help design a bridge

[pixelmonkey]

i want to use all of my yard!

the first attached image is a birds eye view of easements. the section i'm going to build a bridge for is over the blue stripe. the stripe is a concrete storm drain *ditch* thats 15'3" across and the full width of the yard. the ditch is 3' deep which exceeds the flow of a 100 year flood plain.

the red stripe is natural gas pipeline. the blue is storm drainage. the yellow is power and cable under ground.

attached a couple of images to help.

i want sides that i can add vertical post or bolt chain link fence wire to so the pups are safe. the height of the fence is 4' for the section in red. 3' for the section in yellow.

The bridge must be moveable. it needs to be wide enough for a push mower to cross, stable enough a kid cant turn it onto it's side or drag it into the ditch. i cant build a drawbridge due to easements on both sides of the ditch.

lets see ya be creative!
chris<pixelmonkey>

 

[pipehack]

If you need help burning it look me up. I'm excellent at that!!!

 

[rsanter]

how easy does it have to be to move?
move by hand?
move by fork lift? by crane?

bob

 

[Bigrhamr]

Oh Man, I haven't even had my coffee yet and I gotta design a bridge already?

How about a few more parameters, in regards to the easements where CAN it move to and how much room in that direction(s)? Where does it HAVE to move FROM? Just off of the water so aircraft carriers can safely navigate or out of the area completely? And does the easement mean no permanent structure on either side of the Moat? (sounds better than ditch)

Better get crackin' on this, the excavator will be there in the morning and the concrete trucks are right behind him.

 

[buening]

Are you wanting an arch or flat bridge design? If you have the time and patience, you can bend and glue 2x4's to create an arch laminate beam for the structure. Look into wooden hammock stands to see what I'm talking about. Make a few of these beams (I could specify the amount of laminate sections needed) and make a deck out of wood. "Moveability" will be relative considering the span we are talking about. No one will be able to move it by hand regardless of what it's made out of due to it's size.

BTW, bridges are my specialty. That is what I do all day long

 

[T56 Impala]

Boy you hit on a good one! I have nearly the EXACT same situation. Mine would have to be an easy to assemble on site thing. All I'm looking at doing is spanning the gap basically. Maybe a rail on each side? Right now there is a bridge, if you can call it that, made from 2x6's. Its about 2 feet wide and has been there for about 15 years. I'm afraid to walk across it these days. The PO spent a lot of time in the woods and it was his doing.

I know a lot of you guys earn your living doing this kind of thing and giving away free designs isn't what you like to do, but some pictures and ideas would be great. Nothing specific, just an idea would be nice. I know nothing about construction but I can cut a straight line and turn a wrench. I can even use glue if needed! I have a lot of landscape problems being address right now. It would be nice to add something new to the whole thing.

I'll be watching with great interest!

 

[buening]

Another idea for something on the cheap (and removable) is to obtain some used telephone poles, typically two of them, and span the creek. Then take wood planking, such as 2x6 or 2x4, and make your deck. Make the bridge long enough so that the ends of the poles bear onto a few feet of solid ground. This eliminates settling as much as possible. Dig a hole the depth of the pole and planking at the top edge of each bank to make the bridge flush with the approach walkway. I'll see what I can do about pictures of the arch. Not sure about pictures of the telephone pole, but it was done by my parents when I was a young pup and we've taken a riding lawn mower over it. I think it's span was around 23' and was around 8' wide

 

[BECC]

Floating bridge - a minature version of the ones that corps of engineers use in the army. Ramp both sides with platform across the boat portion.

When you dont need the ramp you have a party barge. Also it is not permanent so the county won't make you remove it.

 

[buening]

Wooden arch bridges although mine would use one solid laminated beam that is bent using wood bending procedure that can be accomplished at home.

Telephone pole bridge although ours only used 2 telephone poles spaced at 6'.


Wooden arch that is more along the lines of what I envision. All depends on what you want in terms of aesthetics and ease of construction. Bending the wood members does take patience but it looks much better than telephone pole bridges.

