Suspension bridge design

[schweizer]

Hello friends,

I'm in the design phase of building a suspension bridge (footbridge) across a year-round creek on my land. An earlier bridge got washed away in high water 2 years ago. This one will be higher off the water and stronger. I had designed a wood bridge but now I'm thinking a partial metal design would be better.

While taking a hike in Yosemite this week, I came across this bridge and I liked some of its design: They used 2-3" angle-iron longitudinally under the horizontal planks, and horizontal angle-iron under that for attachment to the support wires. Pretty simple design and easy to bolt together. Lighter than wood. I would make the end posts taller (12-16') to alleviate some of the sag in the middle. I'm including some pics and my calcs. I'd appreciate any input (especially from any engineers).

First bridge (pardon the wife and in-laws on the bridge):



Washed out in high water:



Interesting bridge design in Yosemite:





(yes, that's me on the bridge)



I'm thinking 2 x 2 x 1/8" angle iron would be plenty strong. Here are the calcs:

I = Moment of inertia = 0.19 inch^4 in directions Ix and Iy
E = modulus of elasticity for steel = 30,000,000 psi

Deflection over 4 ft (force at center) of 500 lbs:
= 500 * (48)^3 / (192 * 30,000,000 * 0.19)
= 500 * 110,592 / (192 * 30,000,000 * 0.19)

= 0.0505" or approx 1/16"

Over 2 pieces of angle:

= 0.025" or approx 1/32"

That's seems a very small amount of deflection and would seem to be acceptable?

I'm not quite sure that 0.19 is the correct Moment of inertia. Source:
http://www.engineeringanddesign.com/1/046 Angle Iron Data.xls

The moment of inertia from a different website is 0.9167. Source:
Calculator for Engineers - Moment of Inertia of angle section

(That would give deflection of 0.0104" and 0.0052" respectively, which is even smaller.)

Thanks for any input.

Marcus

 

[Danno1]

.



OK, I wrote a bunch of stuff and then deleted it . But anyway, I think you're using the wrong deflection equation. Can you post a drawing of what you're trying to calculate?



.

 

[CliffordK]

Southern Oregon?

Unless you have a narrow canyon, you will need to do significant work on the approach, which could have the effect of creating a restriction point in your creek, and perhaps also a point to collect debris, making the problems worse.

Do you know if that photo is representative of the high water point? Have you been to Galice, and looked at their pictures of the 1964 Rogue River Flood and the Graves Creek Bridge?

I know that along the Illinois River (Southern Oregon), they have had troubles in the past with the Indigo Creek and Silver Creek Bridges. Both are now quite high up. One of the Silver Creek Bridges was low down, but made out of solid steel, with the idea that perhaps it might be flooded over, but that one didn't last.

Would a well built culvert bridge be more resistant to flooding than other types (with the expectation that it will be overtopped periodically)? Not as spectacular as what you have in mind, but a lot easier to make & maintain, and could be designed for equipment if desired.

Could you design a drawbridge to resist the flooding?

As far as your calculations, I'd design it to hold several people, and perhaps packs or loads. Perhaps 1000 to 2000 lbs (I assume horses won't be crossing it). You will also need to calculate the weight of the steel and the decking. Everything gets distributed along the length of the bridge, but there may be a way to easily approximate the contributions to the weight in the middle of the bridge. Wood can change weight between wet and dry.

 

[CNC Dan]

Marcus, look for and download a program (free) called "beam boy".
It will give you all the info you need about deflection and loading etc.

If you have an android phone there is a simmilar app called EPICFEM, also free.

 

[CNC Dan]

This is the result of a calculation.



It shows .200" deflection for a point load of 500 lbs in the center of a 48" span with simple supports at each end.

 

[BTDT]

Here's link to one I built, spans 121 feet, 15' above the water

http://www.tractorbynet.com/forums/photos/90164-bridge-pics.html

 

[schweizer]

Thank you, BTDT, for the link to your thread. That's basically what I had in mind. You went longer spans than what I had in mind for each segment, both longitudinally and transversely. And I don't plan on driving my tractor across it.

CNC Dan, thanks for the link to EpicFEM. Downloading it now. I'd read about Beamboy, but I don't want to install Windows on my Mac just to try Beamboy.

CliffordK, this is just a small creek that feeds into Williams Creek, which eventually feeds into the Applegate River (tributary to the Rogue). Yes, it is a narrow canyon with fairly steep sides. It's normally a few inches deep and about 5-10 feet wide. The high water we've observed over the past 6 years has been 2-3 ft deep, and about 20 ft wide. Where I want to build the bridge is right next to the spot where we ford the creek with the truck and tractor. It would be a 40' span, 5' above the water.

The 1st bridge I built here was going to be a drawbridge, but then I never got around to installing the drawbridge mechanism. Of course, what happens in winter when you're not at home and the water is rising? Or what about when you're on the other side of the river and the bridge is up? Better to just build sturdy and high enough off the water.

When I calculated a 500 lb load above, that was just for a single 4 ft section lengthwise. I figured that was 2 big guys standing right next to each other in the center of a 4 ft span, and that could be repeated for the entire length of the bridge. The wood x-pieces wouldn't handle a bigger load, anyway. Or would they? I can't get a welder down there like BTDT did with the expanded metal. Everything I do will be just bolted together.

Here are some pics of what I was planning with wood. The concept will be similar except with 2" x 2" x 1/8" angle iron for horizontal and longitudinal support.

 

[schweizer]

Danno1, thanks also for your reply. Here's a quick drawing:



CliffordK, a culvert bridge would require too many permits. A BLM paved road is right next to the creek, through the middle of my land. BLM, Forest Service, etc. have seen the other bridges I built, but as soon as I disturb the creek flow/bed, I'd have environmentalists crawling all over me.

Marcus

 

[schweizer]

I think I entered something wrong in EpicFEM -- result is 0.2 ft of deflection. ~2 1/2 inches? That's huge!

 

[schweizer]

Wait, for S275 steel, 2 x 2 x 3/16 L shape, x 2 pieces: 0.05 ft deflection or 1/3 inch. More acceptable.

I got the same results as CNC Dan with 1/8" thick L.