Friday, June 29, 2012

Knoty Problems

Could designing, planning and building a 32' Town Lattice covered bridge be a valuable learning experience for high school students? I believe so. I would happy to show and explain it to anyone interested in 21st Century education. The first time was a great experience and the learning continues.

For today's work some of the things to consider are:
- Sawing up the kerf in each butt joint for a tight fit.
- Laying out the lattice spacing on the bottom chord.
- Laying out the lattice spacing on the top chord, perpendicular to bottom chord.
- Orienting annual rings in first lattice layer (frown, this time).
- Layout of first lattice pattern piece.
- Cut first layer of lattice to length.

Sawing up the kerf and then driving the pieces together elminates the gap that is caused by the camber put into the chords (shown in the bottom picture). The fit is tested with a playing card used as a feeler gauge. This joint passes inspection and loads will be transfered  through the butt joint like it was a solid piece.

This way or that way? What do you think? These words strike fear in many a high school students today. However I find it refreshing to apply this to tasks in search of new learning and improvement.
On the South Wayne bridge using the lattice pattern piece I cut each as you would if you were cutting rafters for a roof. I could use the pattern to a degree to make the end cuts and avoid knots in the area where the trunnel holes would be drilled. However with the 45 degree angle and 6 inch tail piece on each end this wasn't as straight forward as it would seem.

This time I placed each lattice piece in place on the chords and then shifted them doing my best to keep any knots out of the area where holes would be drilled. I will lay on the pattern and then cut them to length in place.

This will be close. I'll let you know in the next post how "this way" goes.

Tuesday, June 26, 2012

Less is More...

This can only explain my lack of posting. Keeping busy these past few weeks taking care of business around home. New steel roof and repair of some carvings I did several years ago.

Steel roofing was a whole different animal than asphalt shingles when it came to flashing the skylights and chimney. Plus getting the steel under the solar hot water system panels was a challenge. Good learning experience though.

Carved in 2007 these carvings did quite well until the woodpeckers got into the act and did some serious damage. First I put some wire over the hole then covered the area with fiber glass drywall tape. Next a fiber glass cloth mesh was stapled over the entire carving. Two coats of resin and the carvings were weather and bird proofed. Above you can see the restored and repainted carvings mounted in place.

Now with all these distractions taken care of it is on the the business of building bridge # 6 for the cabin land.  I promised myself that I would take my time and enjoy every minute of this project.

Here is a list of several key points that must be considered in the process for today's work:
- Orientation of the annual rings in each piece for the A and B chord layers.
- Orientation of the natural crown in each piece to match the desired camber.
- Relationship of the center of the chord to the false work it rest upon for clamp clearance.
- Relationship of butt joint and false work to location of lattice for hole drilling clearance.
- Stagger of all butt joints in the chords using 4 foot units.
- Placement of parallel block clamps at each butt joint for face and edge clamping.
- Forming of camber into chord layup and blocking in place.

So let the process begin:
False work in place and raw materials ready to go.

Annual ring marked on each end and a nail was driven into the crown edge of the board for reference.

Using combinations of 8'  -  12'   -  16'   or  20' pieces it takes 3 pieces to make each chord layer. Two layers make up the outside chord of each truss. The tricky part is to arrange the different length pieces so that no two butt joints line up with each other. In the picture above you can see a small square sitting on a butt joint in the first chord layer. When the next piece is put in place there will be 8 feet between each butt joint. When the two layers that make up the inside chord are placed in a few days there will be no place where two butt joints line up. This is important to the strength of the truss. Also all butt joints will be away from the location of where the holes for the lattice are to be drilled.

 And here you have it. Outside bottom and top chords clamped, arched and blocked with 4" of camber.

Next will be the task of "sawing up the kerf" of each butt joint and drive them together to make a rock solid butt joint for superior strength and load carrying truss performance.