designing for flood loads using asce 7 and asce 24 william l. coulbourne, p.e....
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Outline
Defining minimum design conditions as required by regulations
Flood load considerations for any floodplain
Issues critical to riverine flooding
Issues critical to coastal flooding
Retrofitting
Community Flood Regulations
Locally adopted flood map (FIRM) and flood insurance study (FIS)
Local flood ordinance
Local ordinance defines: Amount of freeboard if any More restrictive regulations related to foundations, elevation,
construction methods Allowable location for certain types of construction
Type of foundation Height of lowest floor Use of area below lowest floor
Minimum NFIP Requirements
Walled and roofed structures New construction (some additions) Substantially damaged/improved structures Subdivisions and other new development New and replacement water supply
systems and sanitary sewage systems
Affectprimarily
Apply to
Most Restrictive Zone Applies
V zone A zone
A building that has any portion of itsfoundation in a V zone must comply with V zone requirements
All Buildings, All SFHAs
Site reasonably safe from flooding Designed to prevent flotation, collapse, and lateral
movement during flooding Flood-resistant materials Constructed to minimize flood damage HVAC/plumbing equipment designed or
located to prevent water entry
Understanding Flood Maps
Base Flood Elevation
BFE is regulatory flood elevation
Probability of Exceedance = 1% in any given year (100-year flood)
Flood of this probability has 26% chance of being equaled or exceeded in 30 year period (life of mortgage)
Other flood elevations are frequently provided in Flood Insurance Studies that accompany flood maps
Design Safety Factor is increased elevation = Freeboard
Riverine A Zone FIRM
NFIP: A Zone Requirements
NFIP: A Zone Requirements
Enclosures below lowest floor: Used only for parking, access, storage
At least 2 wall openings: 1 in2 for every 1 ft2 of enclosed area subject to flooding Bottoms no higher than 1 ft above grade
Coverings, if used, must permit automatic entry/exit of flood waters
Elevation Example
Issues Critical to Riverine Flood Design
Elevation of expected flooding
Velocity of water
Location relative to floodway
Age of flood maps – accuracy of hazard identification
Scale of flood maps – location within floodplain
Risk of damage by flood-borne debris
Coastal FIRM
Delaware FIS
Flood Zones based on Wave Height
NFIP: V Zone Requirements
Siting: Landward of the reach of mean high tide. Restrictions on alteration of sand dunes, mangrove stands
Elevation: On pilings or columns Bottom of lowest horizontal structural member of the lowest
floor at/above BFE
NFIP: V Zone Elevation
NFIP: V Zone Requirements
Foundation: Piling/column foundations and buildings anchored Erosion control structures may not be tied to building or
foundation
Use of Fill: Not for structural support of buildings
NFIP: V Zone Requirements
Space Below BFE:
Used only for parking, access, storage
Free of obstructions or enclosed only by non-supporting materials
(e.g., insect screening, open lattice, breakaway walls)
Specific requirements for breakaway walls
V Zone Example
Issues Critical to Coastal Flood Design
Elevation of expected flooding
Velocity of water
Depth of waves
Age of flood maps – accuracy of hazard identification
Scale of flood maps – location within floodplain
V Zone vs. Coastal A Zone
Risk of damage by flood-borne debris
ASCE 7 Flood Load Requirements
Chapter 5 – ASCE 7-10
Loads during flooding Hydrostatic Hydrodynamic Waves Impact from debris
DFE – Design Flood Elevation
Load combinations involving flood (Chapter 2 – ASCE 7)
Hydrostatic Loads
Force from standing water – either lateral force or vertical force (buoyancy)
fsta = lateral force per unit width
Buoyancy = γ(Volume of displaced water)
Hydrostatic loads
Hydrodynamic Loads
Formula is from FEMA 55 CCM
Cd = drag coefficient
ρ = mass density of water (slugs)
A = surface area impacted by the water
Hydrodynamic loads
Waves (piles)
Breaking wave loads on piles
Hb is the height of the wave = 0.78ds
CD = drag coefficient
D = diameter of pile
Waves (walls)
fbrkw = unit force on a wall from breaking wave (lbs/ft)
Cp = dynamic pressure coefficient
Impact from flood-borne debris
FEMA 55 CCM has a “cleaner” version of this formula
Where:
CD and CB are depth and blockage coefficients
CStr is a structural coefficient related to rigidity
DFE - definition
And how does that differ from the Base Flood Elevation (BFE)?
