18.104.22.168 Communications Antennae
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This category covers communications antennae, often referred to as satellite dishes, which may be mounted in a variety of ways. Circular antennae used for residential or small commercial applications are typically supported by a single mast that may be mounted on a wall, roof, chimney, eaves, balcony, or freestanding at the ground. Non-penetrating roof mounts that typically rely on ballast are also available.
- Typical Causes of Damage
- Seismic Mitigation Considerations
- While TV antennae have been mounted on roofs for many decades, the appearance of circular antennae on U.S. rooftops is relatively new, and to date, earthquake damage has not been documented. This is in part due to the fact that since antennae tend to be very light, the most severe design loading for circular antennae is typically wind.
- Nevertheless, if antennae have not been mounted to meet seismic loading, they could become dislodged and either the dish or the mast or both could fall.
- Damage to the antennae could disable critical communications systems or television access that may be needed following an earthquake.
Figure 126.96.36.199-1 Most antenna are designed for wind and able to resist seismic loading. In spite of the collapse of the first story of this residential building complex, the roof-mounted antennae appear intact in the 2010 magnitude-8.8 Chile Earthquake (Photo courtesy of Eduardo Fierro, BFP Engineers).
Figure 188.8.131.52-2 Antennae retrieved from roof of adjacent collapsed wing of the Hôpital Saint-François de Sales in the 2010 magnitude-7 Haiti Earthquake (not known if antenna suffered earthquake damage; photo courtesy of Ayhan Irfanoglu, Purdue University). Hospital communications depend on the functionality of antennae such as this.
Figure 184.108.40.206-4 The antenna with guy wires remained upright atop a collapsed building in the 2010 Haiti Earthquake (Photo courtesy of Eduardo Fierro, BFP Engineers). Even though some of the guy wires went slack, the antenna did not fall into the street.
- The Federal Communications Commission (FCC) issues regulations for Over-the-Air Reception Devices to preempt restrictions on the size, mast height, or location of direct-to-home satellite dishes. For instance, Title 47 (Section 1.200) of Code of Federal Regulations which codifies the FCC regulations covers dishes less than 1 meter in diameter with a mast height less than 12 feet above the roofline. In addition, tenant or homeowner association agreements may have restrictions on the size or placement of antennae; check for local code or association requirements.
- The antenna mast may be mounted in a variety of ways, for example to wood or metal stud walls, concrete or solid masonry walls (cells filled with concrete), hollow masonry block walls, freestanding poles, or to roof rafters or a concrete roof slab. Schematic details for installing the mast mounting bracket to a stud or concrete walls are shown in Figure 220.127.116.11-5.
- Some mounting kits available on the internet provide hardware for strapping the antenna to a residential chimney. As unreinforced masonry chimneys are highly prone to earthquake damage as described in Section 18.104.22.168, antenna should not be mounted to unreinforced masonry chimneys. If the chimney is adequately reinforced, chimney mount details may be used for lightweight antennae.
- Hardware and kits for non-penetrating ballasted mounts are also available for purchase. These kits often use standard sized concrete blocks for ballast. Use of multiple concrete blocks for ballast may be heavy enough to trigger the requirement for the equipment to have engineered anchorage. While these ballasted systems can reasonably be used in areas of low seismicity, they could potentially slide and damage roofing or wiring in areas of high seismicity.
- Large or tall antennae need to be properly engineered for both wind and seismic loading. Tower antennae may be anchored with guy wires, or mounted to a specially designed frame. Positive attachments from the antenna to the supporting structure should be provided and one should check with the manufacturer to see if the antenna itself has been designed or tested for seismic loading since seismic forces at the roof elevation are typically much higher than at ground level.
- Communications equipment used for essential facilities may need to be shake table tested and certified. Shake tables operated by the Pacific Earthquake Engineering Research Center (PEER) at UC Berkeley, MCEER at SUNY Buffalo, and others both perform testing of telecommunications network equipment in accordance with NEBS Requirements: Physical Protection (GR-63), protocol to certify that the internal parts and electronic components can withstand seismic shaking.
- As with any items mounted with exterior exposure, components and connectors should be corrosion resistant and roof or wall penetrations should receive flashing and sealant as appropriate.
Figure 22.214.171.124-5 Antenna mast mounted to concrete wall surface at top floor of building (Photo courtesy of (Photo courtesy of Maryann Phipps, Estructure). Note that two wall brackets are used to resist moments produced by wind or seismic forces. This antenna is larger than most standard residential versions.