# Curved Bridge Beams - Construction & Durability



## aero901 (Apr 11, 2012)

I'm working on a few structures which require curved wooden beams. So far, it seems the options are to either steam bend 2x treated lumber or laminate strips of treated plywood or thinner boards to build up the correct beam thickness.

I priced out the cost of the different construction methods/materials and there is a big price increase between 2x dimensional lumber and plywood or 1x boards. The plywood or 1x boards are 3-5 times more expensive given the same size beam.

Anyone have any practical experience on durability/strength that might make up for the cost difference?


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## Wildfire (Feb 4, 2004)

Sorry, not much practical experience but have wondered about this too. I made some curved structures for a boat project out of treated plywood years ago and it ended up looking really fugly and started to delaminate after a couple years. If looks are important, the 2x option will be nicer on the eyes and would have fewer joints between laminations to allow water entry. Would you be gluing the laminates together with epoxy? If so, does the ACQ treatment mess up the glue adhesion? It seemed like it did on mine but that was in the days of the old lifespan-lowering CCA treated wood. Maybe some experimenting would be in order before you commit to a big project.


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## thefriar (Jan 23, 2008)

Maybe MarkE will Chime in, I think the Copper Harbor trails in Michigan did quite a bit of curved lumber to get things going for their trail system.


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## aero901 (Apr 11, 2012)

Looks aren't that important, the beams won't be very visible.

Wasn't planning on using any adhesives between layers, just mechanical fasteners. There are construction adhesives rated for exterior use with treated wood products if necessary. In this area, MCA/MCQ treated wood products are the most common and like many others require corrosion resistant hardware. The literature seems to agree that these types of treatment processes are less reactive to fasteners than the legacy options and I would hazard a guess are less active on adhesives too.

Also planning on capping the top of the beams with a waterproof membrane (roofing felt?) or sealing the gaps with caulk to reduce water infiltration between the layers. I expect this will help prevent delaminaiton from freeze/thaw cycles and reduce rot.


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## epic (Apr 16, 2005)

http://forums.mtbr.com/trail-building-advocacy/curvy-bridge-build-920896.html


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## cmc4130 (Jan 30, 2008)

aero901 said:


> I'm working on a few structures which require curved wooden beams. So far, it seems the options are to either steam bend 2x treated lumber or laminate strips of treated plywood or thinner boards to build up the correct beam thickness.
> 
> I priced out the cost of the different construction methods/materials and there is a big price increase between 2x dimensional lumber and plywood or 1x boards. The plywood or 1x boards are 3-5 times more expensive given the same size beam.
> 
> Anyone have any practical experience on durability/strength that might make up for the cost difference?


Any pictures of the type of thing you're going for??


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## Loren_ (Dec 3, 2006)

Any woodworker with a bandsaw could resaw your 2x lumber into thinner slices which would be easier to bend and shape. You'd still get the benefit of treated, dimensional lumber. I'd definitely plan on adhesive between the laminations.


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## aero901 (Apr 11, 2012)

Thanks for the suggesting Loren. Sometimes the preservative chemicals don't penetrate all the way through a piece of lumber and exposing untreated wood would be undesirable. Treated wood sawdust is nasty stuff too.

I don't have photos yet but the radius on the bends ranges between 10-16 feet with spans from 5-8 feet.


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## jfk (Feb 5, 2009)

I'm the wrong type of engineer and even not a trail designer. However, I'd really like to see a design to comment intelligently. If you are implementing something like epic linked to above, then just use the dimensional lumber. The strength in the span should be very similar, however stress across the bend will be reduced, but that shouldn't be an issue. If you are building are arch bridge (which is fricking awesome, but I question your sanity) then things get more interesting.


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## akpirate (Sep 24, 2007)

There are a couple of considerations to be made when trying to build up DIY laminated beams( curved or otherwise). The process to making a strong durable laminate beam is not something that can be done with a few hand tools and a pile of wood. 

Without having to write up a whole chapter on woodworking and structural engineering I will say that you certainly don't want to use plywood of any kind. Plywood is mfg. by alternating sheets of wood veneer and wouldnt have much structural integerty for what you are wanting to do. 

In order to build an arched beam first you would have to layout and construct a strong enough form with a radius that you would then use to build the beam with. You would need a truckload of clamps or at the very least a few dozen bolts that you would use to clamp the individual boards to the form while the glue/ adhesive cures. Should I mention that the form itself needs to be strong enough to counter the tension of the boards you are bending?

Don't use treated wood. Too may chemicals in TW that will surely inhibit bonding. It would be better to treat the wood once you have finished. Also treated wood is $$$ compared to conventional lumber.

For as tight of radius as you describing 1x lumber should work. You might need to soak the lumber and pre-bend your pieces by blocking the ends up off the ground and stacking enough weights in the middle 
Resawing 2x into 1x will cost you more after factoring the labor involved.


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## aero901 (Apr 11, 2012)

Not making an arched span in the conventional sense. Maybe in the future.  The beams will be curved with the bend axis oriented vertically and 45 degrees from vertical (if that makes sense?). Construction will be similar to the beams in the photos epic posted but they will need to handle lateral and vertical loads. The cross-section will be closer to square to account for this.

The beams will be used in berms that have curved deck planks to create a bowl shaped riding surface. Similar to the overflow trail berm/gap/drop at Copper Harbor. The beams need to follow the arc of the turn unlike conventional wood berm framing which uses straight members to approximate the arc.

