Advice please on bracing timber joists

Hi there. I am building a treehouse. It will be fixed to two joists that will sit either side of a tree. I plan to tighten the two joists toward each other, either side of the tree, but am unsure what bolts / fixings are best to use.

The tree is at least 450mm / 18 inches wide, then each joist is 50mm / 2 inches wide - so the bolt / fixing needs to be at least 600mm / 23 inches long/wide. But I cannot find any hex heads / bolts / coach screws that long on-line. Any suggestions would be greatly appreciated.

Thanks

 

Just buy some lengths of threaded stud.

ebay or orbital fasteners.

 

https://www.screwfix.com/c/screws-nails-fixings/threaded-rod/cat840126

 

You going to need much more than 2 planks of wood held together either side of a tree trunk.
Are you going to rely on them being clamped so tight together that they won't dip if any weight is placed at one end? It needs more timber fitted in such a manner as to be stable when people climb on to it, therefore it will require tobe fixed together in such a way as to be close all round to the trunk.
I would suggest you Google 'Tree House Plans', or something similar, to see what is involved in the construction.
The fact you say, "The tree is at least 450mm / 18 inches wide, then each joist is 50mm / 2 inches wide - so the bolt / fixing needs to be at least 600mm / 23 inches long/wide. But I cannot find any hex heads / bolts / coach screws that long on-line." actually implies you have no building construction experience but are willing to let your/friends children climb over something you have built for their enjoyment.