The purpose of the lid is to support the point load of the actor or scenery, distribute it over as large an area as possible and transfer the load to the frame. The single most common lid material is ¾ thick plywood. While I have seen 5/8 thick and occasionally ½ thick plywood used for platform lids the amount of visible sag was unacceptable and safety was severely compromised. For normal theatrical use, non-dancing, the lid must withstand a bare minimum of 50 psf (pounds per square foot). For any kind of action, several actors close together, 100 psf is needed and for dancing 150 psf is an absolute minimum, 250 psf is better. For tap, clog, Riverdance etc. try to imagine the force of three or four dancers coming down at once. Dancers can generate a ten-fold force during a landing, stomp or tap maneuver. For example, three 110 pound ladies clogging in unison can create 110 lbs. x 3 dancers equals 330 lbs., 330 lbs. times ten equals 3,300 pounds of instant impact on a single platform. The exact load rating of any platform is a combination of all three components and is beyond the scope of todays article. However, for the purpose of this discussion, an average platform will safely support about 100 psf in a static load situation. That means a standard 4 x 8 platform will support about 3,200 pounds evenly spread out over the entire surface. Of course that figure assumes that the legs and bracing and the stage floor below the platform are also capable of and designed to support that load.
The purpose of the frame is to support the lid and transfer the load to the legs and bracing. The type of frame material and its strength will determine the spacing between legs and braces. The frame should support the weight without bending more than 1/360 of the span between supports or a 1 sag in a 30 span. This is about ¼ in an eight foot span or 1/8 in a four foot span. So, if you build a platform and it sags ½ when your actor stands in the middle, you need to cast smaller actors opps .what I meant to say was, you need to use larger framing boards or put the legs closer together.
The purpose of the legs and bracing is to transfer the weight from the frame to the floor under the platform and to hold the platform at the desired height and with little or no lateral movement. The methods of legging platforms are so varied that they can and will fill an entire article by themselves.
In the United States, the single most common size of platform is the four foot wide by eight foot long unit and the most common framing material is the plain old ordinary, SPF 2×4. Why are these the standard? It is simple economics and convenience.
Sheet goods are normally manufactured in the 4 x 8 size. While other sizes are available, they are more expensive per square foot and usually must be specially ordered. A 4 x 8 platform needs no sawing or cutting of the deck or lid material, it comes in that size to begin with. Strangely enough, even in countries using the metric system, sheet goods still are made to what they call Imperial Measurement and are 4 x 8. The measurement is metricized to 1220mm x 2440mm but it still equals four feet by eight feet. It might be interesting in a future article to investigate why sheet goods in those countries stayed at 4x8. Was it human scale, economics, convenience?
Now what kind of sheet goods are we talking about? Generally speaking we are talking about plywood but there are a number of products available with different advantages, disadvantages and cost factors. The following is a brief listing of some materials suitable for platform lids with some of their pros and cons.
| Some Common Sheet Goods Used for Platform Lids in Theatrical Construction
|Name||Actual size||Description||Advantages||Disadvantages||Average Cost as of March, 2000 in US $$
|Plywood, 3/4" AC
||4' x 8' x 23/32" thick
||5 to 7 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. "A" side is smooth and "finish" quality.
||Strong, will support heavy loads if properly framed and supported. Available everywhere. Smooth surface can be painted and become the show surface.
||Cost, March, 2000 price is about $32 per sheet. Moderately heavy at 75 pounds per sheet. May have voids inside that can allow point loads to penetrate.
|Plywood, 3/4" BC
||4' x 8' x 23/32" thick
||5 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. C side is plugged and rough, D side often has surface voids.
||Strong, will support heavy loads if properly framed and supported. Available almost everywhere. Smooth surface can be painted and become the show surface. Price is usually $3 to $5 cheaper than AC Plywood.
||Often very curved, usually has voids. 75 pounds per sheet May have voids inside that can allow point loads to penetrate
|Plywood, 3/4 CDX
||4' x 8' x 23/32" thick
||6 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. "B" side is smooth.
||About 30% cheaper than AC Plywood
||Rarely flat, large voids on D side. Must have some other surface for show side such as homosote plus masonite or luan.
||4' x 8' x 23/32" thick
||OSB means Oriented Strand Board. Wood is shredded and glued up in a resin with the fibers roughly aligned in the direction of the board.
||No voids, generally flat, structurally equal to or stronger than 3/4 AC plywood, but about the cost of CDX.
||About 10% to 15% heavier than equal thickness of plywood, about 85 lbs. Per sheet. Must have a show surface of masonite or luan etc.resin dulls tools rapidly.
|Particle Board, 3/4"||4' x 8' x 23/32" thick, some brands 3/4" true.
||Particle board is made of wood that is ground up roughly and glued up in a resin.
||No voids, generally flat,but usually less than the cost of CDX.
||20% heavier than equivalent thickness of plywood. Structurally weaker than 3/4 AC plywood. Very hard, screws must be pre-drilled for counter sinks to drive flush, edges and corners break off easily.
