steel I beam span question

[nathan.principe]

I know this is a loaded question, probably with many variables, but here goes.  I have a thought to create a footbridge over a pond for a landscape project I am working on.  The span from waters edge to waters edge is 50'.  My thought is to pour 2 footings with deep piers on either side of the pond with a "notch" to receive the I beams perfectly.  Really my questions are these.  Can a steel I beam make a span of 55'-60' in length? and if several pieces were welded together to create the total length are we compromising the structure?  Of course there would be a crane on site to lift these into place.  Also the loads would be that of a 6' wide wooden foot bridge to cross and fish off of at a residence.  Thanks in advance!

[rick91351]

You might down load this and have fun figuring it all out.  Only you know what you have in your head and want to do.

http://www.toolbase.org/PDF/DesignGuides/ResidentialSteelLoad_SpanTables.pdf

Even if you can find a used beam and pay scrap price.  WOW!  Plus the rebar in your piers.

[Don_P]

For the 60' span the AISC safe load tables don't list anything less than a W24x55, A 24" deep I beam weighing 55 lbs/foot. By the numbers a W18x71 works but you're paying by the pound and the deeper is much stronger and they don't list the 18 in 60'. And you want joints. I can see an engineer in that cost somewhere.

Wooden covered bridge? A steel truss bridge? Many smaller lighter parts, that's why they predate the modern deep sections. I drive over a turn of the century former railway bridge frequently that is made of angle iron and rod. They could make a lighter weight of beam because the truss, the effective beam, was 14' tall. A wooden covered bridge is the same thing with a roof on top to keep the wood dry.

A friend here with access to plenty of timber really wants to build a bridge for someone.

[MountainDon]

A friend here with access to plenty of timber really wants to build a bridge for someone.

A wooden covered bridge would be cool. Even uncovered it would be a very interesting array of triangulated components.

The FS here has built a few hiking trail bridges using glulams. Two glulams set on concrete supports and decked with 2x6 about 36 inches wide. No handrails. The longest is about 35 - 40 feet. I love walking them and making them bounce. I do hope the engineer took that proclivity of 'big kids' into account.

[Bob S.]

Would any type of floating bridge work?

[rick91351]

I have wanted to do a cable suspension  bridge sort of a mini 'Golden Gate' bridge.   There are a few of them in the back country build by the USFS  where they use to cross sheep over rivers.  Now they are used mainly by back packers and hose packers.  There are a couple over the Salmon River here in Idaho, have use one or two in Yellowstone when back packing the Yellowstone river.  What I have in mind would not have to be that substantial or heavy duty.  We would use it to cross a wash going over to our blue berry garden about thirty five to forty feet.



   

[MountainDon]

http://www.fs.fed.us/eng/bridges/types/cable_sus.htm

[nathan.principe]

Cable Suspension Bridge........... Now were talkin!  I think this is a much better approach.  Just a little back ground on me and this project.  I am a landscape designer by profession.  I have teamed up with a home builder to design the complete landscape for his most recent project, which happens to include this bridge.  The builder tells me, "just worry about designing it look good and Ill make it work".  While I have no reason to not believe him, I still like to work things out for myself, and steel is uncharted territory for me.  Feel free to check out my website, I dont believe I have posted the link here before.  www.principedesignservices.com

[nathan.principe]

and to answer a previous question, "can it be a floating bridge", here in Tx we have too many droughts.  It eventually might be floating on the ground in summer!

[Squirl]

I know they make pressure treated glulams.  I have read they make them in lengths up to 48 ft. long.
http://www.usglulam.com/products/glulambeams/treated.htm

I'm would assume the manufacturers have load/span tables for their products.  When I first designed a few bridges with wood, I forgot the principle that many hands make light work.  You may need more than two beams to run the span and carry the load.

I believe a foot bridge would require a 40 p.s.f. design requirement.  Wood is generally cheaper than steel and with the design calculations probably preengineered, I would probably veer toward the wood over steel.

