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Building plinths for load bearing wood columns

11/5/20       
Dan Member

Cheers All, I am replacing fake concrete plinths with wood. The challenge is the columns are grouped together in sets of 3, so they are appear too close for individual plinths.I have 2 ideas for a design:
Laminate 2x4’s on end to make a single plank or use 4x4’s and a half lap mitre.
Plinths need to be 31/2” tall 18” wide.
What’s the best method?
Pic files are too large?

11/5/20       #2: Building plinths for load bearing w ...
rich c

Will the plinth be sitting on concrete? Historically accurate design required? Exterior I presume?

11/7/20       #3: Building plinths for load bearing w ...
Dan Member

Yes It will be sitting on concrete with a sheet of epdm underneath.. I don’t think the half lap mitre is historical but I don’t know of a better 45 degree joint to use..

11/7/20       #4: Building plinths for load bearing w ...
rich c

So you suggesting pressure treated 2x4s and 4x4s? No way a miter stays together on that stuff. Tons of shrinkage and movement on PT 4x4s

11/9/20       #5: Building plinths for load bearing w ...
Dan Member

I agree... what would a better choice?
I can’t find 4x4 stock in exotic hardwood. What hardwood laminates well using west system epoxy?

11/9/20       #6: Building plinths for load bearing w ...
dan Member

here is what i am working with... about 3.5" tall and 18" wide..

11/9/20       #7: Building plinths for load bearing w ...
rich c

Some mahogany variation would be my choice.

11/10/20       #8: Building plinths for load bearing w ...
David R Sochar Member

I have made these replacements about 20 times in my career.....

Mahogany (sweitania macrophylla), epoxy.

How do you plan to vent the columns?

11/11/20       #9: Building plinths for load bearing w ...
Adam

We have always used SA mahogany for large paint grade plinths. It’s a good machining rot resistant wood.

What machine will you be running?

11/13/20       #11: Building plinths for load bearing w ...
Dan Member

I plan on using a table saw and some kind of jig to make half lap mitres...
Would using a biscuit and just plain mitres hold well enough?

11/13/20       #12: Building plinths for load bearing w ...
David R Sochar Member

When we build theses, the Square plinth is mitered, and then each miter is splined with cross grained mahogany. Spline cuts are usually 1/2" wide x 1-1/4" deep x as long as we can make it/close to the outer face of the plinth. You want to 'spline' about 1/3 of the miter's surface. 1/3 or more.

The round parts are hexagonal and with double splines, bandsawn and turned on a lathe or shaped on the big shaper.

Of course, the name of the game in that Plinth miter is surface area and glue. More surface area is better. It is worthwhile to do a few calculations to see just how much surface area /glue area different solutions might provide.

Biscuits are not waterproof and you will never get enough meat to hold the joint together.

Asking ".....hold well enough?" Well enough for what? To get paid? To get out the door? Your question implies doing as little as possible. The job requires a certain level of commitment and performance. If you are not willing or capable of exerting that level of work, then then please pass on the job and be honest with your customer. Respect the craft enough to know when to back away.

11/13/20       #13: Building plinths for load bearing w ...
Adam

We’ve done them brick laid and miter/spline. If the builder likes you they will allow you to place them before the columns are installed. Those can be brick laid.The vast majority we’ve done are replacements that are installed long after the columns were installed(100’s of years). So those were miter/splined.

My question was in regards to the profiling. We would make some custom cutters and run them on the shaper with a power feeder.

Are you routing and carving?

11/13/20       #14: Building plinths for load bearing w ...
Adam

We’ve done them brick laid and miter/spline. If the builder likes you they will allow you to place them before the columns are installed. Those can be brick laid.The vast majority we’ve done are replacements that are installed long after the columns were installed(100’s of years). So those were miter/splined.

My question was in regards to the profiling. We would make some custom cutters and run them on the shaper with a power feeder.

Are you routing and carving?

Biscuits are for alignment. Not very structural. Definitely use a high quality epoxy resin like West System or equivalent.

11/19/20       #15: Building plinths for load bearing w ...
Hastings Read Member

Hers how we tackled this recently. Octaganol sections finger jointed with epoxy. Four separate sections turned on a lathe and then joined with Dominoes and epoxy. Bottom section was Accoya, the others Red Grandis.


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