Explained: TIA-222-H Shielding Factors (Ka) for appurtenances attached to headframes/platforms

Welcome to the first of a series of posts intended to help engineers understand specific aspects related to tower analysis and design.

Today, we’re looking at the Shielding Factor (Ka) from TIA-222-H as it relates to groups of appurtenances mounted to towers and monopoles via to headframes/platforms. The values below are for appurtenances only, and not the headframes/platforms themselves, which have separate requirements.

1. Multiple Mounting Frames: For lattice towers, these are individual mounting frames fixed directly to the outside of the tower. For monopoles, they would likely be a Mercedes-style headframe. Appurtenances attached to these headframes can be assumed to have Ka = 0.75 or Ka = 0.8, depending on the mounting arrangement as per Figure 2-8 (extract below). However, a minimum of 3 mounting frames at the same elevation are required to assume Ka < 1.0.

2. Symmetrical Truss Platforms: Triangular and square headframes with a trussed construction when viewed in elevation. These headframes are designed to accommodate people for maintenance and antenna replacement activities. Appurtenances attached to these headframes can be assumed to have Ka = 0.75.

3. Low-Profile Platforms: Any platform constructed from a single members when viewed in elevation. These headframes are usually not designed to accommodate people for maintenance and antenna replacement activities as they have neither a grill-mesh for standing nor safety handrails to prevent falls. Appurtenances attached to these headframes can be assumed to have Ka = 0.8.

4. Circular Ring Platforms: Circular headframes with a trussed construction when viewed in elevation. These headframes are designed to accommodate people for maintenance and antenna replacement activities. Appurtenances attached to these headframes can be assumed to have Ka = 0.8.

5. All Other Cases: Appurtenances attached to all other types of headframes, such a tip-mounted turrets, must be assumed to have Ka = 1.0.

You’ll be pleased to know that our CHECKPOLE software handles all of these cases automatically when you select your headframes and linked appurtenances (which you can of course override), and can lead to significant load reductions for monopoles supporting large numbers of antennas.

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