Framing materials for stock platforms are also varied. The most common is the standard SPF 2 x 4. SPF means that the species of wood varies between spruce, pine and fir. The actual size is 1½ x 3 ½. The reason it is the most common framing material is that it is the least expensive. Two by fours also are very durable, capable of handling a lot of use and abuse. They are heavier than most other types of common framing. Other types of framing materials are 1 x 6 pine and 5/4 by 4 or 5/4 by 6 SPF. Although 1 x 8 and 1x 12, 2 x 6 and 2 x 12 are also used for platforms, these are usually used for specialty or one of a kind units rather than stock units to be used over and over again.
One by six is the stiffest, lightest material. Five quarter is the strongest most expensive material. Two by four is the most durable. One by six makes a rather light but strong platform. However it does not stand up well to repeated banging about into and out of storage. The corners are particularly susceptible to being crunched and fasteners are prone to rip out. Two by four frames are very durable, hold fasteners well, rarely can be the finished frame the audience sees. Five quarter tries to get the best of both worlds, light, strong, and durable but it costs the most.
Framing patterns are the next item in a stock platform. The single most common is the 8 stile with toggles on 2 centers. The plan view is the same regardless of which material is chosen, see Illustration #1.
Occasionally you might come across a platform built with only one or two interior braces but they have a noticeable sag or bounce as an adult walks on them and is totally inadequate for dancing or active theatrical blocking. The reason the particular framing pattern shown in illustration #1 evolved is because it is relatively strong, easy to build and uses the least amount of lumber of any acceptable framing method. While the method serves well, distributes the load evenly, it does have a drawback. The drawback is that it is not the most efficient method of supporting weight. A much stronger and stiffer framing method runs interior member along the long length of the unit instead of across it, see Illustration #2. This style of framing is not as common but is actually superior in most respects. The difference in strength is most noticeable when using 2 x 4 for the framing material. The standard method shown in illustration #1 requires a leg or support at 4 intervals along the sides or 6 legs per unit. The method of framing shown in illustration #2 requires legs at 8 intervals or only 4 legs per platform. It does use four more feet of framing material than the common method shown above.
Another variation is a frame with only one toggle in the center at the four foot mark. While this variation will support 90% of the loads in theatre use, it will visibly sag or deflect between framing members. I do not recommend this framing method and when using 2 x 4 as a framing material, saves only about $2 per platform.
Methods of fastening the frame materials together are varied depending on the type of lumber you are using. Two by fours have been fastened together with 16d common nails for years. One by six or five quarter stock are usually fastened together with 12D nails. A slight variation is to use ring shank or spiral shank nails. The textured shank nails hold much better than the common nails, are just as easy to drive, are available in strings for pneumatic nailers but are about 10% to 15% more expensive. The main drawback is that they are very hard to pull out when you make a mistake. As battery powered screw guns become more common and available, fastening platform frames together with deck screws or drywall screws has become more and more the norm. Screws are roughly 4 times the cost of nails but hold much better, dont work loose and can be easily removed if necessary. A skilled carpenter with a 16oz hammer can drive nails faster than he can drive screws but most ammeter, small or beginning workers can drive screws faster. Fastest of all, of course, is the pneumatic nailer. The nailer is a great equalizer in that the smallest 90 pound woman can drive nails as fast as the largest football linebacker and you can pick your workers for the muscle between their ears instead of in their biceps.
Although some people will swear by the use of glue when building platforms, dont waste your money. It really does little or no good in the frame construction. Soft woods like pine, spruce or fir have very open end grain and end grain is a very poor glue surface. On the other hand when you fasten the lid to the frame, glue can increase the platform strength by as much as 20% depending on the number and spacing of nails or screws used to fasten the lid to the frame.
The last part of this months article is a brief description of the other stock platforms. Along with the standard 4 x 8 unit there are a number of other stock sizes; they are, in no particular order as follows. 4 x 4, 2 x 8, 2 x 6, 2 x 4, 3 x 8, 3 x 6 and the two triangle half platforms, the 4 x 4 and the 4 x 8 left and right. Framing techniques for these units is simply variations of the 4 x 8 stock unit, see illustration # 3.
By using the group of stock platforms shown above, a wide variety of shapes and sizes can be assembled. With the addition of a few special built units the possibilities are endless.
One word of warning however. Stock platforms are not the end all be all of theatrical scenery. There are times when the use of stock platforms will cost more in time or money than building units to fit the size and shapes needed.
Now that we have looked at the basic platforms, the parallel and the stock unit, it is time to move on to more sophisticated and or special use units. Next month we will look at the Triskit and the Texas Triskit.
Until next time, keep the green side up, dont sweat the small stuff .. and remember its all small stuff.