The United States averages about 1,200 tornadoes per year, according to the National Oceanic and Atmospheric Administration. EF0 and EF1 tornadoes account for roughly 80 percent of all tornadoes, according to ustornadoes.com. EF2 makes up another 14 percent, EF3 accounts for about 4 percent, EF4 is almost one percent, and just 0.1 percent of tornadoes reach EF5.
We at PTTG watched with heavy hearts as the South and Midwestern regions were battered by a historic tornado outbreak on December 10-11. Many of our PTTG employees, extended families, friends, and neighbors were affected by the deadly twisters in Western Kentucky.
The twister flattened a 500,000-gallon water tower in Mayfield, which appeared to be in good condition before it was leveled. The level of destruction points to a powerful weather event. According to a preliminary report from the National Weather Service, the Mayfield tornado reached winds of around 190 mph. That makes it a category EF4 tornado – and only 11 mph shy of an EF5 tornado – the strongest on the scale.
Even the sturdiest of structures would be hard-pressed to be left standing after winds as fast as an Aston Martin. Structures and infrastructure suffer wear and tear over the years from everyday exposure to the elements.
Natural disasters – tornadoes, hurricanes, ice storms, earthquakes, and straight-line winds – to name just a few, usually do more than contribute a little bit of wear and tear. Natural disasters wreak damage and destruction on water towers. Here are a few ways you can prepare your storage tank to withstand Mother Nature.
- Remove all debris, brush, and overhanging tree limbs from around or on top of the water tank. Loose tree limbs are more likely to fall and damage a structure if high winds are present. Fallen limbs could puncture a hole in the tank, causing it to spring a leak.
- Build your tank to meet seismic codes or retrofit your old one to ensure it does. Most people associate California with earthquakes, but any state in the United States may potentially have an earthquake. Some states, like California, are more prone to them. Any modern storage tank should be designed according to earthquake design load provisions laid out in AWWA D100. Older tanks should either be brought up to code or replaced with tanks already built to meet seismic codes.
- Install lightning protection on your tank. Lightning strikes are far more common than hurricanes or tornadoes. There are roughly 20 million lightning strikes in the United States annually. As most people know, lightning tends to strike the tallest object in the area Water tanks and towers are typically the tallest structures in many towns, or at least among the tallest, so the chance of one being hit is better than it hitting a house.
Water tanks and towers should be equipped with lightning rods that can absorb a lightning strike and help prevent it from damaging the electrical systems connected to the water tank. According to NFPA 780-2020; 5.4 Metal Towers and Tanks, “Metal towers and tanks constructed so as to receive a stroke of lightning without damage shall require only bonding to grounding electrodes.” Lightning rods can be made from stainless steel, solid copper, or copper-clad steel.
- Tighten windage rods to help stabilize elevated towers. They are designed specifically to handle seismic loads combined with dead and live loads. The rods should also be capable of withstanding 100 mph hour winds blowing in any direction. Loose or broken windage rods can destabilize an elevated tower and, in worst-case scenarios, cause it to collapse. Per AWWA D100-11; 3.1.4 Wind load, windage rods should be tightened to specifications.
- Tighten any anchor bolts so they can withstand wind speeds of 90 mph blowing in any direction, per AWWA D100; 3.8: Anchorage. For reference, an EF1 tornado would have winds between 86 to 110 mph. The majority of twisters have winds less than 90 mph.