Designing a truss (working stress method) Missing Info?

[nuadre]

Homework Statement

"Design a bridge over a valley 300m wide. Road surface is supported by a truss underneath."

My problem: With calculations
Now calculation for buckling says i need to design the truss for compression such that

I > (P. L^2) / (pi^2 x E )
or Pcr > P
where
I = reduction value
P = unfactored load
L = length
E= modulus
pi = Pi.
Pcr= Critical Load

Now the information I'm given doesn't give me a value for I, just the rest. Not sure what I'm meant to do, just estimate a value and insert it in and do calculations from this or another method or if I'm meant to know the value fro plastic piping (?)
- diameter (can work out effective area)
- modulus
- yield strength
- length

Homework Equations

Sum Fx= 0
Sum Fy= 0
Sum M(hinge) = 0

The Attempt at a Solution

Not sure where to start, as it doesn't seem to be enough information?

 

[nvn]

Why not calculate I using your above formula?

 

[Mene]

I would first clarify with the person providing this problem whether they have provided all the necessary information. If they have, then I would suggest looking into the specific design requirements and guidelines for truss bridges, as well as any relevant engineering codes and standards. From there, I would suggest using the given information to make reasonable assumptions and estimates for the missing values and proceed with the calculations. It would also be helpful to consult with other engineers or experts in the field for guidance and advice.