Replies

1. 
   Clint_Searl | Dec 15, 1998 06:25am | #1

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   This past Summer, I replaced a rotted wood bridge over a stream which feeds our pond, intending to fabricate a pair of 32' pressure treated beams, resting on concrete pads on either side, for a clear span structure. I made each beam from paired 2"x8"s with a 2"x6" on the flat for a base. The beam looks like an upside down T. Problem was, it wouldn't support its own weight when only supported at each end. The next idea was to stiffen the beams by attaching 2"x8" light steel C-channel to the inside of the beams. Now the beams would support the walkway of 5/4" PT decking, but there was lots of sag and sway. I finally resorted to setting a PT 4"x6" post at mid-span on either side, and rigged a come-along in such a way as to use the weight of the bridge to sink the posts as far as they would go into the stream bed. After about a week they were fully settled, at which point I made the permanent beams-to-posts attachments.

1. 
   DJA | Dec 15, 1998 08:27am | #2

   *Need some more info....whats the span. What is the bridge width...and what is the live loading. You need to dowel the concrete to the bedrock with steel dowel bars.

1. 
   derf | Dec 15, 1998 08:45am | #3

   *Dear Michael,Lucky you. Bridges are always a fun commission. Your concerns about water freezing about the base of your foundation, really any foundation, are well founded.Streams, as well as other forces of nature tend to resent our guidance. Many jurisdictions recognize this problem and forbid any unpermitted interference with the nature flow of waters. You¹d be surprised at the attention Water Resource people can direct to the tiniest rill. The obvious solution to these prudent restrictions is to build your bridge well above the flood level. Typically, the longer span is more economical and prettier than building down into the water anyway.But, on the off chance that your alternatives don¹t include dry spots for your abutments, and you take extraordinary precautions to protect the local benthos, try building them to accommodate frost action instead of resisting it. One example of this is would be resting the bridge on plates instead of a foundation. These ³feet² could slip vertically on pins set with epoxy in holes drilled into the rock. Warily, derf

1. 
   Rob_Susz_ | Dec 15, 1998 05:02pm | #4

   *Probably the best idea for the situation. I would add that you backfill the abutments with large washed stone or gravel, then place a choke layer of stone on top for your walkway. The large stone will allow for drainage of water to minimize frost heaving. If the abutments are above the water line you could add some weep holes to the base of the abutment to drain water trapped inside. Remember to thread the end of the smooth dowel bars that will be epoxied. There are also products that you could use (waterstop RX) that you can cast into the interface between stone and concrete that will create a gasket. I would still let it float with the smooth dowels though, this situation seems designed to trap water and freeze, so accomodating it rather than defying it is the best approach. Grease the smooth bars before casting the concrete. Exactly where are these abutments relative to the high water mark? What is the span? A nice SYP wood truss would be neat.-Rob

1. 
   John_Wells | Dec 17, 1998 12:04pm | #5

   *What is going over this bridge? Foot traffic? Vehicles? If the latter - going to need some $ and engineering.We used to build foot bridges as part of trail construction for the USFS. I have some design books somewhere that might help. The abutments were normally open timber cribs filled with big rocks (big as we could hump). Freezing was not an issue since they were very open. Getting washed out in a winter flood was an issue.The beams were serious local timbers. Have seen some great suspension bridges done in PT. That's one way to do a long span vs. trying to get the beams beefy enought to support themselves and the bridge loads.Where to place? Above the level of the twigs and flood debris caught in the branches. Even had one or two of these mid stream if we could not avoid it.FHB's Tips & Techniques for Builders has a contour gauge for fitting piers to bedrock on pg. 92.The point regarding permits is well raised. Check it out. The Corps of Engineers, among others, asserts control over all the navigable waters of the US - so if your creek happens to join such - and most do, you just might have them to deal with. We won't ask if fish spawn in the creek. Don't wind up charged as an EnviroCriminal. You will do more time than if you shoot up a shopping mall.J Wells

2. 
   John_Wells | Dec 17, 1998 02:40pm | #6

   *Bridge Builders -Couple of thoughts: GABIONSInstead of concrete, consider using a Gabion - a galv. wire basket with rocks for fill. You can do a web search on "gabion." I found all sort of stuff. This one deals with abutments, etc. and says they are immune to frost heaves. Certainly should be. There is a lot of serious construction done with gabions. Those who have gone thru Snoqualmie Pass in WA, take a look if you are headed west from the summit. That whole retaining wall is done in gabions by Maciferri (Italian firm, I am sure I misspelled the name. They have an excellent catalog with the engineering calcs shown.) The gabions come in sizes and you can mix and match for the desired results.We have been considering them in lieu of concrete for some projects as it's easier and cheaper to deal with volunteers, rocks and wire baskets than forming and pouring concrete. Less skill is required to wire up the baskets - or fill them (however a backhoe or loader is recommended). SPAN TABLE FOR BRIDGESMy trail building handbook has a span table. Forget the 2xs. No way.32' span, round beam - need 12" diameter36' span, rect. beam - need a 10"x12" This sounds spendy.They discuss some neat truss bridges - kingpost, etc. Many photos of timber cribs filled with rock for pilings and abutments. Poor man's gabion. They suggest you cable one end of the bridge to something secure so you can find it after the flood.

