Cd Factor for Formwork Design. 3

[Tinguindin]

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

 

[SlideRuleEra]

I use 1.25, which NDS 2001 calls the (7 day) "Construction Load".

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msquared48]

I from the title of this thread assume you are designing the form system for a concrete structure. The Cd factor for load duration varies with the amount of time the load is increased. In that the majority of concrete lateral stress on the forms dissipate quickly as the matrix hardens - four hours or less - the duration is far less, so the factor should be able to be higher for wall forms.

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

 

[Tinguindin]

SlideRuleEra;

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.

 

[Tinguindin]

Here is something that I found;

 

[SlideRuleEra]

Tinguindin - Since concrete forms, or the materials used to build them are often reused it pays to be conservative. The wood members wind up with bolt hole, damage from being removed (wrecked), use/storage outdoors, etc. They are no longer "new" but still must withstand substantial forces.

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[DRC1]

Dayton Superior has tabularized data for wood form design

http://67.192.65.138/weavecmsresources//Downloads/DS6_ConcreteForming.pdf

Form tables are at the begining

 

[RHTPE]

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

 

[DTGT2002]

As stated above, the right answer comes from the context in which you intend to use the material, and which material you are considering.

What components will you use in your system and what will they be used for?

Daniel