One way shear in pile slab 1

[Aakalim103]

We have a pile slab that is supporting water tank walls and two columns. The piles are fairly evenly spaced but the spacing is decreased around the columns and wall corners. Minimum pile spacing is around 1500mm. Piles size is 350mm x350mm and the pile slab is 800mm thick. The pile slab has been designed for moments and punching shear however the one way shear was never checked. The Fem calculation model is actually giving quite high one way shear values in the slab which exceed the shear capacity of slab without shear reinforcements. I have asked for advice in my office and most of our senior engineers are of the opinion that in such a pile slab we don't check one way shear. I would assume that if the slab is designed conventionally as opposed to strut and tie model, the one way shears should be checked. But I am not sure how to go about checking one way shear in such a slab. Any advise would be appreciated.

 

[HTURKAK]

Dear Aakalim,

I looked for one way shear and performed a small hand calculation ..One way shear, in order to be critical ,water ht should be more than 15 m..I will suggest you to refine your FEM model ( with 350 mm square elements ) and supported on pile springs ( four springs at each pile location) and run again..

Good luck..

 

[centondollar]

Do the shear check; if the analysis model is linear-elastic and design for moment is done by e.g. Wood-Armer ("beam" flexural rebar calculation in two orthogonal directions, with twisting moment included), you obviously also need to make sure that the slab can resist shear force. I would do the shear design for a unit width strip of the slab ("unit width wide beam") using the principal shear force (generates by your FEM-calculation) as the design shear force.

 

[Aakalim103]

I looked for one way shear and performed a small hand calculation ..One way shear, in order to be critical ,water ht should be more than 15 m..I will suggest you to refine your FEM model ( with 350 mm square elements ) and supported on pile springs ( four springs at each pile location) and run again..

Thank you for the response and the quick calculation Actually the pile slab is not just taking the water loads; the top slab of the water tank is part of the building floor system and the floor is quite heavily loaded. so that load will be carried through the walls to the pile slab in addition to the water load. The actual water height is around 3.5m.

 

[Aakalim103]

Do the shear check; if the analysis model is linear-elastic and design for moment is done by e.g. Wood-Armer ("beam" flexural rebar calculation in two orthogonal directions, with twisting moment included), you obviously also need to make sure that the slab can resist shear force. I would do the shear design for a unit width strip of the slab ("unit width wide beam") using the principal shear force (generates by your FEM-calculation) as the design shear force.

Thanks for the reply. Yes the analysis model is assumed to be linear elastic and slab moments are analyzed using the wood and armer method. I have the principle shear forces from the FEM model but the shear should be checked at a distance of d from the support. So does this mean that I check the shear around the pile at a distance d in all directions?

 

[centondollar]

The thumb-rule "check shear at distance "d" from support" relates to the fact that close to the support, shear is transferred primarily in an arching-mechanism (strut and tie model: compression struts towards support) in addition to the other 3 shear resistance mechanisms (dowel action of main rebar, aggregate interlock along cracks, elastic shear capacity of uncracked part of section). If stirrups are needed, the arching action also dominates the capacity near support (compression struts to support at distances "x<d"), and so "check shear at distane "d" from support" is valid advice for that case also. Here, I have used the word "support" to mean a discrete support, such as a pin, roller or fixed boundary condition for single or multiple-span beam or slab, and it is in that context that the thumb-rule can be safely used.

Your slab does not technically have any "support" in the classical textbook sense of the word (think pin, roller, rigid), since it is supported by an elastic support, e.g., a Winkler spring on the slab surface. Therefore, I would not recommend to design against a smaller shear force taken at distance "d" from a column. Shear failure is abrupt, and thus conservative design is preferred. In my opinion, slightly too many stirrups in a thin slab or a "too thick" slab that does not need stirrups are valid design options in tricky cases.

 

[Aakalim103]

Your slab does not technically have any "support" in the classical textbook sense of the word (think pin, roller, rigid), since it is supported by an elastic support, e.g., a Winkler spring on the slab surface. Therefore, I would not recommend to design against a smaller shear force taken at distance "d" from a column. Shear failure is abrupt, and thus conservative design is preferred. In my opinion, slightly too many stirrups in a thin slab or a "too thick" slab that does not need stirrups are valid design options in tricky cases.

I think we will have an elastic support only in the case of a friction pile. In our project, all piles are end bearing pile and are supported on solid rock. I believe in that case we can consider the piles to be pinned supports?

 

[-SPi]

One-way shear is real. Sure, it may not be the critical case, but one cannot just ignore it.
If this was my design, I would:
[ul]
[li]Check both one-way shear (allowing for shear enhancement) and two-way shear.[/li]
[li]If it's a deep pile cap (slab), then use the strut-and-tie method. With a shallow cap, the section method (FEM model with shell elements) is fine. But with a deep cap it may underestimate the reinforcement; a strut-and-tie model will provide the "correct" lower-bound solution.[/li]
[/ul]