Hurricane Resistant Construction Design and Planning
The key message of value to resort and residential developers is that hurricane resistant design and building construction techniques have been refined and do exist. Florida is the primary example of new building codes, which have substantially reduced damages following the onslaught of 2017’s Hurricanes Irma and Maria.
The following article, reprinted from ReidSteel.com, reviews design considerations for hurricane resistant construction:
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.
How can REID Steel 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 minimized.
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.
REID Steel 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 REID Steel hurricane resistant buildings on the Island, all were intact and the contents safe. REID Steel have never had major damage by a Hurricane or Polar Wind in living memory.
C Eng FIStrucE, Director, Reid Steel.