Major Revisions to ASCE 24— A Standard for Flood-Resistant Design and Construction

To view the figures and tables associated with this article, please refer to the flipbook above.

ASCE 24 was last revised in 2014, over a decade ago while major flood events have caused severe damage to the nation’s building inventory and infrastructure. This standard, titled Flood-Resistant Design and Construction was first published in 1998 and was revised in 2005 as well as in 2014. ASCE 24-14 has been adopted into the International Building Codes by reference and is used across the country to guide building and re-building in areas designated as floodplains. This latest revision, ASCE 24-24, has been submitted for consideration in the 2027 IBC and is intended to improve building performance during floods by requiring higher elevations for buildings in floodplains and by strengthening many provisions that address how to construct buildings subject to flooding. This revision also aligns flood design requirements with the recent changes made to ASCE 7-22, Supplement 2 as they relate to more stringent standards being applied to flood load design. This standard revision provides a resource for the practice in the form of a Commentary section that provides examples of buildings in floodplains to illustrate how to use the new provisions, especially as they relate to the new elevation requirements.

Previous versions of ASCE 24 had minimum elevation tables that required a minimum elevation to the Base Flood Elevation (BFE) plus some amount of freeboard. The BFE is FEMA’s regulatory minimum elevation or the elevation of the 1% annual chance flood, which is used for the National Flood Insurance Program (NFIP). Freeboard is an additional elevation that provides a margin of safety above the BFE minimum elevation. The NFIP requires communities to adopt minimum construction standards and in exchange makes flood insurance available for properties. In ASCE 24-14 and prior editions, the required amount of freeboard was dictated by the Flood Design Class (FDC) of the building; the higher the class (from 1 to 4), the greater the amount of freeboard generally. ASCE 24 uses FDC to distinguish building use types instead of Risk Categories used by ASCE 7; while the FDC aligns closely with Risk Category, FDC provides more specificity on the use types in each class.

ASCE 24 strives to meet or exceed the NFIP requirements. The elevation requirements in ASCE 24-24 are now driven by the Design Flood Elevation (DFE) which is determined by applying the higher of the locally mandated elevation requirements and a Mean Recurrence Interval (MRI) flood event. The MRI flood event is different for each FDC. FDC 2 buildings (most commercial and residential buildings) require a minimum elevation to the 500-year flood level; FDC 3 buildings require a minimum elevation to the 750-year flood level; and FDC 4 buildings require elevation to the 1,000-year flood level. Since there is very little information about the elevation of 750- and 1,000-year floods, there are scaling factors provided to help designers estimate those higher flood levels. The 500-year flood level is often provided in Flood Insurance Studies (FIS) and the extent shown on FEMA Flood Insurance Rate Maps (FIRMs). Table 1-2 from the new standard is shown above slightly modified as Table 1 here.

The DFE is calculated as the DFE = FE_com or (FE_MRI + Δ_SLC) where

As noted previously, flood elevations are generally available for 10-, 50-, 100- and 500-year flood events and are shown in the FIS and/or FIRM for the flood source of interest. The needed elevations for 750- and 1,000-year elevations are almost never available unless a site-specific study has been performed. Therefore, two new tables were added that designate how to find the needed flood elevation when only certain pieces of information are available. The methods in these tables accommodate the range of flood data available on reports and maps throughout the country and are listed in order of most commonly available. One table describes the methods for noncoastal flood sources, and one describes the methods for coastal flood sources. The noncoastal flood source table (Table 1-3 in the standard) has five possible methods for each of the four FDCs. Table 2 is a slightly modified version of Table 1-3. The coastal flood sources table (Table 1-4 in the standard) has four possible methods for each of the four FDCs. Table 3 is a slightly modified version of Table 1-4. Tables 1-5 and 1-6 of the standard list the scaling factors necessary to calculate for higher return period events for coastal flood sources when the FEMRI information is not available. The intent of including Tables 2 and 3 shown here is to provide a minimum elevation for the structure that will elevate the structure above the MRI flood event and generally limit the MRI flood loads imposed on the structure per ASCE 7-22, Supplement 2. One example of this comparison is provided in this article in the Example Elevation Section.

