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# WOOD LAGGING DESIGN CALCS

## WOOD LAGGING DESIGN CALCS

(OP)
someone said usually the wood lagging is not required to be designed but usually when doing submittals other engineers reviewing retaining walls, want to see calcs done for the lagging or question the way calcs have been put together.

Up to now I have seem for temporary shoring design (soldier pile and lagging system)
wood lagging design pressure as W=.25*L*gamma as per Silo theory Pressure (please correct me if this is wrong)
then calculate max Moment M=WL^2/8 and find stress on the lagging Fb=M/S< fb=1200 psi
L=wood lagging span
W=Pressure on Lagging
gamma=soil unit weight
DONE

but on the literature I have read as per CALTRAN MANUAL recommends to calculate the required section modulus as S=(M*0.6)/1200

and according to the mathematical model presented on this document http://www.helicalpilebook.com/Files/Article%206%20v4.pdf
max pressure on lagging occurs at a depth=1.4*L
and max Pressure on Lagging is P(max)=Ka(w+1.2*L*gamma)

my question is: which one is more widely used in the industry?
to be more politically correct.

### RE: WOOD LAGGING DESIGN CALCS

Neither (both techniques lag behind the times), as, from what I can see, neither considers or mentions the reducing effect of soil arching, or the use of a CDF backfill, which is being prescribed by many geotechs now.

Mike McCann, PE, SE (WA)

### RE: WOOD LAGGING DESIGN CALCS

It is largely empirical. 3x to a certain depth, 4x beyond that. When I need to do a calc, I either take half the apparent pressure, or the full pressure in a triangular distribution (similar to what you might do for a masonry lintel) tapering down to zero at support. These are the two most common methods I have seen used.

### RE: WOOD LAGGING DESIGN CALCS

SEAONC 1985 seminar has a guideline for lagging design based on arch action.

### RE: WOOD LAGGING DESIGN CALCS

Is the SEAONC 1985 seminar available to view somewhere online? My Google skills didn't result in anything.

Thanks!

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