Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.23
formwork consist of the weight of the forms and the weight of and pressures from
freshly placed concrete. Live loads include weights of workers, equipment, material
storage, and runways, and accelerating and braking forces from buggies and other
placement equipment. Impact from concrete placement also should be considered
in formwork design.
Horizontal or slightly inclined forms often are supported on vertical or inclined
support members, called shores, which must be left in place until the concrete
placed in the forms has gained sufficient strength to be self-supporting. The shores
may be removed temporarily to permit the forms to be stripped for reuse elsewhere,
if the concrete has sufficient strength to support dead loads, but the concrete should
then be reshored immediately. Loads assumed for design of shoring and reshoring
of multistory construction should include all loads transmitted from the stories
above as construction proceeds.
9.18.1 Pressure of Fresh Concrete on Vertical Forms
This pressure may be estimated from
where p � lateral pressure, psf
R � rate of filling, ft/h
T � temperature of concrete, �F
R
p � 150 � 9000 (9.5)
T
See Fig. 9.6a.
For columns, the maximum pressure p max is 3000 psf or 150h, whichever is less,
where h � height, ft, of fresh concrete above the point of pressure. For walls where
R does not exceed 7 ft/h, p max � 2000 psf or 150h, whichever is less.
For walls with rate of placement R � 7,
43,400 R
p � 150 � � 2800 (9.6)
T T
where p max � 2000 psf or 150h, whichever is less. See Fig. 9.6b.
The calculated form pressures should be increased if concrete unit weight exceeds
150 pcf, cements are used that are slower setting than standard portland
cement, slump is more than 4 in. with use of superplasticizers, retarders are used
to slow set, the concrete is revibrated full depth, or forms are externally vibrated.
Under these conditions, a safe design assumes that the concrete is a fluid with
weight w and p max � wh for the full height of placement.
9.18.2 Design Vertical Loads for Horizontal Forms
Best practice is to consider all known vertical loads, including the formwork itself,
plus concrete, and to add an allowance for live load. This allowance, including
workers, runways, and equipment, should be at least 50 psf. When concrete will
be distributed from overhead by a bucket or by powered buggies, an additional
allowance of at least 25 psf for impact load should be added. Note that the weight
of a loaded power buggy dropping off a runway, or an entire bucket full of concrete