Civil Engineering Reference
In-Depth Information
20
Formwork
20.1
INTRODUCTION
Concrete forms are molds into which semiliquid concrete is placed. The molds need to be
sufficiently strong to hold the concrete in the desired size and shape until the concrete
hardens. Since forms are structures, they should be carefully and economically designed
to support the imposed loads using the methods required for the design of other engineer-
ing structures.
Safety is a major concern in formwork because a rather large percentage of the acci-
dents that occur during the construction of concrete structures is due to formwork failures.
Normally, formwork failures are not caused by the application of excessive gravity load-
ing. Although such failures do occasionally occur, the usual failures are due to lateral
forces that cause the supporting members to be displaced. These lateral forces may be
caused by wind, by moving equipment on the forms, by vibration from passing traffic, or
by the lateral pressure of freshly placed and vibrated concrete. Sadly, most of these fail-
ures could have been prevented if only a little additional lateral bracing had been used.
There are, of course, other causes of failure, such as stripping the forms too early and im-
proper control of the placement rate of the concrete.
Although you might think that shape, finish, and safety are the most important items in
concrete formwork, you should realize that economy is also a major consideration. The
cost of the formwork, which can range from one-third to almost two-thirds of the total cost
of a concrete structure, is often more than the cost of both the concrete and the reinforcing
steel. For the average concrete structure, the formwork is considered to represent about
50% of the total cost.
From this discussion it is obvious that any efforts made to improve
the economy of concrete structures should be primarily concentrated on reducing form-
work costs.
Formwork must be treated as an integral part of the overall job plan, and the
lowest bidder will be the contractor who has planned the most economical forming job.
When designers are considering costs, they tend to think only of quantities of materi-
als. As a result, they will sometimes carefully design a structure with the lightest possible
members and end up with some complicated and expensive formwork.
20.2
RESPONSIBILITY FOR FORMWORK DESIGN
Designers of temporary structures such as concrete formwork must meet the requirements
of the Occupational Safety and Health Act (OSHA) of the U.S. government. In addition,
there may be state and local safety codes to which designers must adhere.
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