Duration of Load Adjustmen Factor for Lumber in formwork application 1

[Ingenuity]

Is it permissible/common to interpolate the C_D duration of load factors?

Application is vertical and horizontal lumber (soldiers/walers) for 5' high concrete wall forms, with the 1-minute factor of 1.6 vs 7 day factor of 1.25.

Can I use 1.4 or 1.5 given load duration is less than 24 hours (wet plastic concrete form pressures)?

 

[KootK]

This thread may be helpful: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Is it permissible/common to interpolate the CD duration of load factors?

Technically, yes, but not by interpolation. The duration factor is governed by this equation:

Cd = 1.75192 / (DOL) ^ 0.04635 + 0.29575

Where Cd = Load Duration Factor
DOL = Duration of Load (Seconds)

The values for 24 hours, 1 week, 1 month, etc. are just points on that curve.

Load duration factor, as we know it, evolved over many years. It became the form we use today in NDS 1951. This is explained in the 1997 NDS Commentary, see page 8:

http://www.awc.org/pdf/NDSCommentaryCompressed/Part02DesignValuespp7to19.pdf

The 1951 Forest Products Lab paper that reported on experiments used to derive the equation is here:

http://www.fpl.fs.fed.us/documnts/fplmisc/rpt1916.pdf

Just for fun, I solved the equation for a 6 hour duration and got Cd = 1.40
Note: Also got essentially the same answer when I used my log-log slide rule to if I still knew how to us it for exponentials.

Of course the above is the technical answer. Whether this would be interpreted as meeting any specific code is another question.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ingenuity]

Thank you, KootK and SRE,

Good thread link and great references.

Thanks.

 

[msquared48]

Better watch your deflections though as the load duration factor should not be used. Will give a waviness to the wall if deflects too much.

Mike McCann, PE, SE (WA)

 

[Ingenuity]

Thanks, Mike. Will do.

I am going through an exercise of varying soldier and waler (and through-tie) spacings for a given section size and species...and comparing costs.