Basement Slab Crawlspace 80% 70% 60% 50% 40% 30% 20% 10% 0%

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1 Mitigation of Flood Damage through Raised Wood Floor Construction Catherine Marx Kaake Director, Engineered & Framing Markets Southern Forest Products Association Kenner, Louisiana, USA Vijaya Gopu Distinguished Scholar School of Science and Engineering, Tulane University New Orleans, Louisiana, USA Summary Over the past few decades the percentage of the U.S. population residing in the coastal regions of the country has increased dramatically. This increase has led to a significant increase in the number of homes subject to extreme events. The coastal regions of the U.S. are highly vulnerable to weather related phenomena such as hurricanes and flooding. The recent flooding of New Orleans after the failure of the levees during Hurricane Katrina and the incredible loss of property resulting from this flooding, are clear evidence that the approach of building homes on slabs-ongrade in flood-prone areas has been nothing short of a catastrophe. This paper discusses the raised wood floor construction approach and the myriad of savings stemming for utilizing this construction method. The different methods of constructing raised wood floor structures are discussed along with the details of the connections needed to resist uplift and lateral forces. Introduction Between 1992 and 2006 the number of single-family homes built in the U.S. tripled from 600,000 to nearly 1,800,000. Nearly fifty percent of these homes were built in the Southern part of the U.S... A significant percentage 72% in the South and 56% in the all of U.S. -- of the homes in this country are being built on slab-on-grade which became popular in the late 50 s and early 60 s. Figure 1 shows the steady growth in the concrete slab market share in the Southern U.S Market Share Basement Slab Crawlspace 80% 70% 60% 50% 40% 30% 20% 10% 0% Fig. 1 Growth in the market share of Slab-on-Grade Construction Even in the early 60 s a significant percentage of single-family homes in the South were built on raised floors supported on piers or foundation walls or a combination of them. While the slab-on

2 grade construction approach is appropriate where the land is not subject to inundation, it has evolved into an acceptable and preferred construction method all across the Gulf Coast region where large swaths of land are prone to flooding. The availability of flood insurance at very reasonable cost and the failure of the government agencies in setting minimum slab elevations that ensure minimal potential for flooding have promoted and abetted the construction of homes on slab-on-grade even in areas with high risk of flooding. The numerous hurricanes that have hit the Gulf Coast region over the past two decades have caused enormous amount of destruction of single-family homes either due to flooding or wind. A case in point is the destruction of nearly a quarter of a million homes in and around New Orleans due to storm surge and flooding caused by the failure of the levees during Hurricane Katrina (Figure 2). A very significant portion of the homes that flooded were on slab-on-grade. Had these homes been built on a raised floor system set above the flood elevation, a costly national tragedy could have been avoided. Fig. 2 Flooding of Slab-on-Grade Homes in New Orleans after Hurricane Katrina Given the enormous cost imposed on the society by the choice of a very inappropriate construction approach, it is critically important that the home owners, builders, code officials, floodplain managers and designers be educated about the benefits of raised floor construction and the methods for building these raised floor systems. 2. Advantages of Raised Floor Construction Raised floor construction offers a myriad of benefits to the home owner. These advantages include: Reduced Flood Risk and Lower Flood Insurance Premium: Homes with floors raised above the base flood elevation (BFE) enjoy a lower risk of flooding and an associated lower flood insurance premium. The insurance premium drops by more than 50 percent for single-family homes built a minimum of 3 feet above the BFE. Figure 3 shows raised floor home that is undamaged by flood waters that have inundated the lot.

