Timber Pile Lateral Analysis

[jgeng]

I have a structure supported by a grid of shallow (approx. 4.75ft to 5.75ft) embedded timber piles extending through 10ft of water braced above just above the water line. Some of the methods I am aware of for analyzing piles for lateral loads include the p-y method (I believe this is the method L-pile uses?) and the brohm's method. There appear to be many other methods/approaches as well and they all seem to give you an allowable lateral load capacity based on an allowable lateral deflection limit you set? In analyzing these shallow embedded timber pile I find the deflection is great for very small loads. If the piles are braced a distance above the soil so that the top of the pile could be considered fixed and you neglect soil resistance in providing lateral rotational restraint doesn't the frame boil down to a line of pin suported columns fixed at the brace location? and you can just anlayze your piles for bending and deflection assuming pin supported at bottom? I find it hard to believe the wind loads would drag my structure shearing the soil laterally? Also I was reviewing FEMA's "Design of Residential Buildings in High-wind coastal areas" and they have an example where they just calculate the "point of fixity" of the pile based only on the soil's modulus of subgrade reaction and the properties of the pile. They just use this depth (26.5 in in their example) as the fixed base support location for the column and go on analyzing the pile for bending. What are they skipping over here? What about the total embedment they don't even discuss it? What if I brace all the way to the soil so the pile can be considered fixed at the top...if the structure was braced like this I have even a harder time envinsioning the structure will fail by being dragged directly horizontal shearing the soil by wind loads? Also, as far as I am aware of building codes are silent on limits of lateral building drift with the exception of seismic areas. What are reasonble max limits for designing a residential type structure? Any help would be greatly appreciated.

 

[Splitrings]

ASAE EP 486.1 Shallow Post Foundations covers this topic. This document is available on the ASABE website. A more thorough discussion can be found in an abstract by Meador.

http://asae.frymulti.com/azdez.asp?JID=3&AID=21245&v=40&i=1&CID=t1997&T=2

Like you mention, the point of fixity is approximately H/3 below the ground surface. It can be calulated more precisely using either of the above documents. The required embeddment depth, H, needs to be calculated. Then you need to make sure your pile is stiff enough for the required embeddment. Once you have the H/3 depth, you have a simple frame with piles fixed at the base.

 

[rscassar]

The H/3 seems to be a long way below ground level to assume the point of fixity. 26.5" seems like a good assumption, I have always used 1.5*pile diameter for drilled piles (there is no real engineering science behind that figure). This whole area you are questioning seems to be a dark science, I have asked many questions regarding this in the past and have never received a definitive answer.

As for the building drifts for wind, I use H/250 for sensitive buildings such as residential and H/150 for steel framed industrial buildings.