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round post to round beam joint?

Started by Spruce_Goose, April 02, 2013, 08:43:53 PM

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Spruce_Goose

Hi all. Does anyone have advice on a joint for round post to round beam. The situation is a natural stringer bridge in which we are using vertical posts as center supports. We are using one or two posts and will be joining them to the bottom of the round stringers (i.e. beams). Does anyone have advice on a joint. Strength and simplicity is key (the only hardware we would like to use are spikes).

I thought this would be an easy find on the inter-web, but actually had hard luck.

I have three basic ideas so far.

1) Do a sort of saddle notch in the end grain of the post to fit with the beam and spike straight down through (pre drill for 3/8 spikes, its green spruce)?
2)Create a small flat on the bottom of the beam and just but joint the post, again spike down through.
3)Do a mortise and tenon (tenon in post, mortise in beam) not sure about how to shoulder it up, or if it would be a good idea to be boring a mortise into the structural element (beam). Also seems most complicated.

Jay C. White Cloud

Hi Tyler,

Several ways around this all with traditional joinery.  I have helped with a few AT trail projects that used them, and some down in Seneca Rock WV. 

What is the span?

What are the size of the stringers?

Are the Post round, and do they also form the railing for the bridge?

What is the decking and how thick?

Is it going to be a covered bridge?

Get me that info and we can go from there.  My gut is telling me, a through tenon with wedge will be best.  Very strong and sheds water well. (i.e. it drys out well)

Regards,

jay

If you do google sketch up or can draw a little...post some pics.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Spruce_Goose

Hi Jay.

The bridge is pretty simple. Its a 23ish foot span using red spruce, about 12 inch diameter average. That is the reasoning for the middle supports. Decking is a true 2 inch thick hemlock by 6 inches, 4 feet across. The plan was to stick a nice big stone in the much below (a very light stream, sometimes almost dry) then pin a post to the rock (we have a rock drill). We were going back and fourth between two posts (one for each stringer) or to span across the bottom of both stringers then do just one post in the center. (We found one nice big rock).

So there is no railing and the posts (round) would just be supporting the stringers (also round). No covering.

Tenons might be the way to go. We tend to build as simple as possible though, so if something simpler would suffice, we would probably prefer that (all we have for boring tools are chainsaws, and a 3/8 bit for a bit and brace) we also have a slick.

thanks for the help
Tyler

Jay C. White Cloud

In this case a simple mortise in you cross piece and a tenon on your short post should meet your needs, if I am following you on the intended application.

Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

witterbound

Will you have water rushing against the post when it really rains?

Spruce_Goose

Some, not a hells torrent though. The idea was to place a large rock and pin the post to the rock with rebar to prevent that possibility.

Jay C. White Cloud

Tyler,

Do you have scribing tools, and are you familiar with scribing wood to stone?









"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Spruce_Goose

That's some sweet looking stuff.

We have dividers, which we sometimes use to scribe saddle notches. I also sometimes just use a small stick (its length being the depth of the notch). Do you scribe to stone in a similar fashion as saddle notches? Do you have any particular techniques/materials for pinning the wood to the stone?

The rock we found actually has a large flat face, so in this case we may not even need to scribe.

tyler

Jay C. White Cloud

Hi Tyler,

Yes, you have the right idea, and method, just fine tune it, and you will get a perfect fit.  You don't really want a flat rock.  You want one that is domed on top, and flat on the bottom, (or imbeded into the stream bed is better) it will shed off water better, and get the stone a little higher up.  Both of which will be a good thing.  I will take you through the "big picture" steps:

1.  Place the rock (plinth) in the location you want it to be, in accordance to the bridges geometry.

2.  Choose your post and secure it to a work surface.

3.  Go to this link and do what you see to your post: https://forestryforum.com/board/index.php/topic,64878.msg967960.html#msg967960

4.  Now, take a piece of paper and tack it to the bottom of your post, next tracing around the post edge, make a paper template.  Make sure to transfer the 4 registration marks to the paper.  Before taking paper off, write on the visible surface of the paper, "to stone."  When you take off the paper write: "to post," which is what you will see when you have it in the correct orientation.

5.  Now you have a template to take to the stone to visualise the placement of the post, and where to drill for the "plinth pin," which is the center vertex of the bisecting lines.  Remember, once you have a location selected on the plinth, transfer the 4 lines to tic marks on the stone plinth.

6.  Drill the plinth and secure the pin, which in your case, does not need to be threaded, nor do you need to bolt the post don't.  Gravity and the weight of the bridge will take care of that for you.  In this configuration, it is also called a "plinth drift pin," as it keeps the post from drifting off it's spot.

7.  Drill the post and place it over the pin.  Make sure the snapped lines on the post are all plumb, by putting little wedges under the post, and real close to the tic marks on the stone.  As you fine tune the scribing these tic marks should match perfectly.

8.  Scribe the post to the plinth stone.  Keep fitting till it's perfect.  Your post should have been left a little longer than you need so you can stretch lines for where the bridge will go.

Well that's it in a nutshell.  There is more detail, but that is for the book and a much longer story.

Peace,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

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