King Post wall

[EireChch]

I am doing a preliminary design of a king post wall. I have determined the overturning moment as normal for a 1m width section (into the page). The posts are spaced at 2m c/c. My question is, how do you account for post spacing?

For a 2m spacing, should I multiply the overturning moment by 2?

I’m thinking should I therefore multiply the restoring moment by 2? I don’t think this is correct? Soil arching has something got to do with it. For example if the wall is at 3m c/c each post is taking 3 times the 1m width moment due to the lagging in between. However the passive resistance is based on the arching effect of the embedded portion ?

I would appreciate a good reference. I have yet to come across a text book that provide a good design example on king post walls.

Thanks in advance

 

[SlideRuleEra]

...how do you account for post spacing?

For a 2m spacing, should I multiply the overturning moment by 2?

Yes, multiply the overturning moment by 2.

The "secret" is to understanding what is happening is:

1. Assume the wall is infinitely long.

Then, for randomly placed backfill, any local effects like soil arching average out. Each post will have (more or less) the same force to resist.

2. Carefully select exactly what 2 meters (width) represents.

If the 2 meters (width) is centered on a single post, a sketch makes it clear that each post (on average) must support 2 meters (width) of backfill. This 2 meter assumed length (1 post + 1 meter of wall on each side of the post) repeats along the entire wall. This makes the the 2 meter width of wall the "tributary area" for loading of one post.

Corners or wall terminations are different... but at those locations, the wall details may be also.

http://www.SlideRuleEra.net

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[EireChch]

Thanks SRE - thats what i was assuming too. I have done a king post analysis in Wallap and followed the methodology provided and it works ok. I have had this checked by a senior engineer and he confirmed it. Wallap gave an embedment depth of 2.7m for 1.4m height with a Fos of 2.

However when doing a hand calc as a check, adopting the typical cantilever wall approach doesn't work as its not a continuous wall below the excavation line.

The Wallap method is to ignore the active thrust below excavation line due to soil arching (this is done by setting Ka to 0) and then to factor down the passive resistance as each post is only receiving its passive pressure from a 3d width of soil in front of the post. Kp is then factored to Kp (mod) which is Kp * (d * 3 / post spacing).
So for a Phi of 21 degress and a post hole width 0.5m, at 2m spacing, Kp = 2.11, and Kp (mod) = 2.11 * (0.5 * 3 / 2) = 1.58.

The above seems to work in Wallap but i cant get a hand calc to get an embedment depth/FoS thats in the ballpark.
Quick screen shot of some rough and ready calcs, hopefully not too hard to follow.

 

[Lackoftoein]

What calculation method have you chosen in wallap? Unless you have selected piling handbook method your results will be off. Also the pressure calculation in wallap will give different results to hand calcs. Check wallap output to see by how much? I personally find cads piled wall suite easier by far for king posts if you have access to that?

 

[EireChch]

Lackoftoein,

Thanks for the reply, I have used the recommendations from the WALLAP use manual. Provided below. I have not used piling handbook method, are you referring to the Arcelor Mittal piling handbook ? I have had a quick review and cant seem to find anything?

 

[Lackoftoein]

The way wallap calculates pressures is not something you would want to do by hand. If you look at the wallap output the the active and passive limits should be similar to your calculation?

Regarding the factor of safety I meant BSC handbook method (selectable in FoS Options) as the other methods of calculating stability are not as simple as taking moments. (this is not to say you should be using the BSC handbook method - just it is the only one which will give you a direct comparison).

To be honest you can't expect the same result from the two methods of analysis - I wouldn't expect the same answers when using wallap compared with basic limit equilibrium software or hand calcs. That being said it's important you satisfy both - or at least are confident your wallap analysis is correct.