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# Looking for advice/help on designing a bridge

## Looking for advice/help on designing a bridge

(OP)
I'm needing to design a bridge type structure that is to be used inside of a tunnel. The purpose of the bridge is to allow work to be done on a small section of the tunnel floor while allowing equipment to pass up and over. The bridge will have ramps at either end of the 20' length for small equipment to travel up. The bridge is raised off the floor of the tunnel 4' by 8 angled wheels that will ride on the tunnel wall. The wheels are positioned and angled in such a way that the width of the wheel is tangent to the wall of the tunnel. I have not decided on a design for the wheel brackets yet. I will also probably design some "feet" that will sit on the tunnel wall when deflection occurs. I designed the bridge around the 13' span because it is only supported by the two wheels at the end of the beam. I sized the beams according to the criteria below.

1. Bridge loaded with two skid steers for a total of 24,000 lbs
2. Worst case scenario, assume skid steers are located inline with each other at center of 13' span
3. I'm using a F.S of 2

Calcs:

Loading at center of 13' span with F.S. of 2 = 48,000 lbs
steel max stress = 36 ksi
M = 48,000 * 6.5 = 312,000 lb*ft = 3,744 kips*in
S = M/maxstress = 3,744 / 36 = 104 in3

https://www.engineeringtoolbox.com/american-wide-f...

Looking at the values in the Sx column, the first beam I considered is the W10 x 100 having a Sx of 112. Now obviously two skid steers wouldn't be sitting directly on top of each other at the mid point of the 13' beam span. Is this process correct in providing a good sizing estimate? I also need to decide on the internal structural member size/type. Should I go with beams or square tube to strengthen up the two internal regions? I plan on laying a sheet of steel, maybe 3/8" thick, on top of the frame. Trying to keep weight and cost down. My background is not in structures but I'm looking to learn. Any help would be appreciated. I've attached a PDF showing a rough design.

### RE: Looking for advice/help on designing a bridge

Hmm is the tunnel in rock or soil?? Is it lined. if so how??? Can the tunnel sill withstand the loads you intend applying to it?? A few more things for you to evaluate.

### RE: Looking for advice/help on designing a bridge

You need a structural engineer.

### RE: Looking for advice/help on designing a bridge

Rabbit12 pretty much summed it up. I'll just add that what you're describing sounds like an elevated work platform, which probably kicks in all kinds of OSHA regulations - if you're looking to learn that would be a good place to start.

### RE: Looking for advice/help on designing a bridge

What you need is a motor driven mobile work platform/scaffolding. Steel pipe/HSS is better than heavy structure shapes. You need scaffold designer. We used one for a 20' diameter tunnel.

### RE: Looking for advice/help on designing a bridge

(OP)
I meant to include tunnel specifics. The tunnel is 12' diameter and is in rock, lined with concrete. I will have to double check with the load capabilities of the concrete walls. Unfortunately I was handed this project July 29th and they would like something by August 8th as far as conceptualizing. I'm the only CAD guy at my company and I guess someone on this tunnel project talked with someone else at my company who assumed I did structural stuff.

### RE: Looking for advice/help on designing a bridge

Tell them to talk to a scaffold contractor, they can either handle the design, or refer to the party that is capable of, or may turn out best of luck, that there is one available from other tunnel projects. 12' diameter shouldn't be too difficult for them to handle. Note, this is not just structural, it is rather electrical-mechanical setup with moving parts. It will be a lot cheaper/safer to be handled by whom knows how.

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