Pile cap design - BS and IS codes

[slickdeals]

Folks,
In the BS and IS codes, the shear stress is computed as a function of the amount of tension steel provided. Pile caps can be designed with a shear enhancement when they are closer to the support using (2*d/av) * tau.

However, if I make a section deeper to reduce the amount of flexural steel then I will need more steel added in order to achieve the same allowable shear stress as with a shallower section.

It seems more prudent to provide additional shear reinforcing (for one way shear) than to go through trying to make it deeper.

I am very used to the ACI method and this seems like I have to learn a new approach to design.

 

[ishvaaag]

Quite likely don't like the more fragile failures of scarcely reinforced sections.

 

[asixth]

Don't want to sound like "that guy" who always say that pile caps should be designed by strut-tie methods but I firmly believe that pile caps need to be designed by strut-tie.

When you cut a cross section at the face of the pile or the face of the column and draw a free body diagram of the internal stress distribution assuming beam-flexure behavior, you assume that the concrete develops a compressive stress block at the top of the pile cap which in my opinion cannot be achieved.

The behavior of a pile cap is to fan the load out from the column to the piles. The tension steel at the bottom is designed to take this outwards thrust and the compression stress in the cap needs to be designed using strut-tie checks (?s*Ac*0.6*fc).

Normally the compression stress at the top of the piles is okay because the piles designed using a minimum eccentricity of 2 or 3" are be limited to a stress around 0.15-0.2*fc of the pile. Under the column where the concrete is designed using a minimum eccentricity of 0.05*D (of the column), the stress in the concrete is normally around the 0.25-0.35*fc of the column. As this load works its way through the pile cap addition ties around the column starters may need to be provided to confine the pile cap concrete and increase the design compression stress.

 

[ishvaaag]

Really the traditional ways are consistent with what asixth says, deep pile caps, steel designed by trigonometry, akin to strut and tie method, and significant cage steel at least to the perimeter (for say a limited number of piles, say, 5). In the buildings we practice here I have not had to enter the realm of pile caps with many piles, as for bridge foundations, so I have not an immediate mental idea of any classical way of proportioning the steel for the pile caps, even if having resources out there for such things.

So axisth is in the side of conventional practice for what I remember of most old. Since this surely was also the case elsewhere, if some code prefers more steel it needs be on ductility behaviour being preferred, more or less the same as in corbels where the insistence in having steel in the web parts, both horizontal and vertical, must have been derived from some set of ugly cracks seen along time. Anyway it is unlikely any modern code pushing much the envelope of shear stress on any section, if something, the contrary should be more common, so perhaps they are trying to get some ductility, more than abandoning strut and tie methods.

 

[apsix]

Even though vc is lower due to reduced steel percentage (by BS notation) the increase in section area should more than make up for that and I'd expect the shear force capacity to be greater.
I'd definitely try to avoid shear reinforcement in pile caps.

I assume that you are aware of Cl.3.4.1.1; the flexural method of design is not valid for deep beams where the clear span is less than 2x the effective depth.