Laying of the lattice

After some careful layout and double checking of the bottom outside chord construction and camber (see last post) it was time to pull arcs from the bottom chord to the top chord and establish the perpendicular vertical line at the midpoint of the truss. Then the top outside chord was cambered and blocked in place.

Now the vertical end pieces of the truss were cut to fit and placed at each end. Because the greatest load of the truss is at the ends the fit of these verticals is very important as they help transfer load between the top and bottom chords. The verticals are supported for now with a couple of 2x2's fastened to the underside of the chords.

After laying out the spacing and placement of the lattice the first layer of web pieces were cut and placed on top the the chords. I will have to say this is a bit more complicated that it looks. The end pieces are tricky and the filler pieces take the most time. I started out with a drawing of the truss that showed both layers of the web in place. This caused me all kinds of confusion to say the least! In the end I decided to make a drawing for each layer of the web. This allowed me to focus on each piece and see just what I needed to see. I am sure Alex and any other helpers will appreciate this.

Along those lines I will have to say that I am finding the Town Lattice Truss bridge design to be more and more the ideal project for 21st Century Learning. A good mix of hands on task work and higher level thinking. Something for everyone in a class of Technology students where their performance has real meaning and consequences. Don't get me wrong, sure a coffee table or gun cabinet is complicated and significant to the student that makes it but most high school "shop" projects are not. The students realize this and perform accordingly.

Puzzler - How many students could work on this bridge at a time?

Tech Vocab - Designer, CAD Draftsman, Engineer, Purchasing, Logistics, Material Handler, Carpenter, Helper, Supervisor, Project Manager, Scheduler, Publicity, Accountant

A good days work...

"Even the longest journey starts with a single step." Somebody a lot smarter than me said that but it is true. Today in the picture below you can see... the first step, in the actual construction of South Wayne's 32' Town Lattice covered bridge.

First layer (A) of the Bottom Outside Chord. This consists of a 12'  -  16'   and  8' long 2x12. Next the second layer (B) a 20'  and a 16' long 2x12 is positioned on top of layer (A). The location of the butt joints is critical to the strength of the truss chord and the placement of the necessary clamps that will be needed.

In the picture above I have placed a short piece of 2x2 above each butt joint. The spray paint can marks the center of the bridge truss. As you can see the 4x6 falsework is to either side of any butt joint.

Clamps made from 2"x3" oak and threaded rods can now be placed to hold the layers together and in proper alignment with each other. Then, with some stop blocks and a little help from a "handyman" jack the camber was put into this Outside Bottom Chord. Realize that this bend causes the butt joints to "open up" at the top edge. This must be corrected so there is no gap and the compression forces will be transfered uniformly for maximum bridge strength. In the picture below you can see this butt joint gap.

The technique to solve this little problem is called "sawing to the kerf". In the picture above you can see a handsaw cutting through a butt joint gap. The saw cut will make the ends parallel. Then a sledge hammer to the ends of the chord is used to drive the chord pieces together and close the saw kerf. If you do it right it will look like the picture below.

This is an amazing process and the results will allow the cambered chord pieces to act as a single 32' long piece of wood!

So we end the first day with the Outside Bottom Chord done and the Outside Top Chord layers (A) a 12'  -  16'   and  16' piece and layer (B) a 20'   -   16'   and  8'  piece stacked and clamped. Ready for tomorrow.

Puzzler - What do you suppose will be the next steps in the process?  A look back in the blog at a post or two when we were making the 3/8 scale model will give you some answers.

Tech Vocab - Stop Blocks - Handyman Jack - GRK screws  -  Kerf  -  Butt Joint  -  Compression

Time to go, "All in" as the say in poker

The kiln dried wood for the lattice trusses of the South Wayne bridge project #4 arrived today!

The wood for one truss was dropped off at the COOP in South Wayne where it will be stained. The material for the other truss, pictured above, was brought to Darlington where it will be cut, fit, clamped, drilled and carefully numbered. Then the pieces will be taken to the bridge site, assembled and raised. Then the process will be repeated for the second truss, but the parts will be stained before final assembly.


Material List for One 32' long, 8' high, Town Lattice Truss  (lattice 4' O.C.)

4 - 2x4's - 20' long
6 - 2x10's - 10'
21 - 2x10's - 16'
3 - 2x10 - 20'
4 - 2x12 - 8'
4 - 2x12 - 12'
10 - 2x12 - 16'
4 - 2x12 - 20'

Did a little scale test and can tell you that it looks like the dried pine is coming in at around 2 pounds per board foot. This makes the big boy 20' 2x12's used in the chords of the truss weigh in at 80 pounds a piece!

This coming week will be a busy one as I work to assemble the first real size lattice truss. Wish me luck and keep watching.

Puzzler - Can you tell what the first step in the process was? Why?

Tech Vocab - Annual rings - Cup - Crown - Sawing to the Kerf

It's a long way to... Camber

Luke (an avid bridge blog reader and UW graduate student) suggested that I learn how to put a link in the blog so that readers could find past posts during the design and model building phase that relate to the real construction now going on. So here goes.

If the link works it should take you to the blog post of Feb. 22, 2011 that was titled - "When do we get to work?" In this post I am working on building the camber into the 3/8 scale model truss. Reading and studying this past post may be helpful to understanding the thinking.


Feb 22, 2011 post When do we get to work

In the picture above you can see the real deal. In my research I could fine very little about the design and development of camber being built into Town Lattice Bridge Trusses. What follows are the details of what I have learned and how I am going about it.

