Braced Cofferdam Unbraced Length of Wales 1

[Spellbinder]

I am Designing A braced Cofferdam. The inside dimension is 70.67'x20'. The long wale that i have is 70.67' long with struts at least every 19.33'. If I understand the bracing specification of AISC correctly I see that the sheet pile and cross struts brace the wale in x direction and the top and the bottom flanges are braced against lateral torsional buckling. So unbraced Length for y direction of wale is 70.67', which is affecting the capacity of the section in compression.

But i have seen calculation examples of Wales where the braced length for both x and y direction of wale is taken as the spacing between the struts.

So my question is can i take unbraced length of both x and the y direction of the wale as the max spacing between struts? If yes why?

 

[SlideRuleEra]

So unbraced Length for y direction of wale is 70.67', which is affecting the capacity of the section in compression.

Is the question about axial loading on the 70.67' wale as a column (applied from wales resisting pressure on the 20' long walls)?

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[Spellbinder]

Yes it is.

 

[SlideRuleEra]

For wale bending.
Assume the 71' wale is four-span-continuous.
Although hydrostatic pressure is applied along the full length of the wale, the compression flange is not continuous braced by the steel-on-steel friction between sheeting and the wale (shown in red on the sketch below)

For wale axial compression.
For minor axis buckling of the wale as a column, an unbraced part of the wale will have to slide either "up" or "down" relative to the sheeting.
Again, hydrostatic pressure is applied along the wale's full length creating friction between the sheeting and the steel. However, the magnitude of the friction force is concentrated where the wale is compressed between a strut and sheeting. Friction keeps the wale from sliding. Assuming the distance between struts is a reasonable and conservative approximation of the unbraced length of the wale (for axial loading).

There are advantages to hydrostatic loading, it is both reliable and accurately predictable. Some other loading types (wind, seismic, live load, etc.) can vary with time. The result is that friction from hydrostatic loading is dependable... when the cofferdam is dewatered, friction is working all the time.

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[Spellbinder]

I agree with the design principal of yours. Could you please give me any reference on it?

 

[SlideRuleEra]

See the "US Steel Sheet Pile Design Manual".

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