EPA Contribution of Appurtenances

All appurtenances contribute to the total Effective Projected Area:

(EPA)total = (EPA)structure + (EPA)antennas + (EPA)mounts + (EPA)cables + (EPA)ladders

How It Works

Each appurtenance EPA is:

1. Calculated based on its physical dimensions and shape
2. Projected per wind direction (an antenna facing the wind has different EPA than one sideways)
3. Added to the section where it is installed
4. Multiplied by the appurtenance force coefficient Ca per TIA-222-H Section 2.6.11

Appurtenance Force Coefficient (Ca)

Force coefficients are applied per TIA-222-H Table 2-10 based on appurtenance shape:

• Flat surfaces (panel antennas, flat plates) — higher Ca due to full flow separation
• Cylindrical shapes (pipes, omni antennas) — lower Ca, varies with Reynolds number
• Open frames (platforms, cable trays) — intermediate Ca

Refer to the published standard for specific coefficient values.

Impact on Analysis

Adding appurtenances increases:

• Wind forces on the sections where they are mounted
• Dead load throughout the tower (self-weight)
• Overturning moment at the base (especially for high-mounted antennas)
• Member forces in legs and diagonals near the mounting location

TowerPlot recalculates all of this automatically when you add or modify appurtenances.