How to Make Buildings Resistant to Hurricanes

What do we mean by a Hurricane?

The earth has an atmosphere which extends about 600 miles (1000 km) high, which is dense near the surface but gets thinner as it gets higher. 80 % of the air is packed into the bottom 7 miles (11 km). Most of the weather phenomena happen in this 80 % of air. When the Earth spins, it tends to induce movement of the atmosphere. Close to the Equator, the tendency is for the air to move westward: from Africa towards the Caribbean and the Americas; and from Central America towards South East Asia and Australia; and from there back towards Africa and the Indian sub continent. Further away from the Equator, the return trip for this moving air goes back to the East, in 4 streams. These are at high level, at about 30 degrees and 60 degrees North and South of the Equator. These are very high speed air streams about 7 miles up, called the Jet Streams. The change of direction, from West near the Equator to East further away, induces a little spin to the air masses near the Equator.

The earth is warmed by the sun, more in the Northern hemisphere from April to October, more in the Southern hemisphere from October to April; and more within 7 degrees to 20 degrees North or South of the Equator. Although Ocean Currents move the water around, the tendency is for water near the equator to warm up during the summer months. As the water gets warmer, it can pass more heat to the air above it; and more water evaporates into this air. Air that is being heated becomes less dense and tends to rise. The varying patterns of currents and depths of water and areas of continent mean that the movement of air masses, to the West near the Equator, back to the East within the high, cold Jet Streams, is not smooth but erratic. Where water is wide but shallow, and has little current from colder areas, the temperature difference between the water and the cold upper air can become greater; and it is temperature difference, as well the amount of hot material, that powers a heat engine. It is probable that Global Warming is increasing the amount of heat in the sea, and temperature differences.

Now and then conditions get right for the formation of a tropical storm. The warm water heats the air and its starts to rise, and in doing so, the spiralling tendency it has is increased. The warm air will be very moist; it starts to condense and in doing so, gives up its 'heat of evaporation', the heat the sun applied to the water to make it evaporate in the first place. As the warm air spiral goes up into colder air, it becomes, relative to the surrounding cold air, even warmer and lighter, so it rises and spirals faster. The swirling air, being lighter, weighs down less on the air below, sucking in more warm moist air from the surrounding sea. Sometimes, the Eastward flow of a Jet Stream can catch the top of the spiral and drag air away from it, reducing the pressure in the spiral even more. This enormous heat-engine, powered by the heat in the sea, is a tropical storm.

In addition to the violent wind, the very low air pressures have another trick: they can suck the sea-water towards the core. The sea level can go up to several feet above the normal level. Naturally, the extreme winds whip this extra-high surge of water into extra-high waves. When this surge, and the waves in it, hit a coast-line, the effects on flood defences and on buildings can be more serious than the damage from the wind. The storm has one more trick: all the water in the moist air that is sucked up has to come down somewhere. At first, the air is going up so quickly that condensing water cannot fall down, but is instead carried upwards. Droplets splash together, becoming bigger and bigger; until they are thrown out of the spiral and can fall; or until they become heavy enough to drop through the upwards rush of air. The result is intense rainfall. If the droplets go high enough, big drops can freeze into hailstones (releasing the heat of melting, again helping to power the storm), which can grow to the size of footballs before they drop.

The storm is a swirling mass of cloud, which can be up to 600 miles (1000km) across. At its centre is the 'eye', a spinning tube of clear air whose centre can be slow-moving; but the sides of this eye can spin, spiralling upwards, at enormous speeds, pumping up the moist air. How intense the storm becomes depends on a combination of factors. The right position; a big enough, warm enough sea; the conditions in the upper atmosphere; all contribute to the size and ferocity of the storm. Every ocean in the world, from 7 degrees to 20 degrees from the Equator, can produce these storms (except, oddly, the South Atlantic, where they never occur, perhaps due to the ocean currents, which do not allow the build up of big enough, warm enough, areas of sea). These storms are carried along, fairly slowly, by the air circulation in the atmosphere. The storm dies out only when its heat source is removed, when its core goes over land; or goes over cooler water. Funnily enough, within about 6 degrees North and South of the Equator, just where you'd like a breeze, the air hardly moves at all. The initial circulation needed to start the ball rolling just isn't there.

