Duration of Load Adjustmen Factor for Lumber in formwork application
Duration of Load Adjustmen Factor for Lumber in formwork application
(OP)
Is it permissible/common to interpolate the CD 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)?
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)?






RE: Duration of Load Adjustmen Factor for Lumber in formwork application
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.
RE: Duration of Load Adjustmen Factor for Lumber in formwork application
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/Par...
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....
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.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Duration of Load Adjustmen Factor for Lumber in formwork application
Good thread link and great references.
Thanks.
RE: Duration of Load Adjustmen Factor for Lumber in formwork application
Mike McCann, PE, SE (WA)
RE: Duration of Load Adjustmen Factor for Lumber in formwork application
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.