Bridge Series: Glue-Laminated Beam Trail Bridge Traditional Cache
Bridge Series: Glue-Laminated Beam Trail Bridge
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When I first saw this bridge I jokingly said it looks like it would support a tank but no it turns out that it can safely support only 39,000 lbs and an Abrams M1A2 Main Battle Tank, the USA’s current service tank, weighs in at 68.7 tons, or 137,000 lbs. Even a semi- or concrete truck weighs in too heavy at 80,000 lbs fully loaded. Still, the strength of the bridge is enough to support 8 pickup trucks all stacked on top of each other at the center of the span.
This bridge consist of two 6-3/4” wide by 39” deep x 65’ long glue-laminated beams (GLB). As the name suggests, GLBs are produced by gluing together smaller pieces of lumber. Using small pieces of wood allow for better use of forest resources. The individual pieces are the same thickness, 1-1/2”, as normal house wall studs (though are of a higher quality and are stronger than the generic house lumber) and in fact, you can determine the depth of the beam by counting the number of glued together boards, in this case it is 26 layers deep (26 boards x 1.5” = 39”).
The beam is treated against decay in a process called ‘pressure treating’ during which decay resistant chemicals are literally forced into the wood under pressure. The small rectangular incisions all over the beam assist the process. Pressure treated wood has a distinct smell which you may notice as approaching the bridge.
Some answers to questions you may have:
Q: Why are the bridges built so strong?
A: Most likely to absorb a direct strike by a large falling tree. Also, it’s probably cheaper over time to design these bridges to survive most trees strikes then to replace them each time.
Q: How do they get such large beams up the trail?
A: They don’t, they’re transported and placed by a large helicopter.
Q: How much does each beam weigh?
A: About 4,000 lbs. For comparison, a pickup truck weighs about only 500 lbs more.
Q: The beams are so large that the bridge doesn’t seem to move when I walk across it, does it?
A: Though it’s well beyond what you can feel, your weight in the center of the bridge causes a deflection of about 0.01 inches (one hundredth of an inch) or about the thickness of 3 pieces of standard paper
The Cache:
A container larger than a lock-n-lock but smaller than an ammo can and not very cleverly hidden next to one of the GLBs. A geo-eye will spot the hide while standing on the bridge. You’ll need to jump down and climb up a couple of feet to retrieve it.
Additional Hints
(Decrypt)
bar bs sbhe; abegurnfg gura abegujrfg; fubeg ubc
Treasures
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