There is an old bridge at the resort I do a lot of work for, which as you can see is supported by a large forty year old Douglas Fir log. I am beginning to have my doubts as to it’s structural integrity. I can induce a good one inch bounce in it by jumping at mid-span, and have hammered a screwdriver into it without encountering any obvious decay. Beyond these very specialized tests, any ideas for determining when it is time for it to go?
To give you some background: The tree was placed by a logging excavator when the area was still newly clearcut. As you can see things have grown back and there is no access now. Any replacement would either have to be barged in, or some alternative bridge built – which would be problematic, as the bridge appears on lots of post cards and brochures.
Oh, and don’t worry about my liability. I’m only curious. The owner knows I think it is becoming unsafe.
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Very cool, I believe someone told me about that bridge when I was up there last summer.
I'm not sure that I understand all you are saying.
Are you saying that the bridge is spanning a river or whatever that is large enough for barge traffic and it is supported by one big forty year old log, but you can't access it because a forest has grown up around it.
How much and what type of traffic uses it? Can you post the post card?
The bridge connects a peninsula in the Juan de Fuca Strait with the small island shown in the picture. The peninsula was logged sometime in the 40s, but is now covered with mature trees so the only access is by a walking trail or perhaps at high tide a barge might get close.
Sorry I don't have the post card. There are quite a few good pictures of it on Google Image if you search: Pointnopoint + bridge
As you suggested, I Google it and now see what you are talking about. Neat.
"Tongue-in-cheek" You might, as one of the regular posters on this site often suggest for strengthening floor joist etc., install a hughmongus giant metal strap on the bottom of the log. Check the Simpson catalog.
On the other hand, depending on the overall condition of the log, something along that line may not be so far fetched. Or maybe a moderately unobtrusive steel WF could be placed beneath the log to add necessary additional support.
A fun topic. Could be an interesting project.
I worked for a place that had a similar bridge (though it had a roof). The management had it evaluated by an engineer. I believe they bored into the logs with a device called an increment borer (used by foresters to extract samples of trees). I recall that some of the logs had a "skin" of sound wood, but were rotten on the inside.
"some of the logs had a "skin" of sound wood, but were rotten on the inside."
Just my worry.
It was probably okay before you drove the screwdriver into it. Now it will need replaced for sure. =)
~ Ted W ~
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have some limited real-time experience with 2 ft dia D fir logs.
a. neighbor near cabin at Mossyrock, WA has 2 similar beams (maybe 30" dia) spanning about 20 feet ove Salmon creek. Installed over crushed rock at each end, no pressure treatment. Think he installed it about 1978, pretty sure the tops of the logs have sheet metal over them under the decking - still drives his JD440 across it.
b. Duckabush trail to Marmot lake in Oly Nat park. There is a 2 ft dia D Fir deadfall across the gorge about 10 miles in on the trail jsut past the side falls. An older acquaintence (a founding member of the 'mountaineers' who hiked there in the late 1930s thru the 50's) said that log was there in the 30's. I have not hiked there since '92 or so and dont kow if it is still there, but if anything was going to rot it sure would as it still had most the bark on it. Even rotting it held it's own weight for over 50 years.
c. I have 5ea 18" dia peeled logs in the back yard that I peeled in the mid 1970's, covered with sheets of old pallet plywood, sides open. They are in perfect shape, although I've had to replace the plywood as it starts to rot thru in 15 year.
d. One building I built in the 1970s near Seattle did leave exposed tails of 14 to 18" dia D Fir logs sticking out 4 feet, no cover and they got roof runoff - the one of those that had split on the top had obvious internal rot after about 20 years, serious enough I cut them all off.
e. Covered stacks of D Fir 2x8s - covering had a leak and some water got inside the stack - ants took over, after 10 years a few of the 20 ft 2x8s were now a couple of 8 footers.
So, depends on if the bridge deck is sound and keeps the log relatively dry (no water soaking into cracks. Also if there is no sign of extensive ant damage.
Beautiful spot.........
It is a real treat to do work there. http://www.pointnopointresort.com.
Over the years I built cabins A,B,C,D,E, Otter, Bluejay, Eagle, Orca and did renovations to the remainder.
Looks to be quite a stressful job........
