Theatre Design & Technology - Spring 1981 - 20

Paper
Honeycomb
Sandwich
Structures
Engineering Data

tively, as experience and design parameters dictate. In a test
section, two levels are calculated for each of the three factors;
an allowable, which generally takes laboratory values and
modifies them for safety margins, and a theoretical maximum
level for the trial combination of materials and conditions. If
any of the maximum values exceed the allowable the trial materials fail in analysis: a new selection must be made and analysis started over.
In the next step, the materials properties for the facings and
core may be obtained from tables and charts published by
manufacturers and their associations. Honeycomb Structural
Products (formerly Hexcel Corp.) publishes data on their paper honeycomb products and the American Plywood Association or the Hardwood Plywood Manufacturers Association
both publish manuals with plywood performance specifications. The other values needed are beam coefficients for the
type of beam being analyzed, and can be found in engineering
or mechanical reference works.
For scenic work it is practical to construct only a few types
of beams. In engineering terms all of these are simply-supported or simple beams. This means the beam ends freely deflect upward as the center deflects or sags downward, as if
they were supported on broad-based knife blades or on pivots. The alternate types of beams are labelled "fixed" end
and may be thought of as encased in concrete. They must be
so firmly fastened that no end movement is allowed. Stage
carpentry and welding techniques used on temporary, often
moveable, non-architectural scenic units do not provide such
fixed ends. For a simple beam with a load distributed uniformly across its area the coefficients are:
Maximum shear force (V max) =

by Jack Neveaux
Maximum bending moment (Mmax) =

PL
8

Bending deflection constant (Kb) = 384
Over the past few years, paper honeycomb sandwich panels have produced efficient, cost effective, and design enhancing structures for scenic applications. The engineering
properties designed into the paper core material are well
tested and widely accepted in other industries (e.g., modular
home construction, industrial pallets, and downhill skis). However, the idea of trusting twenty actors or a heavy scenery
piece traversing a twenty-foot plywood arch with nothing but a
paper core still appears inherently problematic, if not farfetched.
To put your mind at ease, I would like to do two things.
First, to describe the mathematical procedure for beam design
analysis for a simply supported plywood and paper honeycomb sandwich "beam." This procedure is based on formulas
contained in Hexcel Corporation's Technical Bulletin 702.
Properly laminating such a sandwich creates a structural unit
which can be analyzed and assigned strength values as would
a 4/1 x 8/1 timber or a steel I-beam. Second, to present, in
four graphs and tables, the results of a computer program
which carried out this same mathematical beam design analysis for many possible combinations of plywood and honeycomb thicknesses, and makes it possible to choose safe, effective sandwich components, but eliminates lengthy
longhand calculations. An understanding of the design analysis will be beneficial, though, so please read on ...
A structural sandwich panel is an assembly consisting of a
lightweight core securely laminated between two relatively thin,
strong facings. (American Plywood Association's Plywood Design Specification, April 1978, Supplement 4, 1.1)

An engineer considers three essential factors when analyzing a sandwich beam design: deflection, facing bending
stress, and core shear stress. The analysis begins with selection of trial facings and core material. These choices may be
made arbitrarily from a known range of materials, or selec-

1
Shear deflection constant (Ks) = 8

P = w L where w = Live Load + Dead Load = Uniform Load (pst)
and L = Beam span or distance between
supports.
Live load is the weight of the people and objects the beam
must support. The weight of the materials in the beam itself
constitutes the dead load. Together they equal the combined
load, which, when divided by the area supporting the load
gives the unit, pounds per square foot (pst).
In the next step, check allowable deflection. Scenery platforms serve the purpose of floors and allowable deflection for
floors according to the Uniform Building Code Table 23-0 is:
L
240
Maximum deflection = Shear deflection
+ Bending deflection.
(The actual equation, using the constants Ks and Kb, incorporates too many variables to explain here, but both the H.S.P.
and A.P.A. bulletins cover it in detail.) If the maximum deflection is less than the allowable. proceed to check facing bending stress.
Allowable facing bending stress =
Yield stress of facing material
(from tables)
Safety Factor
The user determines the safety factor according to the conditions and risks involved, and it usually ranges from 1.5 to 5.

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 1981