3. 
   Michael_Lynes | Dec 17, 1998 10:28pm | #7

   *Thanks for all of the information! I should have provided more background, though I wasn't sure anyone would be listening. Anyway, the bridge crosses an intermittant stream (dry about 5 months of the year) that ordinarily can be accomodated by one 2' dia galvinized culvert. I installed two on the theory that we would eventually need it. As it turned out, the 50 year storm drained 200 acres through our intermittant stream...and blew out the stone surrounding the culverts to a width of about 20' (scrubbed the stream bed clean!) So I had someone from the USDA do a drainage assessment and they provided a bridge design to carry a concrete truck across to the planned building site), but I figure that I need to provide for the water that may seep between wingwall and concrete, though the concrete wing walls are not designed to be in the water except when these floods occur. Steel rod pinned into the granite, then greased and concrete poured around the pins sounds like a manageable plan for a novice like me. Opinions?Thanks very much!

4. 
   David_Rosendale | Dec 22, 1998 12:14pm | #8

   *I have to agree with Rob about trapping water. I am an Army Engineer and trapping water behind abutments causes failure, PERIOD. If it is truely bedrock it is not going to heave in your or your childrens lifetime. The concern is the abutment not moving on the rock so that the load can maintain an even constant load. Use dowels epoxied into the rock with turned or bent ends that point in different directions and are 100% saddle tied to the rebar cage inside the wall. E-mail me with the lengths of the span and the expected load and I can scan some truss or other bridge designs and some span tables that you can use to get the planning started. You will definitely have to take the plans to an engineer in your area and you will more than likely have to check with enviromental laws in your [email protected]

5. 
   John_Wells | Dec 23, 1998 07:35am | #9

   *You were headed in the right direction with the double culvert capacity. Now, about that 100 yr storm. More and bigger culvert. Suspect debris, etc. clogged it, causing the blowout.Even the pros have these problems. They don't like to talk about how the Ventura marina came to be filled to the brim with sand, gravel, clay and other stuff - the boats were "poured in place". Hint - the arroyo is that wide for a reason.If you are going to be sending concrete trucks over this bridge, (what do they weigh - 50K?) you are going to need a Bridge and engineering. Do they charge by the hour if the truck is in the stream bed? I suppose a ford won't do.

6. 
   Fred_Matthews | Dec 23, 1998 09:28pm | #10

   *Careful! As a licensed civil engineer, I always say "Wait a minute" when someone says a "small stream". Do you have any hydrologic data on this "small stream"? Building bridges, even small one for foot traffic requires serious thought and competent engineering. There are USFS publications for small bridge designs, but a word of warning, small streams can carry very high peak flows, resulting in your bridge being carried downstream as a debris pile (and causing some very angry downstream neighbors). As for anchoring, doweling into a very clean, competent bedrock surface is acceptable. A high cement content, grout slurry should be placed on the bedrock surface prior to placing concrete to achieve good bonding. Good drainage design on the backfill side is necessary and achievable with properly sized stone, filter fabrics (Mirafi), and good construction quality control during compaction.

2. 
   Guest_ | Dec 24, 1998 12:38am | #11

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   What about a Mexican-style anti-bridge - pave a section of the stream bed and drive across the resulting 1-2 inches of water? The ones in Baja take huge floods one or twice a year and are other wise dry or have only a trickle. Far cheaper to build and repair (although not as much fun for us engineer types). Water flow can't be blocked by debris and if a huge flood precludes your trip to bowling night once a year, at least you'll be less anxious than if you were waiting for your bridge to wash away.

   "Have you drive a ford, lately?"

   -David

3. 
   Elwood | Dec 27, 1998 10:43am | #12

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   Fastest solution for foot or vehicle traffic , is to use a flat bed railroad car. The wheel trucks come right off. Long span of up to 90ft. Wide enough for most normal vehicles. Pretty ugly till you paint it. Still need to fabricate hand or guardrails if your'e so inclined. Still need abutements, but can do that out of the stream bed. A set of footers, 4ft wide x 16" thick x however long with some re-bar, and an I---I shaped stem wall should do. The decking can be painted steel or decked over with wood planking, making it look more rustic. An asphalt emulsion with pea gravel and fines makes a durable surface as well. Good luck! P. S. if you epoxy re-bar dowels in, make sure you go with the epoxy coated "green bar". That stuff will last forever, standard re-bar can rust even if covered by three inches of concrete. eventually you'll be spalling off huge chunks of concrete.

1. 
   John_Wells | Dec 27, 1998 11:44pm | #13

   *Dave,Had similar idea. Have seen and used a number of these in the south and west. Culverts are just waiting for a plug. If the site would allow it, might be the cheap solution. Also a low-friction approach. Nothing much to clog.May have to cut back the approaches by blasting.J Wells

4. 
   Michael_Lynes | Dec 27, 1998 11:44pm | #14

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   I have to build a bridge to cross a small stream that runs over bedrock. The design that I have uses concrete wing walls to guide the stream under the bridge and to support the cross members. Where can I find (or who can tell me) how to cast the concrete on the surface of the bedrock so that water seepage between the concrete and the granite bedrock will not cause frost heaves of the bridge structure?