 

[Mike83]

If it didn't need to be movable I'd install a culvert pipe and fill over it. A bridge is only expensive fill. Anywho bridges are my bread and butter. This would be easy to do, the whole movable concept makes it more challenging and expensive.

rasanter asks the best question - how will is be moved? At that span you couldn't move anything by hand.

 

[krooser]

Our local snowmobile club has built several bridges using scrap shool bus frames for the main bridge stringers then used treated 2x6's for the deck... but these are pretty permanent structures.

 

[twostory]

I found this site. It has many great ideas & plans on how to build a small bridge.

http://www.pacifi.ca/

Look at the picture below.

 

[buening]

Our local snowmobile club has built several bridges using scrap shool bus frames for the main bridge stringers then used treated 2x6's for the deck... but these are pretty permanent structures.

I've had clients ask me to design the abutments to support railroad flatcars as well as flatbed semi trailers. Both are overkill, but are cheap alternatives to a typical highway-type bridge. Doesn't really fit the scope for the OP's span requirements, but definitely an option for someone trying to span a larger creek in their back yard

 

[kbs2244]

If it has to move but not carry anything heavier than a riding mower I am thinking something made from two of those “Z web” roof trusses.
Make them ride on some kind of rails on the yard side and cantilever over to the house side.
You will need some kind of weight on the far end to counter balance it while it is moving over the ditch.

 

[pixelmonkey]

15'3" is the gap.

6" flat concrete lip on each side. *see image*

PLEASE give your thoughts...

what i have planned is a 8' wide bridge. *yea, so it's overkill*
5 2x10x 16' CCA treated boards for the spanning supports.
2x4x 8' CCA for the floor
4x4x 5' vertical post *4 on each side, see image*
2x4 cap for the top rail
2x4 for the under support for top rail
2x4 bottom edge *again, see image(s)
3' chain link mesh bolted to 4x4 vertical post.

now here is where it becomes interesting... make it moveable!

with all the lumber, it should be sturdy, can withstand being moved without distortion and weigh a ton. *almost literally*

the industrial section of the city is a short drive. from time to time, they will have scrap roller conveyer systems in various lengths. i was thinking about having a couple of these that could be linked together. 1 that spans the "mote" and a second that the bridge can roll onto the grass on. this will still require a wench and some time, but it can be moved!

if other ideas come to mind, post them up!

the "mote" is for the "Castle Rock" sub division... guess it's only fitting it have a mote around it!
chris<pixelmonkey>

 

[RbrtAWhyt]

Buy yourself and AVLB bridge layer. Bet the neighbors would love that...






Seriously though, you could design something movable, kind of like a drawbridge, using a lawn tractor or such and perhaps a winch. Something that could be picked up and moved and put back down...

 

[pixelmonkey]

If it didn't need to be movable I'd install a culvert pipe and fill over it. A bridge is only expensive fill. Anywho bridges are my bread and butter. This would be easy to do, the whole movable concept makes it more challenging and expensive.

i was thinking the same thing!

the deepest i've ever seen water flow during a down pour rain is maybe 9" deep. the depth is 3', so i was thinking of laying 3' diameter culverts into the mote, totaling 4 culverts side-by-side, giving adaquate water flow... but that can not be done.

checking the codes and development documents, the spillway is required for all sub division, commercial and industrial areas covering greater than 40 acres. This was completed prior to the first home being built or lot being sold by the developer of the division. the ditch is designed to carry water equal to that of 100 year flood plain. no alterations to the spillway can be completed. neither intrusive objects nor culverts be installed.

maintenance of the mote is to be completed via joint effort of the property owner and city.

the lead engineer for the city is stopping at my house thursday night to talk about other options. i hope to have the approval to build what i have posted, but if nay is said... i need other options!

chris<pixelmonkey>

 

[nonhog]

Good thread ! I too have need for a bridge. I have a seasonal creek in my backyard and would like to access the back 50-60 feet of my property
Thanks for the ideas so far.

 

[Mike83]

It would take a fair amount of additional design and work, but perhaps you could design the bridge to roll out over the ditch like a toolbox drawer. You would need to have a pretty beefed up support system at the roller end for when the bridge is cantilevered out, and you would have to make sure the roller system is kept clean and greased / protected from elements and dirt. Just a thought - maybe too far fetched?

