Fortified For Safer Living

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1 Fortified For Safer Living Module 15: Protecting Homes Against Floods and Storm Surge An important part of trying to reduce the damage and losses from storm events is to make sure that the home doesn t get wet, or worse destroyed by surge. In this module we re going to look at flood and storm surge issues and how we address these in the Fortified for Safer Living program. The Fortified criteria for surge and flood is to build so that the bottom of the house is 3 feet above the base flood elevation or the advisory base flood elevation if FEMA has come up with a revised set of guidelines based on recent flood history. We would like to see the home above the 500-year flood elevation, but typically the base flood elevation is based on a 100- year return period, so there s a 1% chance on a yearly basis of reaching or exceeding that flood level. We would like to be at the 500-year level by putting a 3-foot addition on top of the 100-year flood level. We think we re getting close to that value and there really isn t a lot of 500-year information out there. People are working on it, doing statistical modeling to try to come up with better estimates of the 500-year flood elevation, but they re not widely available at the present time. Coastal A-Zone and V-Zone Requirements In coastal A-zone or V-zone, if you re building anywhere that can possibly get storm surge, the Fortified program requires you to design the foundation to surge zone criteria. This reflects a recent change as well in the FEMA requirements for the National Flood Insurance program. They found that in the coastal A-zones where you re out of what s called the V- zone, there was still enough water depth to have wave action coming in. So they decided to make sure all foundations are designed the way they would be in the surge zones. So in the areas where you can get this kind of surge and erosion, we want to make sure the lowest horizontal structural member of the lowest habitable floor is above our Fortified design flood elevation. That is the base flood elevation plus 3 feet or the advisory base flood elevation plus 3 feet. In a regular A-zone where there isn t that surge potential, we just require that the finished floor elevation of the lowest habitable floor is above that level (above the Fortified design flood elevation, which, again, is 3 feet above the base flood elevation or 3 feet above the advisory base flood elevation) IBHS or its affiliates, All Rights Reserved Module 15, Page 1

2 The Three-Foot-Above Requirement There are a few reasons for our requirement to go 3 feet above the base flood elevation. First, it s the maximum point in terms of getting a premium reduction from the National Flood Insurance program. You get the biggest reduction 2 feet above the base flood elevation, but there s an additional reduction as you go to 3 feet above. Second, when we start looking at where the information is coming from, flood insurance or flood levels are coming from the flood insurance rate maps. These maps are based on the best available data, but sometimes that is not terribly good. They are also sometimes based on information from the SLOSH model, which is run by the Army Corps of Engineers and is a computer model that s about 20 years old. There s been a lot of improvement in the technology since it was created, so we re a little bit cautious when we start dealing with the SLOSH model and the numbers coming out of that. Third, these base flood elevations are also based on historical flood levels, and this is a point where we want to make a cautionary word that may ring true to people. After data was brought in from Hurricanes Ivan, Opal, Katrina, and Rita, the base flood elevation advisories in Mississippi went up by more than 6 feet. So that gives you a sense that the historical data really hasn t been that great. As we struggle to get a better idea of what our 100-year return period and our 500-year return period flood levels should be, we really need to be on the cautious side. Elevation as a safety factor. Another thing that s important to recognize is that when structural engineers design for wind and seismic and other hazards, they include safety factors. With flood design, the only safety factor you ve got is elevation. Once you re wet, you re wet. And then you have the problems of mold and other kinds of damage that may come from that. The Sea Grant Rules of Flood Design Because of this need for a safety margin in flood design, some friends of ours at Sea Grant in North Carolina put together a general set of rules to follow. Rule Number One. We ve established that the flood elevation is complex to predict. But it s actually easy to design for it by just aiming to be above it. So the first rule is: Don t get wet. Rule Number Two. Now, we know that the official flood level may be too 2008 IBHS or its affiliates, All Rights Reserved Module 15, Page 2

3 low. Insulation and utilities may be in the floor, and there s no safety factor as we mentioned earlier. Once the water gets up to the floor, you re going to have water hitting insulation and utilities in the floor. So the second rule is: Don t even get close to getting wet. That means pushing the elevation up 2 or 3 feet in our case, for Fortified, 3 feet above the base flood elevation. Rule Number Three. There are other issues to consider when you get into floodways and river areas where you may have flowing water coming into contact with the house. Rule Number Three is two-fold, really: Do not build in a floodway where the water may be coming through. And absolutely do not build in high velocity zones or where water flow may come in at a rapid rate. The force of that flow on the foundation of the house may be sizeable. When you do have a situation where flow may be coming in, try to build an open foundation. If you re in a surge area, you definitely need an open foundation a foundation that s designed so the water can move in and out without taking the house away with it. Also, absolutely beware of building on fill because it may settle and affect your foundation dramatically particularly when it gets saturated from a flood. You can get some very dramatic changes in the elevation as well as erosion when you re dealing with a fill situation. Rule Number Four. In terms of hazards in coastal regions, you have really the worst combination of all things. You have a high wind exposure of the wide open expanse of sea and winds coming in over that water. You have flooding and storm surge hazards to deal with. You have waves that can be hitting and as we ll see in a minute, the waves are really a killer. And then you have erosion that can occur around your foundations as storm surge comes in and out around the building. So where we re heading here is to Rule Number Four. Although shallow water depths do limit the heights of waves to less than six feet under most houses, the rule is this: Don t ever get hit by a wave. Designing for Storm Surge with Breakaway Walls There s been a lot of interest in recent years regarding what you can do with the area underneath an elevated house in a coastal area. Can you park cars, create a storage space or something like that? Many people want to kind of enclose this area in with walls of some sort for usable space. FEMA (specifically, NFIP, the National Flood Insurance Program) will allow walls to be put in, provided they will break away with the storm surge and waves and not create loads on the structure. There s been a tremendous amount of debate about how strong we can make a foundation wall below grade and still have it break away. There ve 2008 IBHS or its affiliates, All Rights Reserved Module 15, Page 3

