Building Bridges

[ScottyT]

Been a while since I posted any woodwork on here, but I’ve got a decent job building an oak bridge, the usual start to finish style I’ll post as I go I think, it’s a shade over 5M long, gentle curve in it with lattice in-fills to stop any spills over the side.
It’ll be a good job machining wise as everything will get a good workout, and it’ll involve some practical elements along the way to get over and under a few things.

Pretty decent drawing to get the salient details from and to give you an idea of what the design is. It’ll be a mixture of air dried oak and some kiln dried. The main beams will be 5.1m long and 600 wide by 100 thick. I’ll machine them in one piece, this will present some logistical headaches i reckon but it’ll be no real problem i don’t think (it’s the sheer size of them and being able to negotiate the rest of the machines whilst feeding it on the spindle, I won’t know until I’m in there with the workpiece but using the follower i reckon I can do it)

The tools for the most important job, the rod. I’ve had to make a staging to draw it on as I need it as level as is possible, and I have nowhere that’s flat where I work. I used a traditional site level and some odds and ends of material to create it, and 4 and a bit sheets of 12mm mdf laid down on it ready to roll.

That’s a pretty hefty trammel, 12 metres plus I think to get the first radius, it rises to 350 from nothing so very gentle and it’s going to be superb to machine. I had to Introduce a timber to take the sag out of the stick my trammels are on for greater accuracy.
I haven’t made a curved bridge before so I’m very much looking forward to this one. I’ll post up in the coming days to take you through my finished Rod, it’s the most critical part of the job as always.

Scotty

 

[HOJ]

Very interesting, look forward to watching this.

 

[ScottyT]

Very interesting, look forward to watching this.

Yeah this should be a good one, it’s got a little bit of everything going on and it’s nice and straight forward from almost all aspects, it’s size may be problematic but I still think all will be generally well. Thanks for the interest

Scotty

 

[Tris]

Always learn something new from your posts so looking forward to seeing this one develop

 

[paulrbarnard]

What joinery are you using for the guard rails? Looks like a perfect place for loose tenons.

 

[toolsntat]

Nice job summat to get your teeth into...
Just as an observation I immediately noticed that the lattice work was the same in all four openings. I think I might have gone for two one way and two reversed for visual balance. Then again I've not drawn it out and it might look carp...

Cheers, Andy

 

[ScottyT]

What joinery are you using for the guard rails? Looks like a perfect place for loose tenons.

Everything on it joint wise will be mortice and tenon Paul. It’s big stuff, 4x4 handrail up top, 6x6 O/S posts, 5x5 I/S posts, 3x4 bottom rails, all the lattice work is 3x3 etc etc so I can sink a nice big chisel in everywhere.

 

[ScottyT]

Nice job summat to get your teeth into...
Just as an observation I immediately noticed that the lattice work was the same in all four openings. I think I might have gone for two one way and two reversed for visual balance. Then again I've not drawn it out and it might look carp...

Cheers, Andy

Hi Andy, i don’t get involved In any design work myself. By the time it gets to me the client has signed the final approval and we’re off. This was about design #25 I think

Thanks for the interest

Scotty

 

[niall Y]

A really nice 'Chinoiserie Bridge' - I look forward to seeing it come to life.

 

[deema]

The infill looks unusual, may I ask with the bracing parts mitred into the corners of the hand and lower rail how will you stop the expansion of the hand and lower rail not trying to break apart the joints?

 

[toolsntat]

Hi Andy, i don’t get involved In any design work myself. By the time it gets to me the client has signed the final approval and we’re off. This was about design #25 I think

Thanks for the interest

Scotty

Oh yes I know the scenario.
Hope it goes well.
Andy

 

[Richard_C]

Looks like an interesting project and something satisfying and lasting.

Of passing interest, here's a link to the mathematical bridge in Cambridge. It's in Queens college but easily viewed from Silver Street bridge.

https://www.queens.cam.ac.uk/visiting-the-college/history/college-facts/mathematical-bridge

If you scroll down you will see the cost. Bricklayers and mason together cost more than the bridge itself. 17 shillings and nine pence to the cook for a dinner for the bridge builders men at the end. A tradition you could continue, a feast for 89 pence.

 

[ScottyT]

The infill looks unusual, may I ask with the bracing parts mitred into the corners of the hand and lower rail how will you stop the expansion of the hand and lower rail not trying to break apart the joints?

Hi Deema. Thanks for the interest. There won’t be any ‘crush effect’ on the infills from the top and bottom rails (atleast not enough to upset any joints or promote warping) as there will be ample room in my mortices for them to simply move up in the opposite direction, almost a slide if you like, only around 1/4” or so in the top and bottom respective of the top and bottom rail. I’ll be using 1” and 3/4” M&T’s (including in the corners) and obviously 35mm halvings to cross the two. It’s not bracing it’s pure aesthetics. There are two massive braces under the bridge to keep it square though.
I am not expecting them to expand much anyway, over 70 I should imagine 2-3mm max. Main beams aside it’s all air dried and fairly stable even in our climate.
Whilst I’m on the subject, if your ever messing around with fresh sawn oak, the enemy is moisture escaping too quickly, especially in hotter months, however if you spray a mix of 75% white spirit and 25% tung oil it really reduces the rate the moisture can escape, keeping it from splitting like crazy and staying really stable. I’ve done it a few times in different applications with cracking results. You can barely tell it’s got any finish on it what so ever either.
I hope this makes sense, I know what I’m trying to explain but it may come across like total nonsense

Scotty

 

[ScottyT]

Looks like an interesting project and something satisfying and lasting.