3 Easy Home Improvement: Raised floor constructions offers the flexibility of changing the floor plan at reasonable cost since the relocation of utilities is relatively simple and economical compared to slab-on-grade systems. The crawl space provides easy access to utilities below floor (Fig. 4). Fig. 3 Undamaged Raised Floor Home in Flood Waters Reduced Grading of Land: The raised floor system eliminates the need for cut and fill that is required for slabs placed on sloped lots. Site drainage is easier to provide with raised floor since extraordinary measures are not needed. Adaptable for Poor Soil Conditions: Raised floor systems provide the ability to control and correct movement due to expansive clays or subsidence of organic soils. Fig. 4 Accessible Crawlspace in Raised Floor Construction Extended Living Space: The presence of a raised floor makes it convenient to add a porch or a deck because they are natural extensions of the elevated platform. Better Pest Control: The presence of crawlspace makes it more convenient to inspect and control pests. Also, inspection of the structure for termite problems is significantly easier in raised floor systems. When termites enter a structure through cracks in slab it is difficult to recognize the problem until after significant damage has occurred. Green Building System: Raised floor is a sustainable construction system since wood, a renewable resource, is used in lieu of steel and concrete. Modifications to the structure can be made with relative ease to extend the service life of the structure. 3. Foundation System for Raised Floor Construction The foundation system used for raised floor construction depends on soil conditions bearing capacity and compressibility performance criteria and proposed loading conditions. The foundation system also depends on the depth of the load-transfer member below the superstructure and the type of load transfer mechanism chosen. In most coastal high-hazard areas, the structures are elevated to considerable height to keep them above the base flood elevation

4 (BFE) and from being impacted by possible wave action. In these situations deep foundation systems of driven timber, concrete or steel piles are used (Fig. 5) Fig. 5 Elevated Floor System on Concrete Piles A very significant percentage of homes in the coastal areas that are not subject to storm surge can be adequately protected with raised floors on shallow foundations. These shallow foundations are comprised of footings and piers. The typical footing types include: Spot or Isolated Footings Continuous Spread Footings Grade Beam Footings The piers that elevate the floor system can be built of brick, concrete block or precast concrete. Also, these piers can be isolated or substituted by continuous foundation wall referred to as stem wall -- resting on a continuous footing. Figure 6 shows the various types of footings and piers that are used for shallow foundations in raised floor construction. Steel dowels are used to anchor piers and stem walls to the isolated pads and spread footings. Fig. 6 Footing and Pier Types Used in Shallow Foundations for Raised Floor Construction There are two types of foundations used for raised floor systems; they are pier-and-beam foundation and stem wall foundation. These two foundations are discussed in the following sections.

5 3.1 Pier-and-Beam Foundations Piers in this foundation system are commonly constructed of reinforced masonry (brick or concrete block) resting on isolated concrete footings also referred to as pads -- or continuous reinforced concrete spread footings. The piers are spaced based on the floor framing and location of the bearing walls and partitions. The spacing of the piers is typically in the range of 2.4 meters to 3.5 meters. The open space beneath the floor system permits natural ventilation of the crawlspace and prevents moisture accumulation. The layup of the beams and floor joists at an interior pier is show in Figure 7 and the anchorage of the beam and the floor joists at a perimeter pier is shown in Figure 8. The anchors for the beams and floor joists to the foundation system can be easily designed to resist the uplift forces induced by the wind loads on the structure. The key to proper design of the raised floor system is ensuring that there is load path for both uplift and lateral loads to the foundation. A foundation system with isolated masonry piers supported on a continuous spread footing is shown in Figure 9. The floor beam and joist layout in a typical isolated pier foundation system and the connections to resist uplift are shown in Figure Fig. 7 Interior Pier Detail Fig. 8 Framing Anchorage at Perimeter Pier Fig. 9 Isolated Masonry Piers Supported on a Continuous Concrete Footing Fig. 10 Floor Beam Set on Isolated Piers

6 Fig. 11 Anchorage of Floor Beams to an Interior Pier and Floor Joists to Beams Fig. 12 Anchorage of Floor Beam to Perimeter Pier 3.2 Stem Wall Foundations These foundations are constructed of reinforced masonry or poured concrete, supported on a continuous reinforced concrete spread footing. Stem wall foundations often include interior spot piers to reduce the span of the raised floor system. The stem wall eliminates the need for perimeter beam since the floor joists are supported directly on the wall. A higher level of uplift resistance can be achieved relatively easily for floor systems bearing on stem walls. The details of the anchorage of the floor joists to the stem walls is shown for a typical concrete foundation wall (Fig. 13) and a masonry foundation wall (Fig. 14). A raised floor system supported on stem walls on the perimeter and on isolated piers in the interior is shown in Figures 15 and 16. When stem wall are used to create the crawlspace, particular attention needs to be paid to ensuring this space is properly ventilated to avoid moisture problems. Fig. 13 Connection Details at Concrete Stem Wall Fig. 14 Connection Details at Stem Wall

