Top cords are in compression, pushing out towards the walls. Bottom cords are in tension, trying to be pulled apart by the ends of the top cords. The webs will alternate in compression and tension depending on where and how they’re installed. These keep the top and bottom cords rigid and from bowing away and losing strength…beams are strongest when they’re under compression and tension along their long axis. Those metal gusset plates are there to keep members aligned, as you noted, but the wood does all the work. The extra wood gussets you added will help considerably with reducing twisting due to any overloading.
The issue with adding too much load comes from the dimensions of the material used in the cords themselves, and their lengths, how they’re supported at the walls, etc. Pre-fab trusses like those are engineered to translate a specified dead load, and live load, from the roof to the walls, which carry it to the foundation and then to the ground. They’re usually way over engineered and, as long as you never side load them, you should be fine…but I’d wager an engineer would tell you they’re not rated for what you’re doing. The purlin you’ve added across the bottom cords serves to tie them all together and further distribute the load among the trusses. One truss is weak, many trusses are strong.
My only concern is that you’re adding a compressive load to the bottom cords, which want to be in tension. Again, I think they’re probably strong enough to handle it, especially with the gussets that have been added but it’s not, technically speaking, proper. You’d rather have the load attached to a ridge beam in the middle of the top cords. That way the load follows the natural compressive path downwards. The load you’re adding is transient, and (shouldn’t be) a shock load, so the trusses should be able to absorb it with the modifications you’ve made. If a compressive load were to overwhelm the tension load on the bottom cords, it would change the orientation of the load distribution of the webs, which could cause them to pull apart…again, your wood gussets should help with that. Did you add them to the top cords as well? You might consider that.
I’m not an engineer, but I know some stuff about trusses. If you want to reinforce that purlin you added, get some Simpson hurricane ties to tie the new purlin to the bottom cords. That’ll insure they never shift or pull apart as you add a transient load with the hoist. You need a solid transition from the purlin to the cords to insure load distribution, since you’re kinda loading it in the wrong orientation. And NEVER drill holes in cords or webs of pre-fab trusses without an engineer doing calcs for that…pre-fabs are designed to be light weight, and inexpensive, so they’re not as over-engineered as a cut and stacked roof.
Ryan/// ~sigh~ I blew up another bowl. Moke told me "I made the inside bigger than the outside".
