This year we decided to remodel our back yard. At one end is a circular gravel area with a new fire pit at the center. We needed extra seating and somewhere along the line decided to build a curved garden bench so more people could sit around the fire pit.
The shape of the bench was dictated by the radius of the gravel area in the landscape plan. We selected an internal radius of 83 in with eight 1.5 inch wide rails spaced an inch apart. This gives an outer radius of 92 in and a bench “width” of 19 inches. We wanted the bench to extend for 90 degrees to fit in with the design and accommodate at least 5 people.
Step 1: Design
The bench is built by gluing thin strips of ¼ in thick cedar into rails. The thickness was selected to make sure we could bend the strips and a local cedar product supplier cut and milled the lumber to size.
For this type of glue up you need to build a form so that the pieces can be clamped together while the glue sets.
To get all the dimensions correct and to design the form I decided to model the entire bench and the form using 3D modeling software.
The following picture shows the dimensions sketch which calculates the position of each rail from the internal radius, rail width and spacing. The second screen shot shows the 3D model created from the sketch including the posts and support beams.
Another advantage of creating a 3D model was to create an exact drawing for the location of the posts which helped the landscape contractor to install them very precisely - see the third screen shot.
Step 2: Designing the Attachment Between the Bench and the Posts
This is a large bench – there is probably enough room for 5 or possible 6 adults to sit on it at one time.
I wasn’t sure about how to accomplish this to support the loads where the bench joins the post and how to attach these securely.
Because of this I decided to analyze the potential loads using rigid body analysis and a cool iPad application called ForceEffects.
I was quite surprised by the reaction force or load where the beam touches the edge of the post. The more I thought about this and the more I talked to other people I realized that the shape of the bench is helping me. Because it's curved, the loads are distributed very effectively around the bench to the other posts. The second video shows the resulting loads and confirms that we only need a simple lag bolt at the center of each post to secure the bench.
Step 3: Building the Form
I built the form using 8x4 construction grade plywood screwed to 2x6 support rails. This gives a reasonably flat surface with enough stiffness to resist the clamping forces.
Then I marked out the circles using the dimensions from my 3D model and drilled holes for the ½ inch bolts to secure the fixed clamping blocks.
The fixed clamping blocks were made of handrail sawn into 6 inch lengths. The hand rail gives a nice curved clamping surface that avoids bruising the cedar when you tighten the clamps.
Finally I assembled the entire structure and placed it across two benches to make a secure working surface at waist height – much better than assembling this on the ground
Step 4: Create the Spacers and Clamping Blocks
The bench is built up rail by rail and obviously each rail had a different radius. You could opt to move the inner clamping blocks for each rail but since there are spacers between each rail I opted to create these at this point and use them during the glue up. This enabled me to glue up multiple rails form a single position of the inner blocks. I milled a piece of cedar to a 1” by 2” section and the cut 2 inch lengths on my chop saw.
I used small pieces of cedar to distribute the clamping load. I started out with flat clamping blocks and then changed to curved blocks which I made using a 3” diameter sanding drum on my drill press. These are better at spreading the clamping loads and achieving a tighter joint. You can see the curved blocks in the picture
Step 5: Gluing the Rails
You’ll need a lot of clamps to make this work. We had over 30. If you don’t have enough borrow or scrounge more – you can’t have enough.
Spread exterior grade wood glue on both sides of the two pieces with a hard rubber roller and clamp in place then leave for the glue to harden. Repeat this process with the remaining 4 strips until you’ve built up the entire rail. Then insert the spacer blocks and start with the first two pieces of the next rail. We typically glued one piece in the morning and one in the evening.
With 8 rails and 6 laminations in each rail that’s 48 glue ups which takes over 3 weeks to glue up all 8 rails. The alternative is to use a fast drying contact adhesive – but I don’t like the smell of that stuff… I also realized that with cedar you can get away with thicker rails. If I did this again I’d go for 4 X 3/8 strips per rail.
Step 6: Trimming the Rails
It’s impossible to glue these so accurately that you’ll get a smooth top and bottom.
The easiest way is to trim them with a hand held electric planer. You’ll need to install the “guide”. Because the rails are curved you need to be very careful as you do this to prevent the plane wobbling and digging into the material.
Here I am trying to figure out how to align a bunch of curves..
Step 7: Assembly
I assembled the bench onthe same form that I used to clamp the rails during the glue-up.
The most important part of assembly is making sure that everythign is precisely aligned before drilling the holds for the cariage bolts that hold it together. There are four bolts which are located above each of the posts. After placing and aligning every spacer I clamped everything together and bored the holes with a 24 inch long 1/2 inch drill. To make sure the hole stayed true I also create a steel drillign jig - spee photo. With the holes in the right place it was relatively simple to bolt everything together.
Finally I treated the whole structure with adn exterior stain and water sealant and mounted it on the posts with four 3/8 by 6 inch SPAX exterior lag screws.
I'm really pleased with the way the bench turned out. Now it's time for that barbeque and a case of beer.