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.

 

[CNC Dan]

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

you have to set the default units first.
prehaps that was 0.2 inches.

 

[RedNeckRacin]

Marcus

from my steel manual

for a L2x2x1/8"

wt-1.65 lb/ft
area-.484in^2
X-X I=.484in^4 S=.129in^3 r=.620in ybar=.534in Z .230in^3 Yp=.123 in

Flexural-Torsional properities
J=.00293in^4
Cw=.000789 in^6
Y-Y I=.484in^4 S=.129in^3 r=.620in ybar=.534in Z .230in^3 Yp=.123 in


Fy=36ksi


Dont forget there are alot of other failure modes that need to be checked as well. I.E. Shear strength, Lateral torsional buckling and especially and probably more importantly you connections methods.

We enjoy these questions and it seems like you have a good handle on things, just make sure you use a good factor of safety.

 

[schweizer]

Thank you all. I double checked the calcs manually again. 0.2 inches is the correct deflection #, or 0.1 inches for 2 pieces of parallel angle iron. CNC Dan, I was reading the wrong deflection units on EpicFEM (ft vs. inches). You are correct. Thanks.

I don't think buckling or torsion/twisting will be an issue if wood cross members are jammed into the inside of the "L".

Marcus

 

[RedNeckRacin]

Thank you all. I double checked the calcs manually again. 0.2 inches is the correct deflection #, or 0.1 inches for 2 pieces of parallel angle iron. CNC Dan, I was reading the wrong deflection units on EpicFEM (ft vs. inches). You are correct. Thanks.

I don't think buckling or torsion/twisting will be an issue if wood cross members are jammed into the inside of the "L".

Marcus

Thats very true, but your going to want to check if the holes your drilling in the angles to connect the boards are doing any significant weaking of their capacity though. I think there is an allowance for a perfectly round hole located correctly in the leg of the iron but i dont recall how to figure that out off the top of my head.

 

[sweettractors]

A friend got married last Sat. I noticed the bridge over the creek was made from a retired flatbed semi-trailer. Can be seen in the background. Probably 53 ft long x 8 ft wide. Ken Sweet

 

[gemstarfarm]

CliffordK made a point. Your going to need some pretty significant approaches.

 

[crazyal]

If you're interested there's a cable foot bridge not far from me, maybe 5 miles from her, that I pass all the time. It's part of the state's hiking trail system, built by the state. I looked at it but not too closely. They used composite decking for the treads. I would guess it spans 200' with very little sag. It's a little over built because it has cable sides on it so you can't jump into the river from the bridge, it's a good 40 feet over the river. I would guess it's 4' wide. I can get some pictures and take some measurements if you would like.

 

[schweizer]

CrazyAl, Thanks! Sure, that would be most helpful, if you drive right past anyway. Very appreciated!

sweettractors, That's a unique wedding and unique bridge. Funny thing you mention that, b/c I actually built a covered bridged on a semi trailer on a my parents land a few years back. More that I want to spend (time & money) for this project. Thanks.

 

[crazyal]

So I took some pictures and got some measurements. I'll break the posts out so if you have any questions about a section it'll be easier to answer.

The bridge spans about 140' between the two towers on the ends. The towers are 6" pipe 16' tall.

 

[crazyal]

The two main cables are 1 1/4" in diameter. All the other cables are 5/16". Before I looked at the bridge closely I thought it was professionally done but it looks more like it wasn't. I think the two main cables came from a ski lift from a local ski area. I think they replace them on a regular basis so they could have been donated or sold at scrap steel prices. Not sure if you have any near you but it could be an option to get some very heavy cable for cheap.