Construction of pools near the coast has, for a long time,
been more complicated than the construction of pools at inland locations. For
instance, it has been a longstanding requirement in Florida to have pools
supported by piles when seaward of the coastal construction line.
However, the latest Code updates bring in two new concerns for the Florida pool
engineer. You may be required to increase the uplift loads on the piles,
and place the pool equipment in an elevated location.
Engineers need to know two things before considering these
two issues. You need to know the design flood elevation from FEMA Flood
Maps and the intended elevation of the floor of the pool. You must keep
in mind that flood maps are regularly updated on a county-by-county basis and ensure
you are reviewing the latest online accordingly. The flood maps will give
a zone for each location, identified with a letter and a number. The
letter is the Flood Zone, usually A or V. The number is the Base Flood
Elevation, in feet, relative to NAVD. Think of the NAVD as the zero value
or the typical sea level around the world. The flood elevation is always
above the NAVD, perhaps by 8 feet, perhaps by much more. You need to know
the pool floor elevation relative to the NAVD as well so that the flood
elevation can be compared to the floor elevation of the pool. At the
coast, the flood elevation will typically be higher than the floor elevation of
the pool.
With these two values in hand, you can calculate the flood
load. You need to imagine that the pool is empty when the flood comes and that the floodwaters flow through the groundwater such that the pool will
have an upward, floating force that the piles will resist. Also, compare
the flood elevation to the top of the pool wall. If the flood overtops
the pool wall, the uplift forces are eliminated. Therefore, the maximum
uplift force, the worst case, comes when the pool is empty but the floodwaters
outside the pool are just below the top of the pool wall.
Once the height of the flood water column is known, the loads
on the piles will be based on Chapter 1605 of the Florida Building Code. In a
coastal conditions, the flood load becomes an important consideration.
Chapter 1605 directs engineers to a national standard, ASCE 7, to find the flood
load. The latest edition of ASCE 7 says that flood loads in V zones and
Coastal A zones have higher load factors, compared to flood loads anywhere
else. But what is a “Coastal A Zone”? The flood map will tell you
if the pool is in a V zone or an A zone, but whether the A zone is coastal
might not be readily apparent. Therefore, you need to look for a line on
the map called the Limit of Moderate Wave Action, sometimes labeled LIMWA.
A-zones that are seaward of this line are Coastal. Then multiply the
flood load by the flood load factor and compare the result to the uplift
capacity of the piles.
The second issue to consider is the height at which the
equipment should be installed. The national standard, ASCE 24, was
updated to impose requirements on pool equipment. The 2020 Florida
Building Code adopts ASCE 24, but changes the pool equipment requirements to
say that the equipment must be “elevated to the extent practical” (1612.4.2,
FBC). Building officials are not in full agreement about what the “extent
practical” might be. Some officials are using a rule of thumb that the
equipment must be placed 30 inches above the site grade, regardless of if that
is above or below flood elevation. Why 30 inches? Because more than 30
inches would require an engineered retaining wall and perhaps a guardrail, so
that might not be “practical.” Other building officials simply comment
that everything needs to be above the flood elevation and leave it to the
contractor to respond that this is impractical. For example, the pool’s
design may include a flooded suction pump, and it may not be practical to
elevate the flooded suction pump at all. It is certainly not practical to
elevate a heater to the point that one can’t operate its controls, or a filter
to the point that the operator would need a ladder to perform routine
cleaning. As this is a relatively new requirement, it may often be up to
the contractor to carefully educate the building official during the permitting
process. To help educate builders and building officials, the Florida
Division of Emergency Management website, www.floridadisaster.org, has
published a document called “Interim Pool Guidance” that can be found by major
search engines.