A drawbridge on hinges would be easier if you had a winch to pull it up.

 

[pixelmonkey]

random thought here.... how much weight can a garage door spring support? i'm thinking about the twist type, not the streach type.

chris<pixelmonkey>

 

[buening]

Does everything have to be movable or just the bridge? What is your budget? You can get two sections of steel to span the ditch and use rollers attached to the 2x10 beams. Extend the steel section 17' or so past one abutment so that the entire bridge will have steel to roll onto. Guides will have to be installed on the rollers/wheels to keep them on track. If built correctly, one person could roll the bridge away from the ditch.....but the steel supports will still be there.

The steel sections I speak of can be completely up to you. Square tubing, small I-beams, larger angles, etc.

Go all out and buy a rolling bridge

 

[boiler7904]

How about a drawbridge?

 

[pixelmonkey]

Does everything have to be movable or just the bridge? What is your budget? You can get two sections of steel to span the ditch and use rollers attached to the 2x10 beams. Extend the steel section 17' or so past one abutment so that the entire bridge will have steel to roll onto. Guides will have to be installed on the rollers/wheels to keep them on track. If built correctly, one person could roll the bridge away from the ditch.....but the steel supports will still be there.

The steel sections I speak of can be completely up to you. Square tubing, small I-beams, larger angles, etc.

and now we're to the gray area.

it's written that no fence, or structure can be within 6' of the concrete edge of the spillway. yet, every person on the outer edge has fence up to the concrete. description of the bridge was first presented as being moveable... this was during closing of the home purchase and wasnt given in paper... more for sale promotion without proof than anything.

detailed, it includes bridges must be above the 50 year flood plain. yet the mote design is for the 100 year flood plain. i re-read that 3 times in disbelief. i'm thinking about pouring a concrete anchor in the red area of the first image upload. 1' deep, width of the bridge and 1' wide. it should be enough to keep it anchored... yet if they want to give me flack about it... what about the 6' buffer required for all the fences in the neighborhood? and if they are moved, the fences would be installed on the cable/power.

i'm not out to shoot myself or neighbors in the foot. i hope they agree with the designs and reason.

chris<pixelmonkey>

 

[His200HerScout]

this sounds like a fun project. here's my idea coming out from left field.

you'd build a deck like you described and drew. maybe with K bracing underneath so that it's less likely to warp???

then you'd put posts in every 4' leading up to the edge of the moat. the first 2 posts on either side would have some kind of roller on them that would support the deck/bridge from underneath. the remaining 3 posts on both sides would have rollers under AND over the deck/bridge to prevent it from falling into the moat as you roll it across.

my thought of a roller is a trailer stub axle with a car tire, such as this, which seems like it can easily be bolted to a post: http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200365118_200365118. i'm sure there is something more elegant out there and probably cheaper.

if you attach a pulley to the post closest the moat, you could pull the deck out perhaps by hand or maybe with a garden tractor. like so: tie the rope to the back of the deck (furthest from the moat), run it through the pulley, then back to you. then, as you pull the rope, the bridge rolls out, away from you, over the moat. maybe put in some sort of stop so that the bridge doesn't roll out off the posts...

maybe have something similar on the other side. or just a ramp that you aim for.

note: i'm not an engineer and i have no idea if this is safe.

 

[buening]

Ah, well if you are bound by the 6' limit then that could put a monkey wrench into it. My previous thoughts were in reference to the retractable bridge type, but uses rails to support the bridge during the retraction phase. Typically bridge piers are used to support the bridge while it is being retracted (so you don't have a large cantilever section). If it must be movable by hand, be outside the 6' limit, and not have any remnants in those limits then the only solution that comes to mind is a drawbridge or bascule bridge. The drawbridge would require a pretty significant column at one end of the 6' limit to hoist the bridge section up to a vertical state. The bascule bridge is like two drawbridges (google for more info bascule example), but would require a remote lift for the opposite side of the stream in order to lift/lower it simultaneously with the other section.

