Materials Handling 101-2

Years ago when I was thinking about raising honey bees I read that if the first word that comes to mind when you hear the word "bee" is sting, than maybe raising bees might not be your thing.

Likewise, if someone was thinking about building a covered bridge I would ask them what they though of when they heard the word, jigsaw puzzle.

In this process you have to understand that each bridge truss has an inside and an outside too it. This is because of the cleats and knee bracing from the roof trusses. Also the fact that the six layers that make up each truss will be, assembled, disassembled and then re-assembled in the process. In the best case you want to ONLY handle each piece three times! This requires some thinking and planning if you are to be successful. Oh, did I mention that because of trees in the way at the bridge site the first truss would be raised to vertical from the "inside down" and the second truss from the "inside up" position?

If you look back at past posts you will see the difference. This truss requires that the cleat (piece between upper and lower chords) be placed in the first two layers, then the two layers of web and finally the pieces that make up the outside ( top and bottom) chords. This way when the truss is assembled, clamped and drilled the "inside" of the truss will be down. As layers are taken to the site this time the "outside" layers will be placed on the cribs first, leaving the final re-assembled truss "inside up".  Well this should keep me busy for a week or so.

Let the Cribbing and reassembly begin

My work has changed to site preparation with the setting up of the cribs that will support the butt ends of the truss chords. This requires careful consideration for where the butt joint will be located and where the gin poles for the lift will be placed. The crib material must not get in the way and allow the gin pole to lean forward during the lift. I missed this fine point on the South Wayne bridge and had to make some adjustments after the truss was assembled.

 In this picture you can see the 3" thick slab of Locast that is placed on the top of each abutment pier. This will help hopefully protect the pine wood of the bottom chord from wicking up moisture from the cement. The cribbing must be placed level and to the correct height to support the truss layers as they are reassembled.

First layer "flipped" and loaded onto the truck. That's a 20'   2x12 hanging out there so I gave it some support with a scaffold brace.

I say "flipped" because if you have been following the construction you will notice that the top chord is to the right now. Because of the knee brace cleat and seat on each truss there is an inside and outside to each truss. Also as the 6 layers were stacked A, B, 3, 4, C, D during layout and the process was reversed for reassembly at the site. This reduced material handling.  Any questions? Very tricky but good mind exercise.

Layers D and C in place and ready to be pinned together with the 4" pins. You can see that pesky seat that the braces from the roof trusses will be attached to latter during roof truss placement.

Things are moving along very nicely. You can see web layers 4 and 3 have been added. With each layer a 2" longer pin was used to attach the lattice. At this point in each group of three chord holes there is a temporary 4", 6" and 8" pin holding the layers together. When the next chord layer (B) is added a 10" pin will be  driven into the hole above the 4" pin, driving it out.  For the final layer (A) a 14" pin will be driven into the hole above the 6" pin and then a 14" pin above the 8" pin and so on to finish the reassembly of the cambered Town lattice truss.

Time to make the Swiss Cheese

Things went well with the assembly and clamping of the first truss.

Came up with a good idea for holding the backer block in place when drill the trunnel holes to miminize the damage to the wood surface as the drill bit exits. On the South Wayne bridge I used my hydraulic floor jack to hold the backer piece up tight to the truss. The jack was heavy, bulky and slow, requiring several pumps to raise the jack each time. After some thought and digging through my shed I found an old air cylinder with a 3 inch stroke. Made a base and plumbed in a supply and exhaust valve. Works like a charm. Will be a nice addition to the ol' bridge building tool kit!

Do to the heat again today (over 100 degrees for the past 5 days) had to take a break and put off drilling until things cool off weather wise. Real heat wave this summer in southern Wisconsin. One for the record books for sure. 

Superior method

Without a doubt laying the lattice out in place and then cutting them is a superior method compared to the way carpenters cut roof rafters. A pattern is still layed out and used though.

In this picture you can see the knots in the chord and lattice piece. Taking advantage of the extra length of the lattice this piece can be slid down and the knot in the lattice be placed below the chord.

With all the first layer of lattice in place now the pattern can be used to layout the length and angles. This method was very organized and efficient as each set of tasks were completed in groups, placing lattice, adjusting for knots, tracing pattern, cutting angles. Saved a lot of time and handling.

Second layer of lattice applied and cut the same way. Good days work!

90 clamps latter and truss number one is coming along nicely. Soon I will have to shift gears and start the cribbing prep work at the site.