designing for flood loads using asce 7 and asce 24 william l. coulbourne, p.e....

Designing for Flood Loads Using

ASCE 7 and ASCE 24

William L. Coulbourne, P.E.

bill@coulbourneconsulting.com

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

Questions?

bill@coulbourneconsulting.com