[Don_P]

  Along those lines look at cable stayed bridges, there have been some really neat ones built recently. Steel shines in tension allowing much more slender sections vs the clunky beam approach if the design can incorporate its' strengths. Wood tends to shine in compression.
that said, glulam arches to make the entire span would also work.

http://en.wikipedia.org/wiki/Cable-stayed_bridge

(there are job shops for cast iron fixings if the budget allows, you are not limited to steel fabrications alone)

Whoops Squirl types quicker... I worked the steel beam at 40psf+30psf self weight and that is mighty light... the dead weight can get big as can the loading... everyone will get on the bridge, probably in cadence, for photo ops, just like the whole family gets on the stairs for Christmas pics.

[Squirl]

Glulam Arched Bridge.  I didn't even think of that.

Personally, I find it very attractive.





All of this is little more engineering than I could give advice for though.

[rick91351]

Here is a PDF of a the Rattlesnake Bridge in Montana near Missoula and shows a couple others in Idaho.   

http://www.conservationengineers.org/conferences/2007presentations/RattlesnakeBridge_Miller.pdf

I know they could be built for a lot less than the government specs. 

[firefox]

If the lake isn't too deep at the 1/4 way points, Maybe concrete peers at those points
to support the steel.
Bruce

[nathan.principe]

Thanks for the input Bruce!  If we can help it we are trying to stay out of the water or prevent having to do any draining

[firefox]

You can place concrete in water without draining.
If the lake freezes solid in the winter, you can then cut out a section and place concrete.
Another way is to weld up some supports that you just lower into the water if
there are no bad currents to deal with. Essentially build a reinforced concrete pad
that covers a fair area, lower the pad to the bottom after it cures, and then have connection steel coming up at the four corners
so that you can bolt a framework of steel to it with lots of cross bracing. Sortof like one of those electrical towers. I don't think you would need a lot of steel to do it, just make sure to put plenty of protective coating on the steel. Make sure there are a few holes in the pad to accomodat driving some steel pipe thru to tie
it into the bottom so it doesn't shift. May not be necessary, but the holes are easy to make when you are forming up the concete pad.
Sounds more complicated than it is.
Anyway, just some thoughts.
Bruce

[firefox]

Thought of another thing if you go that route.
Build in a ladder that comes up to the bridge floor so that people can dive off of the bridge and then get back up easily.
Bruce

[Don_P]

We've moved on but I came across a good queenpost truss pic today, so just for general info.



The wooden members are all individual pieces that run from panel point to panel point, there are 8 wooden parts and a steel wire diagonal brace in the center. A single nail through drilled holes at each connection, free to rotate, creates a pin connection. The 1x2 bottom chord could not suport me, certainly not with 2 free pinned splices in it. The angled posts and straining beam in compression form an arch that the vertical struts hang from, supporting the joints in the bottom chord. Compare this to Squirl's picture.

[nathan.principe]

is that bottom chord 1 or three pieces? that pretty impressive.  Ive laid out my design a took a few photos. Ill post them up in a bit

[John Raabe]

I think the bottom chord is three pieces, correct? A great example of the power of triangles. But I'm not sure where the tension wire is.

[MountainDon]

bottom chord...   

.... with 2 free pinned splices in it.

[nathan.principe]

Ahh, I see know........Im guessing that pic was taken around Christmas time! haha

[nathan.principe]

Here are 2 images from the design I decided on.  One is a site view, the other is an elevation.  I usually show dimensions and material descriptions, for this particular project I havent listed them on the plan.  Although I have it all worked out in my head if anyone has any questoins



I should also add that a gazebo on waters edge was requested in the design, I decided to integrate to 2 structures into one.  You must first access the gazebo which will be elevated 24" to access the bridge which will take another 4 steps up to an elevation of 4'.  Im proposing for the bridge to be pt pine that will "grey" and add a weathered look ( as requested) but for the gazebo to be stained cedar

[firefox]

That looks really nice!
Bruce

[roadtripray]

Very nice design.  With this design, does the railing and the webbing between the rail and joist beam(s) become part of the supporting structure as well?  I could see how that could integrate to be sort of like a floor truss.

Ray