There are worked examples for finding the correct minimum elevation for the nine cases (five noncoastal and four coastal) in the ASCE 24-24 Commentary for Chapter 1. The examples use various FDCs, and thus various design associated mean recurrence intervals to provide a range of possible approaches to obtaining the correct DFE. Since in each case, there is a minimum required elevation of either the BFE + freeboard or the DFE, there is a comparison in each example of which is higher—the BFE + freeboard or the DFE.

The elevation requirements carry over into other sections of the standard instead of being repeated and only slightly modified. The previous versions of ASCE 24 had separate elevation tables for Chapter 3 (primarily A zones), Chapter 4 (primarily V Zones and Coastal A Zones), Chapter 6 (Dry and Wet Floodproofing), and Chapter 7 (utilities). Those tables have been removed in ASCE 24-24 and the elevation requirements for these flood subjects are covered fully by Chapter 1 and Table 1-2. The other chapters now simply refer to Table 1-2 when elevation requirements are mentioned in each chapter. To further simplify the determination of the minimum elevation requirements, a web-based tool has been developed by the committee and the LSU AgCenter that will be released in conjunction with ASCE 24-24 in order to aid design professionals and local officials with minimum elevation determinations.

Sections 2.3 and 4.4 of ASCE 24-24 on Elevation Requirements are measured consistent with ASCE 24-14 in that the minimum required elevation for buildings in noncoastal and coastal areas with small waves is for the top of the lowest floor to be elevated to or above the elevation established in the new Table 1-2 (shown here as Table 1), and in areas where the 100-year wave heights are greater than 1.5 feet, the bottom of the lowest horizontal structural member of buildings must be elevated in accordance with the same Table 1-2 requirements. Tables 2 and 3 shown here describe for the practitioner how to achieve the requirements of Sections 2.3 and 4.4 for different conditions related to the flood elevation data available.

The determination of the DFE must now include consideration of sea level change in coastal flood locations. The standard requires that a building service life of 50 years be considered into the future, and that the amount of sea level rise be based on the historic rate of change (over the previous 40 years) at the building site location times the 50-year service life. A sea level change value of less than zero is not allowed. There is information in the commentary that discusses how to find the sea level change for many locations around the country. Data developed and provided by the USACE and NOAA covers many coastal locations around the country. There is no requirement that a site-specific study must be conducted in order to find the rate of sea level change at a specific project site. This requirement to include consideration of sea level change is in line with the requirements in ASCE 7-22, Supplement 2. Designers and owners may wish to consider a greater amount of sea level rise based on projections of future conditions, but such predictions are not required by ASCE 24-24.

Example Elevation Determination Comparison: ASCE 24-24 and ASCE 7-22, S2

Givens:
Building is FDC 2 located on the Gulf Coast
Community has adopted ASCE 24-24 and requires 1 foot of freeboard
FIRM Map indicates the BFE = 13 feet which is to the top of the wave at this coastal location
FIS at transect of interest indicates SWEL₁₀₀ = 10.8 feet and SWEL₅₀₀ = 15.1 feet
Rate of historic sea level rise is 0.015 feet/year and is to be used for a 50-year building life
Elevation at the site taken as 0 feet which is the coastal datum

Find the required minimum elevation in accordance with ASCE 24-24 and the equivalent elevation for loading in ASCE 7-22, S2.
Since the FE₁₀₀, SWEL₁₀₀ and SWEL₅₀₀ are all known, use Method B from Table 3 above to find the elevation required by ASCE 24-24.
1. The required minimum elevation to meet the community regulation (see Table 1) = FE₁₀₀ = 13 feet+1 feet = 14 feet