7 Fig. 15 Raised Floor Structure Supported On Stem Wall and Isolated Piers Fig. 16 Installation of Raised Floor on Stem Wall 4. Vibration of Floors in Raised Floor Structures An important performance issue in raised floor construction is floor vibrations, i.e. bounce. While this is not a safety related issue, it is nevertheless an important performance issue. Since perception of vibration varies from person to person, it s critically important that attention is paid to reduce floor vibrations. There are several measures that can be taken to accomplish this reduction. These include: Proper fastening of sheathing to floor joists: Floor sheathing should be glued and screwed to the floor to improve the stiffness of the floor system and minimize squeaks, vibration, bounce and nailpopping. Failure to do this during construction is a common cause for owner complaints and dissatisfaction. Use of Shorter Joist Spans: Reduced floor spans result in smaller deflections in the floor system and hence less bounce. Reduction in floor spans should be done judiciously since it can add to the cost of construction by increasing the number of interior piers and floor beams. Use of Deeper Floor Joists: Increasing the joist depth by one size can significantly reduce the deflections and bounce in the floor since even a slight increase in joist depth contributes to a sizable increase in the floor stiffness. While this approach does increase the cost of construction, the small premium paid for the deeper joists can be often justified since it does deliver a superior and stiffer floor system. Reduce Joist Spacing: A reduction in the joist spacing leads to smaller loads being imposed on the joists and therefore to smaller deflections in the floor system. Also, since bounce occurs as a result of a foot impacting an additional joist, smaller spacing of joists would allow the shock of a foot to be carried by two joists. 5. Industry Response and Promotion of Raised Floor Systems The damage inflicted by a series of powerful hurricanes over the last decade and the devastation caused by Hurricane Katrina and Rita, have led the North American wood products industry to launch an aggressive effort to promote the use of raised wood floor structures, in particular in the coastal regions that have experienced the wrath of mother nature. Better understanding of the benefits of the raised floor construction on the part of the home owner, builder and the designer is expected to not only benefit the industry but reduce a huge burden from being regularly imposed on

8 the tax payers who end up underwriting the cost of recovery of the region affected by the hurricanes. 6. Summary and Conclusions Raised floor construction is ideal for all structures built in flood prone regions regardless of the whether or not the local codes allow a slab-on-grade system or not. Even minimal flooding in a slab-on-grade home can lead to significant insurance and homeowner losses. Raised floor systems do not demand special skills for construction and provide considerable benefits to the home owner. The availability of a variety of anchors/fasteners makes the design of raised floor structures to resist the uplift forces caused by wind and flooding a relatively easy task. With minor effort the raised floor construction can be rendered superior to slab-on-grade construction. Raised floor construction leads to significant saving in energy since it uses more lumber and less concrete and steel that requires considerable energy to produce. The raised floor homes promote green and sustainable construction. If all the destroyed homes in the Gulf Coast region were rebuilt with raised floors, the demand for lumber would have increased 1.44 billion board feet. If every slabon-grade home built in 2006 had used a raised floor system, then the demand for lumber a renewable resource -- would have increased by 3.8 billion board feet -- a material that is totally renewable. 7. References [1] Federal Emergency Management Agency, "Recommended Residential Construction for the Gulf Coast." FEMA 550, July 2006 [2] Southern Pine Council "Raised Floor Systems." March 2005 [3] Rogers, S.M., "Designing for Storm and Wave Damage in Coastal Buildings." Proceedings of the 22 nd International conference on Coastal Engineering." July 2-6, 1990 [4] American Forest & Paper Association, American Wood Council, "Wood Frame Construction Manual for One- and Two-Family Dwellings." October 2001 [5] Federal Emergency Management Agency, "Engineering Principles and Practices for Retrofitting Flood Prone Residential Buildings." FEMA 259, January 1995 [6] Ayscue, J. K., "Hurricane Damage to Residential Structures: Risk and Mitigation." Natural Hazards Research and Applications Information Center, Institute of Behavioral Sciences, University of Colorado Natural Hazards Research Working Paper #94. November 1996.