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When tops are mechanically prevented from shrinking and swelling,
splitting of the entire top can occur. This principle is illustrated in Figure
24 where a round pedestal table with a split in the top is shown. The top is
supported by cross rails with the end of each rail firmly anchored to the
top. Splits develop in tops supported
as shown here because the top is
prevented from shrinking across the
grain by the cross rail which is
anchored at each end to the edge of
the top with screws. As the top
begins to shrink across the grain, it is
prevented from doing so by the cross
rail. Eventually, a point is reached
where the internal stresses exceed the
tensile strength of the wood, and the
top splits.
This damage could have been
prevented if the screws had been
inserted through slots which would have
allowed slip to take place between the top and the cross rail, Figure 25,
without creating internal stresses in the top. The two screws located in the
cross rail near the pedestal should not cause problems because only a small
amount of shrinking and swelling should take place in the wood between
them. These two screws can, therefore, be used both to locate the rail and
to firmly attach it to the top.
Splitting in rectangular tables with solid wood tops occurs for
essentially the same reason. If the rails are attached to the underside of the
top with screws at the points shown in Figure 26, the end rails will prevent
the top from shrinking if it begins to dry out in service. As a result, the top
will split. With solid wood tops,
these attachments should be made
with connections that allow
movement to take place between
the top and the rails, i.e., with slip
joints. When screws are used,
they should be located in the
middle portion of the end rails as
shown in Figure 27. By locating
the screws in this area, the amount
of shrinkage that occurs between
the fixed fasteners may be held to
acceptable levels.
Figure 23. Splits may develop in the ends of
solid wood tables if the ends dry out too
rapidly.
Figure 24. Splits develop in solid wood tops
when they are prevented from shrinking and
swelling across the grain by cross rails.
Figure 27. To allow the top to shrink and
swell, screws should be located towards the
centers of the end rails.
Figure 25. Splitting can be prevented by
inserting the screws through slots that
allow the top to move.