High School Concrete formwork pressure

[KentG]

I am needing to calculate the pressure that concrete will exert on a cylindrical form for an upcoming project. My question is: if the perimeter forming is constructed as a closed band, will the required tensile strength of that band be equivalent to the hydrostatic pressure against the wall face? The form will be filled with concrete with a specific gravity of about 2.4, and the height of the form is 900mm. Here is a sketch to hopefully clarify.

 

[Lnewqban]

Welcome, Kent!

These are the proportions of your form:

The main problem seems to be to stop the concrete from leaking out between the cylinder and the bottom flat surface.

 

[Baluncore]

What is the form material?

The pressure against the form will be a maximum at the bottom, and zero at the top. You might use the average, mid-point in the calculation of tension, but first you must verify that the form material, is able to transfer or exchange tension from the bottom filament to the top filament of the form. To be conservative, you would assume the maximum pressure was applied at all depths to the form material.

 

[jrmichler]

That's an impressive piece of concrete - 200 $m^3$. I hope that the person in charge is taking into account the thickness and the heat generated during curing.

The pressure at the bottom will be $1000 kg/m^3 * 2.4 * 9.8 N/{kg} * 0.9 m = 21,200 N/m^2$.

To be conservative, you would assume the maximum pressure was applied at all depths to the form material.

This.

The outward force is $21,200 N/m^2 * 0.9 m = 19,050 N/m$. Each meter of the concrete form has 19,050 Newtons of force pushing outward.

The tension force in the concrete form is $19,050 N/m * 16.9 m * 0.5 = 161,000 N$.

Design the concrete form for a total of at least 161,000 Newtons tension force. The tension force is concentrated toward the bottom of the form, so you need to make sure that the bottom is stronger in tension than the top.

 

[KentG]

Thanks for the input, that helps!

To be more specific, the tension could be calculated at 3 points. 1 band would tie it at 100mm from the bottom, 1 band in the middle, and 1 band 100mm from the top. The capabilities of the forming itself could be left out of the equation, the tension would be taken up by these 3 bands/ties.

Another thing that will help here.. The concrete will be poured in layers over approximately 5 hours. So the bottom concrete will be setting up, thereby reducing the pressure. So if the form is designed to withstand the full liquid head, it will be more than adequate for the situation.