Specific modulus

Many common structures are stiffness-driven over much of their use, such as airplane wings, bridges, masts, and bicycle frames.

The problem with this idea is that there would be a significant sacrifice of stiffness, allowing, e.g., wings to flex unacceptably.

The connection details of such structures may be more sensitive to strength (rather than stiffness) issues due to effects of stress risers.

Both stiffness in tension and total mass for a given length are directly proportional to cross-sectional area.

By examining the formulas for area moment of inertia, we can see that the stiffness of this beam will vary approximately as the fourth power of the radius.

By examining the formulas for area moment of inertia, we can see that the stiffness of this beam will vary approximately as the third power of the radius or height.

These buckling modes depend on material properties other than stiffness and density, so the stiffness-over-density-cubed metric is at best a starting point for analysis.

The performance of thin-walled beams can also be greatly modified by relatively minor variations in geometry such as flanges and stiffeners.