 

[Wardster]

If all you are trying to get across it is a push mower, then why does it need to be 8' wide. Three feet would be sufficient and would cut down on the weight considerably since you want the structure to move. From an engineering standpoint, someone needs to run some numbers to size up the elements required to carry whatever its supporting safely. Some may consider that overkill, but you are better safe than sorry.

Determine what your minimum dimensions are, how much you want the structure to weigh, and how much money you have to spend. Then find a reputable structural engineer and pay him to run numbers and come up with a few rough ideas for you.

Coming up with something anyone could do would be alot easier if the structure could be permanent.

-Wardster

 

[Mike83]

A single leaf bascule bridge would be more cost effective in terms of half the machinery and no need to run power to the other side, but the single set of machinery must be larger to move the longer leaf. A counterweight system would be critical for a working design.

 

[twostory]

If the bridge was modular, you could easily disassemble it into parts that 1 or 2 people can move

1) detach the fence/hand rail sections.
2) detach the decking section & remove.
3) Remove the two beams that span the ditch. (or however many beams you need)

As stated above, 3 feet is plenty of room for walking or the lawnmower. The narrower the bridge is, the less the components will weigh.

 

[buening]

A single leaf bascule bridge would be more cost effective in terms of half the machinery and no need to run power to the other side, but the single set of machinery must be larger to move the longer leaf. A counterweight system would be critical for a working design.

Yeah and the problem with bascules is that typically the counterweight and machinery is below grade......which would require quite a large hole to be dug.



In the end it all depends on what the inspector deems necessary for the "movable" requirements, whether it be a bridge that just isn't permanent but can be removed with a loader or whatever terms they require. At that point we will know more with what we are dealing with and can run some numbers on beam sizes, etc.

 

[Mike83]

Yeah and the problem with bascules is that typically the counterweight and machinery is below grade......which would require quite a large hole to be dug.

Yes, that is true. I think we've entered the prohibitive cost region

He could use an overhead counterweight with a truss system for the bridge Doesn't look as pretty, though.

 

[1320stang]

.....move......

 

[Mike83]

...helicopter...

 

[pixelmonkey]

.....move......

the last thing i will do is move to a place like edmond!


chris<pixelmonkey>

 

[babzog]

I saw a site last year (can't find the link now) where they were talking about and building some truss bridges using the Howe truss design. One guy used it for his railroad, another just for ATV access to his property. Love to find the site again but this is a link to a Howe truss plan for a real bridge:

Howe Truss Bridge

Here's one guys bridge.

 

[1320stang]

I didn't mean to Edmond (ever been here?), but to have piece of property like that, that you can't really use (like building a honking shop back there by the trees) seems asanine.

No offense meant though.

I remember you posting about this property before.

If the slope in the ditch wasn't so steep, I'd recommend some sort of trailer axle(s) you could put on it to pull it out, but I can't fathom a cheap (i.e. affordable) idea that looks good. The best I can come up with is a rotating piller that sits between your patio and the power easement and the bridge would be counter balanced on the house side and you could swing it 90 degrees soas not to to be in any easement, but your 3' and 4' fences on either side of your 'moat' screw that up.

The best plan I can come up with still is to move. Sorry man.

 

[pixelmonkey]

today is the day.

i have 3 different designs to propose to the city for approval. 2 of the three comes in under $1000. the third is going to be around $1400

i'll post images of each later in the day,.... and report back with which was approved.

any way i go around this, it's going to be heavy. The floor will take 192 2x4x? boards. the ? being how wide the bridge will be. if going with basic numbers, CCA 2x4x8' boards weigh in around 10lbs. if the full 8ft width of the bridge is created, the floor alone will weigh 1920 lbs. :tard:

divide the weight accordingly for widths...
3 ft wide bridge = 720 lbs for the floor.

then take into consideration the weight of the 2x10x16' supports.
if going with 5 boards for support at an 8ft width of the bridge, it's going to be 50 lbs. per board. thus 250lbs for the main supports under the floor.

the post on the sides will be 4x4x5' . the total will be 8 post. weight = 100lbs

the top cap and supports between the post will be 2x4x?. estimates on the weight for those boards = 100lbs.

