Foot bridge advice

[inwo]

First off I'm not sure if this is the right forum, and second I'm not expecting anyone to sign off on safety or design a bridge for me.

My brother is building a foot bridge for his golf course.
It looked very unstable to me and a search shows no design like this.
I believe he may allow cart traffic on bridge, which means this could be an extreme safety issue.

2' in 60' seems a problem.

I just want to know if my calculation is in the ballpark.

60' span, using 2 light duty mobile-home, I-beams, with the webs heated and arched 2'.

I think you can see where this is going!

Support is from two 3/8" wire ropes crossed in middle, end to end.
No buttress footings are allowed. Cables ties must support the load from the arch.

Google shows 3/8" ` 12,000 lb break and 2500 safe load. (dead load?)
This means the two cables will be stressed to safe 5,000 lb safe load with only 675 lb weight of structure and load. This means present design is barely self supporting.

Breaking @ 3200 lb load in middle.
This may be going up in the next few days. If no answer, I'm going to suggest measurement of cable tension under fixed load to check my math.

 

[bridgebuster]

Another reason why I'm glad I gave up golf.

 

[MikeHalloran]

Check it again for the weight of a police car.



Mike Halloran
Pembroke Pines, FL, USA

 

[bridgebuster]

I didn't mean to be flippant but a 60' span needs to be taken seriously. If your brother's golf course is open to the public he needs to build it in accordance with an accepted code. Based upon your description, or rather lack thereof, the best thing you can do is consult a bridge engineer.

 

[bridgebuster]

I forgot to mention that 1/30 isn't going to give you arch action; bending will control. Also, did you think about compression flange buckling. Hire a bridge engineer.

 

[inwo]

Thanks for response, I'm hoping for a more direct answer to question. Something related to the actual structure. Then I can show him a positive failure senario.

I'm electrical minded and need confirmation that a triangle with a 60' base 2' high will put enormous tension on the bottom tie cables.

He hasn't asked for my opinion. I need one good fact to get him to reconsider this.
Such as, that cable "will" break. Or that "will" buckle. Or even that my weight limit of zero is close.

He has a partner in another business who is a structural engineer. I've suggested consulting him.

Also my daughter is a civil engineer, working in another field, she may still have a program on her computer to back me up.

 

[bridgebuster]

The cable won't do anything because you won't achieve arch action with 1/30. What's a light beam, W14 x 61? It would span 60' but there will be a significant deflection under self weight. That's assuming the compression flange is braced. You're 2' camber will hide the dead load deflection but if you're building to code, the bridge will have a weight limit of one golfer at a time. He better not be too heavy otherwise you'll exceed the AASHTO live load deflection.

 

[inwo]

Thanks, thats something I can scare him with. W14 x 61 for self support.
Light beam, I mean light. Like 16" by 4" wide. Only 3/16 material. Maybe less. I'd measure, but it seems so far off that it won't matter. It is significantly braced.

When you say "no" arch action. I'm assuming you mean the compression load on beam or tension load on cable reaches near infinity? (or huge anyway) That's what I came up with!

Sounds like a death trap.
At this point I just need to have him admit that it won't hold a cart. When he sees how bouncy it is with one person he may relent.

Fortunately it is only crossing a shallow stream may not be life threatening to a walker.
However I believe a collapse with a cart on it would be very bad.

 

[MikeHalloran]

No, he's talking about _trailer_ beams, which are just astonishingly thin, web and flange, for a rolled section. They are just barely strong enough to support the floor of a not very long house trailer, even given considerable bracing.

Mike Halloran
Pembroke Pines, FL, USA

 

[inwo]

Exactly, all I'm looking for is some real facts I can present to him.

Of course, I can tell him "don't do it", or "get engineering", but that's my opinion!

If I can say 675 lb total load is maximum. Or 3000 lb load will buckle beam or snap cables.
Then I have a chance to end this.

tnx again.

 

[bridgebuster]

Let him try cambering it 2'; that will buckle the beam.

Using your dimensions the beam will deflect over 1', under self weight. Assuming the bridge is 5' wide with timber planking with the compression flange properly braced against buckling, it will deflect about 20".

If you use a code mandated uniform live load, the live load deflection will be several feet.

When I said no arch action, I meant the ratio was too small to expect the beam to behave as an arch. A 60' arch would typically have a rise of ~9 to 18 ft. I did a rough calculation and got a very high thrust.

 

[inwo]

Thank you,
I will give him that info.
I am afraid that the cables will stiffen the structure giving a false sense of strength, leading to a catastrophic failure when it buckles or the cables snap.

Beams measure 12" X 4" less than 1/4". 3/16' I'd guess.
4X4 decking will weigh from 600 to 1000 lbs.
My belief is that it can't safely support it's own weight.

Will try to talk him into a test in the parking lot, where a two foot drop should not kill anyone.

I know this might not be the best forum for this. I didn't know where else to go.

tnx for the serious answers!

 

[bridgebuster]

The deflection is worse than anticipated; the dimensions correspond to a W12x14. Good Luck.