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How strong are Hurricane force winds?

On the Beaufort Scale, a Force 8 Gale has wind speeds between 34 to 40 knots (Nautical miles per hour). (Multiply by 1.15 to get miles per hour, 39 to 46 mph; multiply again by 1.6 to get kilometres per hour, 62 to 74 km/h). A force 9, Strong Gale, is 41 to 47 knots; a force 10, Storm, 48 to 55 knots; a force 11, Violent Storm, 56 to 63 knots; and force 12, 64 knots and above, is a Hurricane. (74 mph, 118km/h). The Beaufort Scale gives up here.

The Measurement is taken over by the Saffir-Simpson Scale. A Category One Hurricane has wind speeds of 64 to 82 knots (74 to 95 mph, 118 to 153 km/h); a Category 2, 83 to 95 knots (96 to 110 mph, 154 to 177km/h); a Category 3, 96 to 113 knots (111 to 130 mph, 178 to 209 km/h); a Category 4, 114 to 135 knots (131 to 155 mph, 210 to 249 km/h) and a Category 5, 135+knots, (155+ mph, 249+ km/h). The force of a wind varies with the speed squared. So a category 5 hurricane can be say (135 x 135) divided by (34 x 34) = 16 times as strong as a Force 8 gale!

In addition to the wind provided by a Hurricane or Tropical Cyclone, the conditions often permit the formation of Tornadoes, vortexes which drop down from the storm. These can have speeds locally of over 180 knots, with great suction and pressure zones close together near the spinning inner tube of the Twister. Wind is slowed down when close to rough ground, and goes faster higher up away from the ground. Long open spaces, airfields, lakes, sea allow the full force of the wind. All upwind obstacles tend to slow it down. Polar winds are caused in different circumstances but can be as fierce as Hurricanes.

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Why do buildings and homes fail in Hurricanes?

When a building stands in the path of the wind, the windward wall tends to block the air, and the air pressure here increases. How high is the pressure? Well, in a 135 knot wind (Cat 5) the pressure can be over 65 pounds per square foot (300 kilograms per square metre), which is equivalent to 6 full cement bags per square metre! This force can cave in walls, smash doors or columns or bracing systems, or push buildings completely over. And a hurricane cannot be relied on to attack a building from only one direction. As it passes over, the wind can come from every direction.

On the other surfaces of the building, the same wind applies suction. On a building with 4 walls and 2 roof slopes, it would be normal to find pressure on the windward wall, and various amounts of suction on the leeward wall, the 2 wind-along walls, and on both faces of the roof (unless the roof slope faces windward and has a slope greater than about 30 degrees, when it can also experience pressure). If there is an opening in the windward side, the high pressure can find its way into a building. Here it will blow through partitions, and maybe through the far leeward wall. The whole building can be blown outwards, like a balloon, until something fails and it bursts. The internal pressure is helped by the external suction on all the surfaces other than the windward wall. Whole roofs can be lifted off and dropped elsewhere.

There is a twist in the wind attack. As the wind tries to escape from the dam made by the windward wall, it rushes around the edges of the building, creating a series of small vortexes (or vortices). These are small ice-cream cone shaped spirals of air, which have intense suction at the tip. If the pressure on the windward face was 300 kg/m2, the suction at the tip of a vortex could be more than 600 kg/m2, twice as much. These vortexes are not a constant pressure; they only cover a small area; and they decay (diminish in intensity) as they leave the edge (the corners, the eaves, the barge boards, the ridge). They nag away at these edges like demented housewives. They will tear off tiles and shingles. They will pull loose gutters and corner flashings. If the building has sheeting or plywood sheets, it will tear fiercely at the edges, near every discontinuity.

