Concrete Formwork

[McBridges]

I am checking the design of a form traveler system for a large cast-in-place concrete box girder arch structure. The initial inclination of the arch is 45 degrees from the horizontal. I am trying to determine a reasonable and hopefully accurate estimate of the friction between wet concrete and plywood forms. Does anybody have experience with this or know where I might find reference material on this subject?

 

[37ed]

O.K. speak softer.I know a little about forms, so what is your question in better terms.
ie really break it in to the nuts and bolts.

 

[McBridges]

Thanks for responding.

I will try to simplify. Concrete formwork at 45 degree inclination has 70.7% of the concrete load acting normal to the formwork and 70.7% of the load acting parallel to the form work. A large portion of the load acting parallel to the formwork goes directly back to the previously poured concrete (as it would in a vertical column). However, there is some longitudinal load to be taken with the forms. This force would be related to the friction of wet concrete on forms. I have heard that an estimate of this friction is 5% of the concrete load. Is this reasonable and justifiable?

 

[37ed]

Yes to your question,5% does not sound like to much to me. The form that is at the 45 degree angle WILL TRY TO FLOAT UP,such as a battered wall.
Depending on the form system and a few more elements,like base of wall size,etc
In a battered wall the loads "push out"on the forms.
At the base you really get pressure due to the larger size,and more concrete.Also the concrete in the "water"form is going to as stated above try to float the face form.

hope that helps

 

[McBridges]

Thanks for you insight.

The "float" of the top forms is due to liquid head of the concrete. We have assumed that this is 2000 psf (based on height and pour rate).

The 5% friction is a different matter that defines the amount of "thrust" or longitudinal load in the form system. This is the value that I am unsure of. Again, I appreciate your input.

 

[mapelCE]

O.K.,

Took a little while to understand what you're asking but I think I get it.

If I understand you correctly, your concerned with the "drag" of the concrete down the length of the form panel.

This is tough to explain in memo form so I'll do my best.

Let's start with the top "floater" form:

If your ties are normal to your form face, then the forms will theoretically push straight out. They shouldn't have a tendancy to float up along the face of concrete (F.O.C.)or be pushed(dragged) down the face of concrete. As you know nothing is perfect, so if you're restraining those forms in the parallel to F.O.C direction at 5% of concrete load you should be more than fine. When we place exterior stem forms for concrete bridges, the ties are perpendicular to the form and only the form weight is used to resist this action. I have yet to see that be a problem.
If your ties are not perpendicular to the form face, then you will have to account for the load component running along your form face that is created between the angle of your tie and the perpenducular load on your form.

The bottom form:

Whole different animal.
This form typically can see a fair amount of compression. Because these forms have to be supported from beneath to hold the dead weight of the concrete the actually act as beam-columns. The ties are rated based on the form pressure perpendicualar to the panel as are the plywood, studs and walers. The supports (scaffolding/kickers) are rated based on the vertical dead load of the mass of concrete that the are supporting, not the liquid pressure of the concrete. Whis this in mind you can break the vertical load into a component going down your kicker and down your form panel. If your supports are truly vertical, then the load transfers directly down them and your assumption of 5% friction transfer on the bottom form is good to go. I would use 5% of the concrete weight not 5% of the form pressure. The concrete is a fluid at this point and it will shear internally before transfering 5% of 2,000 lbs. down the face of your form.

Bottom line is the number is pretty small, and compared to the dead weight and form pressure is is unlikely that it will drive your design.

This is probably a really wordy answer to a relatively simple question so I hope there was something in here that you find usefull.