#### schweizer

##### Gold Member

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