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

78 Chapter Four
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Connections for Species of Wood for Heavy Formwork
In the preceding sections, values were given for nails to fasten Southern
Pine, Douglas Fir-Larch, and Hem-Fir species of wood. These are
the materials that are commonly used for formwork. However, for
some projects it is necessary to build formwork from larger members,
with species of wood that have higher densities and strengths, and
with fasteners that are stronger than common nails.
Screws, bolts, or timber connectors are often used to fasten larger
members or members with high-density wood. The following sections
present information for these types of fasteners for Southern
Pine, Douglas Fir-Larch, and Hem-Fir wood.
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Lag Screws
Lag screws are sometimes used with formwork, especially where
heavy wood members are fastened together. These screws are available
in sizes varying from ¼ to 1¼ in. shank diameter with lengths
from 1 to 12 in. The larger diameters are not available in the shorter
lengths.
Lag screws require prebored holes of the proper sizes. The lead
hole for the shank should be of the same diameter as the shank. The
diameter of the lead hole for the threaded part of the screw should
vary with the density of the wood and the diameter of the screw. For
lightweight species of wood with a specific gravity less than 0.50, the
diameter for the threaded portion should be 40 to 50% of the shank
diameter. For species with a specific gravity between 0.50 and 0.60,
the diameter of the threaded portion should be 60 to 75% of the shank
diameter. For dense hardwoods with a specific gravity greater than
0.60, the diameter of the threaded portion should be 65 to 85% of the
shank diameter.
During installation, some type of lubricant should be used on the
lag screw or in the lead hole to facilitate insertion and prevent damage
to the lag screw. Lag screws should be turned, not driven, into the
wood.
Withdrawal Resistance of Lag Screws
The resistance of a lag screw to direct withdrawal from wood is related
to the density or specific gravity of the wood and the unthreaded
shank diameter of the lag screw. Table 4-25 provides the withdrawal
design values for a single lag screw installed in species of wood that
are commonly used for formwork, including Southern Pine, Douglas
Fir-Larch, and Hem-Fir. The values for the lag screws are given in
pounds per inch of thread penetration into the side grain of the main
member. The length of thread penetration in the main member shall
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