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 in[sup]3[/sup]
Looking at the values in the S[sub]x[/sub] column, the first beam I considered is the W10 x 100 having a S[sub]x[/sub] 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.
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 in[sup]3[/sup]
Looking at the values in the S[sub]x[/sub] column, the first beam I considered is the W10 x 100 having a S[sub]x[/sub] 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.