Terzaghi Braced Cuts and Hinge Method

[vulcanhammer]

One of the curious things I have noted in classical sheet pile design is the fact that treatments of Terzaghi's techniques to analyse braced cuts and the strut loads generally insist on the hinge method to analyse the sheet pile wall itself, i.e., to assume hinges at each strut location except for the uppermost.

Except for the fact that this renders the sheet wall statically determinate, there seems to be no reason to do this. The wall itself is certainly not hinged at the supports; it's probably best modelled as simply supported.

Hoping to get some answers by going back toward the source, I took a look at Terzaghi and Peck (1948), but this simply states the following: "At the elevation of each strut except the uppermost, the piles are assumed to be hinged, and at the bottom of the cut they are assumed to bear against a knife edge." And that's about it.

Any thoughts on this?

 

[BigH]

I have an old British text on "Deep Foundations and Sheet-Piling" revised in 1961 but originally written in 1945 by Donovan H. Lee - a friend of mine who is the son has let me borrow the book. I'll take a look into it - being of the same era, it might have some information. Did you try Terzaghi's 1943 text on Theoretical Soil Mechanics?

 

[vulcanhammer]

Did not. Only have Terzaghi and Peck.

Clarification--the beam is obviously simply supported using the hinge method. My basic question is, why can't the beam be continuous?

 

[PEinc]

The beam can be continuous. However, the design would need to use a more sophisticated design method or computer program to solve a probably statically indeterminate problem. Given the facts that 1) for years, the simplified method has worked successfully, 2) there is much assumption and round-up in the soil values and pressures, and 3) because of the variability of soils, there are relatively high safety factors being used; a highly sophisticated design process for the soldier beams is, in my opinion overkill. It would be similar to calculating earth pressures to 4 decimal places. The age-old design methods are pretty simple and they work. Why pretend to more accuracy than is achievable?

 

[jdonville]

Remember that at the time that these methods were developed (or at least published), the use of electronic computational aids (computers and calculators) was probably not terribly widespread in private practice, by reason of the expense required to use such resources in terms of capital outlay and effort required to operate the equipment. Thus the need for a reasonably accurate and computationally simple approach that anyone could perform.

Jeff

 

[vulcanhammer]

Thanks much for your responses. I figured that the reason the hinge method was employed was for computational simplicity, but I wanted to see if there might be another.

As far as computational simplicity goes, a good finite element/beam analysis program could be used to do the computations, eliminating the problem of the statically indeterminate beam. That's obviously a relatively "new" option. I demonstrated that in Sheet Pile Design by Pile Buck.

 

[PEinc]

I have rarely seen anyone use FEA to design sheeting. And those who I have seen use FEA seemed to know more about FEA than they knew about sheeting.