Lateral pressure of fresh concrete on wall formwork or any liquid pressure on walls 8

[struggle67]

Hi

This afternoon, I was calculating the lateral pressure of fresh concrete on wall formworks. Conservatively I was using ρ H (density x height) without considering placement rate, temperature, etc.. then I came to realize that the wall thickness (i.e. width of the fresh concrete) does not affect the lateral pressure on wall formwork. Even if my wall is only 5mm thick I will still get the same lateral pressure as 1000 mm thk wall. It is quite hard to accept for me. Does it true or is it just a simplification or conservative approach? If so any way to account for it? Should it be density x height x wall thickness?

Thanks

 

[JoelTXCive]

Think about water in a glass. The water body only exerts pressure on the glass at the perimeter where it is in contact with the glass surface.

If you pick a point in the middle of the glass, then all the opposing pressures cancel out.

 

[jjl317]

I would recommend reviewing ACI 347 "Guide to Formwork for Concrete" - it should help clarify any appropriate design lateral pressure.

And it does not consider width of concrete

 

[SlideRuleEra]

JoelTXCive - That is the best explanation I have ever come across.

http://www.SlideRuleEra.net

 

[dik]

I've always wondered why the glass didn't accelerate across the table.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

The width of the element doesn't alter the lateral pressure on the forms in their fully concreted state. However, the width of the element absolutely effects the design because concrete placement is much harder to control (and thus less reliable) in wide elements.

For example, I just designed / poured an elevated 26' long x 7' high x 5' wide beam (see here). I designed the form for a loading envelope including A) full typical hydrostatic head and B) partial lateral pressure on one form face only. The latter being a pretty significant condition because the ties are unreliable unless concrete is on both sides (so forms themselves need to work to span the full 7' where deflection was the governing factor naturally), and loading on one form face only induced a lateral force in the elevated deck that needed to be kicked down somewhere.

I would never do that for say a 12" wide element because it is virtually impossible for any significant amount of concrete to exert lateral pressure on one side and not be in contact with the other. On the other hand, when you pour an ultra wide element the likelihood that things are going to go wrong, and concrete is going to pile up in significant heights worth worrying about (on one form face only), is pretty likely. I was at this particular pour and that absolutely happened!

EDIT - At the pour the finisher was attempting to dump about 6' worth on one of the closing ends without any concrete on the other end of the beam. Since I was there his plan was swiftly altered and I "politely" told him to get his hose moving and pour it in lifts as instructed prior to the pour. Though that wasn't strictly necessary because I had actually designed for that condition. On the other hand, I didnt want to test an elevated form carrying 136,000lbs unnecessarily. But if you don't design for that and you are not at the pour, bad things can definitely happen during placement even if the form is designed for full head.

 

[struggle67]

Thanks, Everyone

Think about water in a glass. The water body only exerts pressure on the glass at the perimeter where it is in contact with the glass surface.

If you pick a point in the middle of the glass, then all the opposing pressures cancel out.

Thanks, Joel

I can imagine that the pressure being equal and cancel out at the center of the glass. It was just hard to accept the fact(I will exaggerate a bit) that the lateral pressure on the perimeter of 5 mm diameter & 100 mm deep glass and 500 mm dia & 100 mm deep glass are the same.

 

[SlideRuleEra]

struggle67 - If pressure was somehow dependent on width (it is not), at what width does this value of width become a factor... and why is that critical width value such a perfectly kept secret?

http://www.SlideRuleEra.net

 

[hetgen]

@stuggle67 You're not alone it is hard to accept intuitively. Here is another related thread.

 

[HTURKAK]

STRUGGLE,

The lateral pressure of fresh concrete on wall formworks is basically function of concreting speed ,type of blend, fresh concrete density, slump... and does not change with the thck. of wall ,column , dia of column etc for reasonable thicknesses..

The water pressure on the perimeter of 5 mm diameter & 100 mm deep glass is less than the 500 mm dia & 100 mm deep glass due to capillary action.

I will suggest you to look ACI 347-04 and the following doc.

 

[struggle67]

Hi Hetgen,

Thanks. Haha good to know I am not alone.

HTURKAK,

Thanks. It really helpful.

 

[XR250]

Think about sitting on the bottom of the swimming pool.
The pressure on your head does not change depending on how wide the swimming pool is.
Same with atmospheric pressure.