Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

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Economy of Formwork 13
to fit around column tops. Wide columns with narrow beams are the
most expensive intersections to form by far because beam forms must
be widened to column width at each intersection.
Economy in Formwork and Sizes of Concrete Beams
Cost savings can be accomplished by selecting beam widths that are
compatible with the standard sizes of dimension lumber. Consider a
concrete beam 18 ft long with a stem size below the concrete slab that
is 16 in. deep and 14 in. wide. If 2-in.-thick lumber is used for the soffit
or beam bottom, it will be necessary to rip one of the boards in
order to provide a soffit that has the necessary 14.0 in. width. However,
if the width of the beam is increased to 14.5 in., two pieces of
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lumber, each having a net width of 7.25 in., can be used without ripping.
Thus, two 2- by 8-in. boards will provide the exact 14.5 in. width
required for the soffit. The increase in beam width from 14.0 to 14.5 in.—
an additional 0.5 in.—will require a small increase in the volume of
concrete as shown in the following equation:
Additional concrete = [(16 in. × 0.5 in.)/(144 in. 2 /ft 2 )] × [18 ft]
= 1.0 cu ft
Because there are 27 cu ft per cu yard, dividing the 1.0 cu ft by
27 reveals that 0.037 cu yards of additional concrete are required if
the beam width is increased by 0.5 in., from 14.0 to 14.5 in. If the cost
of concrete is $95.00 per cu yard, the increased concrete cost will be
only $3.52. The cost for a carpenter to rip a board 18 ft long will
likely be significantly higher than the additional cost of the concrete.
Also, when the project is finished, and the form lumber is
salvaged, a board having its original or standard width will probably
be more valuable than one that has been reduced in width by
ripping.
There are numerous other examples of economy of formwork
based on sizes of form material. For example, a 15.75-in. rip on a 4-ftwide
by 8-ft-long plywood panel gives three usable pieces that are
8 ft long with less than 1 in. of waste. A 14-in. rip leaves a piece 6 in.
wide by 8 ft long, which has little value for other uses. With 6 in. of waste
for each plywood panel, essentially every ninth sheet of plywood is
thrown away.
This is an area in which architects and engineers can improve the
economy in designing concrete structures. Designs that are made primarily
to reduce the quantity of concrete, without considering the effect
on other costs, may produce an increase rather than a decrease in the
ultimate cost of a structure. Additional savings, similar to the preceding
example, can be achieved by carefully evaluating the dimension lumber
required to form beam and column details.
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