Loads

A load is a weight which will be supported by a prop. A prop should be built to withstand any type of load that will potentially be applied to it. You can think of loads in a few different ways. A static load is one which is applied gradually and remains constant. The weight of a turkey that sits on a table is a type of static load. An impact load is applied suddenly, like when a prop sword strikes a wall. A repeated load is one which is applied and removed over and over again. Repeated loads may weigh less than a static load which the prop can support, but will cause the prop to break due to fatigue over time. You can also describe a load by the size of the area it is applied to. A concentrated or point load is applied to a single, small area. A distributed load is applied over an area.

You need to know what types of forces and how much weight will be placed on your props and build them so they can withstand the loads applied to them during a performance without failing. An impact load occurs even when a force is applied gently. For instance, a person who plops down on a chair can apply an impact load of nearly twice their body weight. A sandbag dropped from a mere six inches can apply an impact load of seven times its own weight.

A prop may need to withstand loads in ways that real objects are not designed to. A common example is a chair. Often, directors want to lay a chair on its side and have an actor stand or dance on the legs. Normally, a chair leg is only designed to support a static load of around a hundred pounds (45 kg) from its side. But an actor who weighs 160 pounds (72 kg) can easily break or bend the leg just by standing on it, and once you involve dancing, you can see that this type of prop chair needs entirely different design considerations than just your normal dining room chair.

Failure

So what happens when a prop fails? It can break; either two parts can break apart from each other, or the material itself can crack in a way that will no longer support weight or remain rigid. It can bend; some materials can be bent back into shape, but continual bending and rebending may fatigue the material until it eventually breaks. It can be crushed or compressed.

Your prop is only as strong as its weakest point, so you want that point to be able to withstand any potential loads and forces. However, you cannot always make a prop completely immune to failure. First, make sure that a failure does not cause injury to the actor or audience. Second, make it so the prop’s weakest point is easy to repair.

You can buy bolts that are rated to withstand forces and weights up until a certain point, after which they break. If money was no object, shouldn’t you just buy the highest-rated bolts you can? No. If you somehow apply enough force to a prop to break it, it would be better to break the cheap and easily replaceable bolt than to break the pretty prop that took several weeks to build.

Prop builders are not engineers, and so we should not be constructing anything whose failure can endanger a lot of people (in some locations, you need a license to perform actual engineering). Still, we do have to build chairs and tables, and they need to withstand a certain amount of weight and force without breaking. Whenever possible, figure out how and where the prop might fail, and see if you can construct it in a way where you only have to fix a cheap and easily replaceable part, like a bolt.

The same is true of parts that will wear out over time. This is why we often put pads on the bottom of a piece of furniture. When furniture moves around a stage floor, it will wear the floor down until you have to repaint or refinish it. If you put a pad on, however, the pad will wear down; when the pad wears down too much, you simply replace it with another cheap and tiny pad. If you use pads that screw on, then you can simply unscrew them and screw on new ones. If you glue them on or make them integral to the prop, you will have a tougher time replacing them later on.

Sometimes, it may just not be worth it to try and make your prop completely indestructible. Maybe it compromises the appearance too much, or makes it too heavy. Maybe the potential for the prop breaking is unlikely, and it will hold up in the normal course of action. In these cases, it may make more sense to provide multiple backups of the prop, and just replace it when it breaks.

Rigid versus Flexible

Is papier-mâché stronger than fiberglass? Fiberglass is used in all sorts of applications, such as outdoor sculptures or large television props. Anyone who has felt fiberglass knows how strong it is. But papier-mâché can be stronger in some circumstances. Upon impact, the papier-mâché will flex a little bit, whereas the rigidity of the fiberglass does not allow it to. Flexing absorbs some of the energy, and the material bounces back to its original shape. With rigidity, the material will either break or it won’t; there is nothing in between. Think about this when you have to build a prop that will be subject to strong forces or impacts. Is it worth trying to make it as rigid and unbreakable as possible? Or is there a way to put some flex in it so it has a bit of “give”?

Figure 7-4 to 7-6: The skeleton and inner form of a dead animal prop uses a tricky mix of hard and soft shapes to give it an accurate shape, while flexible joints confine the movement to a realistic range of motion. La Donna del Lago, Santa Fe Opera, 2012. Scenic design by Kevin Knight.

Cushions and padding serve a similar purpose. Rather than allowing one object to impact another suddenly, a cushion will absorb and spread out the impact. Cushioning certain sections of a prop where an actor needs to strike it will help keep the actor from being injured as well as protect the prop itself. Stunt weapons for film and television are often made from a single piece of cast foam rubber so the entire weapon is essentially a shaped cushion; this both protects the actor during rigorous stunts as well as keeps the weapon from breaking apart.

