It’s no secret that more and more Americans have been moving to coastal states in the past few decades. In fact, today the population in coastline counties now accounts for about 50% of the US population, increasing at a rate that is more than double that of the nation’s population as a whole. In just the last year, 5 of the 6 fastest growing states were coastal.
Source: The U.S. Census Bureau
Of the 254 coastline counties stretching across parts of 23 states, 185 of these are susceptible to severe hurricanes as shown by the maps below (Atlantic – 129, Gulf of Mexico – 56).
· The yellow highlighted area (>100 mph) indicates a small missile debris-prone region.
· The orange highlighted area (>110 mph) indicates a large missile debris-prone region if within 1 mile (1.6 km) from the coast or anywhere where the design wind speed is 120 mph.
This growth in coastal areas presents many challenges to the construction and insurance industries. Coastal construction in these areas are inherently risky due to higher exposure to natural catastrophes from not just severe windstorms, but flood, and surge. In addition to these exposures, the forces of weathering and corrosion are particularly harsh in coastal areas. Buildings near water are exposed to erosion and scour, breaking waves, high velocity storm surge flow, moving floodwaters, and floodborne debris. Emerging hazards associated with rapid climate change such as sea level rise and wider temperature swings also need to be accounted for in these regions.
So, what are the coastal construction challenges and how should they be evaluated in 1. Existing construction and 2. Future construction, to ensure structures are adequately designed for the hazards? Let’s examine some of the major factors.
- Wind
- Design. As seen in the map (Fig. 2) above, wind speeds are higher closer to coastal areas and require stronger building cladding, framing, and foundations to handle the additional load. Wind loading for new structures should be per either
- FM Global Data Sheet 1-28, or
- ASCE 7-22, using Risk Category III or importance factor (I) of 1.15, as recommended by FM.
Wind loading requirements have changed significantly in the past decade and older structures may not meet today’s standards. To understand why, reference https://risklogic.com/windstorm-standards-how-did-we-get-here/.
- Existing Construction should be evaluated for:
- Roofing. Roof uplift assessment, uplift testing or other testing (ie bonded pull test) should be done for all roofs older (more than 5 years old) to confirm uplift pressure resistance per FMDS 1-52 or FBC TAS 124, particularly for roofs that were constructed per the older perimeter and corner definitions.
- Roof Decks. An assessment should be done ensure proper deck design and load path securement to building frame. The condition of fasteners and welds should be checked and repaired as needed.
- Fenestration pressure and impact. Windows, doors, and wall openings should be designed for the expected wind pressures and small and large impact resistance where needed. The wind (Fig. 2) map above shows areas prone to missile impact.
- Impact protection can be provided using approved shutters, fabric systems, or replacing with impact resistant products.
- Deficient garage and dock doors need additional bracing to meet TAS 202, ASTM E330 or ANSI/DASMA 108 or equivalent local test standard, as recommended by the manufacturer or a registered structural engineer.
- Fenestration leaks. Windows and doors are prone to leakage. Joints, gaskets, seals, caulking, etc. fail over time due to thermal movement, UV degradation, and separation from the glass and/or frame. All windows and doors should be inspected annually for any signs of leaks around them. The seals, door gaskets, and weather stripping conditions should be checked for shrinkage, cracking, gaps, etc. per ASTM C1392 “Standard Guide for Evaluating Structural Sealant Glazing”. If there are any signs of leaks or any defective seals, correct them as soon as possible as recommended by a building envelope consultant and using ASTM C920 compliant sealant.
2. Flood
- Design. Based on the specific location (surge exposed oceanfront Zone V, coastal setback Zone A, or inland Zone A), extra measures may be required. The diagram below from Federal Emergency Management Agency (FEMA) P55 shows when possible wave effects that may to be accounted for.
- Zone V areas. Challenges to designing coastal foundations include wave action and ground erosion from storm surge which results in more damage compared to inland flooding. When floodwaters destroy weaker buildings in coastal areas, they expose the foundations of other structures to floodborne debris. Erosion and scour can destroy foundations and cause the building to fail. Therefore,
- Buildings must be elevated high enough to avoid flooding and the foundation must be designed to protect the building against flotation, collapse, and lateral movement.Building design must minimally meet National Flood Insurance Program (NFIP), as well as state and local floodplain management, requirements.
- Existing construction. If existing structures are exposed to projected flood levels, several protection options exist such as sandbags, flood barriers, and engineered flood protection. See https://risklogic.com/flood-protection/ for protection measure options.
- Climate Change. Global mean sea level has been rising at long-term rates (over 6 inches total during the twentieth century) and rates of mean sea level rise are higher in some regions such as along the Louisiana and Texas coasts, as well as portions of the Atlantic coast (IE, as high as 3.03 feet per century in Grand Isle, LA).
- Structures should be designed to withstand anticipated erosion and scour throughout the entire life of the structure and to factor any anticipated sea level rise for the life of the structure. FEMA does provide some prescriptive residential design guidance for coastal areas that can also be applied to commercial construction. FEMA P-55 and P-499 includes many topics including planning, foundations, load paths, wall systems, openings, roofing, and attachments:
- Saltwater corrosion. Exposure to salt in coastal areas is magnified which can lead to accelerated decay of rebar, spalling and concrete erosion and possible failure as in the June 2021 Surfside building collapse which led to 98 deaths ( https://risklogic.com/saltwater-corrosion/).
The issues with salt exposure are many including:
- Salt water is an excellent electrolyte contributing to an aggressive corrosive environment.
- Saltwater corrodes metal five times faster than fresh water
- Salty, humid ocean air causes metal to corrode 10 times faster than air with normal humidity.
- Bacteria in ocean water also consumes iron and their excretions turn to rust.
- Electrochemical Corrosion – Metal ions dissolve in water and saltwater conducts electricity and contains ions, which attract ions from other compounds. Saltwater attacks the metal and corrosion occurs.
- Anaerobic Corrosion – metal is exposed to saltwater for an extended period, leads to hydrogen sulfide produced which then corrodes metals. Between the ions, sulfates and bacteria, metal is attacked from all angles when it is in salty air/water.
- More permeable concrete and higher the calcium aluminate concentration, will be more susceptible to damage.
How to Inspect? The Surfside tragedy led to a rule for buildings to go through a “milestone inspection” certification process when reaching 30 years of age, or 25 years if the building is located within three miles of the coast, and then inspected again every 10 years afterward. This is a good rule of thumb to follow.
Prevention. Potential issues could be avoided as follows:
- Proper concrete and rebar design when exposed to seawater.
- Proper drainage, waterproofing, to prevent pooling of salt water.
- Immediate remediation when problems are found.
- Other factors. Coastal stability depends heavily on sea level and may be impacted by changing infrastructure, loss of land, marsh migration, flooding impacts on land and infrastructure, social and economic impacts, saltwater intrusion, bank and bluff failure, and coastal erosion. Continued monitoring of these factors is necessary to plan for any mitigation steps that may be needed in the future.
Conclusion
So, what can be done to mitigate the risks of coastal construction? As laid out in this article, new and existing construction can be designed or improved for the many factors that lead to losses in these regions, primarily wind, exposure to windborne debris, wind-driven rain, flood, and corrosion. A detailed evaluation would benefit any facility in these regions to accurately assess the windstorm and flood risk. Risk Logic offers an advanced survey service that can accurately assess the risk and provide necessary recommendations for improvement based on current conditions and standards.
References:
https://www.nar.realtor/blogs/economists-outlook/where-people-moved-in-2022
https://toolkit.climate.gov/topics/coastal-flood-risk/building-resilience-coastal-communities