& criteria for maximum elevation of residential buildings in floodplains william l. coulbourne,...

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CRITERIA FOR MAXIMUM ELEVATION OF RESIDENTIAL BUILDINGS IN

FLOODPLAINS

William L. Coulbourne, P.E.Applied Technology Council

[email protected]

Manny Perotin, P.E.

Atkins [email protected]

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Why This Topic

Currently no technical guidance on limitations

Applications submitted with little to no engineering

Federal dollars sought for elevation in locations where there should be limitations on the extent of elevation

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A Simple Example

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Current Guidance

FEMA HMA Guidance Limits on height derived from FEMA 550

IRC Minimum elevation is DFE V Zones and Coastal A Zones – minimum

DFE is BFE + 1 ft. Unsupported height of 8” RM walls – 8’

ASCE 24-05 Minimum freeboard reqd. of 1’

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Technical Issues

Flooding Concerned with hydrodynamic flood forces

and flood-borne debris Wind

Non-hurricane wind speed areas Earthquakes

Elevating increases seismic vulnerability Snow

This study limited snow load to 30 psf

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Design Scenarios

Floods, winds, earthquakes, snow can all occur in separate events

Floods and winds can occur simultaneously

Earthquakes and snow can occur simultaneously

Floods and earthquakes or wind and snow have a very low probability of occurring simultaneously

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Limitations

Flooding Flood level not above top of foundation Flood velocity is equivalent to depth (3’ of

water = 3 fps) No hydrostatic pressure Flood-debris does not control design

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Limitations con’t

Wind Not in hurricane wind zones Used ASCE 7-10 wind maps and speeds Assumed 2-story building with 5:12 roof

pitch Exposure B condition Building is enclosed

Map shows the shaded area excluded

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Limitations con’t

Earthquake Short period acceleration is 60% g Building weight is 10 psf for floors, 12 psf for roof, 9

psf for walls Foundation weight assumes concrete Ground motion taken from ASCE 7-10 maps Response value for foundations:

RC walls = 4, RM walls = 2, RC columns = 1.5, wood piles = 1


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Limitations con’t

Snow Upper bound snow load is 30 psf ASCE 7-10 snow map used


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Constraints

Foundation wall thickness – 8” to 12” IRC limitations on height of wall or

columns applies Pier spacings limited to 6 ft. and 8 ft. No cross-bracing between piers included Limits on amount of reinforcing steel not

exceeded

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Height (ft)

Reinforced Concrete

Walls1

Reinforced Concrete Masonry

Walls1

Reinforced Concrete Piers2 Wood Piles3

3 Wind – 8”0.5 in2

Wind – 8”0.5 in2

Wind – 16”x16” x 10’ – 2.6 in2

Wind – 12” dia.x 8’

Seismic – 8”1.02 in2


Seismic – 16”x16” x 10’ – 13.6 in2

Seismic – NP4

5 Wind – 8”0.6 in2

Wind – 8”0.6 in2

Wind – 16”x16” x 10’ – 3.0 in2




Seismic – 16”x16” x 8’ – 20.4 in2

Seismic – NP4

8 Wind – 8”0.8 in2

Wind – 8”0.8 in2

Wind – 16”x16” x 10’ – 4.2 in2




Seismic – 24”x24” x 8’ – 22.4 in2

Seismic – NP4

10 Wind – 8”1.1 in2

Wind – 8”1.1 in2

Wind – 16”x16” x 10’ – 5.7 in2




Seismic – 24”x24” x 8’ – 26.9 in2

Seismic – NP4

12 Wind – 10”1.3 in2

Wind – 10”1.3 in2

Wind – 16”x16” x 10’ – 8.1 in2

Wind – NP4



Seismic – 24”x24” x 8’ – 30.4 in2

Seismic – NP4

15 Wind – 12”1.8 in2

Wind – 12”1.8 in2

Wind – 24”x24” x 10’ – 9.4 in2

Wind – NP4


Seismic – NP4 Seismic – 24”x24” x 8’ – 34.1 in2

Seismic – NP4

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Conclusions

Structures elevated on walls are limited by height to thickness ratios

Structures elevated on piers are limited by pier dimension to height ratios

Designs are controlled by the prevailing hazard – wind or earthquake

Large segment of country could be covered by these study constraints and limitations

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Proposed Funding Restrictions Project may be proposed when technical

parameters presented here are met Applicant must demonstrate knowledge of

the prevailing hazard Any parameter not met must be determined

by engineering analysis Buildings in V Zones must be designed Building elevation must be compliant with

most restrictive of ASCE 24, IRC or local floodplain ordinance

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Questions?

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