Yes! Trial-and-error, plus established calculations/formulas. Why would you want to go thru trial-and-error again when it's already been done & is now well documented by registered engineers that are ready & willing to sell you their knowledge & design along with the actual trusses at a price that is less than you can build the trusses for yourself?
Sure they have, but does that mean you want to toss out engineering altogether & go with the ol' best-guess, I-done-it-this-way-for-a-long-time-so-it-must-be-good-enough method?
We're talking about very, very heavy loads directly over peoples' heads. Why gamble?!
I don't know about where you are but here that is not the case. I have plenty of time and the tools needed to do the work so with that in mind, no I cannot go out and buy the truss for my building cheaper than I can build them myself. I priced them from 5 different places here and they all were within a few cents of each other. When asked what I was going to use them on all they asked for was the span, what if any overhang I wanted, the pitch, and how many I planned to use. Other than that it was pretty much cut and dried.
If you look at the finished product at all these places for the same 32' truss you find that they are all built different in some form or another. Some use 2x8 on the top and 2x4 on the bottom with 2x6 bracing. a couple used 2x6 for everything but for the most part 2x6 top with 2x4's every where else was the call of the day.
I know there are all kinds of different factors that go into these systems and every company has their own tooling, materials and work standards they go by but if the standards are so cut and dried and must be followed so closely how could all these guys come up with so many different variations of the same simple 32' span truss system and all be so closely priced?
As I said the vast majority of people will go out and have someone else build the barn and any good contractor with any sense will buy ready made truss from a reputable company just for the liability factor if nothing else. Also it is not cost effective for a builder to have a crew build the truss for a building because if it was he would have used the old tried and true rafter type building method to begin with. All the customer wants is a barn and if the builder can do the job for what the customer is willing to pay then everybody is happy.
On the other hand there are people that for one reason or another just like to do things themselves rather than have someone else come in and do it for them. For some it may be the money thing, for others it may just be they get joy out of taking the bull by the horns and making it happen on their terms and time frame.
The OP wanted a little helpful insight on his 40x50 barn and asked what I would call a dog gone good question because if the truth be known there are probably many people around here that have been faced with the same question. Some just want to learn about these things and others want to put their ideas to the test but for what ever the reason these kinds of questions do get asked all the time and from what I have seen these questions are hard to get reasonable answers for because of the far reaching effects of the internet. What will work in one place may not be right for another because of different wind and snow loads for the given area or other factors that come into play.
With all this in mind I would think that anybody that would be willing to tackle building a 40'x50' anything would have some basic knowledge of the task at hand and the tools to do the job effectively. All that is left is to fine tune the plan and ask questions of others that have either had this done or did this kind of work themselves so the plan can be implemented. To me a 40' span would probably be the limit that I would try and build myself and anything longer I probably would not try to bridge. After that there are really a lot of factors that come into play that would be hard to overcome and do yourself with any reasonable and effective amount of success. Deep down I think we all know our limitations of what we are capable of and I don't think it's wrong for someone to try and expand those limitations before he gets too old to enjoy his life's work.