Strength Design Load Combinations
ASD Load Combinations
FEMA 55 CCM Flood Load Combinations
ASCE 24 Flood Requirements
FEMA funded first pre-standard effort
Wanted more flood provisions in building code
First standard came out 1998
Intended to be for all types of construction and flood conditions
ASCE 24 referenced in IBC
Current version is ASCE 24-14
Scope
Buildings subject to bldg code requirements
Located in flood hazard areas
Applies to new construction or substantial improvements or repairs
See Figures 1-1 and 1-2 for further details
Classification of Structures Table 1-1 is different than ASCE 7 classification table
Table 1-1 titled Flood Design Class of Buildings and Structures
Use or Occupancy Flood Design Class
Buildings and structures normally unoccupied and pose minimal risk to public or community (temporary or storage facilities)
1
Buildings and structures that pose moderate risk to public or community should they be damaged or fail during flooding (most buildings including residential, commercial and industrial)
2
Buildings and structures that pose a high risk to public or community should they be damaged or are unable to perform normal function after flooding (large assembly buildings)
3
Buildings and structures that contain essential facilities and services for emergency response and recovery or that pose substantial risk to community at large in event of damage or failure by flooding (hospitals, EOCs, emergency shelters, fire, police, etc)
4
Table 1-1 Flood Classification
Section 1.5 Basic Requirements
Account for:
Elevation relative to DFE
Foundation and soils
Prior damage
Enclosures below DFE
Flood-resistant materials
Structural connections
Floodproofing
Utilities
Egress
Adverse impacts on others
IBC and ASCE 24
Appendix G covers Flood-Resistant Construction and references ASCE 24
IRC covers flood resistant design issues in Section R323
Crosswalk document between codes, ASCE 24 and NFIP
IRC References
Topic IRC Section
Basic Flood Resistance R322.1
Flood Loads and Conditions R301.1, Table R301.2(1), R322.1.2, R322.3.3
Lowest Floor Elevation R309.3, R322.2.1, R322.3.2
Foundation R322.2.3, R322.3.3, R401
Use of Fill R322.1.4.2, R322.3.2(3) and (4), R401.2, R506.2.1
Basements R322.2.1, R322.3.2
Use of Enclosed Areas Below DFE R309.3, R322.2.2, R322.3.5
IBC References
Topic IBC Section ASCE-7, ASCE-24
Basic Flood Resistance
1612.1, 1612.4
ASCE 24-05: 1.5
Flood Loads and Conditions
1605.2.2, 1605.3.1.2, 1612.4, 3102.7
ASCE 24-05: 1.6
ASCE 7-05: 2.3.3, 2.4.2, 5.3.1, 5.3.2, 5.4
Lowest Floor Elevation
1603.1.7, 1612.4 ASCE 24-05: 2.3, 4.4
Foundation 1612.4, Chapter 18 ASCE 24-05: 1.5.3, 2.5, 4.5
Sections 1.5.2 and 1.5.3
Elevation requirements References to other sections related to specific situations
Foundation requirements Design considerations for foundation walls Use of fill
Anchorage and connections
Section 1.6 Flood Loads
Refers to Chapter 5 – ASCE 7
Load combinations reference to ASCE 7
Combining various flood loads is not covered except in FEMA 55 – CCM
Extensive discussion in ASCE 7 Commentary for Chapter 5 regarding loads (covered later)
Chapter 2 Requirements for A Zones
Elevation – Table 2-1
Fill
Slabs on Grade and Footings
Openings requirement
Table 2-1 Min. Elevation of Top ofLowest Floor
Flood Design Class Minimum Elevation of Lowest Floor
1 DFE
2 BFE + 1 ft or DFE
3 BFE + 1 ft or DFE
4 BFE + 2 ft or DFE
Chapter 4 Coastal High Hazard Areas
V Zones
Coastal A Zones
Section 4.1.1 Commentary – Hazard ID important
Page 51 – explains rationale for Coastal A Zones
Elevation requirements
Foundation requirements
Breakaway walls
Table 4-1 Min. Elevation of Bottom of Lowest Horiz. Structural Member
Flood Design Class Min. Elevation of LHSM relative to Base Flood Elevation (BFE) or Design Flood Elevation (DFE)
1 DFE
2 BFE + 1 ft or DFE
3 BFE + 2 ft or DFE
4 BFE + 2 ft or DFE
ID Coastal HHA and Coastal A Zones
Coastal High Hazard Area has SWL 3.8 ft or more above eroded ground elevation which supports a 3 ft wave
Coastal A Zones have water elevation 1.9 ft minimum above eroded ground elevation which supports a 1.5 ft wave
Section 4.5 Foundation Requirements
Designed to minimize forces on the system
Foundation system must be free of obstructions
Structures supported on piles, columns, or shear walls
Top of spread footings, mat, or raft foundations must be below eroded ground surface
Piles must be used in erodible soils
Free-of-Obstruction T.B.
