Theatre Design & Technology - Fall 2009 - 28

reacts (strain) to a force (stress) placed on it. How a wood panel reacts
to the stress is the Modulus of Elasticity and is one of the more revealing
indicators of a panels strength in structural applications. However, while
all wood composites have MOE data for their products, this is not meant
to represent material performance in structural applications. In fact,
within wood based composites, only plywood, OSb, COM-PlY, and specific particle board classifications approved for manufactured housing,
are considered structural panels suitable for applications in stage scenery
as platform decking and other situations where they span between structural support beam. Structural applications of OSb and Plywood are well
covered in materials available through the Engineered Wood Associatation (apawood.org) where the Plywood design Specification (five supplements now exist) is available. Specific stage scenery examples of plywood
structural applications exist in the text, Structural Design for the Stage.
In wood composites, MOE is largely an indicator of the strength of
fibers and the ability for those fibers to absorb and transfer stress and
strain from wood particle to wood particle. The size and shape of wood
fibers are key characteristics when assessing MOE. For example, a sheet
of OSb has large particle flakes and is a strong structural panel, while
standard hardboard is comprised of very small fibers and has an MOE
about half that of OSb. This illustrates how important wood fibers size is
to the MOE and overall stiffness of wood composite panels.

Product 1

Grade

Sheet Weight 2
(lbs)

Particleboard Underlayment
Particleboard for manufactured
home decking

PBU
D-2

80 to 100
-

Standard

94 to 100
33 to 53 (4˝ basis)

Tempered

40 to 53 (4˝ basis)

SkyBlend (premium-blend Part. Bd.)
Hardboard

LDF (low density fiberboard)
MDF (medium density fiberboard)
HDF (high density fiberboard)
GlacierGreen (Premium "Green"
LDF-MDF-HDF)
Wheat Straw Board
Homasote
Fiberboard
OSB
Plywood

140
.118˝ to .354˝ thick
.354˝ to .551˝ thick
PBU
440 Sound Barrier
Celotex-Soundstop

80
90 to 94
106 to 120
110
98
76-84
34 to 37 (2˝ basis)
26 to 41 (2˝ basis)
72 to 85
65 to 75

MOR - Modulus of Rupture

Modulus of Rupture (MOR), is the measure of strength at the instant of
failure. Taking strength testing beyond that of measuring the elasticity and
flexibility of a material (MOE), the MOR measures this breaking point
and the force necessary to do so. Fiber flexibility is less of a concern at
this point, as the MOR is measuring the adhesive strength between the
adhesive and fibers under stress. In some instances, this bond is of little
concern as with fiberboard insulation (soundboard, or Celotex) where
almost no adhesive is incorporated into the panel. Tempered-class hardboard has an MOR 25% higher than standard hardboard, reflecting the
harder and stiffer characteristics of tempered hardboard. Therefore, a
high MOR typically equals a hard and stiff, but often brittle, wood panel.
Hardness

Hardness measures the dent resistance of a -material at the surface. The
test procedure uses a solid steel ball, one-inch in diameter, and presses
it into the material face. The force, depth of the dent and impression left
by the ball are both measured. Harness relates directly to the durability of
a surface. The harder the surface, the more durable that surface will be.
Unfortunately, Hardness data for hardboard, the most popular floor covering for stage scenery, is not published. However, experience tells us that
hardboard is very smooth verses most particle board panels which have
a rough surface texture that telegraphs through most paint treatments.
These surface characteristics can be considered useful when specifying
certain painted finishes. For example, a "sandy" or heavily spattered floor
treatment may be improved when applied to a particle board floor because the panel's texture could improve the visual texture of the treatment
enhancing the final look. The Hardness of underlayment particleboard, at
500 pounds per square inch suggests that it would be a durable flooring
material as if it were painted onto standard hardboard. Since Hardness
relates directly to surface durability, Hardness can predict the durably

theatre

design

technology

2009

of a flooring material. The higher the Hardness, the more durable the
surface. Of course, this does not bring into account the slipperiness of
that surface, which will increase with Hardness. However, since surface
roughness is not a published specification for most wood composites, it is
up to the end-user to gain personal experience to interpret which material
meets the needs of each application on a case-by-case basis.
Screw Holding

The force necessary to pull a wood screw out of a panel both parallel
and perpendicular to the panel face is known as Screw Holding. Of all
published specifications, Screw Holding is most common among all composites and provides a measurable, predictable, and intuitive sense of
material strength. Manufacturing facilities continuously perform Screw
Holding tests to maintain consistent products and processes. While Screw
Holding tests are standardized, the specific screws and installation methods used in stage scenery construction introduce many variables. Particularly with homogenous non-structural wood composites of particle
board and MdF, it is important to pre-drill screw holes roughly two-thirds
the diameter of the screw threads themselves. When performed properly,
screwed joinery together with liberal glue can yield extremely strong material connections.

An Overview of Non-Structural Wood-based Composite Panels
Particleboard

articleboard is comprised of hammermilled wood particles
that are easily visible on the surface. Over fourteen different classifications of particle board exist, but PbU,
or particleboard underlayment, is most prevalent and
locally available nationwide. Particleboard may be the

http://www.apawood.org

Table of Contents for the Digital Edition of Theatre Design & Technology - Fall 2009