Theatre Design & Technology - Spring 2008 - 62

4´
6´

Figure 6. Isometric view, 4´x6´ foam-core sandwich panel, top skin cut away for clarity.

to be 1/2˝ CD, exterior grade plywood, and the stringers to be
#2-grade spruce-pine-fir (SPF) 2x2 placed on 1´ centers. Constructed in this manner, a 4´x6´ stressed skin panel can support
an evenly distributed load of 54 psf. (A 4´x6´ panel is the longest panel, built at this thickness, which passes our criterion of
being able to carry at least a 50 psf uniform load. Although an
8´ panel would seem to be more cost effective-after all, why
waste that 2´x4´ chunk of plywood?-the thickness of the plywood and stringers required would have to increase to support
the load across an 8´ span, increasing the overall cost.)
Similar to a stressed-skin panel, a sandwich panel is
designed to maximize strength by combining the structural
properties of plywood skins with those of interior materials.
In a stressed-skin panel, the interior is made up of individual
members; in a sandwich panel, this idea is extrapolated to the
extreme: instead of relatively thin, evenly spaced members, the
interior of a sandwich panel is made up of, essentially, one
member, the width of the entire panel. Using lumber as the core
of a sandwich panel would result in an incredibly heavy panel;
instead, a lightweight material which resists crushing and which
resists shear can be utilized. Common choices for the core of
a sandwich panel include pre-manufactured paper honeycomb
sheets, polystyrene foams, and polyurethane foams. Assuming a
strong glue bond between the core and the plywood faces, the
top skin resists compressive forces due to bending, the bottom
skin resists tension forces due to bending, and the core resists
shear (see fig. 6).
Along with creating a strong glue bond between the core
and the skins, other construction and application methods
must be taken into consideration when examining the strength
of a stressed-skin panel. Stressed-skin panels are strongest
when supported fully across the ends running perpendicular to
the face grain of the skins (when the unsupported span is parallel to the face grain). Additionally, methods of staggering the
seams in the skins must be carefully chosen; as with stressedskin panels, above, however, this analysis will be restricted to

panels 8´ in length or shorter, negating any concerns about
seams in the skins.
The advantage of a sandwich panel lies in the fact that
thin panels can carry relatively heavy loads. Consequently, for
this analysis, we'll examine a thin panel, assuming the plywood
skins to be 1/2˝ CD, exterior grade plywood, and the core to be
commercially-available extruded 2˝ polystyrene foam insulation
(specifically, Dow Chemical STYROFOAM brand Square-Edge insulation board). Constructed in this manner, the overall allowable
load for a 4´x6´ sandwich-core panel is 40 psf, determined by
deflection. Although this panel's overall maximum allowable load
is below 50 psf, we can reasonably include this panel in our comparison, because this allowable load is determined by deflection,
not a catastrophic failure. Deflection is an arbitrary criterion, and
the allowable loads for bending and shear for a 4´x6´ sandwich
core panel are above 100 psf. Similar to the stressed-skin panels above, an 8´ panel length might seem more cost-effective, but
would necessitate thicker skin and core materials to achieve the
same structural strength, increasing the overall cost even while
reducing waste, making a 6´ panel a logical choice.

Part II: EffIcIEncy analySIS

ow that we have identified panel/platform designs
which perform their structural functions with relatively similar efficacy, we can examine each design in
terms of a number of different categories: material
cost, manufacturing labor required, installation labor required, reusability, and storage requirements.
In addition, as the above structural analysis assumes
a rectilinear, fully-supported deck, some attention
will be paid to each technique's structural advantages and disadvantages for non-rectilinear decking
applications, as well as those decking applications requiring
overhangs and cantilevers. This analysis will assume a "trial
deck" 24˝ high, 32´ wide, and 24´ deep.

S P R I N G

2008

theatre

design

technology

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 2008