Most trusses are built of 6063-T5 aluminum tube with a 1- or 2-inch outer diameter (OD) or HE30 alu-minum alloy with a fairly heavy inside wall thickness.
Chrome-moly can be used because it is less expensive and easier to weld; however, the added weight (about twice that of aluminum) makes the use of chrome-moly undesirable for touring purposes. If the trusses are for semipermanent installations, chrome-moly
could be considered for its cost savings, but the addi-tional weight must be factored in. Its other advan-tage is that it can be arc welded, whereas aluminum requires the use of the more diffi cult heliarc method.
The welder does not have to be as highly skilled to arc weld as he does to use the heliarc method.
Although steel welding is less expensive with regard to materials and equipment and labor is more readily available, steel is seldom used in the United States, largely due to its weight and to the contro-versy surrounding the employment of amateur or semiqualifi ed welders, which can create a tremendous liability problem and should be avoided. The poten-tial for a wrongful death or injury suit is substanpoten-tial.
Touring trusses should be built by companies that specialize in entertainment structure design.
A list of some of these companies is provided in Figure 13.12 . Unless your design requires custom fab-rication, most rental lighting companies will have a complete collection of different style and size trusses in stock. Make sure you are provided with certifi ed mechanical stress test information ( Figure 13.13 ) or x-ray records if you are simply renting gear your-self and are going to assemble the lighting. Because the rental house is not the person attaching lumi-naires or other things on the trusses, they cannot be held responsible if you misuse the truss. Many manufacturers now provide stress data information in their brochures ( Figure 13.14 ).
The engineering of trusses is critical and is prob-ably the biggest reason why one-off shows should not try to contract for special built trusses. Figure 13.15 shows the basic procedure used to test the trusses for load. Most rental companies I have surveyed say they do this procedure about once a year, although if the trusses come back from a long tour they will some-times test them before sending them back out. The trusses can be sent to a lab that does stress analysis, but some rental houses keep the equipment in-house to check for the manufacturer’s recommended defl ection.
A few admit to only doing a close physical inspec-tion, looking for stress lines in the welds. Even fewer actually have the trusses x-rayed for very fi ne cracks.
There is no industry standard here. Maybe the Entertainment Services and Technology Association (ESTA) will get involved and set American National Standards Institute (ANSI) standards.
James Thomas Engineering www.jthomaseng.com
LiteStructures www.litestructures.co.uk
Milos Structure Systems www.milos.cz
Prolyte Products www.prolyte.com
Tomcat (The Vitec Group) www.tomcatglobal.com
Total Solutions www.totalsolutions-group.co.uk
Tri-Lite Truss www.opti-kinetics.com
Tyler Truss www.tylertruss.com
Xtreme Structures www.rigging.xsftruss.com
FIGURE 1 3.12 Truss m anufacturers.
FIGURE 1 3.13 Stress t est cer tifi cate. (Reproduced by permission of Penn Fabrication, Inc.)
Prolyte Products Group, a Netherlands-based company, has published a good booklet called Prolyte Black Book , which discusses rigging meth-ods, safe spans, stress, and types and use of hard-ware. Remember, though, that this is only a guide, and an experienced rigger should always be in charge of any rigging.
It is best to lease trusses from an established concert rental company. If you are considering con-structing your own, I recommend you use only cer-tifi ed welders. Actual construction time could be as much as 5 days for a 40-foot truss, but it is essential to take the time to have a certifi ed structural engi-neer design or check your plans. The added cost and time are other reasons to lease if your project is short term. Be sure to ask the company for certifi cation of the structural stress and load capacity. This can be
done by specialized engineering fi rms and should cost less than $1000, depending on what procedures they use and how far you carry the tests.
Spans
As there is a large difference between the strongest and weakest truss, a certifi cate of load capacity is very important. Moreover, the clear spanning capability of the truss must be determined. Some trusses can only be supported up to a clear span of 40 feet. Not only length but also the size of the tubing and design of the truss are important considerations. You must also add in the weight of the luminaires, cable, someone focus-ing the luminaires, and maybe truss-mounted follow spots and operators. Generally, truss sections come
FIGURE 1 3.14 Example of an allowable loads data sheet. (Photograph by Pyolyte P roducts.)
FIGURE 1 3.15 Stress procedure. (Provided by Rigstar Rigging, Inc.)
in 8- and 10-foot lengths. The rental companies will help you fi t your design into the lengths they have in their inventory. Even better, contact the company for a breakdown of what is available before you begin a design, then have an Entertainment Technician Certifi cation Program (ETCP) Certifi ed Arena Rigger create a rigging plot.
Now that most trusses are fl own, the full 40 feet of a standard portable stage are usable (ground sup-port lifts reduced the usable width to 32 feet). That is why 40 feet is considered the average truss length.
Larger productions, however, are now calling for 50- and 60-foot lengths.
It is rare to see lengths over 40 feet being fl oor supported. If your design requires a truss over 40 feet wide, it is time to consider a fl own system. Why
can a longer truss be fl own when it cannot be ground supported? The solution is found in placement of the rigging pickup points. Bridles are assemblies that enable the suspension of an object at a particular loca-tion; they are composed of two overhead load points of lesser load-bearing capacity that are joined to lift a heavier load. They help to distribute the weight evenly. It is common for a 40-foot truss to have two motor or winch pickup points. These, in turn, will usually be bridled about 4 to 6 feet apart. The proper bridle confi gurations for a given load must be deter-mined by a qualifi ed rigger. Figure 13.16 shows some of the calculations needed to determine the stresses;
however, the method for making these calculations is much more complicated and should only be done by an experienced rigger.
FIGURE 1 3.16 Rigging br idle cal culations.
(Provided by Rigstar Rigging, Inc.)
Ground -supported trusses are generally not con-sidered as safe as fl own because they are subject to ground movement. No, I am not talking about a California earthquake. Out of necessity, the ground-supported trusses are at the mercy of many factors, such as portable stages with uncertifi ed construc-tion that could collapse under the weight. In several reported cases, lighting companies have refused to set up a ground-supported truss because they felt the stage was unsafe. If the towers supporting the truss are on the cement off the portable stage, there is a better chance that the system will be safe, but do not let down your guard; make sure the structure is level and securely attached to the truss. If the event is outdoors and the support tower is on grass, use extra caution; for example, thick plywood squares to spread the load could be placed under the tower (see Chapter 14 on lifts and towers).