Capacity Checks & Unity Ratios
Member capacity is checked per AISC 360-16 (Specification for Structural Steel Buildings).
Check Types
| Check | Formula | Standard Reference |
|---|---|---|
| Tension | Pu / (φ × Pn) | AISC 360-16 Chapter D |
| Compression | Pu / (φ × Pn) with slenderness | AISC 360-16 Chapter E |
| Combined | (Pu/Pc) + (Mu/Mc) | AISC 360-16 Chapter H |
Compression Capacity
Compression capacity depends on slenderness ratio (KL/r):
Slenderness = K × L / r
Where:
- K = effective length factor (depends on end conditions)
- L = unbraced length (distance between connection points)
- r = radius of gyration of the member profile
Higher slenderness = lower compression capacity (buckling governs).
Unity Ratio Ranges
| Range | Meaning | Action |
|---|---|---|
| 0.0 – 0.5 | Under-utilized | Consider smaller profile to save weight/cost |
| 0.5 – 0.85 | Efficient design | Optimal range |
| 0.85 – 1.0 | Acceptable | Pass, but limited margin |
| > 1.0 | FAIL | Increase profile size or change bracing |
Common Failure Patterns
| Pattern | Cause | Fix |
|---|---|---|
| Bottom legs fail | High axial + overturning | Increase leg profile in bottom sections |
| Diagonals fail at base | High shear transfer | Increase diagonal profile or change to Type 2/3 bracing |
| All members near 1.0 | Tower at capacity limit | Major redesign needed (wider base, more sections, or lighter loading) |
| Only one direction fails | Unbalanced loading | Check antenna placement — may need to redistribute across faces |