Designing a truss (working stress method) Missing Info?

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SUMMARY

The discussion focuses on the design of a truss bridge over a 300m valley using the working stress method. The key equation for buckling is provided: I > (P. L^2) / (π^2 x E), where I is the reduction value, P is the unfactored load, L is the length, and E is the modulus of elasticity. The main issue raised is the lack of information regarding the reduction value (I), which is critical for calculating the critical load (Pcr). Participants suggest estimating I or using values from plastic piping to proceed with the calculations.

PREREQUISITES
  • Understanding of structural engineering principles, specifically truss design.
  • Familiarity with the working stress method in structural analysis.
  • Knowledge of material properties such as modulus of elasticity and yield strength.
  • Proficiency in applying equilibrium equations: Sum Fx= 0, Sum Fy= 0, Sum M(hinge) = 0.
NEXT STEPS
  • Research methods for estimating reduction values (I) in truss design.
  • Study the properties of materials used in truss construction, focusing on plastic piping.
  • Learn about critical load calculations and their implications in structural stability.
  • Explore advanced structural analysis techniques, including finite element analysis (FEA) for truss systems.
USEFUL FOR

Structural engineers, civil engineering students, and professionals involved in bridge design and analysis will benefit from this discussion, particularly those focusing on truss systems and buckling calculations.

nuadre
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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?
 
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Why not calculate I using your above formula?
 

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