I suppose you could always helicopter a new bridge in.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
I'd examine the ends, where it contacts the rock, and under the decking (at the cross pieces), where moisture would be trapped. If there's minimal visible/probeable rot in those places then it's "pretty good" odds that the log is sound.
Of course, the owner may want better than "pretty good" odds, but that would be expensive.
Fingersandtoes
Just east of you in the San Juans. As others have pointed out 40 years for a DF log unsupported span is probably mid life. Give or take. Considering that the bridge is baked by the summer sun and splashed with salt spray I would think that it has lots of life left. If no obvious fungus or mycellium on exposed cut ends and only an inch of bounce it might be pretty good. The only real test is to borrow an increment borer of sufficient length to reach the heart and check out the cores. Replug the little holes when done. Given the salt environment it is likely that the fungal rot is in the heart. Doubt that you will find carpenter ants at all. As you know the connections of lumber to the crown of the log are most likely to be the entry point for moisture and rot. Hope that it is good for a few more decades of memorable photos.
Stache
You are probably right. I guess it is the unpredictability of wood then it rots that troubles me. I have a fairly good idea of new wood's structural properties. Because most construction sizes members to deal with the problems of deflection, I don't worry much about structural collapse. But when a beam or in this case a log ages I just don't have a handle on how quickly its strength deteriorates.
I have spent a fair amount of my time at the resort replacing beams and posts rotten until I could dig through them with my hands. In most cases wood frame structure adapts very well to the loss of strength of one element, but not this bridge. Too late to do anything anyway: It's covered by tourists until late September.
I'm guessing the sag at mid span, and bounce are both over an inch, or you probably wouldn't have noticed.You could calculate the sag, for the diameter at mid midspan using your body weight, and then set up a level to check the actual sag with your body weight to asses how strong it really is. As someone else said, if there is any, the rot will be where the moisture gets trapped. The bottom at the supports, and under the cross pieces at for the deck. They start to sound hollow when tapped with a hammer if they have interior rot. Quit driving screwdrivers into it. If you really want to do it right you need a coring bit. But, you could get a general idea with just a long drill bit. If you drill it come from the side, and fill the void with epoxy when you are done. But, it shouldn't cost too much to fly in a new one by helicopter if it does need replaced. There is quite a bit of heli-logging done in BC, and Washington, and one log will be quite light by their standards. Just guessing, but it might be less expensive, to have a new log flown in, (either by helicopter, or dirigible), than it will be to have a thorough structural evaluation done. I think that if I were flying a new one in, I would build at least the new deck and probably railing while it was sitting on supports, so I could walk on the ground while building it. And, then lift it into place.
Helicopter might be just the way to go. I used one to install a hot tub and it was under $400.
You can see from the picture why I find the discussions here about unsafe railings and guards so funny. The cabins and trails are all bordered by precipitous drop offs. The bridge may have nice child friendly guards but the ends sure don't!
Like I said, it would probably be less than having a structural engineer to a real evaluation of the integrity of the existing log. You may have to get the new log bridge signed off by a structural engineer though. Not sure of codes on such things where you are. But, the lessened liability in the event of an accident, might lower the insurance rates enough to cost it out in a short period though. If you have the structural evaluation done on the existing bridge the engineer would have to do almost all the design work to do the certification on the existing log, and then do the field work. So, it should cost less to just do the design. That is, if one is required.
Nothing to add on the soundness of the log, but, from your description of the trail, maybe a mule or ox could snake a replacement in there.
Greg
Yeah Greg, like we all have an ox or a mule handy. Or maybe you just run down to the rental place. I see them around all the time. (Just joking and having a little fun)
One ox, one mule, a dirigible, a barge and two helicopters.
For some time my secret wish was to own a Pygmy Rhino that I saw in the Dan Diego zoo. The possibilities for pulling things and personal security seemed endless.
I'm surprised that no one suggested an elephant. They use them in India just for for that purpose. Pulling logs out of the forest.
I like the idea of your pigmy rhino. At least you would have someone to talk to while you were eating lunch.
Reminds me of when I was a kid (and that was a very long time ago). My dad would let the cows into the yard to eat the grass. Always said that he didn't have a lawnmower but he had a shovel.