FEMA 543, Design Guide for Improving Critical Facility Safety from Flooding and High Winds (Risk Management Series, January 2007)

Overview

The guide is a Risk Management Series document, shaped by the damage of Hurricane Katrina, that aims to keep critical facilities functioning under flooding and high winds. It has four main chapters: an overview of design considerations, protection from flooding, protection from high winds, and observations on how hospitals, schools and emergency response facilities performed in Katrina. The appendices give acronyms, a glossary and FEMA grant programmes. The target audience is owners, managers and designers of critical facilities.

Chapter 1 stresses that risk reduction starts with site selection and then facility design, and introduces performance-based design. Four flood performance levels are defined: operational (no damage), moderate impact, high impact and severe impact. Most critical facilities fall into ASCE 7 Occupancy Category III or IV, with IV covering essential facilities such as hospitals, emergency services and power stations. The guide recommends basing risk decisions for critical facilities on the 0.2%-annual-chance (500-year) flood, not only the 1%-annual-chance (100-year) flood.

The flood chapter explains the design flood elevation (DFE), the base flood elevation (BFE) and freeboard, often 1 to 3 feet for critical facilities. Loads are hydrostatic, hydrodynamic, debris impact and scour; hydrodynamic loads from water moving under 10 feet per second can be treated as equivalent hydrostatic loads. Probabilities are given: a 100-year flood has a 26% chance over 30 years and 50% over 70 years; the 500-year flood has 18% over 70 years. Elevation, resistant materials and dry floodproofing are covered for A Zones, V Zones and Coastal A Zones. Utilities and generators should sit above the DFE plus freeboard, at least 2 to 3 feet above the DFE.

The wind chapter defines the ASCE 7 basic wind speed as a 3-second peak gust 33 ft above grade in Exposure C: 90 mph over most of the U.S. and up to 170 mph in Guam. It covers the envelope, roof coverings, rooftop equipment, ductwork and equipment screens, wind-borne debris and lightning protection attachment. It recommends anchoring rooftop equipment for design wind loads and, where possible, placing it in penthouses. Separate best-practice sections cover hurricane and tornado regions and retrofit of existing facilities.

On power outages, the guide recommends for facilities needed during or within weeks after a hurricane emergency generators rated for prime (continuous) power, standby generators interconnected through manual transfer switches, and an exterior cam-lock box for a portable generator. Generators should sit in buildings resistant to wind-borne debris, not outdoors or in weak enclosures. The document dates from 2007, rests on U.S. codes and is not specific to data centers, so its criteria must be checked against local regulation and current standards.

Key points

1. Most critical facilities fall into ASCE 7 Occupancy Category III or IV; Category IV covers essential facilities such as hospitals, emergency services and power stations.
2. Critical facility risk decisions should be based on the 0.2%-annual-chance (500-year) flood rather than only the 1%-annual-chance (100-year) flood.
3. A 100-year flood has a 26% chance of occurring over 30 years and 50% over 70 years; the 500-year flood has an 18% chance over 70 years.
4. A freeboard of 1 to 3 feet above the flood elevation is often applied to critical facilities.
5. Hydrodynamic loads from floodwaters moving under 10 feet per second can be treated as equivalent hydrostatic loads.
6. Building and utility systems in flood hazard areas should be protected to at least the 500-year flood level or, at minimum, 2 to 3 feet above the DFE.
7. Generators installed at grade are inundated in a flood; underground tanks can float and should be checked for stability assuming they are empty.
8. ASCE 7 basic wind speed is a 3-second peak gust at 33 ft above grade in Exposure C; most of the U.S. is 90 mph, with 170 mph in Guam as the highest.
9. Facilities needed during or shortly after a hurricane should have emergency generators rated for prime power, standby generators interconnected via manual transfer switches, and an exterior cam-lock box for portable generator hookup.
10. Generators belong in wind-borne debris resistant buildings; outdoor siting or weak enclosures is not recommended.

Using it in design

• For site selection, look beyond the 100-year flood to the 500-year (0.2%) flood and the facility's occupancy category, and avoid flood hazard areas where possible.
• Place critical equipment (electrical rooms, UPS, switchgear, chillers, generators) above the DFE plus freeboard; target the 500-year flood level or 2 to 3 feet above the DFE, and give the ground floor non-critical uses.
• Keep inlets, fill openings and vents of fuel tanks above the DFE and check buoyancy stability of underground tanks assuming an empty tank.
• Require anchorage to design wind loads for generators and rooftop equipment, and house equipment in debris-resistant enclosures or penthouses where possible.
• In the backup power architecture consider prime-rated generators, standby generators tied through manual transfer switches, and an exterior cam-lock box for a portable generator.
• Write the flood performance level (operational, moderate, high or severe impact) into the design criteria, and use the guide's vulnerability checklist for existing facilities.

Who should read it

Designers and risk managers who handle siting, architectural and electrical-mechanical design of mission-critical facilities such as data centers and who assess flood and wind risk.

Document contents

1. Foreword and Acknowledgments
2. Chapter 1 - Critical Facility Design Considerations
3. Hazard Mitigation: Site Selection and Facility Design
4. Performance-Based Design
5. Chapter 2 - Making Critical Facilities Safe from Flooding
6. Flood Loads: Hydrostatic, Hydrodynamic, Debris Impact, Scour
7. Floodplain Management Requirements and Building Codes
8. Requirements and Best Practices in Special Hazard Areas (A Zones, V Zones, Coastal A Zones)
9. Utility Installations
10. Risk Reduction for Existing Critical Facilities
11. Checklist for Building Vulnerability of Flood-Prone Critical Facilities
12. Chapter 3 - Making Critical Facilities Safe from High Wind
13. Critical Facility Design Considerations for High Wind
14. Best Practices in Hurricane-Prone Regions
15. Best Practices in Tornado-Prone Regions
16. Chapter 4 - Observations on the Performance of Critical Facilities in Hurricane Katrina
17. Appendix C - FEMA Grant Programs

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Publication details

Authors

Federal Emergency Management Agency (FEMA)

Publisher

FEMA

Year

2007

Document language

English

Source type

Government / public

Access

Open licence — downloadable

Licence

U.S. public domain — U.S. Government work; free to reuse

Licence evidence

www.copyright.gov

Citation