Single sided wall formwork 4

[haideeb]

I am designing the bottom tie support of a single sided formwork with A-frame. I have calculated the horizontal reaction force due to concrete lateral pressure. Can anyone lead me to a reference on how to calculate the vertical force due to uplift?

 

[BridgeSmith]

I don't think I understand the question. Where does the uplift come from?

 

[civeng80]

The uplift of the forms comes from the reaction of the prop in compression on the form, therefore pushing the form upwards.

It is very important to realise this uplift in form construction and design.

 

[Lomarandil]

Exactly. The vertical force in uplift is a function of your brace angle, since the horizontal brace force is known, and the brace must carry load as axial compression.

Although I'm not sure how a single-sided form has a "tie". That term is usually in reference to a tension-only member (like a steel rod) that passes through both sides of a two-sided form. A sketch would help.

----
The name is a long story -- just call me Lo.

 

[Ingenuity]

I am designing the bottom tie support of a single sided formwork with A-frame.

Is it something like this form system photo?

If so, you have calculated the lateral form pressure, and the resulting lateral force equivalent and distance - take moments about the leading edge screw jack of the form frame, and divide by the distance between the front screw jack and the proposed form tie back ==> vertical force.

It is very important to realise this uplift in form construction and design.

That is the truth. Even where no theoretical uplift is 'applicable', over zealous concrete placement and/or vibration where concrete can get under the form bottom plate...bingo..forms lift up...

Although I'm not sure how a single-sided form has a "tie".

Some single-sided wall form systems are tied using these sacrficial form ties:

 

[Lomarandil]

Neat stuff, thanks for sharing Ingenuity!

----
The name is a long story -- just call me Lo.

 

[BridgeSmith]

Ah, I think maybe I get it now...it's what we'd call an overturning load in a retaining wall?

 

[haideeb]

Yes, Ingenuity, that's exactly what I was referring. That's very helpful.

 

[haideeb]

Thank you!

 

[OldBldgGuy]

I'm not sure how you calculate it, but haideeb is absolutely correct, you must account for uplift. 45 years ago I rushed to a jobsite with others in our company (I was a summer student labourer at the time) to shovel wet concrete and tear apart a single sided form that lifted and blew out, filling an existing loading dock with concrete. If we didn't shovel it, it was jackhammers tomorrow. That was an early and important lesson in the uplift force on single sided forms which I've never forgotten and I've seen similar disasters as recently as last year on jobs where my advice and warnings were disregarded as the words of an old woman.
I'm not sure how to calculate the exact force, so I make sure I allow enough. The buoyancy of the forms in a fluid 2.5X the density of water is a start.

 

[BridgeSmith]

The the concrete is contained behind the forms, correct? If so, buoyancy doesn't apply. The uplift would be from an overturning moment, would it not? Conservatively neglecting friction between the forms and fluid concrete, it's just the moment due to the resultant of the lateral fluid pressure on the forms acting the distance from the bottom to the centroid of the pressure (1/3 the depth of the fluid concrete. That moment must be resisted by the weight of the form facing plus either a hold-down force at the bottom of the facing (Uplift = M / width of the A-frame base) or ties such as the ones Ingenuity showed in the sketch above (Tie tension = M / height to the ties from the A-frame bearing elevation).

 

[haideeb]

Thanks for all your inputs

What is the usual or the good practise rate of concrete rise to use for a wall with single sided form like this? 2m/hr? Slowing it down, say 1m/hr could lessen the pressure but could that introduce a cold joint?

 

[Ingenuity]

My personal experience (similar to OldBldGuy - i.e. from past projects) and having constructed many wall forms is that on both single or double sided vertical forms (or column forms too), if there is a somewhat significant gap at the bottom of the form such that wet concrete can 'seep' under the formwork, and with a significant 'head' of wet concrete with continued vibration/consolidation, there is uplift. If the bottom plate of the forms are not secured they will lift! Hard work to restrain this after the fact.

Often wall forms - and column forms - are constructed with 'kickers' at the bottom - to both align the forms but also helps in reducing potential uplift by reducing gaps. I still secure the bottom form plates when we use 'kickers' - small cost for some comfort.

 

[jjl317]

Two comments -

One sided systems like Ingenuity posted typically rely on threaded rods cast into the previously poured footer, at an angle relative to grade (often 45 degree). So the horizontal concrete pressures are resolved into a tension load on the threaded rod, and a vertical reaction at the frame leg furthest away from the new concrete. Rest is statics.

In terms of concrete pressure, in my experience, the because of the structural limitations of the one-sided assembly (typically provided by the manufacturers), the allowable pour pressures (and typically pour rates) are lower. Here in the US, ACI 347 defines the pour pressures based on temperature, chemistry, pour rate, consolidation technique, etc.

 

[BAretired]

My very first project when starting my own consulting business was a treasury branch which involved a concrete vault with 18" thick walls. The vault was on the property line and there was an adjacent building on the neighbor's property line, so the contractor used a single sided form on his side and the neighbor's wall on the other side. The form was well built and stood up very well, but the neighbor's wall collapsed during the pour which was unfortunate because the neighbor was the mayor of the town and also a lawyer.

BA