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\technote{4}{Dan Ports}{2004/03/11}{drkp@mit.edu}
\tntitle{Triscuits}

\section{The design}
\label{sec:design}

The triscuit is a type of panel for use in decking,
developed by the Yale School of Drama. Its stress-skin design gives it
several advantages over the traditional platform decking. This note
presents the basic design and the structural rationale behind it, a
brief comparison to other designs, and some notes on common
variations.

The triscuit is a stress-skin platform. In the Yale design, the
dimensions are four feet by four feet square, with a thickness of 2
$\frac{3}{8}$ inches. A frame is constructed from $\frac{5}{4}$ nominal pine
lumber, as in Figure~\ref{fig:frame}. The $\frac{5}{4}$ boards are
laid flat (i.e. in their weak direction), and glued and stapled or
screwed together. Five \emph{stringers}, pieces of $\frac{5}{4}
\times 2''$ (true) are used as the structural elements of the frame (three at
$3' 7 \frac{7}{8}''$ and two at $3' 9 \frac{7}{8}''$), and two pieces
of $\frac{5}{4}'' \times 1$ (true) serve as endcaps, holding the others
together but otherwise non-structural..

\begin{figure}[htbp]
  \centering
  \includegraphics[width=5.5in]{21m735-tn4-frame}
  \caption{Frame construction layout}
  \label{fig:frame}
\end{figure}


The two faces are constructed of $\frac{5}{8}''$ plywood, generally
CDX, cut to $3' 11 \frac{7}{8}''$ square. The triscuits are assembled
at slightly under their $4' \times 4'$ nominal dimensions in order to
allow for a $\frac{1}{8}''$ gap between adjacent panels in a deck that
prevents squeaking during use. The faces must be carefully attached by
covering the entire frame surface with a glue coat, then attaching the
plywood with nails or screws.

The strength of this platform comes from its stressed-skin design.
When the platform is loaded, it has a natural tendency to bend under
the applied force. Because the top and bottom skins are securely
attached to each other through the stringers, the top skin must
contract and the bottom skin must expand in order to bend. The result
is that the top skin resists compressive stresses, the bottom skin
tensile stresses, and the stringers shear stresses. The wood
effectively resists these stresses, and therefore the panel as a whole
can withstand greater loads with minimal deflection than any of the
individual pieces could alone. With this design, the platforms can
support a load of at least 50 psf. However, it is critical that the
plywood faces are carefully attached to the lumber stringers.
Moreover, the plywood must be oriented such that the face grain is
parallel to the $2''$ framing members. If these conditions are not
met, the result will not have the strength of a stress-skin platform.

A triscuit deck must be supported by a system of studwalls, typically
made from cheap, readily-available $2\times 4$ lumber. The studwalls
are placed $2'$ apart. Unlike standard
platform legs, the studwalls allow the platform to be evenly supported
along its length. It is essential to ensure that the studwalls are
placed perpendicular to the surface grain of the plywood, as in
Figure~\ref{fig:frame}. Since the endcaps are only $1''$ wide, if
properly oriented all five of the stringers will be placed over the
studwall's top plate; if oriented incorrectly, not all of the framing
members will be able to bear directly on the studwall, weakening the
platform considerably. Holes are drilled in the triscuit at each
corner, and four bolts or lag bolts are used to secure it to the
studwall.


\section{Advantages and disadvantages}
\label{sec:advantages}

As mentioned above, the triscuit can support a large load with a small
amount of materials used, relative to a traditional platform
design. This is its principal advantage: its high strength-to-size
ratio.

This translates into several practical advantages. Listed briefly:

\begin{itemize}
\item Each triscuit unit is small and light enough to be handled by
  one person.
\item It takes less space to store a set of triscuits than a set of $2
  \times 4$-framed platforms of equivalent area.
\item The limited materials required make triscuits cost-effective.
\item Triscuits are thin. This has obvious design advantages. In
  particular, the visual effect of a projecting overhang is magnified
  when the deck is so thin.
\end{itemize}

One disadvantage, however, is that each unit is more complex to build
than a standard platform. Two faces must be attached to the frame, and
they must be glued as well as screwed on. Moreover, the deck must be
supported using a studwall; the normal $4\times 4$ or $1 \times
3$~\textsf{L} legging cannot be used. Fortunately, however, it is
straightforward to attach the panels to the studwall.


\section{Some variations}
\label{sec:variations}

A common problem encountered when building triscuits is the
unavailability of $\frac{5}{4}''$ lumber. This has led various
technical designers to develop alternatives.

A simple variation uses standard $2\times$ stock instead. The $2\times$ lumber
is ripped down to the $1''$ thickness of the $\frac{5}{4}$ lumber,
to match any other triscuit platforms. The quality of the lumber is
important: the cuts must be made avoiding any knots in the wood. The
$1.5''$ true dimension of the $2\times$ nominal lumber is used in
place of the $2''$ true dimension of the $\frac{5}{4}\times 2''$ in
the original triscuit design.

Since the load is resisted by the skin, it is also possible to create
a stress-skin platform using foam in place of the stringers. At Penn
State, a $1''$ thick panel of extruded polystyrene was used, laminated
to OSB skins. A plastics manufacturing company created similar
platforms by injecting \textsf{A/B} foam between two panels. This must
be done very carefully in order to ensure that the foam is distributed
evenly throughout the platform and at a consistent thickness, in
addition to all the usual safety issues involved in working with
foams. The advantage is that the resulting platform has impressive
strength and weight properties, and a truly uniform internal
structure.

The Texas triscuit is a variation introduced at Trinity University.
The design has similar advantages in terms of strength and weight.
However, though the layout is similar, it has some important
differences structurally. Most obviously, it uses a frame made from $1
\frac{1}{2}''$ 16-gauge steel tube, in essentially the same shape as
Figure~\ref{fig:frame}. More significantly, it is not a stress-skin
platform, as it has no bottom skin. This means the stress-skin
structural analysis mentioned before does not apply; instead it relies
upon the strength of the steel used. Helpfully, it also means that it
is possible to attach a normal compression leg to the frame, instead
of using a studwall.


\section{References}
\label{sec:references}
\begin{itemize}
\item \emph{The Triscuit-Studwall Deck System}. Yale Tech Brief No. 1249. Don
  Harvey, Yale Repertory Theatre. April 1993. Available at\\
  \texttt{http://www.yale.edu/drama/publications/tech_brief/triscuits/triscuits.html}
\item \emph{The Triscuit and Texas Triscuit}. Michael Powers,\\
  \texttt{http://www.hstech.org/howto/carpentr/plats/mpplats3.htm}
\item Stagecraft mailing list archives
\item \emph{Structural Design for the Stage}. Holden and Sammler,
  Focal Press 1999
\item http://www.nabiscoworld.com/triscuit/
\end{itemize}


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