Hello,
I am a landscape architect and i am looking for some help in designing a curved pergola for a feature element in a garden. Basically, i am trying find out how to bend or arc a 2″x10″ stk cedar board into a 13′ radius and 25′ length on the inside and a 21′ radius and 32′ length on the outside (the pergola is 8′ wide) to create the beams for the 2×8′ arbor joists to span. I need to draw up construction specs. for my carpenter, but a, stuck on how to get the wood the bend on that radius. My problem is that i can’t get a single board long enought so i need to create a joint. remember is is for outdoor use which may complicate things. thanks so much for any ideas.
steve w
Replies
Steve, I don't have any specific ideas, but I have a couple of thoughts on where you might have some success getting information:
First, you might want to consider posting this over on "Breaktime," the Fine Homebuilding discussion board. This seems to be somewhat more of a construction issue than a woodworking issue.
Second (and I apologize for my foggy memory), it seems to me that about 6-18 months ago there was an article in Fine Homebuilding about making curved beams for the construction of a home or boat landing or a small residential bridge that, because of the curvature, used the bent lamination method to create the beams. Also, if my poor memory serves, the length of the beams was longer than the boards used in the laminations, so there was some discussion about making a structurally sound joint(s), or brick-stacking the laminations to distribute the weak end-to-end butt joints between boards. The article, as I remember, had plenty of photos about how the beams were laminated, clamped, glues that were used, etc.
Anyway, sorry for being so vague, but checking out Breaktime and searching the FHB index might ring a few bells or lead you in the right direction.
David
Look, I made a hat -- Where there never was a hat!
David,
thanks for the tip. i will check it out.
Steve,
This is a perfect place for a curved lamination. The trick is getting the individual laminates 25' long. You could just butt boards together and stagger the joints, but that would make for a tough glue up and compromise strength. I'd use 3 layers, 3/4" in the middle and 5/8" for the outer layers. Create 25' long layers using finger joints created with a router or shaper and make sure the joints are staggered when the lamination is glued up. Finally, use a good 2 part epoxy glue which will give you plenty of time for the glue up, resist creep, and is completely waterproof. Glue up the individual laminates at the finger joints before the curved lamination glue-up. The form for the curved lamination can be built fairly simply depending on how accurately you want to create a circular arc. Hope this gives you some ideas.
Steve
thanks for the ideas. it seems like laminating is the way to go.
I suggest a glue laminated beam. It could be made in one of two ways,
1 thin full depth plies bent to the radii you require, joints could be butt or scarf, but in either case they have to be staggered. Butt joints would be strong enough (the glue bond between the plies will allows adequate shear transfer so the joint wont fail), but scarf joints, especially on the face layers would be neater. There was an article in FWW #145 on making a table with a laminated curved apron. What you want to do is the same, just on a larger scale. The major cost is likely to be constructing the bending form.
2 short length 2x4s butt jointed as though you were coopering the curve, then trim to final shape. Your glue lamination (before trimming) looks like a stepped 4in wide 10in high beam made of 5, 2in high layers. This method doesn't require a bending form. The way I would approach the task is draw the final shape full size on a floor, cut 4x2 pieces to fit arround the circumference, repeat for subsequent layers, staggering the joints - ideally no joints line up through the beam. Advantages - you dont't need a bending form, the centre layers can be a lower grade lumber compared to the top and bottom layers. But you will need a lot of clamps
You'll also need to use a good waterproof glue.
Edit at 02:03 UT 18 Jan: BTW, why a 10x2 beam. If you've selected this size from a span table be aware that the eccentric loading a curved beam is subjected to may invalidate the table's loading assumptions. ie the beam may not be strong enough.
Edited 1/17/2003 9:02:26 PM ET by ian
the is actually a double 2x10... one set on each side of the 10" column creating a 6" space in between the two curved boards. Just like a support beam on a deck.
Steve, I agree that you should post your question on the Breaktime forum,but you might also try contacting a custom stair shop and perhaps a boat builder to give you some ideas.
Bill Bleiler
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