2. The elevation required to comply with ASCE 24-24 for FDC 2 buildings = SWEL₅₀₀+[CMRI₁₀₀(FE₁₀₀-SWEL₁₀₀)] = 15.1 + 1.35 (scaling factor from Table 1-5 in standard)*(13-10.8) = 18.1 feet

ΔSLR must be added to this elevation to determine the final FE₅₀₀ for DFE comparison, so 18.1 feet + 0.015*50 = 18.9 feet

3. Compare FE_com to FE₅₀₀ to determine the DFE and then compare to minimum elevation requirements. FE_com = 14 feet < FE₅₀₀ = 18.9 feet = DFE. DFE = 18.9 > BFE + 1 feet (13 + 1 foot = 14 feet).

The required elevation for ASCE 24-24 = 18.9 feet

4. The elevation for loading in ASCE 7-22, S2 is determined using Equation 5.3-1 (ASCE 7-22) or df = (SWEL_MRI-Ge) +ΔSLR where df = design stillwater flood depth, Ge = ground elevation and ΔSLR = change in sea level elevation. In this example, df = (15.1 – 0) + 0.75 feet = 15.85 feet

We do not know the wave height for the FE₅₀₀ event; we only know the SWEL₅₀₀. Therefore, we must find the FE₁₀₀ wave height and use a scaling factor from Table 5.3-3 (ASCE 7-22).

We must find the top of wave elevation so the result can be compared to the ASCE 24-24 result. The controlling wave height above the SWEL₁₀₀ is the BFE = 13 – 10.8 feet = 2.2 feet and the controlling wave height above the SWEL is 70% of the total wave height so the total controlling wave height for the FE₁₀₀ = 2.2/0.70 = 3.1 feet

The total controlling wave height (HC₅₀₀) for the 500 year MRI = 3.1 feet1.3 (scaling factor) = 4 feet. The wave portion above the SWEL = 0.704 feet = 2.8 feet so the DFE (FE₅₀₀ elevation equivalent) = 15.85 feet + 2.8 feet = 18.7 feet

5. Therefore, the required minimum elevation using ASCE 24-24 is 18.9 feet which is greater than 18.7 feet (ASCE 7-22 DFE) and so it may not be necessary to increase the minimum elevation of the building to minimize loading.

If the ASCE 7-22 DFE was greater than the ASCE 24-24 required minimum elevation, then a designer may consider elevating the floor system to minimize the flood loads on the building.
In addition to changes in required elevations for structures, there were changes to several other important sections in ASCE 24-24. These changes primarily deal with flood proofing methods other than elevation and materials used for flood-prone structures. The change in minimum elevation also required addressing the extent of the 500-year floodplain.

ASCE 24-24 expanded the floodplain from the requirements of the previous version. ASCE 24-14 only required the provisions of the standard to apply to the delineated Special Flood Hazard Area (SFHA) or the 100-year floodplain as shown on FIRMs. A significant change was made to ASCE 24-24 to expand the floodplain to also include the Shaded X Zone (500-year floodplain) where it is mapped. The standard also allows communities to delineate their own floodplain if it is more restrictive than the SFHA and Shaded X Zone shown on the FIRM. This brings ASCE 24-24 further into alignment with ASCE 7-22 Supplement 2.

Significant language is added regarding the use of materials in salt-laden environments and expanded on materials and material standards that should be consulted when the building is to be located near a coastline where salt exposure is prominent. Steel and concrete material especially received additional discussion, particularly as it relates to metal connectors and reinforcing steel for concrete or masonry.

Two new ASTM standards are discussed regarding flood damage-resistant materials. ASTM E3075-24 is a Standard Test Method for Water Immersion and Drying for Evaluation of Flood Damage Resistance, and ASTM E3369-24 is a Standard Specification for Determining the Flood Damage Resistance Rating of Building Materials. This is the first time that such an evaluation method for flood damage-resistant materials has been available to designers. This will help address the NFIP requirement that all materials installed below the minimum elevation be resistant to flood damage. It will also provide manufacturers with a method of certifying that new products are indeed flood-damage resistant.

