Forum Index > Trail Talk > 1-person suspension bridge at MRNP?
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peter707
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peter707
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PostMon Jan 16, 2023 11:00 pm 
I had a random thought. If this bridge is only rated for 1 person at a time, how can it hold the weight of snow in the winter?

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Randito
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Randito
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PostMon Jan 16, 2023 11:09 pm 
Have you ever walked across one of those bridges? They are pretty bouncy and having multiple people on the bridge at once wouldn't be a lot of fun with it bouncing around. Not to mention the complications involved in having travelers in opposite direction having to pass each other.

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peter707
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PostMon Jan 16, 2023 11:33 pm 
That makes sense, thanks. It moved around when I crossed it, especially because I was trying not to rely on the wooden planks, instead standing on the spots where the wooden planks were supported from below. There was no one around when I was there in mid October although perhaps in summer it gets crowded. From a weight perspective I was puzzled. Just 4 inches (1/3 foot) of snow * 3 ft wide * 100 ft long = 100 cubic ft of snow. If the snow is 'cascade concrete' probably at least 10lb per cubic foot. So the 1st day of winter already puts the bridge at 1000lb, well above a human.

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vogtski
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PostTue Jan 17, 2023 5:31 am 
Perhaps the newer bridges are different, but I helped build several prototype suspension bridges back in the 70's. Randito is correct that the sign is to reduce bouncing but snow load could easily cause such a bridge to fail. A large pulley was built into one of the cable attachments on the trailhead end to slack the line with a big come-along so the deck could be steeply sloped for the winter. A harness and two biners were needed to safely cross in the off-season. While the original Tahoma Creek bridge was under construction in 1976, the base of the tower and the main support cables were splashed with mud overnight. This was ninety feet above the normal stream level!! I hiked partway up to Emerald Ridge and saw that the source of the flood was a rockslide in the gorge through the extensive moraines. It had dammed Tahoma Creek until enough water pressure built up to flush it. Some 'outburst floods' may indeed start within the glacier, but I think rockslides in the unconsolidated moraines are a more common source.

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peter707, ChinookPass, reststep
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Bowregard
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Bowregard
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PostTue Jan 17, 2023 7:19 am 
I am definitely not a bridge expert but support systems that account for motion often need to account for many times the expected static load. You would be surprised at the loads harmonics can generate if the load is applied at just the right (or in this case wrong) frequencies (think of how high you can get a swing or raft swinging if you shift your weight at just the right time). They probably also figure that capacity signs like that will be ignored by some.

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Randito
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Randito
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PostTue Jan 17, 2023 8:29 am 
Also remember snow isn't going to sue the NPS if the bridge collapses. But people (or at least their insurance company) will. But if the bridge collapses while two people were on it, the NPS's legal case would be much easier.

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sarbar
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sarbar
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PostTue Jan 17, 2023 2:24 pm 
OK, I swear long ago they used to drop the plank (foot area) in the winter and it hung. Does anyone else remember this? I remember reading about that in a book from long ago.

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Malachai Constant, Riverside Laker
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Riverside Laker
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PostTue Jan 17, 2023 4:48 pm 
sarbar, I've read that somewhere also. In fact once I was going to hike one of those bridges, but read the bridge planking was out for winter.

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Malachai Constant
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Malachai Constant
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PostTue Jan 17, 2023 6:44 pm 
I also heard thy roll up the planking in winter back in the day.

"You do not laugh when you look at the mountains, or when you look at the sea." Lafcadio Hearn
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Bruce Albert
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PostTue Jan 17, 2023 9:52 pm 
peter707 wrote:
From a weight perspective I was puzzled. Just 4 inches (1/3 foot) of snow * 3 ft wide * 100 ft long = 100 cubic ft of snow. If the snow is 'cascade concrete' probably at least 10lb per cubic foot. So the 1st day of winter already puts the bridge at 1000lb, well above a human.
Here are a couple of snow load factoids for the good of the order: With respect to freshly fallen snow, like your typical Stevens Pass study plot stuff, it may be coarsely assumed that 1" of water equivalent precip falling as snow will yield 12" of snowfall. Yes there are a million modifying circumstances, but that's a pretty OK rule of thumb for a typical system, which means that a cubic foot of fresh (or a "board foot" of rain) will weigh in at around a little over 5 pounds. Snow in the seasonal pack slowly gets denser and denser and reaches a weight per cubic foot, I'm led to believe, of about 50% of an equivalent volume of water. This too is subject to a wide variation but is not a bad rule of thumb which yields a weight per cubic foot in the neighborhood of 31 pounds or so. As snow becomes neve becomes glacial ice weight rises, but ice, even ice made in laboratory conditions, never equals the weight per volume of water since maximum density is achieved above freezing beneath which point volume increases. The last time I saw the number, King County design snow load for Stevens Pass was a whopping 660 pounds per square foot, meaning that one's roof would be expected to hold twenty-two feet of the gnar before collapsing. I wonder what the design factor was for the Tahoma Bridge?

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yorknl
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yorknl
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PostTue Jan 17, 2023 10:04 pm 
This may not be the passage others are remembering, but: Ira Spring and Harvey Manning, 50 Hikes in Mount Rainier National Park, Third Edition, 1988. Page 32, (Hike 5) Indian Henry's Hunting Ground. "On the way, cross a limber suspension bridge 80 feet above the raging water of Tahoma Creek. To save the bridge from winter's tons of snow, the wooden walkway is removed from late October to late May."

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ale_capone
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ale_capone
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PostWed Jan 18, 2023 4:33 am 

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BigBrunyon
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BigBrunyon
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PostWed Jan 18, 2023 11:24 pm 
They need to widen these bridges up. Widen em up to road standsrds. Known to be about 8 feet!!! It's unacceptuble for the Big Horse Groups. You're seeing more and more of these large groups!!!! Need to widen up these bridges for the Big Horse Groups!!!

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gb
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gb
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PostThu Jan 19, 2023 5:48 am 
Big hoarse groups can still just cross one at a time, or are you suggesting they all cross sideways or something?

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Randito
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Randito
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PostThu Jan 19, 2023 11:25 am 
In Mt Rainier National Park horses are only allowed on the Pacific Crest Trail along the eastern boundary of the park. So those suspension bridges along the Wonderland trail are designed for human usage only.

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