stability bracing for reducing unbraced length

[mikeCTE]

i am examining the capacity of existing timber pile.

we are demolishing a portion of a timber pier, leaving behind a few timber piles and pier. this will reduce/remove some of the inherent stability of the overall structure since the whole thing was previously tied into existing rigid structures. now, our strip of timber pile and deck will be free-standing.

to remedy this loss of overall stability, we are looking at adding x-bracing to the piles to decrease the unbraced length of each pile and effectively increase the allowable stress for the pile.

these piles only have vertical loading.

my question is: what load do we need to design the stability x-bracing to handle? i cannot find anything in the codes (AASHTO, NDS) that would give guidance on this. others in my office think 10% of vertical load seems like a reasonable number. i agree with that. but, is there code guidance on this subject?

thanks!

 

[WARose]

Even though it's for steel (I don't work with wood much), AISC gives guidance as far as required strength and stiffness goes.

If I understand you description correctly, it sounds like you are counting on the piles to brace themselves......if so, you'll need to account for that in checking them (i.e. the brace force will be an additional lateral load to consider as it will be a net force in a direction that could be additive to something like wind or seismic).

 

[SlideRuleEra]

You could go with the "2% Rule" (see below), but I would increase the value to at least 5% since timber piling are not as uniformly shaped or accurately placed as typical columns.

The single best step is to rigidly fix the pile butts into the superstructure. This nearly doubles the pile's buckling capacity.

In the past, design of pile bracing has be based on experience and engineering judgement, not code requirements.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[daranguiz]

Hello, maybe this can help. In some cases when you take into account imperfections, the 2% rule may not be enough.

This is from Stability of steel beams and columns - SCI.

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[Lomarandil]

I concur, to provide a brace point for the piles I'd design for something better than 2% of axial load (5% sounds good).

Big picture view question, are you looking to make the structure stable with this bracing, or only to increase the capacity of the piles?

You mention separating the pier from it's lateral support, but then say these piles only carry vertical load. Without some other stability system (moment frame, battered piles, etc), pile stability bracing may not be adequate for global stability of the structure.

(If you have that handled separately, excellent)

 

[mikeCTE]

Lomarandil:

we are trying to ensure the structure remains stable AND maintain adequate pile capacity.

it's one of those odd things when working with old structures that still somehow function despite calculations indicate they shouldn't. in this case, the piles are overstressed under the current load rating for the structure(HS-15). however, since the overall timber pier is functioning and stable in its current state, but we are removing some redundancy simply by removing portions of the pier, we're just trying to make sure we keep stability and timber pile capacity.

i'm sure it's overkill, but we don't like to make the news.

 

[Lomarandil]

By current load rating -- do you mean with or without being tied back to the rigid structure that was previously providing lateral support?

If the former, you potentially have a bigger issue of needing to provide some alternate global stability system.

If the latter, we're back to the bracing discussion -- and I think the advice above stands.

Regarding the discrepancy between the real-world and what calcs tell you, what are your geotechnical assumptions (and how confident are you in them)?

 

[Shu Jiang PEng SE]

There are both strength and stiffness requirement for bracing per appendix 6 "STABILITY BRACING FOR COLUMNS AND BEAMS" in AISC 360.