US Steel Sheet Piling Design Manual 1

[btomcik]

Question on the USS Sheet Piling Design Manual, version 1974:

Net earth pressure diagram for a cantilever system in granualar soil is shown on page 21. I understand how it is arrived at. But why when calculating the section modulus are various pressures both part of the net pressure diagram and not part of the net pressure diagram calculated and used?

Design example #1 on page 86:
Pressures as part of the net pressure diagram at points A1 and J are calculated. Ok - I understand. But pressures not part of the net pressure diagrams at points A2 and E are calculated. Why?

If the net pressure diagram is bounded by the points B-A1-O1-A-B; O1-O-C-O1; and O-J-F-O, why are points A2 and E utilized?

Thank you.

 

[SlideRuleEra]

btomcik - As a follow up to the beginning of this question in thread255-226465 I'll take a stab at it, in the most general terms:

Looks like the book has presented the graphical diagrams using the "Conventional Method" (see Fig. 15a on page 20), but the math is based on the "Simplified Method" (Fig. 15b, page 20).

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[btomcik]

I thought about that too but the math is done for the conventional method.

Another question - can surcharge pressures simply be added to the net pressure diagram? I read in the CalTrans Trenching and Shoring manual that they can't - that an equivalent height of earth must be used. Agree/disagree?

 

[SlideRuleEra]

btomcik - I agree with the CalTrans T & S Manual... for your work as an employee of your "client" (Bridge Contractor). If I may, I'll offer some opinions, in good faith:

From what I see in the T & S Manual, it looks like the equivalent height of earth greatly simplifies the calculations. The simpler the better; you will be designing temporary structures for conditions that change from day-to-day as construction progresses. Unlike some permanent structures, you can't really predict what the loading will be. The Contractor's field forces will have to do what it takes to get the job done (like walk a crane right up to the top edge of wall and lift "something" unexpectedly heavy out of the excavation). Be conservative in your assumptions and stay away from designing for "precise" loads.

Performing engineering for a Contractor's temporary structures is nothing like designing a permanent facility. Here are some reasons why:

1. The economics are all "twisted". Usually the Contractor will already have certain structural "materials". Your job will be how to make THOSE do the job, not to select the BEST for the job. Also the labor to work with those materials is at "cost" not with the typical markup that a client will have to pay.

2. The loads that you design for are both "real" and immediate (unlike the statistical probability of wind or seismic). What you design for, in most cases, is what WILL happen (and soon). Again, be conservative.

3. Considerations that are important for a permanent structure are often irrelevant for a temporary structure. For example, defection of beams can usually be ignored or compensated for (as long as allowable stress is ok).

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[PEinc]

In my opinion, the design calcs in the USS Sheet Pile Manual are not very good. In addition, if I remember correctly, there are typos in the calcs. If you are trying to design for a contractor on highway work, you may need to use LRFD and use the FHWA recommended unsymmetrical trapezoidal pressure diagram for anchored walls which some state DOT's are also requiring.

Additionally, it is a rare occurance when I can find a design example in the USS Manual that is very similar to what I am designing. The Manual has good information (charts, tables) in it but I am not a big fan of the design examples. For the most part, they are sometimes cryptic and hard to follow. The Manual is also very old and has been updated as the Pile Buck Steel Sheet Piling Design Manual.

From my experience, designing temporary excavation support and retaining structures for bridge and highway work is pretty much like designing a permanent structure because the DOT or its consultant who reviews your design is used to working on permanent structure. They don't usually think "temporary." Also, they rely heavily on the FWHA and AASHTO publications and design manuals which are geared toward permanent structures.

Different design methods may be required for different type of projects for different owners. Private, state highway, and RR projects can all require different design criteria and methods. You need to know several different ways to design these walls.

 

[vulcanhammer]

Actually, the Pile Buck Steel Sheet Piling Design Manual was updated again as Sheet Pile Design by Pile Buck.

Although I used several examples from the USSSPM and PBSSPDM, I reworked them all, comparing them with SPW 911 results.

As far as the T&S manual is concerned re equivalent soil height, it says that "It will often be convenient to convert a long uniform surcharge to an equivalent height of soil, increase H by that amount, and then analyze the shoring with one pressure diagram." That doesn't sound like a mandate.

How the use of surcharge or equivalent soil height (which in effect biases the active pressure diagram) works out depends on how the equations are set up.

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[DRC1]

To answer the orginal question, in order to compute the location of C you need Z. They metod computes the gross pressur envelopes and sums them to set them equal to 0. For a given D, Zis computed to yield the net envelope.
Once you are below the dredge line, you move from net active on the backside to net passive on the front side to net passive on the backside. In order to cantilever, youneed to develop support at two points on the pile. Hold a stick about a third of the way from the bottom. Now have someone grab the stick at the top and pull. in order to keep the stick from moving, you need to grab the stick at the bottom and pull back. Most of the force is in the bottom hand. This is why the passive pressure develops on the back side. Using the gross pressure envelopes allows you to solve for the net.
As for the Cal trans question, the answer is yes and no.
By using an equivent surcharge, the math is not significantly changed. However if you look at it, the area above the mudline can be replaced by a resultant at the proper location to obtain the required moment, with A1 being the lateral pressue at that point. However, and I think this is an answer to another post, if you try to continue the surcharge pressure below the mudline, The math can get complicated, although the principals remain unchanged. However, generally the pressure at the mudline is small enough to ignore, or compensate by increasing A1 slightly.
One final hint, for checking iterations for M, several of the terms will be small and can be dropped from the iteration computaions. The final answer should be checked with the full equation.