Compression

You can build stronger props if you take advantage of compression strength wherever possible. The legs in Figure 7.7 are just the most common example; the principle itself can be applied to many other situations. With props, force may be applied in many directions, such as when someone sits on a prop, pushes on it from the side, or lifts in the air. Compression in the direction of the length of a beam is the least likely way for a material to fail. Whatever direction the force comes from, your prop will be stronger whenever it relies on the compressive strength of your materials rather than just the joints.

Figure 7-7: If you place a platform on top of a leg, the leg itself would need to be completely crushed for the platform to hit the ground. This kind of leg is known as a compression leg, because it relies on the compressive strength of the entire length of the leg (you rarely see the leg being compressed because it happens on a microscopic scale). If you attach the leg to the side of the platform, it will only hold as long as the joint does not fail. Whether it is bolted, screwed and glued, or welded, it would take far less force to break that joint than to crush the leg.

Spanning a Gap

A platform does not just rely on how you attach the legs; you also do not want the surface between the legs to break or sag. In props, you are not always dealing with platforms and legs; there are any number of situations where you need the surface of a material to hold its shape without collapsing.

A wale (sometimes called a waler or whaler) is a ridge or a strip spanning a length meant to stiffen, straighten, or otherwise help support a flat (the ridges on corduroy fabric are also called “wales”). A very thin piece of wood or metal added to a surface for strength or support is known as a cleat.

Any shape or span that you want to strengthen can benefit from a ridge running along the length where it is weakest. You can see this in armor and shields, where ridges are added along the middle of long curves. This is not merely decorative; it also adds strength. You can also see it in small plastic items, such as the cases for your cellphones and other gadgets. Take them apart, and you can see small ridges added at curves and sides to give strength and structure.

The strength of adding a ridge comes from the fact that it gives a small amount of material pointed perpendicularly to the surface of the main material. A board such as a 1″ by 4″ laid down flat is easy to bend up and down in the middle. If you turn it on its edge, it will flex back and forth. If you attach a flat board to a board laid on edge (creating an “L” shape), it will not flex in either direction. Theatre carpenters call this type of beam a strongback.

A beam has to support its own weight in addition to the weight of whatever is placed on top of it. A solid beam weighs more than an I-beam or a piece of hog’s trough, so the additional weight a beam can support (the weight of whatever is placed on top of it) is actually less than the additional weight an I-beam can hold. It also means all the legs and beams supporting that beam need to be stronger. In other words, using lighter pieces with the same strength means you ultimately need less structure to support the whole prop.

Figure 7-8: A wale placed along the back of a stud wall helps stiffen and straighten the wall.

Figure 7-9 to 7-11: A piece of paper sags under its own weight. With a single fold, it can span the same distance without sagging. With three folds, it spans the distance without sagging and remains flat. These folds form a ridge, which is used frequently in manufacturing to stiffen an otherwise flexible material while retaining a low profile.

As you add material, the weight of an object increases faster than its strength; if you scale up the size of an object, even if you use the same materials and maintain the same proportions, you will reach a point where it gets too big to support its own weight. This is why an ant can lift fifty times its own weight, while a human can only lift the same amount of its own weight (and that’s with proper training and exercise).

Figure 7-12: Top left: Two lengths of lumber joined perpendicularly form a strongback. Top right: A similar configuration rolled from a single piece of steel (or other metal) is known as angle iron. Bottom left: Three lengths of lumber joined in a U shape may be called a hog’s trough. Bottom right: Three lengths of steel are typically joined (by welding, bolting, or riveting) together in an “I” shape; this is called a “plate girder.” When the shape is rolled from a single piece of steel as in this diagram, we call it an I-beam.

Figure 7-13: A piece of strongback has nearly as much strength and rigidity as a beam with the same outer dimensions, but it uses far less material, making it cheaper, easier to work with, and lighter in weight.

Figure 7-14: A truss is essentially a solid beam with all the nonstructural material removed to make it lighter.

Triangles

Many of us are taught at an early age that a triangle is a strong shape, and looking around any prop or scene shop, you may see carpenters adding diagonal braces to tables and platforms to give them some more strength.

You can see why; even a triangle made of parts attached by flexible joints cannot change its shape. A square, on the other hand, can fold up. Even if the joints are extremely rigid, the leverage you get when you push on the top of a square creates a lot of force on the joint at the bottom of the square, making it more likely to fail.

In order to keep this from happening, we can add a diagonal beam from the top corner of one side to the bottom of another corner. This creates a triangle in our structure. In construction parlance, this diagonal member is known as a brace.

