Reinforcement in pile caps greater than 36"

[BAGW]

Hi,

If a pile cap is greater than 36" will skin reinforcement per deep beam ACI section be required? Pile cap is 10'-0" x 10'-0" x 4'-0".

Seems like the skin reinforcement is more for beam section where the width is much smaller. CRSI manual has design examples where the pile cap is 54" deep and no skin reinf

Thanks

 

[dik]

I'm not sure what the ACI requirement is... but, I wouldn't think it necessary. With concrete mass over 3' thick, you may have to look at the requirements for mass concrete and use thermocouples to track temperature differentials.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[WARose]

As far as I know, the skin reinforcement requirements of ACI are applicable to things like beams, walls, panels, etc. (Although the requirements for walls and panels generally govern.)

I've typically avoided this issue by having the slab cantilever far enough out (or, in the case of mats, be big enough) to where the forces/stresses/strains are low enough to where cracking won't be a issue at the edges.

 

[HTURKAK]

ACI 318 requires skin reinforcement in all beams that are deeper than 36 in.

This is copy and paste from ACI 318 -14

( 9.7.2.3 For nonprestressed and Class C prestressed beams with h exceeding 36 in., longitudinal skin reinforcement shall be uniformly distributed on both side faces of the beam for a distance h/2 from the tension face. Spacing of skin reinforcement shall not exceed s given in 24.3.2, where cc is the clear cover from the skin reinforcement to the side face....)

IMO, protection of pile cap surface with protective coating or membrane protection is more meaningful...

 

[r13]

I usually wrap the pile cap in a steel cage. It is just a personal preference, not required/directed by code.

 

[BridgeSmith]

Don't know about ACI, but our reading of the AASHTO provisions is that the cap is a flexural member, so skin reinforcement is required if the cap is over 36" high.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[skeletron]

I think it still applies and would treat it as @r13 is suggesting (or how I interpret it). Wrap the top bars down and the bottom bars up to create a cage.

 

[HTURKAK]

I agree with r13, skeletron and bridgesmith providing reinforcement cage to all faces in both ways is common practice and provision of AASHTO . Just for curious, ACI defines skin reinforcement as additional longitudinal reinforcement in deep beams, between the neutral axis and the tension face. I do not know ,is it correct to interpret the reinforcement cage as skin reinforcement or not..

 

[r13]

Note that deep beam provisions has limited applicability for pile caps design, as cap for pile group is more like two way slab. My reasoning for the "steel cage" is to increase the confinement on the highly compressed/stressed concrete core, so to lengthen its service life.

 

[BridgeSmith]

I do not know ,is it correct to interpret the reinforcement cage as skin reinforcement or not..

Not sure how it reads in ACI, but AASHTO is fairly clear that the skin reinforcement may supersede other requirements, but the requirements are not in addition to other steel requirements. It also is clear that, unlike the shrinkage and temperature steel, it's only longitudinal steel.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BAGW]

thanks all

 

[MSUK90]

So, for conclusion, skin reinforcement in pile cap is not described in code and it can be avoided right?
I was following this post to clear this issue; as for big projects, it may result in a lot of cost saving.

 

[HTURKAK]

My interpretation of ACI 318 for the pile cap design is, satisfy the shear strength requirement for one way or two way shear ( if two way is applicable) and choose the reinforcement for flexural. That is, design for for flexure and shear at assumed critical sections .

If a pile cap is designed and satisfy two conditions, ACI will be a good shield . But , IMO,in order to sleep with comfort, a reinforcement cage should be provided for pile caps having more than 3 piles and for two piles case, the cap should be designed as beam element and minimum transversal shear reinforcement should be provided.

 

[BridgeSmith]

My understanding of AASHTO is that if the pile cap is designed as a flexural member, (with flexural reinforcemnt), the skin reinforcement provisions apply.

As close as we put the piles to the edge of our pile caps, I wouldn't be comfortable without reinforcement in the faces for confinement of the piles, to at least as high as the top of the piles.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[HTURKAK]

I have an old versİon of AASHTO bridge design specification.

My interpretation is , the strut-and-tie model is proposed for the design of deep footings and pile caps which the distance between the centers of applied load and the supporting reactions is less than twice the member thickness and Crack Control Reinforcement is defined for the footing which have been designed in accordance with STM . The minimum steel ratio in the vertical and horizontal
direction 0.003.
That is , IMO, the designer is free to choose STM and not to provide proposed crack control reinforcement .

Pls correct me if i am wrong..

 

[BridgeSmith]

HTURKAK, the STM design is a whole different animal than traditional flexural design. I haven't really dug into it yet, but I remember some from a class I took last year. I don't think the skin reinforcement provisions (or crack control) from the traditional flexural design apply to STM. STM has it's own provisions and requirement to provide adequate confinement and crack control.

STM has been recommended for design of deep beams and footings, and could be required in the just-released 9th Edition of AASHTO.

Rod Smith, P.E., The artist formerly known as HotRod10