Rebuilding a wooden bridge... how to cut BIG slots?

**SupafastR** · 5th April 2010, 08:12 AM

G'day mates. I have worked on various small woodworking projects in my spare time for many years. Right now I have a monster-sized project on my hands... rebuilding a bridge.

I need to figure out the most efficient way to cut two 3 cm deep by 35cm wide slots across 25 or so ~22cm wide Timbers that are the bridge "planks". The slots will fit into the two metal I-beams that stretch between the shorelines of the bridge.

I looked into Mortise and Dado blades, but the maximum width I could find was only about 3cm across and the feed rate was pretty slow.

I was thinking it would be more efficient to remove the majority of the whole 3cm x 35cm x 22cm block of wood at one time instead of trying to "chip" away at it. The only saw I could think of maneuvering and fitting sideways across the 120+kg Timbers to cut the bottom of the slot is a chainsaw! I was thinking of using a skillsaw to cut 3cm deep for the slot sides, then make some more cuts 3cm deep inside that than use the skillsaw at about a 45 degree angle to start cutting sections out so I can start to cut away enough room to use the chainsaw to clear away enough room so I can start to run the chainsaw sideways at 3cm in depth.

The slot does not have to be perfectly flat, just fairly flat so the timbers will lie at the same height when placed on the I-beams.

Is this a good way to do this? Any other ideas or tools I could use to get the job done efficiently?

- SupafastR

**artme** · 5th April 2010, 09:48 AM

How about using a router and doing multiple passes?

**nihilism** · 5th April 2010, 11:11 AM

1) circular saw set to 30mm depth
2) make a whole stack of parallel cuts
3) biggest chisel you can find.

**nihilism** · 5th April 2010, 11:20 AM

Alternatively, https://www.woodworkforums.com/f155/n...1/#post1132782

**Artesano** · 10th April 2010, 09:51 PM

SupafastR - how many slots do you have to cut? A circular saw set to 30mm depth , as suggested by nihilism , then followed by router to clean the rest with give you perfect results - only I'd hate to make 320 slots this way

. The other question is , is this a hobby thing , or you get paid? - if you can justify some expenses , take a look at this baby- it's a chain morticer , and probably could be adjusted to clean the rest of the timber between circ. saw cuts in no time and without too much effort

**martrix** · 10th April 2010, 11:31 PM

Originally Posted by nihilism

1) circular saw set to 30mm depth
2) make a whole stack of parallel cuts
3) biggest chisel you can find.

yep circular saw and the biggest chisel you can find. Once you've done all the cuts, the timber left will easily chip away. Love to see some pics of what you're up to.

[ame="http://www.youtube.com/watch?v=B0GJw7y9vzc"]YouTube - Using a Circular Saw : Circular Saw Notch Cuts[/ame]

**Harry72** · 14th April 2010, 02:53 AM

Originally Posted by nihilism

1) circular saw set to 30mm depth
2) make a whole stack of parallel cuts
3) biggest chisel you can find.

Yep

Except make a straight guide up for the saw and cut 5 or so boards at once, wouldnt bother a chisel... use a heavy chisel/bolster end crow bar(a good 5 footer)to knock out the chips.

**joe greiner** · 14th April 2010, 10:26 PM

As I see it, the only purpose of the slots is to position the planks between the steel longitudinal beams. Although they might also help to maintain position of the beams, if they're not otherwise restrained. Unfortunately, the reduced thickness weakens the shear strength of the planks at the locations where the shear stress is highest.

This is not the best way to build a bridge, and I've built a few.

The usual practice is to secure the beams at the abutments, with cross-wise end diaphragms, and allowance for longitudinal motion at one end. Depending on the span, additional cross-wise diaphragms or bracing may be needed to preclude buckling of the beams.

To secure the planks, I would use cleats screwed to the planks from below at the interior face of the beam flanges. These could be all at once from below, or individually from above, with the planks flipped over to their final position. This method also preserves the full thickness of the planks.

Your application may require the services of a consulting engineer. I'm not licensed in Australia.

Cheers,
Joe

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