One of my biggest fears would be to build a bridge and then have the dead weight load of the bridge itself cause the bridge to sag. Looking at the design of flat bed semi trailers one can see the arch or camber built into them and this is what I thought would be needed in a bridge. Bridge #1 was supported with knee braces. The bridge deck was jacked up and the braces installed. For 24' long Bridge #2 the bottom chord was arched. The center was 2" higher and then pulled down 1/2" per each "vertical" in the truss. This rate of drop was linear and not a true arc but did the job. Bridge #3 (also 24' long) was a mortise and tenon design and there was no reasonable way I could see cutting a camber into the joinery. Just doing it straight would be enough trouble for me and the students!

Milton's book described the camber in two of his bridges. The 140' Union Street had 12" of camber and the 240' long Zehnder's bridge in Frankenmuth, Michigan had 14" of camber. This worked out to between .342" and .233"  per 4' interval of the truss. The 32' long South Wayne bridge with 8 - 4' intervals at .342" each would come to 2.75" of camber. I rounded it up to 3"so there should be plenty of camber.

This time I wanted the points for arc of the camber to follow the true arc and not be linear. I would be working with the chord of a circle that would form the camber. The length of the chord would be 32' and the rise at the midpoint would be 3". That was the easy part. Finding what the radius for drawing this circle would be a dandy math problem! Then doing the calculations to determine the rise from the chord to the arc at 4' intervals would be a killer. However, using the CAD program was the easy way out and worked like a charm. Next was to determine if the arc for the top chord used the same center point (were the circles concentric just increased by the height if the bridge truss)?Again the CAD program made quick work of this and I determined that the top and bottom chord arcs had to be identical. Keep in mind that the top chord would be 40' long because of the angle and way the lattice continues at each end of the truss.

In the end the measurements for the camber, going from the center of the truss to the end in 4' intervals  are:  3.000"  to 2.875" to 2.250"  to 1.375"   to 0.000" in the deck  to -1.750" in the top chord.
Interval drop #1 = .125"    #2 = .625"    #3 = .875    #4 = 1.375   (deck total = 3" ) plus  #5 = 1.750"

 Puzzler - On page 85 of Milton's book he states, "The base intersections of the lattice having been laid out for 4' on centers, a radius is projected from the extreme intersections to the top of the fourth chord. A measurement along the top chord will determine the amount of excess to be added to each 4' multiple without the inaccuracy of projecting additional radii." What did this mean?

Tech Vocab - Perpendicular, Perpendicular bisector, Arc, Radius, Chord, Circumference  

Rookie mistake...

 The day started out great! Alex called and said he wanted to start applying the finish to the bridge material we had stored at the COOP in South Wayne. Alex got there before I arrived. He was set-up and already working (in the trades they refer to workers like Alex as "keepers") when I got there!

Oh, oh... 

I should be flogged and then keel hauled! Thinking (or actually not thinking). I believed it would be a good idea to store the green rough sawn lumber inside at the COOP. Stickering the boards would allow for air flow and drying. So they would not mildew. I should have known better. As you can see in the pictures above that did not work. With no direct sunlight and little air circulation in the closed shed the fresh sawn green pine did it's thing. This would need to be corrected.

Using a wire brush and then rolling on a 75% bleach 25% water solution and putting them in the sun to bake would do the trick.

A lot of extra work. Note to self: In the future green wood is stickered and stored outside until finish is applied.

Puzzler - How would you design and layout the camber for the bridge?

Tech Vocab - Chord, Circumference, Concentric, Arc, Rise, Diameter, Radius, Offset 

All hands on deck...

I guess that should be all hands on abutment not deck. Saturday morning and all three bridge building helpers were there anchoring the treated 2 x 12's that will act as a barrier between the concrete abutments and the bottom chords of the bridge trusses. This separation (I can never spell this word but my wife the teacher says there is "a rat" in the middle) will minimize moisture wicking up from the concrete and rotting away the untreated chord material.

Alex, Bruce and Lynnan installing some split/drive cement anchors.

After watching the process, Lynnan got right in to it. Next we'll be at the COOP applying the finish to the rest of the bridge material and then putting their math skills to work on the bridge false work in preparation for the camber and truss building.

Puzzler - How many gallons of finish will to take for the bridge project?

Tech Vocab - Split-Drive anchors, hammer-drill, Carbide-drill bit, portable generator

The "Tree Nails" have arrived!

The Trunnels (Tree Nails, pegs, or whatever you want to call them) have arrived and they are some dandies! 342 - 1.5" diameter Oak Trunnels (32-4",  98-6", 40-8", 48-10", 124-14") that's a lot of pegs!

The plan is to layout and drill the trusses here in Darlington. Then the trusses will be un-clamped and the pieces taken to South Wayne where they will be re-assembled and fastened together with the Trunnels. Because of the camber the trusses will have to be assembled in layers. That is the reason for the different lengths of Trunnels. In an earlier blog post it was explained how the shorter Drift Trunnels will be driven out by longer ones as the layers are added. In Milton's book he talked about using "heavy cup grease" to aid in the process.

We used a bottle brush to apply petroleum jelly but have come up with a nice applicator and some Biobased Lithium Grease for the real job. Above you can see the 1.5" diameter hole BEFORE greasing.

Above is the applicator I made hooked to a power grease gun, and next you can see it being pulled from the hole.

The results speak for its self in the picture above. A nice even coating of grease throughout the hole. Pretty "slick"!

Puzzler - How many different length of Trunnels are needed to make a Town Lattice Truss?

Tech Vocab - Tree Nail - Trunnel - Cup Grease - Gin Pole - 20:1 Ratio - 2nd Class Pulley - Choker - Worm Gear - Falsework

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