Quote (wickedone)

Simple question; does the steel plate have to extend to each support??

It doesn't have to but, if it doesn't, some special detailing is required as you clearly have anticipated.

Quote (If not, how to design the connection at the termination of the plate??)

1) In a perfect world, you would force the curvatures of the plate and the glulam to match at all locations along the span. This speaks to moment connections at the ends of the plates to get that done. The same connections would have to dump the shear out of the plates locally as well.

2) Given the inaccuracies involved with this kind of work, I favor a simplified approach where I consider the plates to be end supported and loaded only at the middle or third points etc. Then, with that model in hand, follow through with free body diagrams and shear only connections to suit.

How much deterioration? A flitch beam depends on the wood to more-or-less brace the plates. If the wood is on the way out, the plates may not be stable enough to do the job or may not be for long. You could easily get rot concealed behind the plates.

This article might give you some ideas.

Killer article Agent, thanks. This comes up regularly and is always kind of a pain to explain.

@OP: The method described in Agent666's article is basically the same as my #1. It's also analogous to the MQ/I method of "developing" partial length cover plates in steel work. We discussed that in depth in this thread: Link

Be aware that glulam beams don't always like bolts with loads perpendicular to the laminations, at least without special consideration to edge distance and force reductions. to transfer the moment out you will be imposing perp. to the lamination forces.

If possible, using a channel that is long to keep the moment force down will be beneficial. the flanges will help brace the plate from buckling when not bolted back.

In your scenario you also have to consider carefully the fact that the load is all in the Glulam member, so adding the plate has less of an effect than if it was applied before the member was loaded during some original construction period. .