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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. Why This Topic. Currently no technical guidance on limitations
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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
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
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’
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
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
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
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
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
Limitations con’t • Snow • Upper bound snow load is 30 psf • ASCE 7-10 snow map used • Map shows the shaded area excluded
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
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
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