For a self-supporting panel, a shaped slip would normally be made
to suit the panel profile when the panel is stabilized at 55% RH. If environ-
mental conditions remain stable, good contact should be maintained with
the shaped slip on all four edges, and only small, balanced reaction forces
will result (Fig. 1a). Problems can arise, however, as soon as conditions
change (Fig. 1b, c).
Figure 1 shows a panel’s response to environmental changes.
Differential absorption or loss of moisture content in the panel, due to
changes in RH, cause it to warp (Thomson 1978:208–10). The opposing
forces illustrated in Figure 1b and 1c may result in bending stresses, which
in an already weak panel could result in fracture. These adverse effects are
further accentuated when the grain runs horizontally, because the weight
of the panel resting on its supporting edge causes frictional resistance to
the movement needed to accommodate a change in curvature. In large
panels, forces can be magnified by leverage to produce dangerously high
concentrations ofstress some distance from where the resistance to move-
ment occurs. If an area of weakness exists, failure is likely to occur there.
Under these circumstances, the use of one of these types of secondary
supports would not be satisfactory.
Having fully assessed the condition of a panel, the panel conserva-
tor must make a decision as to whether an attached support will be neces-
sary. After removal of a cradle or damaging support from a panel, it would
be preferable not to have to make any further attachment. However, there
are circumstances in which this measure cannot be avoided.
As a general rule, if an unframed panel cannot be handled
confidently or will not safely support its own weight when placed horizon-
tally on a surface, then an attached support should be considered in order
to provide the required reinforcement. It is almost impossible to reinforce a
weak panel without using an attached support. But an attached support can
be designed to ensure that it is in sympathy with the panel’s requirements.
Areinforcing structure is required to help strengthen a weak panel
and assist in spreading stresses more uniformly. The other function of a
secondary support in this situation should be to act as a restraint by allow-
ing changes of curvature to take place in a controlled manner and within
predetermined limits. Therefore, the secondary support should be flexible.
This design concept was successfully established by Simon Bobak (see “A
Flexible Unattached Auxiliary Support,” herein) for the unattached support
ofsmall panels but would need considerable development before it could
be applied to an attached support for large, heavy panels.Batten design
In an attempt to design an attached support that would fulfill these basic
requirements, an analysis was first made of the effect of attaching a uni-
form rectangular-section batten to a curved surface. It was hoped that this
would also provide a better understanding ofwhy some cradles, even if
they allow movement, still have a damaging effect on panels.
The simplified representations in Figure 2a–d show curves
achieved by loading a uniform rectangular-section batten. The curvature of
the panel is shown exaggerated as an arc with a constant radius of curva-
ture (Figs. 2a, b). The batten is deflected within the arc by the application
of a force at its center (Fig. 2b). This situation may also be represented dia-
gr ammaticallyas a simplysupported beam loaded at its center (Fig. 2d).384 Marchant
5 5% RH
aBelow 55% RHbAbove 55% RHcFigure 1a–c
Apanel retained against a slip profile using
spring framing clips. The magnitude of pres-
sure at these points and the reaction of the
profiled support is shown by the arrows. At
55% RH (a), minimum pressure is applied at
four retaining points. Belowand above 55%
RH (b, c), the magnitude of pressure increases
and the reaction of the profiled support is
concentrated at the contact points. These
forces would be similar if other unattached
support systems were used as a method of
retention.