It is not a steady pressure but shaking, like a Terrier with a rabbit. If a fixing or a bit of cladding fails, a corner of the sheet will lift and flap. The vortexes now have a bigger, protruding, edge to work on, and the next fixing will tear, too. The bit of material being tugged gets bigger, the forces on the next fixing are bigger, too, and the whole sheet can fly off. The next bit of sheet becomes the new leading edge. The entire cladding of a roof or wall can be torn off, a bit at a time; or even rolling up like a kitchen towel. If there is an overhang near the roof line, on a wall exposed to pressure, then the overhang gets not only the tearing upwards from the vortex, but the full whack of the pressure from underneath. Hits from flying bits of building are a major cause of death and injury; and bits of building become weapons, which help smash other buildings, too.

The rain-fall by itself can damage structures by washing away poor quality walls; and by overflow cutting under foundations; or the shear weight of water on so called flat roofs. Hail can smash down even quite strong roofs. The rain is travelling horizontally in the wind and gets in everywhere, damaging all interiors. Rain causes floods and floods cause land-slides. Hurricanes cause sea surges and waves. The effect of these threats can be more damaging than the rain. Each has the effect of thrusting a wall of water towards a building. The water is much slower than the wind, but of course much heavier. The effects are similar, though. The pressure can push in walls and doors and windows and frames. The suction from the vortexes around the corners can rip off cladding. Whole buildings can fall over or be shoved into the next street. People drown.

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How can we make buildings and homes resist hurricanes?

The cladding has to be designed to resist the maximum wind pressure from all directions. The cladding has to apply the pressure loads to a structure. The structure has to be strong enough to resist the cumulative loads from all the cladding. It has to be braced or framed to pass all these loads to the ground. The foundations have to be big enough to resist uplift and sideways load. It helps to reduce some of the loads if there is some sort of venting for internal pressure, which can be caused by failures of windows and doors on the windward side. A length of ridge vent, away from the building ends, always has suction on the outside, so can help reduce unwanted pressure; but remember that any internal suction has to be added to the external pressure on the windward face.

Suction loads generally are less than the pressure loads. But most components of buildings are stronger at resisting pressure than at resisting suction (tiles can easily be blown upwards, but it is much more difficult to blow them down through the roof. Pressure would have to crumple sheeting through the purlins or rails, but Suction only has to tear at the fixings. Purlins and rails have their compression flanges restrained when pressure is pushing on the sheets, but do not, and are weaker, in Suction. Beams and columns are similarly restrained by the rails on their outer flanges). So the cladding fixings, and the cladding, and the purlins and rails, and the frames and bracing system, just have to be strong enough to resist the forces.

Suction loads in the vortex zones can be very high, locally. These zones are close to every discontinuity: the eaves, the corners, the gable peaks, the ridges, at slope changes. Bits that stick out, such as chimneys, get the worst combinations of loads. In these zones it is not just suction but a constant shaking, which can tear at exposed edges and the fixings near these edges. The materials and their fixings have to be more robust and more numerous around these edges. All the structural members have to be strongly fixed to the frame and then to the foundations, to prevent them flying off, and becoming missiles. A good roof slope will help shed water; and can reduce wind forces.

How a building can resist flooding is best demonstrated by the 2004 Tsunami. All the fragile shacks built at ground level were simply washed away. Multi storey buildings that were weakly built with no side-sway resistance were badly damaged. Some multi storey buildings had their lower wall pushed in on one side, and out on the other as the wave went through, but otherwise, survived. Some buildings were pushed along where they were not fixed firmly to firm ground. But well-built buildings survived in the middle of areas that were otherwise completely devastated.

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How can REIDsteel make Hurricane Resistant Buildings?

