code (IS 456) suggests that the pile cap to be designed using the sectional method (by the bending action). International codes, based on recent research are recommending the Strut-and-Tie Method. (see Section 15.9.1 of my RC Design book, which also contains an example designed by both these methods). The publication (M.S. Thesis) at http://publications.lib.chalmers.se/records/fulltext/132140.pdf,


gives extensive report of the struct-and-tie method of analysis for pile caps, some recent solutions and examples.


Naturally, the detailing of reinforcements in these two methods are different. Grid type reinforcement is used in sectional method and bans of reinforcements in strut-and-tie method. (see also Fig. 15.46 of my book).


While detailing according to sectional approach, a common mistake made bydesigners is to underestimate the need for anchorage of the flexural reinforcement atthe supports. One of the main advantages of the strut-and-tie model is that it clearlyindicates to the designer that tension is resisted in nodes, and thus the need ofanchorage is obvious.  Different anchorage solutions can be provided depending mainly on thegeometry of the pile cap and the state of stress close to the nodal zone. The designershould try to avoid cracking of concrete in nodal zones as well as cracking of theinclined strut when detailing the anchorage region.

To reduce the cracking in the nodal zone it is better to spread the flexuralreinforcement a bit around the node, thus confining the node and transferring aportion of the steel bars out of the nodal region. This arrangement will reduce tensilestrains and thus the tendency to crack  in the nodal region. It is important to note that the designer has to consider that spreading the reinforcement will create extratransverse tensile stresses in the vicinity of the node for which additional U-bendshave to be provided. The node region can also be confined using vertical stirrups,which is good for the anchorage.

It is important to keep the compressive strut with no or little cracking, as long as possible. If thecompressive strut is badly cracked, it will fail in a combination of splitting andcrushing before the full capacity of the main reinforcement can be used, resulting in ashear failure. A way to reduce cracking of the inclined strut in the web is to use stirrups with lowbond in the area were the stirrups cross the strut. This kind of reinforcement has apositive effect of reducing cracking in the web, but has a greater need for anchorage inboth the compressive and tensile chords. However, space is usually not a problem inpile caps were a lot of volumes with good confinement can be used for anchorage. More details of these confining reinforcements and the anchoring can be found in the cited thesis, which also has examples.

Best wishes
NS


ghoshuttamkumar wrote:
Hello SEFIANS,
I need your kind help to solve an issue of pile cap reinforcements. As per SP 34 , horizontal ties are required to resist bursting. In clause no. 6.9.4.2 it is mentioned that the reinforcements should be provided around outer piles. In the reinforcement details sketches (fig. 6.10) these rebars are provided like side face rebars. In fig 6.12, binding steel around projected pile bars are shown separately for a 3-piled cap. My question is whether both the binding steel(around pile bars) and bursting steel(as side face)are to be provided-if so there will be congestion of rebars. For your ready reference two pages from SP 34 are enclosed. Kindly help.

With best regards,

Uttam K Ghosh
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N. Prabhakar
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Joined: 25 Apr 2009
Posts: 436

PostPosted: Thu Jun 28, 2018 5:50 am    Post subject: Reply with quote

Dear Dr. Subramanian,

Thanks for your posting on Pile cap design,

The link http://publications.lib.chalmers.se/records/fulltext/132140.pdf, you have given in the posting for MS Thesis Publication shows error message when it is opened.  Please inform us the correct link.

With best wishes,

Yours truly,

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Fri Jun 29, 2018 2:11 pm    Post subject: Reply with quote

Dear Er Prabhakar,

Remove the comma at the end as below:
http://publications.lib.chalmers.se/records/fulltext/132140.pdf

It works

Regards
NS


N. Prabhakar wrote:
Dear Dr. Subramanian,

Thanks for your posting on Pile cap design,

The link http://publications.lib.chalmers.se/records/fulltext/132140.pdf, you have given in the posting for MS Thesis Publication shows error message when it is opened.  Please inform us the correct link.

With best wishes,

Yours truly,

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208
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ghoshuttamkumar
SEFI Member
SEFI Member


Joined: 25 Jan 2013
Posts: 5

PostPosted: Wed Aug 29, 2018 11:42 am    Post subject: Re: Binding reinforcements and bursting reinforcements of pile cap Reply with quote

Dear NS Sir,
Thank you very much for your reply with explanations.
Regards,
UKG

Dr. N. Subramanian wrote:
Dear Er Ghosh


The Indian code (IS 456) suggests that the pile cap to be designed using the sectional method (by the bending action). International codes, based on recent research are recommending the Strut-and-Tie Method. (see Section 15.9.1 of my RC Design book, which also contains an example designed by both these methods). The publication (M.S. Thesis) at http://publications.lib.chalmers.se/records/fulltext/132140.pdf,


gives extensive report of the struct-and-tie method of analysis for pile caps, some recent solutions and examples.


Naturally, the detailing of reinforcements in these two methods are different. Grid type reinforcement is used in sectional method and bans of reinforcements in strut-and-tie method. (see also Fig. 15.46 of my book).


While detailing according to sectional approach, a common mistake made bydesigners is to underestimate the need for anchorage of the flexural reinforcement atthe supports. One of the main advantages of the strut-and-tie model is that it clearlyindicates to the designer that tension is resisted in nodes, and thus the need ofanchorage is obvious.  Different anchorage solutions can be provided depending mainly on thegeometry of the pile cap and the state of stress close to the nodal zone. The designershould try to avoid cracking of concrete in nodal zones as well as cracking of theinclined strut when detailing the anchorage region.

To reduce the cracking in the nodal zone it is better to spread the flexuralreinforcement a bit around the node, thus confining the node and transferring aportion of the steel bars out of the nodal region. This arrangement will reduce tensilestrains and thus the tendency to crack  in the nodal region. It is important to note that the designer has to consider that spreading the reinforcement will create extratransverse tensile stresses in the vicinity of the node for which additional U-bendshave to be provided. The node region can also be confined using vertical stirrups,which is good for the anchorage.

It is important to keep the compressive strut with no or little cracking, as long as possible. If thecompressive strut is badly cracked, it will fail in a combination of splitting andcrushing before the full capacity of the main reinforcement can be used, resulting in ashear failure. A way to reduce cracking of the inclined strut in the web is to use stirrups with lowbond in the area were the stirrups cross the strut. This kind of reinforcement has apositive effect of reducing cracking in the web, but has a greater need for anchorage inboth the compressive and tensile chords. However, space is usually not a problem inpile caps were a lot of volumes with good confinement can be used for anchorage. More details of these confining reinforcements and the anchoring can be found in the cited thesis, which also has examples.

Best wishes
NS


ghoshuttamkumar wrote:
Hello SEFIANS,
I need your kind help to solve an issue of pile cap reinforcements. As per SP 34 , horizontal ties are required to resist bursting. In clause no. 6.9.4.2 it is mentioned that the reinforcements should be provided around outer piles. In the reinforcement details sketches (fig. 6.10) these rebars are provided like side face rebars. In fig 6.12, binding steel around projected pile bars are shown separately for a 3-piled cap. My question is whether both the binding steel(around pile bars) and bursting steel(as side face)are to be provided-if so there will be congestion of rebars. For your ready reference two pages from SP 34 are enclosed. Kindly help.

With best regards,

Uttam K Ghosh
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