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Gregory Wilson, CFM
Adrienne Sheldon, PE, CFM
Amit Mahadevia
FEMA 259 UpdateEngineering Principles and Practices for
Retrofitting Flood-Prone Residential Structures
2
Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures
FEMA P-259
• Provides guidance for selecting and implementing flood retrofitting measures that are feasible and cost effective.
• Describes:• Elevation• Relocation• Barriers (floodwalls and levees)• Dry floodproofing• Wet floodproofing
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Organization of the Design Manual
Chapter Title
I Introduction to Retrofitting
II Regulatory Requirements
III Parameters of Retrofitting
IV Determination of Hazards
V General Design Practices
VD, VE, VF, VR, VW Types of Flood Retrofits
VI Case Studies
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Elevation
• Structure is raised so lowest floor is at or above the Design Flood Elevation (DFE)
• Protects structure from damage in a base flood
Base flood elevation (BFE): Flood having a 1-percent chance of being exceeded in any given year.
Design flood elevation (DFE): Regulatory flood elevation adopted by a local community. Typically, the DFE is the BFE plus any freeboard adopted by the community.
FEMA P-55, Figure 2-14
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Residential structures that can be elevated
Houses over a crawlspace• Elevated on either solid or open foundation walls
Houses over basements• Elevated on either solid or open foundation walls
Houses on piers, post, piles, or columns• Temporary relocation of home may be necessary
Slab-on-grade houses• Wood frame vs. masonry; different elevation
methods
Harder
Easier
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Select New Site (Step 3)
Examine potential sites for:
• Location outside of floodplain
• Utility extension feasibility
• Accessibility for both the house movers and the new site construction crews
• Permitting feasibility for the existing house on the new lot
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Prepare the Structure (Step 6)
• Disconnect utilities
• Cut holes in foundation wall for beams
• Install beams
• Install jacks
• Install bracing
• Separate structure from foundation
Structure is separated from its foundation (photograph courtesy of Wolfe House Movers)
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Preparing the New Site (Step 7)
• Design foundation
• Design utilities
• Excavate and prepare new foundation
• Construct support cribbing
• Construct foundation walls
New foundation wall construction begins (photograph courtesy of Wolfe House Movers)
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Move the Structure (Step 8)
Trailer wheel sets are placed beneath the lifting beams(photograph courtesy of Wolfe House Movers)
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Dry Floodproofing
The portion of a structure below the flood protection level (walls and other
exterior components) is sealed to make it impermeable to floodwaters
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Dry Floodproofing Examples
• Watertight shields for doors and windows
• Reinforced walls
• Membranes and sealants
• Drainage collection systems and sump pumps
• Check valves
• Anchoring
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Select and Design Sealant System
• Selection centers on the ability of the manufacturer’s product to be compatible with the length and depth of flooding expected as well as the type of construction materials used
• Processes for selection of the following systems are presented in the manual
• Coatings• Wrapped Systems• Brick Veneer Systems
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Select and Design Shield System
Selected material must be able to:
• Structurally secure the opening
• Be compatible with existing construction materials
• Be responsive to the duration and depth of flooding expected
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Select Drainage Collection Systems
Typical sump detail
Underdrain systems: reduce flood loads for short duration flooding by moving floodwater away from building foundation
Sump Pumps: used to prevent accumulations of water within residence, typically around important utilities
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Select Backflow Valves & Emergency Power
Backflow valves: helps prevent backflow through sanitary sewer and/or drainage systems
Emergency power: if used, observe proper guidelines
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Dry Floodproofing Pitfalls
• Active mitigation
• Increases buoyancy
• Increases lateral forces potential
• Potential for total house failure
• Power dependency (sump pump)
• Maintenance requirements
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Wet Floodproofing
Involves modifying
structure to allow
floodwaters to enter
in such a way that
damage is minimized
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Wet Floodproofing Concepts
FEMA 259 discusses the following concepts related to wet floodproofing:
• Protecting the structure
• Designing openings for intentional flooding of enclosed areas below the DFE
• Use of flood-resistant materials below the DFE
• Adjusting building operations and maintenance procedures
• Emergency preparedness for actions that require human intervention
• Designing protection for the structure and its contents, including utility systems and appliances
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Use of Flood Resistant Materials
All materials exposed to floodwater must be:
• Durable
• Resistant to flood forces
• Retardant to deterioration caused by repeated exposure to floodwater
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Protecting Utility Systems
• Electrical systems
• Heating, ventilation, and air conditioning (HVAC) systems
• Fuel supply/storage systems
• Sewer systems
• Water systems
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Floodwall Design
Types of floodwalls include:
• Gravity wall
• Cantilever wall
• Buttress
• Counterfort
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Levee
Home protected by a levee
Levee: A manmade compacted earthen structure built parallel to a waterway to contain, control, or divert the flow of water.
System may include non-earthen components, such as floodwalls, floodgates, and pump stations.
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Levee Design
Standard levee design criteria was established to provide a conservative design while eliminating several steps in the USACE design process, thereby minimizing design cost
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Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures
FEMA P-259
• An important resource for engineers, architects, and local code officials in flood-prone areas since the first edition was published in 1995.
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FEMA Building Science
Flood/Wind Building Science Helpline:
866-927-2104
http://www.fema.gov/rebuild/buildingscience