Q.: In the American Concrete Institute's Formwork for Concrete, the formula for computing lateral pressure on wall forms when the placing rate is 7 to 10 feet per hour is:

p = 150 + 43,400/T + 2800 R/T

where:

p lateral pressure in pounds per square foot

T = temperature of the concrete

R = placement rate in feet per hour 0

In the same book, the formula for pressure on column forms where lifts don't exceed 18 feet is:

p = 150 + 9000 R/T

How do you calculate form pressures if the rate of placement for walls will exceed 10 feet per hour or the lift height for columns will exceed 18 feet?

Also, is there a length- or width-dimension combination that distinguishes a wall from a column? For instance, is a 12-inch-wide by 36-inch-long section of 10-foot-high concrete a short wall or a wide column?

A: We contacted Formwork for Concrete author Mary Hurd for some answers. She says the form-pressure formulas for walls and columns were developed from experimental data, and their application is limited to conditions matching those under which the supporting data were observed. For conditions outside those stated with the formulas, ACI Committee 347, Formwork, recommends that you revert to the formula for a full liquid head:

p = wh

where:

w = density of the fresh concrete in pounds per cubic foot

h = depth of fluid, or plastic, concrete

She says to keep in mind that the depth of the fluid, or plastic, concrete isn't necessarily always the height of the form.

Hurd says ACI Committee 347 doesn't give guidelines for differentiating between walls and columns, but a definition from the British Standards Institute may be helpful. According to BSI, if none of the cross-section dimensions is greater than 2 meters (6.6 feet), it's considered a column.