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Vancouver, BC
Canada
874 Posts |
Posted - 05/10/2012 : 08:27 AM
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quote:
Originally posted by Stoked
quote:
Originally posted by weedWhacker
Wood floats with about 10% of its volume above the water.
Therefore its density is 0.9 kg/l or 900 kg/m3.
The volume of a 6x6 beam, 30 feet long =
15cm x 15cm x 10m =
0.45 m3
Its mass will be
0.45 m3 x 900 kg/m3 =
405 kg
Cargo limit for Bell 206 jetRanger =
1200 lbs = 545 kg.
Is 900 kg/m3 for dry wood? Or saturated wet wood?
900 kg/m3 is based on Archimedes principle and the assumption that 10% of the log would be above the water. It is a rough estimate that should be good enough for planning helicopter loads.
Wood floats => its density must be less than 1000 kg/m3. |
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Vancouver, BC
Canada
874 Posts |
Posted - 05/10/2012 : 08:47 AM
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quote:
Originally posted by Stoked
quote:
Originally posted by scottN
I thought there was a rock in the middle of the stream at the bridge site?
Yes, I'd thought of a couple of ziplines, one on each side angled downwards.
You can get by with one zipline with a string to retrieve the seat. It has been suggested for the Hat Hilton too. I am not sure about liability if it fails though. |
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North Vancouver, BC
Canada
16 Posts |
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962 Posts |
Posted - 05/10/2012 : 12:07 PM
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quote:
Originally posted by path finder
Is this the spot you're referring to Paul?
   
That's the one. Your pics? How's it going Don?
The flat rock gets overswept in the spring freshet. Wulf thinks the force of the water flow snapped the log. Basically, the log gets floated but is secured in place by steel cables. That doesn't prevent it from rotating so the natural bow in the log faces downstream and the water force over 10 years caused it to snap. That's one theory. The other is it just snapped from snow load.
We would put some steel mesh cribbing on the rock and the opposite bank to elevate the bridge one metre. That should elevate the bridge above the freshet. The cribbing would have to be anchored to prevent it from being shifted. |
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962 Posts |
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Vancouver, BC
Canada
1421 Posts |
Posted - 05/10/2012 : 1:43 PM
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A vertical load bearing post is an example of pure compression.
A bending beam experiences compression on the top of the beam and tension on the bottom of the beam, and shear throughout the inside of the beam. The longer the beam, the greater the forces involved. |
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North Vancouver, BC
Canada
16 Posts |
Posted - 05/10/2012 : 1:52 PM
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I don't grok the section modulus calc but the materials datasheets is interesting. Am I wrong in assuming that the "Compressive Yield Strength" is what resists a vertical force on the member such as a snow load? If so, the compressive yield strength of "Plastic Lumber" significantly exceeds that of "American Coast Douglas Fir".
[/quote]
Section modulus is how we account for how the cross sectional shape changes the strength of our beam
The beam normaly fails in tension (the value of interest is often just called the yield stress) as the fibers on the bottom will be pulled apart as the beam bends (there are more complex explanations of failure involving shear and states of stress but we dont need to go there )
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Dewdney, BC
Canada
472 Posts |
Posted - 05/10/2012 : 2:12 PM
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quote:
Wulf thinks the force of the water flow snapped the log
Not true Paul ; Wulf thinks that snowpack movement (flow or creep) is the main problem here and vertical snow load is much less of a factor . And that is exactly what I was trying to say during our last discussion .
The original log as shown in Pathfinders photos is straight . After being subjected to side pressure from snowpack movement the log deformed (bent) in the downstream direction and subsequently rotated so the belly was on the bottom . That is what we fixed the time I was up there , and at the time I did not clue in to the cause , so we just rotated the log back to it`s original position and secured it there .
Water pressure , especially with a debris flow could certainly break that log , but is unlikely to deform it in that manner . That takes heavy pressure applied over a longer period .
Wulf
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Edited by - Wulf2 on 05/10/2012 4:18 PM |
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Vancouver
1808 Posts |
Posted - 05/10/2012 : 5:40 PM
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quote:
Originally posted by Stoked
quote:
Originally posted by sgRant
An example of a bridge with angled braces not resting on the streambed is the latest bridge at the outflow of Garibaldi Lake. That bridge has a fairly long span and is perhaps 10 years old now, so maybe it could be used as an example.
