Thursday, June 2, 2011

Schools out but learnin' goes on...

"Regular" school is out for the year but Alex is doing a bit of free lance learning.  This past Sunday we took advantage of a little rain to look at the 270 pictures of the bridge project so far. Pretty interesting to see how Alex can now see the details and relate to the inner workings that go into a community bridge project like this. 

Then it was time to give working with the CAD program and making some beginning drawings a try. I think he now has a better appreciation for the working drawings of the bridge.  He realizes the time and effort needed to make them.

Over the years I have worked with several students on CAD and Alex is one of the best! After going through the menus, commands and a brief demonstration he took right to it! He dove right in and with minimal help made a nice 3 view orthographic drawing of a Button. If you recall these are the blocks that will be used with a threaded rod to hold the roof rafters and floor joists to the bridge trusses.

After an hour or so we "took a break". It was still raining so we did a bit of work with the framing square and laid out and cut Primary and Secondary rafters for a 3/8 scale model.

Alex knows how to have serious fun! It stopped raining so we went out and worked on the False Work where the Town Lattice Trusses for the bridge will be built. The two 4 x 4 rails had to first be straightened. Then the 4 x 6's had to be placed 4' on center at right angles to the rails. This was accomplished by measuring the diagonals. After screwing each 4 x 6 in place to the 4 x 4 rails we went about the task of leveling and placing support blocking. This was no small task as the False Work surface must be level in both directions. Using three 4' levels this was accomplished nicely.
In the picture above you can see Alex sighting over the tops of the 4 x 6's and admiring his work.
Having a sturdy working surface that is rigid, level and true is a must so the 4" camber can be built into each lattice truss. The final step will be to brace and anchor the 4 x 4 rails to the ground. We will then be able to jack and pull the 2 x 12 chords of the 32' lattice truss into place and form the 4" camber. Quite a process, requiring considerable thinking, insight and understanding. Something that again Alex and the other students working on the South Wayne Community Bridge Project #4 will gain for sure. Plus they will have a 32' Town Lattice Truss covered bridge to show for it!

Puzzler - If one 32' truss weighs 6,000 pounds how much force will be needed to raise it to vertical?

Tech Vocab - CAD - Offset - Line - Extension Line - Dimension Line - Front View - Top View - Layer - Erase - Cut - Extend - Doddle - Construction - Hidden Line - Center Line - Line Block


  1. I'd guess it would take 3000 pounds of force. Less the higher you raise one end as more weight is transferred to the end on the ground. That is, if I understand the question correctly.

  2. Yes. I agree, 3000 pounds to lift for starters. Although, because of the design at the ends of the lattice truss the top chord is 8 feet longer than the bottom chord. Overall it is symmetrical. This would place the center of gravity, and slightly more weight on the top half of the truss. Sharing this 3,000+ pound load between two Gin Poles will further spread out the load. Another consideration will be the "actual dry weight" of the wood vs the calculated green weight (2.35 pounds per bd ft). So all the weights are being calculated at the high end for safety. I will also explore the possibility of using hydraulic jacks and blocking for the first few feet of lift if the landscape allows it.