The requirements for dry floodproofing are allowed in only A Zones (primarily riverine flood areas) and Shaded X Zones (500-year floodplain areas) and similar noncoastal locations and require flood opening barriers to have passed the tests and be certified as described in ANSI 2510, American National Standard for Flood Mitigation Equipment. This means that in order to be used in flood mitigation projects that require compliance with ASCE 24-24, all opening barriers such as shields, flood barriers, closures over doors and windows must meet the ANSI 2510 standard. That also means that designers who will certify compliance, must assure themselves of such compliance.

Designers who certify compliance with the dry floodproofing requirements will need to make sure flood barriers have been tested to the flood depth required of a specific design; they will need to know how the flood barriers are to be installed and design attachments for such flood barriers; they will need to conduct a flood vulnerability assessment of an existing structure where flood barriers are planned and specify how to seal up any other small openings that might exist in the structure’s exterior walls.

There is more specific information provided about what type of shields are allowed for each FDC for both new construction and for substantial improvements. Additionally, more discussion is provided about the use of dry floodproofing techniques for non-residential buildings and non-residential space in mixed-use buildings. Distinctions are made between a barrier for a building opening (i.e. door or window) and a “temporary floodwall.” Floodproofed wall systems must be marked to indicate the level of floodproofing sealant to minimize the potential for unsealed penetrations to be made through a sealed wall system, which would compromise the floodproofing.

There are new inspection, maintenance, and operations plans required to be filed with the Authority Having Jurisdiction (AHJ), and actual deployment of equipment and exercising the operations plan is required annually (or as stipulated by the AHJ). Post-disaster assessment indicates that most flood damage to dry floodproofing systems occur because of poor design, poor installation, not knowing where the floodproofing shields or devices are located, and the owner or support personnel not knowing how to install measures properly. There are maximum times allowed for the deployment of active dry floodproofing systems based on Flood Design Class.

For wet floodproofed buildings, exceptions were added for the location of flood vents based on restrictions imposed by the geometry of the spaces with only one exterior wall and on sloped sites.

Additionally, two other changes were made to the standard based upon post-disaster assessments. The standard now addresses elevating existing slab-on-grade buildings, which has been a common practice in some parts of the country as a way to elevate the lowest floor of a house and bring that house into compliance with the NFIP. However, there have been structural slab failures when this has been done, since most slabs have minimal thickness and minimal to no reinforcing steel. Additional language has been added in ASCE 24-24 to make sure the existing slab is assessed for strength, strengthened as needed and connected properly to the new foundation to be able to resist uplift and buoyancy as required. There were also changes to the standard to require that when there is an entrance from the elevated building into an enclosure below the building that the door must be an exterior grade door. This was added based upon elevated buildings being damaged during flood events due to the failure of doors leading into enclosures, leaving the buildings open and unable to be secured.

The new revisions to ASCE 24-24 were primarily influenced by the need to make this Flood-Resistant Design and Construction Standard compatible with ASCE 7-22 Supplement 2 and ten years of post-disaster flood assessments. Many users will recognize the increase in the floodplain extent and the additional elevation requirements. The committee considered the use of the standard for both design and floodplain management in development of the requirements. Engineers will need to consider the minimum elevation requirements in ASCE 24-24 and the minimum elevation for flood load consideration in ASCE 7-22 Supplement 2 and should use that when determining how high to elevate buildings. The material, slab, and enclosure access provision should further reduce damage to elevated buildings. Innovations in the dry floodproofing requirements should reduce the potential risks for both new and substantially improved floodproofed buildings. The changes are viewed as a major improvement to flood resistant design and are intended to reduce flood losses with increased building flood protection and increased national resiliency. ■