4 been attempts to try to make the connections weak enough that the wall would blow out under a 20-psf load, but then the walls could blow out in a strong windstorm, even if you don t have a surge event. So how strong can you make the connections without having it create a problem for the structure? The good news is that this issue has been solved. There was a lot of testing that was done looking at breakaway walls, looking at the effects of waves hitting things. And FEMA developed Technical Bulletin FIA-TB-9 provides a lot of the information on how these walls should be designed and built. The FEMA design coming out of this guideline was based on peak storm surge, not breaking waves, and using a 10- to 20-psf range. North Carolina State University worked in cooperation with Oregon State University, and their testing on breakaway walls looked at rising storm surge, breaking waves impacting the walls, and wood-frame construction. They compared these elements and asked: How can we design the wall so it will stay in place with 120 mile-an-hour gusts according to ASCE-7, but break away in waves? They found that they could in fact nail it on all four sides, they could beef things up a bit. They wanted a single wall system that would break away, but they found that they could do a lot more in terms of attaching and still have it break away. Looking at what happens when a wave impacts a flat wall of a structure, you have a still water elevation, a crest some distance above that still water elevation, and the total height of the wave is typically some fraction (about 60% or 70%) of the still water height. So to estimate the total wave height of a 5-foot surge, for example, you would add 3 1/2 feet on top of the 5 feet, for a total of 8 1/2 feet, with a certain part of it above the Stillwater elevation and a little bit of a valley below it. Now what is really dangerous is when air is trapped and impacted on a flat surface. Then you get a very large force being applied to that flat surface. There is much less load when a wave comes around a round pile or a round cylinder than when a wave hits a flat wall. That kind of wave creates tremendous forces IBHS or its affiliates, All Rights Reserved Module 15, Page 4

5 The Strength of Waves Here we re going to have a pop quiz: What is the equivalent force from a 120-mile-an-hour wind? What kind of wave would it take to create the force from that 120-mile-an-hour wind? As we look at trying to keep a breakaway wall in place with 120-mile-an-hour winds, what kind of wave would really take that wall out? The answer is it only takes about a 1 1/2-foot breaking wave to knock out that wall, to create the same force as 120-mile-an-hour wind. So for any waves beyond that (3-, 4-, 5-foot waves), it s not a question of whether it s going to break away, but it s a question of what it s going to take with it. So you want to be sure that you re designing the wall in a way that the wall can let go and not take the columns that are supporting the structure above and so that there is not undue load on the other elements that you want to stay in place. Wave Tank Testing This part of our discussion brings us back to the North Carolina State University research and the resulting design criteria. With the wave tank test and the nailing they went to for the system (nailing around four sides), they ended up with failure with 1 1/2-foot breaking waves. As soon as that wall began to break away, the forces on the structure dropped dramatically and very little was transferred into the piling above ground. So their low moments on the foundations really did reduce things quite nicely once the wall started to break away. Break-Away Wall Design In FEMA Technical Bulletin FIA-TB-9, there is a cookbook for developing breakaway walls that will help you walk through the design by function and not by load. You will try to make sure that the wall breaks away and doesn t apply loads to the rest of the structure. And you want to design the wall to exceed the local code wind loads, because when the waves come in and hit the wall, they re going to take it out anyway. But you want the wall to fail at or near that point so that it won t load up the structure more. So you don t want any real heavy connections; i.e., no bolts. You can connect the wall on all four sides but you also certainly don t want to embed any utilities into the wall structure. When it breaks away, you don t want it damaging utilities. If you ve got any vertical runs for sanitary pipes and so forth, you want to make sure that those are kept out of the wall and are not attached to the wall that s going to break away in any way, shape, or form. You can attach utilities to the pilings above the base flood elevation, but you don t want to have them embedded in the wall IBHS or its affiliates, All Rights Reserved Module 15, Page 5

6 Materials. You should use flood-resistant materials. You can use 2x4 studs 24 inches on center. A single top and bottom plate would be typical, as well as 1/2-inch exterior sheathing and 1/4-inch interior sheathing. The sheathing can overlap the piles and still break away. Other details. In terms of a concrete slab for parking cars on, it should be no more than 4 inches thick. You don t want it to be so strong that it creates a lot of loads on the piles and the house should it get lifted up by the waves or rotated underneath the house. Residential garage doors are okay. Flood vents are not typically required for breakaway walls in the V- zone like they are in other areas. However, some local ordinances and insurance ratings may still require them even in the V-zone, so you need to check local ordinances and local requirements. So the coastal A-zone damage mystery is this: Wood-frame buildings fail if the wave height exceeds 1 1/2 feet. That s the summary coming out of this. Piling foundations and breakaway walls are recommended in all coastal A- zones and V-zones. The ASCE 24 is the guide that Fortified recommends. It is also recommended by FEMA and is the national standard now for developing pile foundations IBHS or its affiliates, All Rights Reserved Module 15, Page 6