Suspension bridge design

[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.