CRITERIA FOR MAXIMUM ELEVATION OF RESIDENTIAL BUILDINGS IN FLOODPLAINS William L. Coulbourne, P.E.

1. CRITERIA FOR MAXIMUM ELEVATION OF RESIDENTIAL BUILDINGS IN FLOODPLAINS William L. Coulbourne, P.E. Applied Technology Council bcoulbourne@atcouncil.org Manny Perotin, P.E. Atkins Global manny.perotin@atkinsglobal.com

2. 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

3. A Simple Example

4. 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’

5. 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

6. 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

7. 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

8. 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

10. 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 • Map shows the shaded area excluded

12. Limitations con’t • Snow • Upper bound snow load is 30 psf • ASCE 7-10 snow map used • Map shows the shaded area excluded

14. 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

16. 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

17. 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

18. Questions?