Concrete Formwork Pressure - Walls 15' and taller

[BJMZCSEngineering]

What is the justification/reasoning for almost doubling form pressures for walls over 15' tall per ACI 347 Sec. 2.2.2.1?

E.G.: 690-psf for a 14' wall or column of any height VS. 1190-psf for a 15' wall (pour rate of 3'/hr and temp. of 50 deg.)

Thanks in advance for your help

 

[BAretired]

I don't know the answer to your question, but could you please clarify what you mean by "a 14' wall or column of any height".

BA

 

[BJMZCSEngineering]

ACI 347 gives one equation for walls 14' or less and columns of any height, and a different equation for walls over 14'.

(Columns are defined as any element with a plan dimension not exceeding 4'6".)

I have a 12' wall and 15' wall I am designing, and I can't justify two completely different form designs when the same pour rate will be used.

 

[BAretired]

I cannot see why there should be any difference, but I am not familiar with ACI 347.

BA

 

[dhengr]

BJMZCSE:
If you do a lot of this work ACI has a real good book “Formwork for Concrete,” by M.K. Hurd, which you should have on your book self. My copy is pretty old, and I’m getting too old to learn real many new tricks, so I don’t know exactly what the state of the evolution is of the formulas you are looking at.

There has been a good deal of testing done on this issue, some seat-of-the-pants, but much engineering experience and judgement; all to arrive at a less than absolute scientific set of formulas and judgements on the matter. Then throw in all the variables in concrete mixes, plasticizers, fly ash, etc. etc., plus temp. and pour rate in ft./hr. (larger pumps, etc.), leading to an initial set time for the conc. vs. full height of fluid conc. Finally, your own judgement as an engineer, and your ability to control the above variables, plus placement and consolidation methods which lead to the realization that a form blow-out is a real disaster. Then, throw in the fact that a 100' long wall form acts differently (one way action, infinite length), except at corners, etc., whereas a column form with a 4-6' footprint (two way form action); and finally they do tend to fill a wall form in 3 or 4' lifts over an extended period of time (3 or 4' initial set-up) before the next lift; whereas column forms tend to be filled and consolidated full height in one shot (no initial set-up at 4 or 6', just keep pourin).

You don’t end up with something that lends itself to a formula with one exact answer. Form design has not been my forte, but if I were to guess, the 10 & 15' heights come from long, good, experience and earlier testing programs. There probably should not be some absolute 15' mark where pressures double, IF you can control everything else. Look at slip forming, for example. Maybe ask these questions of a member of ACI Committee 347.