Final answer:
To find the average normal stress in the bars of a truss, consider the axial load, and use the formula σ = F / A with the cross-sectional area calculated from the given diameter.
Step-by-step explanation:
To determine the average normal stress in each of the 20-mm-diameter bars of the truss, you would need to consider the axial load (A) acting on the bars. Normal stress is calculated as the axial load divided by the cross-sectional area of the bar. The formula to find the normal stress (σ) is σ = F / A, where F is the axial force and A is the cross-sectional area of the bar. Since the bar has a diameter of 20 mm, you can calculate its area using the formula for the area of a circle, A = π(d/2)^2, where d is the diameter. For shear, bending, and torsional loads, different stress calculations are applied.
Shear stress is relevant if there are shear loads (B) acting perpendicular to the axis of the bar. Bending moment (C) would impose bending stress, and torsional load (D) would imply torsional stress; both of these are beyond the simple axial stress scenario. However, these stress types are not related to calculating normal stress under axial loads.
For a given axial load, once you know the cross-sectional area, you can calculate the normal stress. To illustrate, if a bar is under a tensile axial load of 1000 N, the area calculated as π(0.01 m)^2 (converting 20 mm to meters) would yield the normal stress value when this area is used in the stress formula.