[This is the third post in a month-long series on stage decking and platforming. Check out this article in the December issue of Stage Directions, as well as this discussion forum.]

The typical 2x4 platform design is based on the strength of joist and girder construction: the joists are the members running perpendicular to the grain, while the girders are the members running parallel to the grain. The design of a stressed-skin panel derives its strength from the same concepts that give an I-beam strength to resist bending over long distances: concentrating the cross-sectional area at top and bottom of the member helps resist the compression and tension stresses that build up at the top and bottom of a beam that is under bending stress. In a stressed-skin panel, stringers run along the grain and are bonded to a top (and usually also a bottom) lid; the combination of the stringers and the lids form a series of I-beam shapes across the width of the panel.

The calculations for determining the allowable load of a stressed-skin panel can be a little intimidating, but can be completed using simple algebra. However, there are a lot of places where a small error could result in seriously over building or under building a panel; a project I typically assign the students in my structures class is to build a spreadsheet that performs the calculations so as to reduce the chance of mathematical errors. (I’ve got one that I show them [here]. You enter in the geometric properties of the panel—width, span, number of stringers, etc.—the geometric properties of the components, and the pertinent species properties of the components; the spreadsheet then calculates the multiple different allowable loads that describe the strength of the panel. The lowest of these is the governing load.)

Based on my spreadsheet, a 6’x4’ panel comprised of a ½” BC plywood (5-ply) top and bottom lid and 5 stringers (on 12” centers) 1 ½” x 1 ½” (#2 spruce-pine-fir) can carry a load of 43 pounds per square foot, roughly equivalent to our 2x4-framed 4x8 platform. (The governing load is “top skin deflection,” or how much the top plywood will flex between the stringers of the panel—all other tests exceed a 50 psf load. A load of 50 psf will cause the top skin to deflect slightly more than 1/8”, which while greater than the standard acceptable deflection in this situation, is probably not a deal breaker.) Additionally, a 4x4 panel, comprised of similar materials with 5 stringers will more than satisfy this 50 psf load considerations (with the same compromise on deflection in the top skin). For the purposes of this week’s post, I’d like to look at both panels.

Material costs:

Unlike rigid-framed platforms, stressed-skin panels require the use of stud wall supports, increasing the overall cost of materials for a deck. In some situations, this may overtake any savings to be had over material costs for individual platforms/panels. Using 4x4 panels will require the same number of stud walls as rigid-framed platforms (which require supports every 4 feet); using 4x6 panels will result in a slightly lower number of stud wall supports.

Construction Labor

4x4 panel

Approximately 2 person-hours, moderately-skilled (requires precise planing and shaping of stringers, counter-boring holes for installation bolts)

4x6 panel

Approximately 2 person-hours; moderately-skilled (requires precise planing and shaping of stringers, counter-boring holes for installation bolts)

Load-in Labor

4x4 panel

Requires 2 people to carry and install safely (although, in a pinch, 1 person can carry a panel—though this leads to a significantly faster rate of fatigue)

When using stud walls, can be very quick—four bolts/lags in the corners

4x6 panel

Requires 2 people to carry and install safely

When using stud walls, can be very quick—four bolts/lags in the corners

Long-term “save-ability”

4x4 panel

Very durable, especially is care is taken in the assembly process, and in setting counter-bored holes for installation.

4x6 panel

Very durable, especially is care is taken in the assembly process, and in setting counter-bored holes for installation.

Rep-ability

4x4 and 4x6 panels

Swiftly shifting a large deck requires multiple people (2 people are needed to carry each platform); weight of 4x6 panels can lead to fatigue

Weight

Last week, I mistakenly indicated ¾” plywood weighs 50 pounds per sheet; the actual weight of ¾” plywood is closer to 70 pounds. This would mean that a 2x4-framed or 1x2-steel-framed platform would weigh a total of about 115 pounds.

Volume/Size

4x4 panel: 4’x4’x0.21’ = 3.36 cubic feet

4x6 panel: 4’x6’x0.21’ = 5.04 cubic feet

Other considerations

Like their rigid-framed cousins, stressed-skin panels perform exceptionally well when creating rectilinear-shaped decks. When used to tackle organic profiles, stressed-skin panels present greater difficulties: their strength relies on support at either end, which means building an organic-shaped support. Barring that, stressed-skin panels can be overhung a stud wall support that is inset from the edge of the organic profile; however when used this way, the plywood lids must be supported along the entire width of the panel at the end—similar to how one might frame out an organic shape on a rigid-framed platform.

Like rigid-framed platforms, having a stock of stressed-skin panels makes building a deck relatively inexpensive, as all you really will need to purchase materials for will be stud wall supports and any suicide panels that are required to complete a particular shape.

Unlike rigid-framed platforms, once a stressed-skin panel is manufactured, there is no access to the framing. This makes supporting point loads that do not fall on stringers or stud walls impossible—if they exceed the loading capabilities of the plywood between the stringers, you’ll get deflection or even fracturing of the plywood skin. Careful planning is required to ensure boom pipes, piano wheels, and other large point loads land on stringers and/or stud walls.

The benefits of stressed-skin panels are pretty obvious: they are slightly less expensive (compare $21 for a 4x4 panel to $45 for a 2x4 platform) and lighter than rigid-framed platforms (74 pounds versus 95 pounds). Additionally, they take up significantly less space to store (compare 3.3—or even 5!—cubic feet to 10 cubic feet). However, they require a level of carpentry experience that may put their construction outside the capabilities of some shops, as well as a time-on-task for construction that may not be available even if the skills are. In a situation where I don’t have a lot of time to dedicate to construction, I would likely lean to 2x4-framed platforms just because I know I can get them built. If I’ve got the time, though, I’d personally build stressed-skin panels over rigid-framed any day—they’re cheaper, lighter, and smaller.

Next week: a look at sandwich-core panels that can carry a 50 psf load, and how they stack up in these categories against the rigid-framed platform and the stressed-skin panel.