You're an engineer, which means you're never really not doing or thinking about engineering. It's now summer and the great thing about summer vacation is you can design and build something for fun and adventure.
At the same time, these summer projects can teach you
something about yourself.Image may be NSFW.
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That's what I learned on last summer's engineering project: That engineering not only solves problems, it stretches and strains our personal, physical, and intellectual boundaries.
So what did we do? We decided one evening, well after the stars blinked into the night sky and the beer bottles were drained, that it would be a fantastic idea--bordering on genius, really--to build a footbridge across a creek at our cabin. Decades after their predecessors were washed away in a cold winter of Biblical rains. In the middle of nowhere.
Cast of characters
Lee Delano is a civil engineer by profession; a friendly, funny
fellow by nature. Lee, being the quintessential engineer, builds things in his
spare time, like small airplanes, some of which occasionally take off without
him and land with unhappy results.Image may be NSFW.
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He is the U.C. Davis engineering-school roommate of my cousin's husband, Roy Fray, a mechanical engineer who built nuclear power plants back in the day (and not, he is quick to point out, Three Mile Island; if he had helped design it, you never would have heard of it).
Also involved in the dirty-fingernails aspect of the job were me, my brother Kirk, and Roy's wife, Joan. Moving the financial levers was sister Lina. There were many others, often surprised they were gang-pressed into service, who aided and abetted.
The challenge
We had to span about a 45-foot-wide creek, bank to bank. It rages in the winter rains and dries out by August. We had to take into consideration the awful ground for anchor piers (even the Indians called the valley rocky and inhospitable). The heat (100 degrees F and more every day in the summer); our small workforce (usually 3-4 at any given time); our timeframe (get it done before they fixed the Bay Bridge on Labor Day).
Roy was dubious when Lee, who has built many bridges before and just completed a 40-foot truss design for his airplane club, shared his AutoCad draft.
According to Lee:
"Roy was skeptical of the rotational stiffness of this configuration and also asked about the span and abutment locations. We visited a footbridge near the Pescadero Beach in early March: a footbridge made of fiberglass spanning approximately 100 feet. The cross-section of this bridge included a bottom truss which also supported the left and right vertical trusses, a typical cross-section for this type of bridge. Even though this substantial bridge displayed a significant amount of deflection horizontally due to moving live loads at mid-span, we determined that this type of cross-section should be considered."
Joan shared photos of some of our old wood bridges and saw comforting similarities. After some tweaks, Lee was back in June with the following:
Image may be NSFW.Clik here to view.
1. 52-foot span bridge
2. 4'4" bay spacing of the vertical and horizontal trusses
3. Live loading of 40 lbs/ft^2
4. Change of material from 18 gauge to the thicker 16 gauge
5. Euler formula (member buckling) calculations for the highest stressed compression
6. Estimate of the total length of material required
7. Total weight of the bridge estimated at just over 2,000 lbs. without wood decking; 3,000 lbs. with 2x decking
Calculations completed on June 21, 2013, showed the following calculated Factor of Safety (F.S.):
1. Compression stress: F.S. = 2.4
2. Euler buckling: F.S. = 1.87
3. Screw pull-out force F.S. = 3.0
4. Compression member constraint required at 2% = 364 lbs, F.S. = 2.60
5. Bridge deflection including Live Load and Dead Load = 0.91 inches (DL only = 0.23 in.). A F.S. of 2.0 was determined to be adequate, a camber of 2" was built into the vertical trusses.
Work begins
Around July 4, I hauled up some 6,000 pounds of material in
my truck over a dreadful, washboardy dirt road, and we moved almost everything
by foot and hand across the creek to begin construction. We have no
electricity. No cell service, and the nearest Home Depot is an hour's drive.Image may be NSFW.
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Lee and Roy measured and cut the steel members, setting the field afire just once with sparks from the generator-powered saw (who needs fireworks with those guys?). Roy devised a wood platform upon which to erect the bridge, and the members screwed together reasonably quickly.
In 100-degree weather, we hammered away into the rocky soil to form and pour the two abutments for the bridge. Here, we encountered one of those humbling problems that gave rise to the phrase "measure twice, cut once" (in this case, measure twice; pick axe, pry bar, hoe and shovel once). Roy made a final measurement from the finished east-side concrete slab to the wooden form on the west side. We were off by exactly a foot. What the heck?
Work ground to a halt as Roy reviewed the drawings, re-measured and re-measured, sat and stared, shook his head and finally drove five miles to the nearest pay phone to call Lee.
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Turns out the looong tape he'd used was an engineer's tape, with exactly 12 inches of extra metal on it. Mystery solved! Except that we had to pull up the stakes and the form and re-dig the western foundation. Sigh. Beer tasted extra delicious that afternoon.
Lessons learned
The project taught us about the peculiar feel of hot metal shavings falling on one's face during drilling; stressed our endurance in the face of heat and schedule pressures; introduced some of us to the wonders of camber (fill in various schoolboy jokes here); impressed upon us the essence of keeping a shovelful of dirt in the air at all times.
Finally just before Labor Day, the 52-foot green bridge was constructed but needed to be moved 100 feet into place across the creek (here's a time-lapse video in 2:47).
With our eyes on the prize of beating the Bay Bridge construction deadline of Monday night, we devised a system to move it by hand, using a clever Teflon-topped beam system and a come-along to crank the bridge into place. I lobbied to pull it with my truck, but the ever-cautious risk analyst Roy demurred. We built a falseworks to support the 2,000-pound beast - in essence, a second bridge - in a day and then spent two days inching the bridge closer to the creek and then across it into place.
At 5:30 p.m. on Labor Day, the structure was in place above
its foundations. Kirk and I tore the falseworks down in two and a half hours
and enjoyed a beer in the gloaming as we stared at what we'd all wrought. We
had built a metaphor, it turned out. And we beat the Bay Bridge builders by
three hours. Image may be NSFW.
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Moral of the story?
Lee and Roy called this project "The Bridge to Innovation," an homage to an electronics-industry campaign Kirk and I participated in called the Drive for Innovation, which was in itself a homage to engineering and engineers.
With the bridge, we all learned first hand what we saw on the Drive to Innovation, which is that teamwork and the human spirit can combine to achieve pretty much anything.
Ours was involved but not particularly complicated as engineering projects go. What's your summer engineering project? I'd love to hear about it!
Here's Kirk's time-lapse video of the construction and movement of the bridge:
Brian Fuller
Image may be NSFW.
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Clik here to view.