approximate total bridge weight with design 1 at 8' width = 2370 lbs.

cutting the width down to 3' takes the weight down to = 1070 lbs.

going with my third, yet more expensive option at 8' width = 2200 lbs.

yep
chris<pixelmonkey>

 

[buening]

For a 16' bridge length, I'm getting around 55 boards if there are no gaps between the 2x4. You don't need 4" depth for the deck, the 2" side should be the depth (laid in 4" lengths). Keep in mind the nominal dimensions are 3.5" but there should be some gaps between the boards for drainage purposes

For the 2x10 beams, assuming you use higher grade wood (like No 1 grade) you could get by with only 2 beams as long as you tie to two together (like purlins between trusses) to keep them from kicking out. 2 beams will support a 1000 lb load at midspan, plenty for a medium size riding lawn mower and the person driving it. You could go with 3 beams if you want to be over conservative.

For the rail posts, I think 4x4 is too builky. You could get by with 2x4 for the posts and if you want extra width for the rail you could use 2x6 for the top rail. You can go smaller than 2x4 for the posts if the rail height is not too high.

With these suggestions, I think you will be able to reduce the weight of the structure. With a 4' wide structure and 3 beams, you will be around 400lbs without the railing taken into consideration. The 4' width won't handle a very large riding lawn mower deck, so keep that in mind.

 

[pixelmonkey]

For a 16' bridge length, I'm getting around 55 boards if there are no gaps between the 2x4. You don't need 4" depth for the deck, the 2" side should be the depth (laid in 4" lengths). Keep in mind the nominal dimensions are 3.5" but there should be some gaps between the boards for drainage purposes

For the 2x10 beams, assuming you use higher grade wood (like No 1 grade) you could get by with only 2 beams as long as you tie to two together (like purlins between trusses) to keep them from kicking out. 2 beams will support a 1000 lb load at midspan, plenty for a medium size riding lawn mower and the person driving it. You could go with 3 beams if you want to be over conservative.

For the rail posts, I think 4x4 is too builky. You could get by with 2x4 for the posts and if you want extra width for the rail you could use 2x6 for the top rail. You can go smaller than 2x4 for the posts if the rail height is not too high.

With these suggestions, I think you will be able to reduce the weight of the structure. With a 4' wide structure and 3 beams, you will be around 400lbs without the railing taken into consideration. The 4' width won't handle a very large riding lawn mower deck, so keep that in mind.

16' bridge length.
x12 inches per foot
/ 3 1/2" per board
= 55 boards

WOW! when i posted that the caffeine had not yet kicked in!

so lets re-do the weights.

2x4x8' 55 boards x10 lbs. per board = 550 lbs for the floor
2x10x16' 5 support beams x 50 lbs. per board = 250 lbs
4x4x5' 8 vertical banister x 12.5 lbs per board = 100lbs
2x4x8' 16 boards for rail cap & supports x 10 lbs. per board = 160 lbs
4' x 15' chain link mesh x2 sides of the bridge = 35lbs

TOTAL = 1095 lbs.

riding lawn mower will never be needed. have a honda push mower that will cross the bridge. i like the idea od having a bridge wide enough to push a wheel barrel across... or wheel chair.
chris<pixelmonkey>

 

[1320stang]

Use 5/4 decking boards instead of 2x4s or some of that composite decking, dunno the weight on it but it's 'green' and will last longer.

 

[Mikea57]

I don't have any engineering data to back this idea up I'm just thinking outside the box a little. How about a bridge that has a pivot post on the near side of the ditch? (Let's say as an example on the right side of the bridge as you approach to cross it.) The left side of the bridge (on the same side of the ditch) would have a concrete pad and a wheel. Across the ditch would be another concrete pad and wheels attached to the frame perpendicular to the framing of the bridge. Grasping a rope that is connected several feet out on the span maybe it could be pulled around so that when it was "stored", it would rest parallel to the ditch. I don't know how you'd frame it or make the walkway light enough to pull back around to store but that would make it movable. How do you get over there now?