AASHTO LRFD Design for Sheet Pile Cofferdams with Water Pressure (WA)

[PEinc]

Has anyone designed sheet pile cofferdams per AASHTO using LRFD where the major lateral load is water pressure? The load factor for water pressure is 1.0 whereas the load factor for earth pressure is 1.35 to 1.5. If the major load on the sheet piling is water pressure, the sheeting and bracing will be designed using a load factor of 1.0 and a sheet pile bending resistance factor of maybe 0.9 to 1.0. Therefore, the sheeting is being sized using, or very close to, the yield stress of the sheet pile steel. Essentially, there will be little or no safety factor in the design.

IF YOU HAVE DESIGNED WATER COFFERDAMS USING LRFD, please comment on how you handled the load factor to provide a proper safety factor. Thank you.

http://www.PeirceEngineering.com

 

[FixedEarth]

Try getting hold of the Karl Hanson at

http://www.dcalc.us

Also try posting on Linkedin's Deep Foundation Institute forum.

 

[wmta]

using a water loading with a FS of yield stress in the sheet pile makes now sense even if the code recommends this.

This code is not conservative for this condition asce standard 7-05 requires factor load of 1.4(D+Fluid)

also another load condition is 0.9d + 1.0 E + 1.6 H(lateral or water pressure)

Min factored load should be 1.6 plus seismic as a minimum

 

[Doctormo]

I have noticed the same problem with AASHTO LRFD. The WA load factor is 1.0 for all load combinations which does not make a lot of sense if designing for water pressure alone such as a dam or cofferdam.

I am pretty sure that the LRFD concept is that there is no variability in the water pressure calculation (H*62.4 PCF) so a very low load factor is mandated, 1 in this case.

There is a similar situation with an impact barrier where the collision LF is 1.0 and there are no other loads. The resistance factors are only 0.9 or 0.8 so fairly low equivalent factors of safety are used.

I think in a cofferdam there would be increases in WA load due to waves and surges that might drive the load up a bit above a static load but that would be up to the designer.

It seems that the WA = 1.0 may be ok for water pressure in soil but not so good for stand alone water pressure. I suspect the ASCE values may make more sense.

 

[PEinc]

Thank you, all.

http://www.PeirceEngineering.com