## Allowable Stress Design/Rating of Driven H Piles

## Allowable Stress Design/Rating of Driven H Piles

(OP)

Hi all, I'm looking at an old bridge pier foundation, with a pile cap supported on H-piles. The piles go mostly through sand and are tipped in shale. On the structural capacity side, using allowable stress design, the AASHTO Std. Specs (which I'm less familiar with compared to the LRFD) limit you to 0.25Fy (9 ksi for A36 steel, or a little higher if certain conditions are met). My impression of the 0.25Fy limit has more to do with installation of the pile (e.g. providing an extra factor of safety of a little more than 2 over 0.55Fy), but does anyone know if that is the case? It seems to be a bit restrictive compared to 0.55Fy allowed on a compression member that's ~fully braced.

## RE: Allowable Stress Design/Rating of Driven H Piles

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Allowable Stress Design/Rating of Driven H Piles

The maximum allowable stress on a pile shall not exceed the following limits in severe subsurface conditions. Where pile damage or deterioration is possible, it may be prudent to use a lower stress level than the maximum allowable stress. For steel H-piles, the maximum allowable stress shall not exceed 0.25Fy over the cross-sectional area of the pile, not including the area of any tip reinforcement. The maximum allowable stress may be increased to 0.33Fy in conditions where pile damage is unlikely.This is taken from a very old, 1992 AASHTO manual. For new work, you probably need to use newer or current AASHTO manual specifications.

www.PeirceEngineering.com

## RE: Allowable Stress Design/Rating of Driven H Piles

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Allowable Stress Design/Rating of Driven H Piles

https://www.fhwa.dot.gov/bridge/lrfd/us_dsp.cfm#de...

Or in Bethlehem Steel's old H-Pile Manual where for a pile with "entire length embedded in any soil or combination of soils, other than virtually fluid material...the pile is supported throughout its length, and no reduction in load is required because of slenderness ratio." And so they end up a with a capacity based on a fully supported column. I think normally in the past when I've looked at stuff like this I've assumed an unbraced length based on soil-structure interaction results from something like LPILE or FB-Multipier using one of the many methods to determine a theoretical point of fixity, or assuming some depth to a point of fixity and finding the effective unbraced length using an assumed K between 1.2 and 2.1 depending on the pile head fixity.

PEinc -- right, if this were for new design definitely I would use the newest edition of the LRFD specifications. We're just being asked to evaluate these old existing H-piles using a service load/allowable stress methodology. I guess I was wondering if anyone had any insight as to the "why" of 0.25Fy (or 0.33Fy) for piles specifically.

For example, was it the case that engineers used to just find the pile reaction from simplified analysis (Pile Reaction = P/# piles + M*x/Iy_pilegroup + M*y/Ix_pilegroup) and they used a lower allowable stress because they weren't accounting for any soil-structure interaction or moment in the piles? In the "pile damage is unlikely" case where you can use 0.33Fy, it seems inconsistent that you'd allow such a low percentage of yield, where if you considered these piles as a typical compression element, the allowable stress would be a function of 0.55Fy and the slenderness ratio, which will probably result in an allowable closer to 0.55Fy than 0.33Fy. So I was thinking the 0.25 (or 0.33) allowable may've been some way to conveniently account for those kind of effects, or to sort of guide the engineer toward choosing a pile that wouldn't have issues during driving.

## RE: Allowable Stress Design/Rating of Driven H Piles

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Allowable Stress Design/Rating of Driven H Piles

The resistance factors for LRFD are similarly low. We use 0.5 Fy, based on the driving measurement techniques we specify.

## RE: Allowable Stress Design/Rating of Driven H Piles

## RE: Allowable Stress Design/Rating of Driven H Piles

## RE: Allowable Stress Design/Rating of Driven H Piles

## RE: Allowable Stress Design/Rating of Driven H Piles

If that could be assumed, there would be no reason to limit the design capacity to the.25A*Fy in the first place. I'm fairly sure the design assumption is that the .25A*Fy is the geotechnical resistance limit after driving.