Steve, how do those angled braces work? I know the bridge but it was under snow the last few times I was up there.
Someone must have pictures of it. On each side of the creek, there's a brace that angles out from the footing, and connects to the bottom of the bridge deck about 1/3 of the way across. So the whole span is divided and supported in thirds, and there is no support in the creek itself.
I used to wonder how Slim Creek could be bridged, and one idea I had was a double-ended drawbridge set up with rigging so it could be operated from both sides. Probably too finicky though. And some people would leave it down, of course. |
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Delta, B.C.
Canada
457 Posts |
Posted - 05/10/2012 : 6:12 PM
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| Are there suitable trees creekside to construct a Burma Bridge? |
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Grouse Grinding, GPS carrying, lawn chair packing, bike riding North Shore tech addict who stares at Crown Mountain from his office window all day
North Vancouver
Canada
1905 Posts |
Posted - 05/10/2012 : 6:16 PM
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Garibaldi Lake Bridge:
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 Trail blazin', backcountry bushwackin', pine huntin', photo takin', long winded story teller
3838 Posts |
Posted - 05/10/2012 : 11:09 PM
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Just my 2 cents but if you use 3 or 4 logs side by side and bolted in with decking that should stop side drift and snow load issues.
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521 Posts |
Posted - 05/11/2012 : 06:46 AM
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quote:
That's the one. Your pics? How's it going Don?
It's going well Paul. And those are my pics. They were taken when the log bridge was put in place October 2002. So looks like we got close to 10 years out of that one!
Another issue to consider is that the wood will rot over time (wet environment) which will also weaken the log(s). Pressure treated wood will stand up much better, but then you have the environmental issues with it near a riparian area.
I think the cable span idea is a good one. You could also install decking that could be removed before winter. |
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Squamish, British Columbia
Canada
1002 Posts |
Posted - 05/17/2012 : 11:16 AM
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Bridge condition as of last weekend. Looks pretty much unchanged from last summer. Needless to say, we where unable to test it though.
Taken: 05/15/12
Taken: 08/27/11
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Vancouver, BC
Canada
874 Posts |
Posted - 05/22/2012 : 12:03 PM
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I just heard about a material that might be ideal for this project.
http://en.wikipedia.org/wiki/Aerated_autoclaved_concrete
Pros:
* 20% of the weight of normal concrete so it is light enough to be backpacked
* 50% of the strength of normal concrete so it is strong enough to support beams
* completely weatherproof, never rots, can be submerged indefinitely
* can be cut and drilled with ordinary hand tools
Cons:
* very hard to find in Vancouver.
* only available as precast blocks or boards
You could use it for the decking or as a mid-span support, possibly bolted onto the large flat rock in the middle of the creek.
I would like to use it for some structural repairs for the Hat Hilton. However, I have not been able to find a supplier. Does anybody know where to buy this stuff?
Note: this is not the same material as "foamed concrete" or "aerated concrete". |
Edited by - weedWhacker on 05/22/2012 12:44 PM |
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962 Posts |
Posted - 05/22/2012 : 4:10 PM
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quote:
Originally posted by path finder
[quote]Another issue to consider is that the wood will rot over time (wet environment) which will also weaken the log(s). Pressure treated wood will stand up much better, but then you have the environmental issues with it near a riparian area.
BC Parks prefer untreated wood over stream courses. |
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962 Posts |
Posted - 05/22/2012 : 5:28 PM
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quote:
Originally posted by Steventy
Some great information here:
http://www.fs.fed.us/eng/bridges/index.htm
I took a good look at the Trail Bridge Catalog produced by the USDA Forest Service. It looks like two bridge designs are suitable for Sigurd Creek - Single Log Stringer and Solid Sawn Timber Stringer. I used the design factors for Pacific Northwest (region 6) as the closest approximation to Tantalus Range. The highest design factor for Ground Snow Load is 200 psf and I used it. It corresponds to a Pedestrian Load of 85 psf, again the highest PL.
