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Concrete Formwork Design Question 1

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cepe01

cepe01

Civil/Environmental
Jan 17, 2017
18
IMG_2429_sxzwir.jpg


I have to check the following concrete formwork layout. It is to be used for walls and footings. All the studs, braces and wales are 2x4's and the sheathing is 3/4" plywood. I have checked the wales and studs for bending and the braces for compression and they seem okay but I can't figure out how to check the plywood. Is there any chance of the plywood failing? The formwork is only 5 feet high but the spacing is 3' wide.

I have been looking at the ACI formwork guide but I can't figure out what they are doing. Can someone explain to me what "Rolling Shear" is? Is there any chance of the plywood failing in shear? would this be a "blowout"?

Please see attached sketch for reference.

Incase the image doesnt work: Wale (vertical) spacing is 2.5', the stud spacing is 3'. The design does not use ties but instead 3 braces at different angles per stud.

Any help would be much appreciated!!! Thanks!
 
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I think you will be able to read the deflection of the plywood and Wales in your finished product.
If you can live with that, you should be ok. However, I always use ties and 24" max spacing with 3/4 inch ply and 2x4 walers.

Mike McCann, PE, SE (WA)


 
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Draw yourself a section view. Show the pressure diagram from he concrete horizontally and show the anchor point. Then compute the overturning moment rotating about the anchor point. Then show what friction there may be, acting downward on the plywood from fresh concrete resisting that over turning moment. Once it starts to rotate will that resistance change? Also once lifted up a little what is the upward force from that concrete that passes out under the form? Consider the form weight as negligible. These vertical forces or resistance from the concrete may or may not be there of significant value.
 
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The success of plyform depends on the face grain direction, the concrete placement rate (vertical feet per hour), the temperature (how fast the concrete takes it's initial set).

Plywood has an odd number of layers. To get best performance (shear, rolling shear, moment, deflection) orienting the face grain perpendicular to supports (wales) will align the maximum number of layers in the correct direction.

Rolling Shear: The alternating layers of wood that are glued together to form a sheet of plywood guarantee that some of those layers will be positioned in the worst possible way. In the layers that are perpendicular to the horizontal shear the fibers tend to "roll" over each other.

I have not performed any calcs on your sketch, however, the plywood is overloaded. It probably won't actually fail but the results won't be "pretty".

[idea]
[r2d2]
 
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Thanks for the responses!

I will lower the spacing to 24".


How do I calculate the maximum moment on the wales when there is a non-uniform load? In this case the lateral pressure of the concrete. I can't figure out the load (w) is for Mmax=(wL^2)/10
 
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SRE is correct. Get ahold of some formwork design manuals from formwork suppliers like Dayton Richmond. These manuals contain engineering info on plywood, other form lumber, form tie systems, etc.

 
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The Dayton Richmond manual looks great! lots of good info in there. Thanks.
 
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APA (US) has two free publications that can be downloaded. Design/Construction Guide: Concrete Forming V345 (May 2012) is for plyform and the Plywood Design Spec Y510 (1997 ?) for plywood. They both have design allowables and design examples.
 
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The ACI's SP4, "Formwork for Concrete" by M. K. Huro (ACI committee 347) also has a lot of good information. Kinda the bible for formwork design.

My copy is the fourth edition and was published in 1981 as the revised second printing.

There is probably an updated edition.

Mike McCann, PE, SE (WA)


 
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What are you anchoring to? Keeping formwork from sliding can be a challenge.

Five feet tall isn't the tallest form in the world, but you don't want your form to slide an inch into the site either.

"We shape our buildings, thereafter they shape us." -WSC
 
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