Timberframers call theis a "brace".
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at the fourth page of this document (upper left). It illustrates brace joinery which meets your need to install into a preexisting T-shape. The bottom mortise for the brace is extra tall. The brace it inserted into the lower mortise, and lifted straight up so that the upper tennon inserts into the cross-member mortise. A block is then inserted below the lower tennon to fill the oversized mortise.
The document has a few other approaches to brace joinery that might also meet your needs.
If you need specific pointers on how to to the layout for these, just ask I've done a "barn-load" of them.
Cheers,
Steve
Sketch of joint is on:
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there any technique for doing this type of joint (don?t even know what they are called), some posts will have one gallows bracket ? others will have a pair .. in both cases there are galvanised steel plates through bolted providing structural strength. Sketch of double one is at :
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do not have the option to move posts form their now vertical permanent position, so I am looking for technique for cutting in-situ, maybe it?s just a case of getting in there with a chisel, but thought I would ask.
Intent is to glue the joint as well with quality polyuretane glue ... but mainly to prevent water ingress, steel plates will provide the strength.
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