Steel plate lagging for soldier piles

[reggeo]

Doing a shoring design for a 16' deep UST excavation. Design is almost done. Client wants to use steel plate lagging b/w W14x132 soldier piles; I usually rec'd 4" X 12" DFL timber lagging and am a little "fuzzy" on steel plates.

1) Do these plates typically come in standard "nominal" sizes...the soldier piles are 8.0' o.c. Should I order 7.5' wide plates to give the contractor a little "wiggle room" b/w the webs of the 8' offset piles? Or should I allow a tolerance in the pile offset of a few inches and require 8.0' wide A36 plates?

2) I have calc'd the Sx as follows:
M = 4736 lbs/ft.; Sx = M/Fb = 4736 x 12"/(36K x 0.66) = 2.37 in3

How do I determine the thickness of the steel plate lagging...I figure a 5/8" thick plate will do but this design will be peer-reviewed by LA City and I need an equation to determine thickness

 

[PEinc]

For a 16' high, triangular, active earth pressure, without any surcharge loads, your lateral earth pressure at subgrade could be about 600 psf. For an 8' lagging span, your bending moment would be about M = 1/8 x 600 x (8 x 8) = 4800 ft-lbs. Above 16', the earth pressure would decrease and so would the plate's bending moment.

At 16', S = (4800 x 12) / (0.75 x 36,000) = 2.13 in.3

S = 2.13 = 1/6 x b x (t x t) = 1/6 x 12 x (t x t)

Therefore, t = 1.03 in. at 16'

If the lagging and the soldier beams are installed propery and the soil is not soft or very loose, there could be some soil arching which could reduce the bending moment.

At some depth above 16', you could use a thinner plate.

However, at 16' your 5/8" plate is too thin and too expensive. Use wood, probably 3" nominal thickness mixed hardwood or better.

If your client wants to use steel plate for lagging, I suspect he also doesn't understand how to install lagging properly. In addition, 16' is pretty deep for a cantilevered sheeting wall. I suggest you slope off the top few feet and try to keep your lagged wall height to 12' or less. It will deflect less and perform better than a 16' wall.

Designing the steel plate and designing the timber lagging require the same type of calculations. Only the allowable stresses are different. With all due respect, if you do not know how to size the thickness of a steel plate, you probably should not be doing the design.

 

[reggeo]

PEinc,

Thanx for your prompt and thorough reply. The soldier piles will be set in predrilled 2' diameter holes, which will be backfilled with lean concrete. Embedment was calculated per the "flagpole" UBC formula to be 20'.

You are correct in assuming there is an arch effect for the piles. For a W14x132 encased within a 2' borehole, we usually allow a passive width, b, of 2x pile diameter, or 4'in this case.

Based on an active EFP of 37 pcf and an allowable passive EFP of 300, I have calc'ed a max moment of about 400 ft.-lbs/pile. This was cross-referenced to my Civiltech shoring program which rendered near-identical results for embedment depth and Mmax.

This is a very transient shoring structure; it will be in the ground just long enough to replace USTs (~2 weeks)and then the excavated area around the tanks will be backfilled with pea gravel.

It turns out that my initial formula for plate thickness, which I considered suspect, was identical to yours; we just started with different "b" values. However, in response to the caveats you mentioned, I will require the Client to provide 1" thick 8'wide steel plate b/w the soldier piles. He has these in his warehouse.

Thanx again for the help.