Design Considerations for Economical Formwork - Simplified Design of Reinforced Concrete Buildings - page 257

9.2.2 Repetition

Whenever possible, the sizes and shapes of the concrete members should be repeated in the structure. By doing
this, the forms can be reused from bay to bay and from floor to floor, resulting in maximum overall savings.
The relationship between cost and changes in depth of horizontal construction is a major design consideration.
By standardizing the size or, if that is not possible, by varying the width and not the depth of beams, most
requirements can be met at a lowered cost, since the forms can be reused for all floors. To accommodate load and
span variations, only the amount of reinforcement needs to be adjusted. Also, experience has shown that changing
the depth of the concrete joist system from floor to floor because of differences in superimposed loads actually
results in higher costs. Selecting different joist depths and beam sizes for each floor may result in minor savings in
materials, but specifying the same depth for all floors will achieve major savings in forming costs.

9.2.3 Simplicity

In general, there are countless variables that must be evaluated and then integrated into the design of a building.
Traditionally, economy has meant a time-consuming search for ways to cut back on quantity of materials.
As noted previously, this approach often creates additional costs—quite the opposite effect of that intended.

An important principle in formwork design is simplicity. In light of this principle, the following questions
should be considered in the preliminary design stage of any project:

(1) Will custom forms be cost-effective? Usually, when standard forms are used, both labor and materials costs
decrease. However, custom forms can be as cost-effective as standard forms if they are required in a
quantity that allows mass production.

(2) Are deep beams cost-effective? As a rule, changing the beam depth to accommodate a difference in load
will result in materials savings, but can add considerably to forming costs due to field crew disruptions and
increased potential for field error. Wide, flat beams are more cost-effective than deep narrow beams.

(3) Should beam and joist spacing be uniform or vary with load? Once again, a large number of different
spacings (closer together for heavy loads, farther apart for light) can result in material savings. However, the
disruption in work and the added labor costs required to form the variations may far exceed savings in materials.

(4) Should column size vary with height and loading? Consistency in column size usually results in reduced
labor costs, particularly in buildings of moderate height. Under some rare conditions, however, changing
the column size will yield savings in materials that justify the increased labor costs required for forming.

(5) Are formed surface tolerances reasonable? Section 3.4 of ACI Standard 347 9.2 provides a way of
quantitatively indicating tolerances for surface variations due to forming quality. The suggested tolerances
for formed cast-in-place surfaces are shown in Table 9-1 (Table 3.1 of ACI 347). The following simplified
guidelines for specifying the class of formed surface will usually minimize costs: a) Class A finish should
be specified for surfaces prominently exposed to public view, b) Class B finish should be specified for
surfaces less prominently exposed to public view, c) Class C finish should be specified for all noncritical
or unexposed surfaces, and d) Class D finish should be specified for concealed surfaces where roughness
is not objectionable. If a more stringent class of surface is specified than is necessary for a particular
formed surface, the increase in cost may become disproportionate to the increase in quality; this is
illustrated in Fig. 9-1.