The fact that long span trusses are complex from an engineering perspective, and difficult to fabricate correctly, cannot be overstated. As with all principal member framed structures, whether wood, steel or concrete, the loads developed in supporting large areas of loading (big floor or roof areas) are tremendous. Because of this concentrated load condition, joinery, materials, design and fabrication skills have to be the best available. These factors combined begin to explain the lack of economy that is inherent to long span trusses.

It can be very rewarding to consider the need for a long span truss as suspect to begin with. Often the goal is to emulate a picture in a magazine (normally residential) of a great room with a grand sprawl of clear span timbers. Other cases are much more real. Like an airplane hanger.

Unfortunately Ben, Ed, Joel, and Grigg can support the fact that many of the whimsical creations, seen in the coffee table magazines, designed to fill these requirements, are in trouble before they ever leave the shop. Sometimes it's a case of empowerment by observation (yeah, I can do that) but more often it's a lack of understanding of the real value of professional engineering services.

It is important to look at the real needs in your construction project, before assuming gigantic trusses are required. For instance: Will 30' trusses (much more economical and obtainable) in a central bay, flanked by to 15' bays work as well? If so, not only is the difficulty in engineering reduced, but the bay spacing (center to center location of trusses) is as well. This can often lead to a significant decrease in cost.

One other aspect to consider is allowable deflection. That's how far things can sag and still meet code. In a roof structure l/240 is often acceptable (in a simply loded condition) this means that a timber spanning 20' can sag a full inch and still be within the design parameters (for deflection). Push it to 30', and it's 1 1/2" etc. As deflection (sag) increases, so do the problems associated with it. Common practice says "let's hold this total to 1/3 inch or less". This can be a very real challenge in long spans.

All this is not meant to say "Don't do it!", but rather to say "Do you really need to do do it?" and if so, make sure you understand what it takes to do it right. It's kind of like driving in Montana. It's legal to drive 100 MPH, but is it a good idea?