I haven't seen this type of construction method used on a wooden berm before so there are a lot of unknowns at the moment.


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## Wildfire (Feb 4, 2004)

*Curved beams*

You're probably aware there are companies that manufacture curved beams.







These'uns are double-curved.







These could be used as an arch or horizontal. 







There's a lot of good info if you do a Google search for "curved glulam beams". There's also a lot of information, videos, pics, etc. on building your own.

Pretty cool stuff.


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## aero901 (Apr 11, 2012)

Wildfire, the beam in the middle photo, with the double curvature, looks exactly like a few needed for this project (though much smaller for our needs). The glulam technique definitly looks promising and I'm currently reading up on it. Building such a beam wouldn't be out of the question but how to treat it afterwards to hold up to an outdoor environment is a problem I'm not sure we have the resources to effectively solve. It looks like commercial beams are laid up and cured before going through the pressure treatment process. We would be forced to built a beam from pretreated lumber from the get go. This method would work if the glue will adhere to the treated wood (a lot of it comes wet though which is a problem). The goal is for these structures to last 15-20 years before any structural pieces would need replacement.


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## akpirate (Sep 24, 2007)

aero901 said:


> Not making an arched span in the conventional sense. Maybe in the future.  The beams will be curved with the bend axis oriented vertically and 45 degrees from vertical (if that makes sense?). Construction will be similar to the beams in the photos epic posted but they will need to handle lateral and vertical loads. The cross-section will be closer to square to account for this.
> 
> The beams will be used in berms that have curved deck planks to create a bowl shaped riding surface. Similar to the overflow trail berm/gap/drop at Copper Harbor. The beams need to follow the arc of the turn unlike conventional wood berm framing which uses straight members to approximate the arc.
> 
> I haven't seen this type of construction method used on a wooden berm before so there are a lot of unknowns at the moment.


Well, arch or curve, the process is nearly identical only the length and width are changed. What you are wanting to construct is actually rather complex and would be insanely expensive and time consuming. While you would definitely win the award for fanciest wooden trail feature it's a lot of time and resources that could be used on other trail improvements.

The reason you don't see glue lam construction outdoors is because they don't do very well outdoors ...especially if they are exposed. There are plenty of examples of wooden berms/ ramps/ bowls at indoor bike/ skate parks. (I swore I saw several pics posted early yesterday of examples ) I can't think of any of them that aren't build using conventional framing techniques.

I don't mean to sound discouraging but what you are wanting to build isn't something that is going to be cheap or easy. Just to attempt to build up a curved beam depicted in that second pic you would need almost as much wood to build up the form as you'll need to build the beam itself. (Granted the form can be used mutiple times)


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## jfk (Feb 5, 2009)

akpirate said:


> While you would definitely win the award for fanciest wooden trail feature it's a lot of time and resources that could be used on other trail improvements.


aero, while akpirate it probably right, I want to see you build it and I'm about the most risk adverse person there is. If this is what motivates to build, then "build it and they ride it." However, a test construction that isn't in the field would be helpful.

I hope you have been dissuaded from a laminate based solution. No matter what direction you go, I want to see some pictures.


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## dubthang (Apr 2, 2009)

Perhaps another construction method would be better. You could always make treads from 4x4 stock, drill holes in them, and feed them onto bent metal tubing. The tubing would take place of the beams.


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## epic (Apr 16, 2005)

Wildfire said:


> These'uns are double-curved.
> View attachment 956016


How the......


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## aero901 (Apr 11, 2012)

Definitly not discouraged at this point. Challenging projects are the most rewarding. We have 3 more months of winter and frozen ground so our trailwork schedule is wide open.

Still working towards a solution that will allow the double curvature but I have at least two options now to experiment with (steam bending and glulam or a combination of the two). The single curvature beams should be a piece of cake by comparison.

I came across a brush on wood preservative (Copper Green - [Copper Naphthenate (pdf warning)]) which may make diy outdoor glulam beams possible. This stuff could also be used to retreat sawn faces if slicing treated lumber into thiner pieces for better flexibility. May even be able to soak the wood in it to give better penetration.

As for adhesives, commercial glulam beams use waterproof adhesives like epoxy, resorcinol, or polyurethane and most of these are not that hard to find. Polyurethane glues are moisture activated so bonding damp treated wood should be possible.

Here is a concept for one of the berms I described earlier.





​


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## Wildfire (Feb 4, 2004)

So you're trying to turn 2-dimensional lumber into 3-dimensional curved shapes? So far, so good. Looks like the double curved beams will work for you if you can get them sealed. Maybe wrap them in 40 mil EPDM pond liner or bituthane and leave the bottom open for ventilation? The stuff should pretty much seal up any holes created by fasteners. 

How about the reverse-arched decking, are you going to bend or laminate that, too? I'm thinking not only does it have to be bowed somehow but if you want to get fancy (no danger of that), you're going to have to cut the decking planks into reverse-hourglass (gore) shapes to have a consistant gap along the edges of the decking since they baloon out in the middle.

Minor technical difficulties aside, it could work. Keep us posted.


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## aero901 (Apr 11, 2012)

The decking will be steam bent using a jig to get the proper radius. Individual planks will be partially tapered to account for the turn radius but gap consistency isn't a huge concern; they just need to fit together without interference. When we finish, I'll be posting more details about the project itself.


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