calculating loads on concrete forms

[delfina]

Anyone know how to calculate a load due to concrete ( poured )on forms. Is it as simple as if it was water? ( density x height for pressure ? )

THanks

 

[PEinc]

Yes, if the concrete has not started to set. However, concrete pressures are also a function of temperature and pour rate. I suggest you get a book on formwork design and get some formwork accessory catalogs from suppliers. These catalogs frequently contain an engineering section with design information.

 

[LPPE]

Make sure you use the density of concrete, not water.

 

[delfina]

So the load on the forms is not dependent on the width of the wall correct?

 

[jheidt2543]

One of the best investments I made years ago was buying the book Formwork Design by Mary Hurd from the American Concrete Institute (ACI). You REALLY should do the same.

Your original question says "...load due to concrete (poured) ON forms." To me that implies a deck or suspended slab. Your second post implies a wall - there is a BIG difference!

 

[PEinc]

As hard as it may be to believe, wall thickness does not affect the pressures. There is still a column of wet concrete creating a vertical pressure and therefore a horizontal pressure. Whether it's PSF or PSI, it's still the same pressure. I've never seen a reduction for thinner walls.

 

[delfina]

Thank you for all your help. My ? was a bit confusing. It is a wall and I will get that book.

Boy it is hard to believe that the same lateral load will be experienced by the forms regardless of wall thickness. I know theory dictates it but when you look at it in a practical sense you would think that a 2" thick 5ft high wall would exert a lot less lateral load on the forms than a 10" wall.

Thank you again!

 

[Bbird]

If the concrete is fluid enough the pressure will get into the 2" wall laterally and so its pressure is no different from a 10" thick wall.

 

[jheidt2543]

Just a follow up to give the correct book citation:

Hurd, M.K., "Formwork for Concrete, SP-4", 6th Edition, American Concrete Institute, Farmington Hills, MI, 1995, 500 pp.

Also, the October 2003 issue of "Concrete International", the magazine of the Amercian Concrete Institute, on pages 41-46, had an article titled "Using Interactive Spreadsheets for Teaching Concrete Formwork Design". A good article on formwork and an example of speadsheet design for formwork calculations.

 

[Ginger]

For those in the UK, the definitive guide is CIRIA Report Number108. This gives design tables and design guidance on formwork pressures based on the height of pour, rate of rise up the shutter, concrete temperature, cement content etc etc. It incorporates factors for walls and columns also.

Regards

 

[AlohaBob]

These thoughts seem to presume that concrete is a fluid like water. This in turn would give very conservative predictions for design of formwork for instance, since concrete doesn't behave like this. I just don't think it's so clear cut.

Internal frictions can induce a 2 ft high 8" wall poured over a slab and free to flow out at the bottom. If it were fluid, it wouldn't stand up to this head pressure. It would all flow out the bottom.

I'm sure there's many factors involved. Including mix design and choice of agregates and admixtures. Age of mix, temperature, consolidation, vibration.

I wouldn't so quickly discount the form thickness either. I'm sure a 2" thick wall would stand much higher than an 8 inch wall. It may have something to do with surface friction. In which case, the wall pressure of an 8" wall would be higher than a 2" wall.

The form designers have rules, but they aren't as simple as suggested.

 

[SlideRuleEra]

Another factor is the size & spacing of rebar in a wall. Sometimes it takes a lot of mechanical vibration to get the concrete thought the rebar mat where it can exert any pressure directly on the forms (if there is surface honeycomb there must not have been much pressure on the wall forms).
As stated above, treating concrete as a fluid (in a wall) is very conservative; the "slump" of water is 12 inches, compared with 3 to 6 inches for "typical" fresh concrete.