Of passing interest, here's a link to the mathematical bridge in Cambridge. It's in Queens college but easily viewed from Silver Street bridge.

https://www.queens.cam.ac.uk/visiting-the-college/history/college-facts/mathematical-bridge

If you scroll down you will see the cost. Bricklayers and mason together cost more than the bridge itself. 17 shillings and nine pence to the cook for a dinner for the bridge builders men at the end. A tradition you could continue, a feast for 89 pence.

That is awesome

 

[ChrisWiduWood]

Looks like an interesting project and something satisfying and lasting.

Of passing interest, here's a link to the mathematical bridge in Cambridge. It's in Queens college but easily viewed from Silver Street bridge.

https://www.queens.cam.ac.uk/visiting-the-college/history/college-facts/mathematical-bridge

If you scroll down you will see the cost. Bricklayers and mason together cost more than the bridge itself. 17 shillings and nine pence to the cook for a dinner for the bridge builders men at the end. A tradition you could continue, a feast for 89 pence.

It just so happens i built a bridge based on the mathematical bridge in Cambridge.

You can find it on The West Wycombe Estate (Sir Edward Dashwood) National Trust site.

The original bridge was built in Iroko and was a replica of the aforementioned bridge but sadly only lasted 25 or so years, we were asked to replicate it again for Sir Edwards Birthday, this time in Opepe (https://www.wood-database.com/opepe/)
It was a giant sudoku puzzle as we only hade what remained of a rotten bridge to work from..


Good luck to the OP on his build!

Chris.

 

[ScottyT]

It just so happens i built a bridge based on the mathematical bridge in Cambridge.

You can find it on The West Wycombe Estate (Sir Edward Dashwood) National Trust site.

The original bridge was built in Iroko and was a replica of the aforementioned bridge but sadly only lasted 25 or so years, we were asked to replicate it again for Sir Edwards Birthday, this time in Opepe (https://www.wood-database.com/opepe/)
It was a giant sudoku puzzle as we only hade what remained of a rotten bridge to work from..


Good luck to the OP on his build!

Chris.

Bravo. What a marvellous job.

 

[MikeJhn]

One of the most interesting threads on UKW for a long time, just one question, out of interest, why physically setting it out rather then using trigonometry to work out the relative heights at each section from the base and use a parabola curve?

 

[ScottyT]

EOne of the most interesting threads on UKW for a long time, just one question, out of interest, why physically setting it out rather then using trigonometry to work out the relative heights at each section from the base and use a parabola curve?

Hi Mike, I rod everything out I make 1:1 and put every detail you can imagine onto it. Position of mortice and tenons, depth of tenons, haunch sizes, it gives me my cutting list, positions of all components and how and where they land (particularly important for curved work) all my radii measurements, obviously my overall sizes height /width etc. but most importantly it allows me to make the job and make all my mistakes before I actually make the job and any mistakes (i still make mistakes though)
I’ll level with you Mike, you’ve lost me with the Parabola curve, i won’t pretend to know
I rod everything out 1:1 btw, be it a door or window (only need a height and width obviously) it’s Autopilot from there. Many thanks for the interest and I hope you enjoy the thread

Scotty

 

[ScottyT]

I have the rod finished and the template for the 2 main beams out ready to go. I need to run the hand rail and the bottom rail out Monday, I got sidetracked with my planer this afternoon and ran out of time for this.
It’s been very enjoyable drawing it out, with the bottom curves working out to 10M so it was fun wrestling with the trammels marking it all out.

I need to mark in all my mortices on here, i am thinking 3/4 shoulders on the tenons, keep things pretty loose to help me when I come to put it all together.

I apologise if you can’t see much on that mdf, I use a 4H pencil that’s always sharp and I don’t push down to hard! It’s clear in the flesh, but a bit faint on the photos. There is no detail on my rod yet I just wanted to get the cutting list sorted. You’ll see all that when I’m marking out.

Here’s the router on the end of a very long plywood guide. This is the method I use to get all my single curve work templates out.
It’s a simple process using a tape measure and the inside and outside of the routers cutting edge to both marks, it’s imperative you stick to the radius point here.

I get a snap line to mark the router guide so I have a known reference point from the cutters edge, this ensures that I don’t wonder away from my first pass on the second pass. It’s all to do with being as accurate as is possible.

And here is the rod and the main beam template done and certainly very MDF dusted. It’s worked a treat thus far, I need to add a little bit on to it but that’s not a problem and was always the case, the railway line that holds the shed up was just too narrow to do the whole lot in one run, so I’ll add 8” and spoke it in. It’s this process here where the Job is one or lost.

Scotty

 

[Jacob]

One of the most interesting threads on UKW for a long time, just one question, out of interest, why physically setting it out

basically because you have to physically make it.

rather then using trigonometry to work out the relative heights at each section from the base and use a parabola curve?

Basic trad woodwork is to do a "rod". Workpieces laid directly on to it and measurements marked straight off.
If you did it by calculation it would give you pages and pages of figures with no check on accuracy until you made the thing, and incredible complications if you wanted to change anything.
Even of you did it all by CAD you would still need to draw up a rod to take marks/measurements from.