Figure 7-15: This table for Henry IV at PlayMakers Repertory Company has sturdy legs, but it will still collapse if pushed or leaned on from the end. We added diagonal metal rods connecting the top to the legs to brace the table and keep that from happening.

Figure 7-16: The back of a theatrical flat.

In many cases, you do not need a diagonal brace to run the whole length of the square. Look at a theatrical flat: the same principle applies to the keystones placed in the corners. These are the bare minimum needed to keep the rectangular shape from collapsing.

You will also notice that the keystones on the middle beam are made of thin plates attached on top of the beams on both sides of the joint. These introduce another means to strengthen joints.

Gusset Plates and Scabs

A gusset plate is a sheet of material used to strengthen and/or join two or more beams together (not to be confused with a gusset used in sewing, which will be covered in Chapter 12). It is kind of like a very short stiffener or wale used for a single joint.

A wooden gusset plate used to strengthen an end-to-end butt joint (usually for making a long beam out of several smaller beams) is known as a scab. If it is made of metal, it is known as a fishplate.

Figure 7-17: A section of roof truss illustrates several uses of gusset plates (shown transparent here for clarity). They quickly reinforce joints that otherwise do not have enough contacting surface area.

Figure 7-18: On top, a fishplate joins two 2 by 4s. At the bottom is a scab. For more strength, a scab or fishplate can be added to both sides of the joint.

Cantilevers and Through Rods

Many props will have long, thin pieces extending from the main body, such as antennae, flagpoles, or branches. These are prone to breaking off, and can be difficult to attach securely. A cantilever is a rigid beam or rod which extends out from a vertical surface without additional bracing. In construction, a cantilever can support weight or force by having part of it extend on the inside of the surface, known as the “backspan.”

Though the term is most often used when talking about floors or airplane wings, props often use cantilevers to support weight. The outstretched arms of a statue, the antennae on a spaceman transmitter or the spikes on a morning star are all thin extensions that can be strengthened by extending them into the main body, or by having a hidden rod that extends into the main body (known as a through rod).

Figure 7-19: This fantastical walkie-talkie for Santa Claus is made of various beads, shapes, and rods. The rods go through all the beads and into the main body, which is a far sturdier construction than gluing each piece together end-to-end.

Figure 7-20: Embedding a sturdy rod or dowel inside a sculpture, as shown in red, helps strengthen the extremity.

Grain

Some materials consist of long strands, fibers, or other directional particles that we call grain. When you think of materials with a grain, you usually think of wood, though other materials have grain as well: paper, fabric, stone, and even some metals that have been forged or extruded. A material with a grain has more strength in the direction of the grain than across it.

Testing Your Props

Much of what you have read in this chapter takes a lot of intuition and guesswork. We are not engineers who can measure and calculate all the parts of our props to determine strength and stability. We do not have testing machines that can swing our prop sword against a wall a thousand times to determine its exact breaking point. We need to build our props and get them to the actors pretty quickly. They trust us that our props will not break or fall apart, or if they do, it will be in a safe manner and with plenty of warning.

So before you hand your prop off, test it out. It helps to know how they are planning to use it. If they are dancing on it, jump on top and try dancing on it yourself. If they hit it or throw it against the set, throw it around the shop yourself. Make sure you are not holding back; actors will not be worried about breaking the prop once they are on stage surrounded by audience with lights blinding their eyes; they will be worried about maintaining their character and remembering their lines. They will punch your props with more force than they ever did in rehearsal. Don’t build a prop that requires the actor to take their attention away from acting in order to avoid breaking it. It needs to remain rock solid while being used in the manner that the scene requires.

Figure 7-21 and 7-22: Imagine the tiny chair is a force that can break a bar of material in half (known as “fracturing”). The grains of orzo pasta show a material without any grain; that is, the individual particles are positioned randomly. In order for the chair to pass from top to bottom and split the material, it can travel in nearly a straight line. Now imagine these pieces of linguine show a material with grain. In order to split it in half, the chair would have to take a fairly circuitous route. This is why materials are stronger across the grain.

For those props that have the potential to break, it is helpful to get them in rehearsal as soon as possible. It gives the prop a few days or weeks of use to start showing wear and tear. You can see where cracks start forming or where parts start coming loose. If the prop does break, at least it does not happen in front of an audience.

As you can see, choosing the proper materials for your prop can be a complicated balancing act. The strength of your prop is dependent not only on the type of material you use, but also the shape and configuration the material is used in. The weight of your material will affect how strong your prop needs to be. You also have to consider the kind of forces that will be applied to your prop and their directions. Finally, you need to determine whether you need your prop to be absolutely impenetrable, or whether it makes more sense to have it bend or even fail at certain points.