As I've started to look at real estate, something in the back of my mind as a radio nerd is where I'd put a tower.
In the course of doing some reading, I've learned a fair bit that I thought I'd try to summarize here.
Beyond obvious stuff like height, the big issue with towers is wind loading. This is typically measured in square feet, but not much of what is attached to towers is actually a square cube. Antennas or microwave dishes therefore have an "effective projected area" (EPA) in square feet, identified by the antenna manufacturer.
Towers have a certain maximum EPA windload rating, but in reality, it's usually stated multiple ways. The applicable standards are in TIA-222, which is... a lot. But here are some of the summaries.
For a long time, Revision F ("TIA-222-F" or "Rev F") was the applicable standard, but Revision G ("Rev G") came out in 2005. It substantially changed how wind loading was calculated. Rev F used a "fastest mile" measurement for wind loading: tower manufacturer Rohn describes this as, "the average wind speed over the time required for one mile of wind to pass the site." Rev G switched to using a 3-second wind gust. Even though Rev G is 16 years old at this point, windloading specs typically still show both Rev F and Rev G ratings. Note that the linked Rohn page includes a link to a PDF explaining all of this in more depth.
These specifications are then stated for different wind speeds, typically 90 to 130 mph, with allowable EPA dropping off as 3-second gust speed increases.
The other big consider is structure classification.
Class I structures specifically list ham radio towers as an example, and have a 25-year return period (probability of occurrence, more or less). They're towers where, although it would stink if they collapsed, it's not going to affect any mission-critical systems.
Class II structures are based on a 50-year return period and are specified as including things like broadcast TV/FM towers and cell towers. There's probably some redundancy in the systems: your cell phone might fail over to a further cell tower, or you might have to get news in an emergency from a more distant TV station.
Class III structures have the lowest risk tolerance, based on a 100-year return period. They include things like emergency radio systems or navigation beacons, and they may include systems where there's no redundancy available.
Most tower specifications I've seen only show Class I and Class II loadings. There can be a tremendous drop in allowable wind loading from Class I to Class II.
Another consideration is Exposure Category, based on surrounding terrain. They range from Exposure B (least exposed) to Exposure D (most exposed); there is apparently no Exposure A. Exposure B is an urban, suburban, or wooded area; winds at ground level are typically low, broken by homes and trees. Exposure C is more flat, open terrain; buildings or other obstructions are less than 30', and the tower therefore sees greater wind speeds. Exposure D is reserved for shorelines and the surrounding area, where there is no protection from wind.
The loading for Rohn RSL self-supporting towers (PDF) is a good illustration of some of this.
With Class I loading, a 70' RSL70L40 can support 80 square feet EPA designed for a 3-second gust of 90mph, or 56 square feet at 100mph. Designing for a 140mph 3-second gust drops you all the way down to 5 square feet of allowable surface area.
Things change drastically if you design for Class II loading, though. On the same 70' tower, you're only allowed 35 square feet at 90mph, and the maximum wind speed with any loading is at 110mph, where you're permitted 8 square feet.
Hapco, which appears to be a manufacturer of light poles and such, has a 3-second gust map on their site. Much of the country is in the 90mph category, but those of us within perhaps 100 miles of the Atlantic (or Gulf) coastline are exposed to higher wind speeds.
Cover image photo credit: Thimo Pedersen on Unsplash, showing a site in Zürich.