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?