pile bracing requirements

[struct_eeyore]

Hi all.

For those of you familiar with the requirements for pile bracing... I've previously interpreted 1810.2.2 of the IBC, in a grade-beam/pile constriction context, as requiring the grade beams to be able to take lateral forces out from the pile at right angles. In particular, in any soil requiring piles (that soil is just assumed to be air), all the piles should be locked-in in this manner, outstanding the exception that allows you to stagger piles 1' o.c. along a grade beam; or, if you have a slab directly attaching to the piles/caps. Previous company I've worked at, this was the interpretation as well, and all our pile buildings were designed with this in mind. Yesterday I was reviewing a set of drawings that has gone out, and noticed the piles were not braced. The building is on organics. I brought it up with the senior engineer, and he dismissed this as not needing any additional bracing, stating that only piles above grade need lateral. In particular, the design as of right now has an ~40' grade beam, with piles equally spaced (the only grade beams that would provide bracing are at the ends of the beam); on top of the grade beam is a 6-7' stem wall with back-fill behind it - the stem wall is capped with a slab. The backfill presents another problem, in that I'm getting about 3k of lateral at the top of the pile, whereas the allowable (from geotech) is about 2. The total gravity load acting on the stem wall is about 3400 plf, so that will result in additional lateral as well from any eccentricity. Piles are augercast. So naturally, my question is: is there anything at all that I'm not considering that will make this ok? Can anyone comment on the likelihood and means of failure of such construction? And on a personal note - how much should I escalate this?

Thanks in advance

 

[jayrod12]

If you are confident in your numbers and your load path, I'd be taking it back to the senior engineer and explaining how you've analyzed it and why you are worried. You appear to have thought it out well, but when you discussed with him the first time did you have the analysis done or were you just commenting that you felt it needed bracing? If the latter, he may have just assumed that the piles could take the load laterally (i.e. less than 2k applied). In most cases a senior engineer's eyes are pretty keen, but every once in a while there's some quirk about a specific situation that catches them off guard.

How tight did you check your numbers? If you sharpened your pencil on both the applied loads and resistance calculations could you make it work?

 

[MIStructE_IRE]

Tomlinson’s foundations book states that single piles shall be tied in both directions. I always go by this. A single pile on its own is not adequately restrained in my view unless connected to an insitu slab.

A group of 2 piles (under pile cap) need only be tied one direction. A group of 3 piles (under cap) is considered inherently restrained.

 

[SlideRuleEra]

...is there anything at all that I'm not considering that will make this ok?

Does paragraph 1810.2.1 apply?

Use of augercast piles implies two things to me:

1) Soil must be pretty good, at least not "fluid", or augercast piles could not be constructed.

2) Augercast piles are probably constructed in undisturbed soil (not backfill), so lateral support from soil is available along the entire length of the pile.


Of course the piles have to be able to resist lateral load, but that is not the same thing as lateral bracing.

http://www.SlideRuleEra.net

 

[Phil1934]

does it meet exceptions of 1 or 2 story?

 

[BAretired]

The building is on organics.

That doesn't really tell us very much. As one geotechnical engineer once said to me "the mere fact that you can grow potatoes in it doesn't mean that it is not a good foundation soil". There are many cases in my area where there are organic materials in the soil, yet the soil appears to be firm and sufficiently competent to provide lateral bracing for drilled piles; it may even be adequate to support grade slabs or footings. Obviously, if the soil contains a very high percentage of organics, that could be a different story.

Usually, topsoil is removed from the site before construction begins but occasionally, there may be exceptions. In the end, it is a judgment call on the part of the geotechnical engineer.

BA

 

[struct_eeyore]

@ SlideRuleEra - 1810.2.1 is about the bracing of the pile for buckling - 1810.2.2 is about stability (and the fact that it is not a subsection makes me think the provision is independent of the one preceeding it). The way I interpret it is that having grade beams in two directions creates a pin condition at pile top - vs having grade beam in a single directions allows the pile to cantilever out in the non-braced direction. Using a soil as a brace against buckling, imo, does not imply that the soil can carry lateral loads, in particular those generated by any eccentric loading at pile cap, which would be redistributed as axial loads when a brace/grade beam is added.

 

[SlideRuleEra]

Structee - I agree that the two paragraphs are independent, and said so in my previous post. However, the two paragraphs work together... if the piles adequately braced for bucking (1810.2.1) but remain insufficient for applied lateral load (1810.2.2) it is time to consider changes to the design. Two straightforward revisions come to mind:

1) Increase the number of (vertical) augercast piles use to increase total lateral load resistance. Of course minimum pile spacing has to be maintained.

2) Since batter augercast piles are typically not practical, use driven piling for the project (some or all battered) to resist lateral load.

Since you know all project details, you can probably come up with other (and better) solutions. The point is to "move on" with the design. Appears the building code is doing it's job... warning that the proposed design will not work.

http://www.SlideRuleEra.net