The 2013 Halloween Cemetery Arch Part 2

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Based on the experiences of last year, I think I have a better design for my Cemetery Entry Arch. Last year I used a combination of PVD pipe and hard insulation made of extruded polystyrene foam.  The load bearing structural elements depended on firm bonds between the two materials. Neither the PVC not the foam completely bore the weight.  After the comedy of trying to erect the structure, it fell over and the glue bond between the materials broke. Well, not exactly. The glue maintained a bond to the PVC and the surface of the foam. But the surface of the foam pulled off.

This year, I am building the structural elements from 1” PVC pipe and only using the foam for the exterior, non load bearing facade.

Each side support is 8’ tall and breaks down into two 4’ tall sections. Those section further divide into a front half and back half. The front and back sections are joined by 1.25” PVC sections slid over and bolted to 1” PVC tabs. The top and bottom halve are zip-tied together by at least 2 zip-ties per corner.  

The facade is .5” extruded polystyrene hard foam insulation. I chose this material because it is light weight, is fairly strong for its mass and is easy to work with hand or power tools. It will attach to the support frame with rare earth magnets that lock to small steel plates epoxied to the frame.

Last years’ arch was constructed from 1” PVC elbow “T“ fittings with 5” of pipe sticking out. 2 “T”s together middle to middle make an “I” shape. 1.25” PVC pipes with elbow fitting slip over and bolt to the “I” pieces. The result is a double vertical arch that I can attach coroplast letters to. All the joints allow me to store it easily and still span about 12 feet. The design and arch span desires an article unto itself.

There are problems with that design. First, it is hard to attach to the supports. Sliding pipe sections into each other at an unforgiving angle while supporting the whole structure is a bit of a pain. Second, the braces are only in the vertical plane. I’m not entirely comfortable with that. Finally, the “I” braces visually interfere with the letters.

At first, I planned to use an additional run of pipe to make it a 3 pipe span with the pipes forming a 45-45-90 triangle when viewed from the side. That is still the backup plan. Right now I am thinking that I could just use a 2 pipe span but make them horizontal to each other instead of vertical to each other. That would still allow me to mount goodies off the back and I think the coroplast letters are stiff enough that I could use a coroplast runner and pop rivet or zip-the letters to that before zip-tying  the runner to the arch.  For some reason, I think the dynamics of that arrangement might work better. First of all, the weight is balanced front to back so its less likely to get unbalanced and tip the whole thing over. Then the axis of flex is in the vertical instead of the horizontal. As long as the arch is self supporting across the span, I think will be a lot more stable. The math is a lot easier too.

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