Common Design Considerations

All extreme weather conditions have the potential to create damage to buildings, increase risk to people, or both. Not all do it in the same way, however, nor do they all pose the same types of threats. The key, from a design standpoint, is to understand the differences between various weather conditions and the ways to help assess, quantify, and address the potential concerns.

Flooding

As mentioned, flooding is one of the most frequent and potentially most damaging events related to extreme weather that people and buildings need protection from. The National Flood Insurance Program (NFIP) managed by FEMA is the only place that most properties can get flood insurance, therefore, it is a significant source of information and data on flood events. Of particular interest to NFIP are the specific costs of flooding which can help to identify where to focus ways to reduce damage and hence cost. One of the best known recent flooding events occurred when Hurricane Sandy battered the northeastern United States in October of 2012. Statistics have shown that NFIP claims filed within the four months following that storm were 93 percent for building damage as opposed to 7 percent for contents damage. That may be different from what a lot of people think when they see streets piled with ruined furniture, refrigerators, etc. after a flood. The reality is that building and structural damage claims were the predominant issue in Hurricane Sandy, with claims in the range of $10,000–$100,000 each. The bottom line is that such structural damage from flooding can add up quickly, making it extremely worthwhile to design for reducing building damage in known flood zone areas.

Designing to mitigate flood damage starts by consulting the FEMA Floodplain Maps for the location where a building is sited. There you can determine the Base Flood Elevation (BFE), which is the calculated level that floodwaters will rise to during a base flood event. The maps may also show Special Flood Hazard Areas (SFHA), which are high-risk floodplain areas. If the building site is located in any area on the map that shows a BFE or a SFHA then building designs need to meet local, state, and federal floodplain construction requirements. The degree of flood mitigation required may depend on identified zones in SFHAs. Type A zones (AE and A1-30 zones) are areas with established BFEs and a perceived low impact from waves. For example, in a coastal area, these zones are typically found about two to three blocks inland from the coastline. Type V zones (VE and V1-30 zones) have a greater risk due to impact from storm-induced waves. Typically, these zones are right along the coastline as in “beachfront property.”

Using the maps to determine the BFE and zoned degree of risk, the first mitigation step is to focus on keeping occupied space above the flood level. The term used to describe this is “freeboard,” which is defined as the elevation of a building’s lowest occupied floor above the base flood elevation. Many municipalities may have freeboard provisions already incorporated into their floodplain management ordinance, indicating, for example, +1 or +2 or +3, meaning that the lowest occupied floor for a home or building must be 1 foot, 2 feet or 3 feet higher than the established base flood elevation on the map. This approach does two things. First, it provides a bit of a safety factor, allowing for the underside of the structure of the lowest floor to remain above floodwaters or to guard against occasions when floodwaters may exceed the calculated BFE. Secondly, the actual floor elevation compared to the freeboard elevation is used as a basis to calculate flood insurance premiums. If the built floor elevation is at or above the freeboard elevation, then damage is expected to be less, meaning flood insurance premiums can be less. If the floor is below the freeboard elevation, then damage can be expected to be higher and premiums can go up drastically as a result.

This freeboard approach is good and certainly helps protect building contents and the building structure from the floor up, but we all know that something has to hold up that lowest occupied floor. In some cases it is a crawl space, while in others it could be a full-height car port or other non-habitable space under the building. The available height between grade and the first floor level will certainly influence how that space is used, but so will the building type. A residential building will create different opportunities and limitations compared to a commercial (non-residential) building. One thing that is similar, however, is the need to protect the structure in all cases from the forces of floodwater. Damage to the lower or foundation level is never good for any building since it can telegraph upward and impact the entire building above it. Hence, proper mitigation of the effects of flooding at the lowest level is critically needed.

Photos courtesy of Smart Vent Products, Inc.

Commercial and residential buildings located in flood plains need to have proper flood mitigation as part of their design, such as the flood vents shown at the base of each of these buildings.