Wave Loads on Solid Walls:
This is why the CCM promotes
V Zone open foundation
requirements in Coastal A Zones
NJ – Sandy – Loads on Solid Walls
NJ – Sandy – Coastal A Zone
Chapter 5 Materials
Metal and corrosion
Other materials
FEMA T.B. on corrosion
Corrosion example
Chapter 6 Flood proofing Methods
Dry flood proofing
Wet flood proofing
Need for human intervention important
Not allowed to bring noncompliant residential buildings into compliance
Dry flood proofing permitted for commercial buildings with licensed professional seal
Chapter 7 Utilities
Electrical service
Plumbing systems
HVAC systems
Elevators
NY – Sandy – High-rises
Other sections
Building access
Decks, porches, etc
Garages
Pools
Tanks
ASCE 24 RE: ASCE 7 Flood loads
Chapter 5
Structural systems shall be designed to resist flotation, collapse and permanent lateral displacement
In flood hazard areas, design must be based on the design flood
Effects of erosion and scour shall be included in calculation of loads
Loads on breakaway walls designed for largest of: Wind load Earthquake load 10 psf Not to exceed 20 psf unless:
Wall designed to collapse from flood that is less than design flood Foundation designed for load combination specified in Chapter 2
Have found by calculation that small waves (1.5 ft or less) will not govern in areas where wind speed is a minimum of 140 mph; walls would have to be designed for wind
Flood loads should be applied to walls as unit loads not equivalent forces at the Stillwater level
Breakaway wall loads
FEMA T.B - Breakaway Walls
Refer to FEMA Technical Bulletin 9
NJ – Sandy
Breakaway Wall Design Methods
Prescriptive
Simplified
Performance
Breakaway Walls – Prescriptive Method
1. Walls subject to > 10 psf and < 20 psf
2. Wall heights between 6 and 9 ft. Columns or piers spaced between 8 and 12 ft.
3. Wind speed does not exceed 110 mph (ASCE 7-05) except 100 mph for areas within 4 ft of corners
4. Method permitted for all seismic design categories
5. Walls serving as backup for brick or stone veneer that might be damaged by deflection may not be designed using this method
Breakaway Walls – Prescriptive Method
Breakaway Walls – Prescriptive Method
Breakaway Walls – Simplified Method
1. Wall heights between 6 and 9 ft. Columns or piers spaced between 8 and 12 ft.
2. Wind speed between 110 mph (ASCE 7-05) and 140 mph
3. Method permitted for all seismic design categories
4. Walls serving as backup for brick or stone veneer that might be damaged by deflection may not be designed using this method
5. Use Tables to look up designs
Breakaway Walls – Simplified Method
Breakaway Walls – Simplified Method
Breakaway Walls – Performance Method
1. Design must be performed by design professional
2. Method is always permitted
3. Expected to be used when neither of the other 2 methods can be satisfied