The single log stringer is the simplest design. A 30 foot span would require an 18 inch diameter No. 1 Douglas Fir log for GSF 200 and PL 85. As there is not Douglas Fir on site I would only be able to get Mountain Hemlock or similar log. Therefore, I would have to increase the diameter to say 20 or 22 inches. The biggest drawback to this design is the weight of log. It might be almost impossible to maneuver it onto the platform. It would certainly require a heavy jack and come along.
A solid sawn timber stringer bridge requires three sawn timber stringers. For Hem-Fir or Mountain Hemlock, the stringers will need to be 6x22 inches for a 30 foot span and GSL 200/PL 85. These could be sawn on site using an Alaskan MK-III mill from Lee Valley and a 30" or 36" chain saw. I don't have either of these so they would need to be purchased. I would guess that once cut, the stringers could be manoeuvered by four to six people. Alternatively, they could be sawn elsewhere and flown in. Does anyone know how much a 30-foot 6x18 Douglas Fir stringer weighs? Or 6x22 Hem-Firm? There would also need to be decking laid down. I suppose it could be flown in or sawn on site.
It would remain to raise the level of the bridge to get it above high water. High water may occur in spring freshet or during winter storms. I recall one November or December there was a massive flood event. It was during this memorable event that the Upper Squamish gauge recorded something like a meter of rain in 24 hours or something like that. The one thing that is probably impossible to design for is a damming of the creek upstream of the bridge by avalanche. The surge after dam burst will likely sweep away everything in its path. This likely occurred the winter of 1998/1999 when the original bridge was washed 200 meters downstream of the current crossing.
To raise the bridge level, we will need to lay gabions (wire mesh baskets) on each side to raise the bottom of the bridge say one to two feet above the level of the top of the flat rock. |
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Burnaby, BC
Canada
667 Posts |
Posted - 05/22/2012 : 5:36 PM
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I was thinking the same things. There is big tree that if felled in the right direction would be perfect. But maybe that is not legal or some thing.
It would definitely need a few people, lots of rope and tools.. not an easy job.
I hate that creek crossing!
Why wouldn't you use what is there? Maybe even find a new spot to cross with a couple trees that could be roped tensioned and felled into place? the bolted together? There are tones of Good trees on the other side of the creek .. A little up creek and I am sure there could be a better spot. |
Edited by - MatthewBaldwin on 05/22/2012 5:43 PM |
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962 Posts |
Posted - 05/22/2012 : 5:56 PM
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quote:
Originally posted by MatthewBaldwin
I was thinking the same things. There is big tree that if felled in the right direction would be perfect. But maybe that is not legal or some thing.
It would definitely need a few people, lots of rope and tools.. not an easy job.
I hate that creek crossing!
Why wouldn't you use what is there? Maybe even find a new spot to cross with a couple trees that could be roped tensioned and felled into place? the bolted together? There are tones of Good trees on the other side of the creek .. A little up creek and I am sure there could be a better spot.
Why do you hate it?
There are a couple of considerations about the location. The large rock in the middle is about halfway across the creek so it acts as a pier and effectively halves the crossing width. There aren't many locations along the creek like that. Secondly, locations further up the creek require crossing a large wet meadow immediately above the current crossing and there is a second wet meadow beyond it. That's the principal reason for crossing at the big rock. We don't want a trail like the one to Tricouni that is essentially a long stretch of mudhole. As soon as any number of people cross a meadow like that, it turns to mud and then everyone starts stepping around it and making it wider. It's a real environmental mess. No one wants that.
And at some point it's necessary to cross the creek to avoid a big rockslide with huge rocks that cuts off access to the upper valley. |
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Vancouver, BC
Canada
874 Posts |
Posted - 05/22/2012 : 6:05 PM
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Kerrisdale equipment will rent an alaska mill for $80/week and a 36" chainsaw for $300.
http://www.kerrisdaleequipment.com/catalogframe.html?action=category&category=21
Are the stringers simple beams or are they struts? If the former, then the weight will be volume x density as discussed above. But either size sounds like it will be too much for a jetRanger, so you might be into big bucks to rent a bigger machine.
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Snow load/footbridge replacement