A suitable shape is a good start. Parapets all round to hide the roof slope provide abrupt discontinuities at the highest position on the buildings. They will suffer from enormous vortex loads trying to rip them to bits, and can more than double the effects of the wind on the frames and foundations. Internal eaves gutters should be avoided because no gutter or down-pipe system can cope with the intensity of rain or hail in a hurricane. This is even worse where parapets place a dam across the rain (which is horizontal) or can trap hail. Valley gutters also will prove a problem. If a parapet is deemed essential, it should be on a stand-off, allowing external gutters and some ventilation gap. A good roof slope is desirable, preferably between 12 and 14 degrees. But on wide spans this has the detrimental effect of making the building too high so 8 degrees may be a minimum. Curiously 10 degrees appears to make wind loads more severe, so should be avoided.

If there is a need for exposed canopies, and if they are at roof level, then they will suffer from very high pressure from underneath coupled with very high suction loads above. But if these canopies are lowered to say 2/3rds of the height, they are all within the fairly static air dam zone and will have fewer loads. The lower the building, the less the wind intensity and the less the area on which it acts, un-necessary height should be avoided. A ridge vent well away from either end of the ridge will help reduce internal pressures. It should not be placed close to the gable peaks because it will be blown off. It should be able to resist very heavy rain in low wind. In high wind, air is always passing upwards through such a vent, which helps reduce leakage. Alternatively self-opening vents on all sides may help.

A common style in hurricane countries is to have hipped roofs. The reason for this is that these buildings have survived. The hipped ends are lower than a tall gable, so get lesser wind loads on a lesser area; and the hip rafters make an effective wind bracing system. A 14 degree slope is best. Most structures are designed for downward loads, and are then tested for wind resistance. Often the wind members are of the nominal variety with poor connections and load paths. In Hurricane areas the main design is for wind resistance. This has to be seen as a proper engineering design, with correctly made connections and straight-forward load paths to bring the loads to ground; to foundations that are designed to resist these loads.

Doors or openings should be kept away from the intense vortex zones within 5 feet, 1.5 m, from corners or other discontinuities. All unnecessary discontinuities should be avoided. A building which looks simple and sleek to the eye will look that way to a hurricane. The cladding should be strong and supported at close intervals. The number of fixings needs to be greater than the number required to resist the maximum suctions. Remember that it is the constant shaking caused by transient vortexes which tears the cladding off. At each discontinuity there should be even more fixings (two in each trough at the eaves, for example). If there are flashings, at the corners and up the gable peak barge boards, then these should have very frequent fixings on both legs, and the areas of flat sheet should be minimised.

To resist the rain, steeper roof slopes (not less than 8 degrees, possibly up to 14 degrees) should be used. Gutters should be designed so that they overflow outwards for the full length. If they overflow only at a low point, the volume of falling waterfall can wash away the soil. They should never overflow to the inside, and one cannot rely on any internal or sub surface drainage system being able to remove all the water. If internal or valley gutters are needed they must be over-sized and must have big overflow systems at the ends. To avoid floods and surges, the building should be built out of the projected water path; and this may mean building it on legs with a suspended lower floor level. Even if the elevation of such a floor is modest, the forces from rushing water will be much less if the water can go under the building as well as round it. Such suspended floors will also act as bracing for the whole structure, and will help it resist seismic loads as well as Hurricanes. The cost may even be less than a reinforced concrete slab on the existing ground.

REIDsteel hurricane resistant buildings have been built all around the world in all the high wind areas: Pacific Islands, Mauritius and Madagascar, Philippines and South East Asia, all over the Caribbean and Central America (As well as Iceland, Greenland, the Falklands, South Georgia and the Antarctic). They always embrace the best design principles, and they survive Hurricanes. The Island of Antigua was hit in close succession by Hurricanes Luis and Marilyn. These were intense, went all the way round through 360 degrees, and lingered a long time. Almost every building on the Island was damaged. But of the 40 odd REIDsteel hurricane resistant buildings on the Island, all were intact and the contents safe. REIDsteel have never had major damage by a Hurricane or Polar Wind in living memory.

Rollo Reid
C Eng FIStrucE, Director, Reid Steel.

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