pad to a spring bridging strip, with feet at each end for mounting on the
backboard (Fig. 6a).
This arrangement, which allows both increase and decrease of
curvature to take place in the panel while it maintains contact with the
support, was retained in principle but modifiedto suit the new lattice
design (Fig. 6b).
It was considered that one function of the action of the bridges
could be improved if they were inverted with both feet mounted onto the
battens, thus providing two reasonably spaced points of pressure against
the battens. This arrangement would encourage return movement
equally of the top and bottom of the panel to a neutral position, when
the curvature reduces, rather than the panel pivoting on the center pads.
In order that both feet could be mounted on a surface with variable cur-
vature, the timber pads were given a Plastazote^8 foam core, allowing
them to adjust to the changes. The pressure pad, which would now be in
contact with the backboard, was also made into a timber-foam sandwich
so as to prevent the creation of a rigid area being in the center of the
spring strip. The modification to the pads improves the overall cushioning
effect and allows differential changes of curvature, dimension, and align-
ment to be absorbed.
Another advantage gained by inverting the bridges is that a nar-
rowbar can be used to bear against the pressure pads. Previously, if a bar
were used, it would have had to be wide enough to engage both bridge
feet, or else a backboard would have had to be rigid enough to take the
spring pressure without bowing.
If a retaining bar is used to take the spring pressure, then the
backboard can be reduced in thickness and weight (which may be consid-
erable on a large panel) and can then act purely as a lightweight environ-
mental barrier (Fig. 6c illustrates this later development). The position of
the bar should be such that it engages to produce a slight preload of the
spring bridges just adequate to retain the panel against the slip profile.
(Note: Most pressure will occur against the bar during high RH, when the
panel will tend to flatten, producing a far greater deflection of the spring
bridges than when curvature increases.) In this particular case, the back-
board was a single sheet of plywood with a reinforcing section of timber
glued to the underside to stiffen it (Fig. 6b). In later supports, the improve-
ment of a rigid framing bar was adopted.Evaluation of batten flexibility
Throughout this development, probably the most difficult judgment to
make was to determine the degree of stiffness or flexibility of the sup-
port lattice to match the panel’s requirements. With experience, it is pos-
sible to make a reasonable assessment of the strength of small panels,
but when a panel is so large that it cannot safely be lifted, handled, and
flexed by one person, this becomes very difficult. Even when it is within
a manageable size, it is not easy to evaluate hidden weaknesses resulting
from small fractures, compression damage, and structural deterioration
resulting from age. Evalues (modulus of elasticity) cannot be used to
assess strength (resistance to bending) in the cross-grain direction. Tables
ofEvalues for timber only apply to bending at points along an axis par-
allel to the grain.^9390 Marchant