Cd Factor for Formwork Design.
Cd Factor for Formwork Design.
(OP)
Hello:
I would like to know which factors have you guys used in the past for Load Duration.
The 2004 -APA Concrete Forming Document states that I can use 1.625, but the ACI 347 says to use a 1.25.
I think that 1.625 is just high.
What do you guys think?
Thanks
I would like to know which factors have you guys used in the past for Load Duration.
The 2004 -APA Concrete Forming Document states that I can use 1.625, but the ACI 347 says to use a 1.25.
I think that 1.625 is just high.
What do you guys think?
Thanks






RE: Cd Factor for Formwork Design.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Cd Factor for Formwork Design.
However, if the forms support vertical loads of concrete, then the 1.25 is reasonable as the forms remain in place longer with the curing of the concrete matrix not drastiocally affecting the load seen by the forms.
Where did you see this in ACI 347? I have the fourth edition.
Mike McCann
MMC Engineering
RE: Cd Factor for Formwork Design.
ACI 347-04.
3.7—Removal of forms and supports
"3.7.2.3—Because the minimum stripping time is a
function of concrete strength, the preferred method of determining stripping time is using tests of job-cured cylinders or concrete in place. When the contract documents do not specify the minimum strength required of concrete at the time of stripping, however, the following elapsed times can be used."
In my case, I have a 20' High wall;
This is what they have....
Walls*........... 12 h
Columns*......... 12 h
But going back to the NDS, They have a Load Duration of:
Two Months = 1.15
Seven days = 1.25
Ten Minutes = 1.6
So I guess if you want to be conservative you should use seven days with a Cd = 1.25. Although it is only 12h.
Thanks guys.
RE: Cd Factor for Formwork Design.
RE: Cd Factor for Formwork Design.
Mike pointed out that the concrete sets and in a few hours the hydrostatic head of fluid concrete is gone. I agree that this is true... sort of. The deflection that occured while the concrete was fresh remains after the concrete sets. IMHO, if the members retain the deflection then the stress in the memebers also remains until the forms are wrecked (often 7 days). That is why I consider the 7 day figure for Cd to be the best choice.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Cd Factor for Formwork Design.
htt
Form tables are at the begining
RE: Cd Factor for Formwork Design.
Tinguindin: The 1.625 factor in APA's Concrete Forming Guide is NOT a duration factor alone - it is the 1.25 factor (7 day loading) times an arbitrary "experience" factor of 1.30. This factor is included in some of the values that you find in that guide, so read carefully.
Generally speaking, for plyform (IF the contractor actually does buy plyform), performance (i.e. deflection) always governs. Never seen large sheets of plyform fail by bending or shear - it's always a case of too much deflection. Keep in mind with multiple reuses fatigue of the fibers connected to the glue line will occur and deflection will increase (for the same load & span) as the number of uses increases.
Best thing to do is consider the dimensions of your facing material (say 4' x 8') - 8' divides into an integer number of spaces only so many practical ways - resulting in support spaces of say 8", 8.73", 9.60", 10.67", 12", 13.71", etc. - and plan your back-up member spacing accordingly.
Ralph
M.ACI 347 Committee
RE: Cd Factor for Formwork Design.
What components will you use in your system and what will they be used for?
Daniel