Rfreund:
WOW... If that’s the only weld or detail on that sketch that you have a question about, you better rethink the whole concept. If you need to reinforce that 20H7 for moment out in the middle, or for deflection, will you have enough shear (reaction) cap’y. at or near the bearing ends? Do you also need to reinforce the last diagonal and the bearing seat detail on the existing jst.? How many of these are there? How long, what loads, moments, reactions? What you’ve got is good for about 500"kips, 5.4kip max. reaction and I = about 185"^4. You have to shore each of the 20H7's before you apply the reinforcement and do any welding. Right now you have a mixed up mess, such that you will never be able to follow a clean load path from a new element to an existing element, at any welded load transfer point.
Why not think of this more in terms of needing to add some moment of inertia, maybe some stronger end diags. and bearing cap’y.? You need to add some top and bot. chord material, and you probably need to add some new diags. to tie them together, since the existing diags. are tough to analyze and pin down, you don’t know their welding either. The three will share the total load in proportion to their moments of inertia. So, let’s make two new light trusses, one for each side of the existing 20H7, and each will have .6(needed Mof I) 50 - 60% of the needed additional moment of inertia. Then we’ll also need to make a stab at the shear flow (welding) needed to tie the new and the old together, to act as one structural unit.
The new trusses will be identical, except one left and one right side. I’m just pulling sizes out of the air to draw my word picture for you, you have to actually size things, check the stresses, and design the welding. The bot. chord on the existing jst. is made up of round bars ~.78" in dia., but the diags. get smaller in dia. toward mid span, correct? I think your .95" dia. must be the end diags. You may want your new diags. to vary in a similar way, to pick up the increasing shear. The new trusses are going to be about 19.5 or 20" deep; to fit under the existing top chord angle horiz. leg and tight against the existing bot. round bar, but still allow shoring. Lay down a .25" x 2 or 2.5" bar for a bot. chord, lay down a .5" x 2 or 2.5" bar for the top chord. These t&b chord bars should have mill edges, not sheared or flame cut edges. These chord bars are laying flat on an assembly table. Use square bar stock or round, and bent as the big boys do it, for your diags.; they could just be straight pieces too. Lay these diags. on top of the t&b chord bars, and weld them together. Your diags. on the bot. chord bar must fit above the existing .78" rounds, and must be below the top of the top chord bar. Your top chord bar may have to be heavier out near the ends for shear/bearing considerations, as may your last couple diags. These new trusses get rolled into place, top first, on either side of the 20H7, with your new diags. nearest the 20H7. Your new bot. chords gets stitch welded to the existing .78" rounds, down-hand and from the opposite side of the truss. Your top chord bars get stitch welded over-head to the existing top chord angles horiz legs.
If you are trying to pick up a new roof load in a specific location (over a specific length, near mid span), you could provide a top chord member which will take an axial loading from each end. Then just put two vert. chord members (compression strut members) at your new load area; then just harp a bot. tension chord member under these struts and up to the end bearing assemblies. Maybe this bot. chord tension member has a turnbuckle btwn. the two struts. These little post tensioned trusses get tied to the existing 20H7 for some stability.
