Designing Against Devastation
Contrary to popular belief, a hurricane in itself is not a natural disaster; rather, it’s an uncontrollable natural event which, if unleashed upon an unprepared community, can lead to a destructive, costly and deadly disaster. Thankfully, in our technologically advanced times, destruction due to these kinds of natural hazards is predictable and largely preventable. When it comes to home building, properties can be protected from damage if careful thought and attention is given to good design, quality materials and decent construction.
A hurricane has huge destructive potential due to very high wind speeds, the likelihood of torrential rains producing flooding, and possible storm surges affecting coastal areas. The most obvious and feared component of a hurricane is its severe and complex wind forces, which present special problems for protecting a building. Sustained hurricane-force winds can last for several hours, becoming turbulent and continually changing direction. Every face of the building may be impacted. Last year’s Hurricane Earl was a case-in-point, where the winds in the BVI started in the northeast and gradually swung around the north until the most fierce forces were being thrown at us—some six hours later—from the south west. Under these conditions, no aspect of the structure is spared.
As long as the outer skin, or envelope, of the building remains intact, the wind will flow over and around the structure. As hurricane-force winds intensify, positive pressure is created on the windward faces, and as the wind flows around the building, it causes huge lift or suction—negative pressure—on the leeward faces. The uplift forces from hurricane winds have been known to pull buildings completely out of the ground; hence, the design of the foundations can be as critical as the design of the walls and roofs. Perhaps the most common area of failure is the roof cladding; metal sheeting, tiles or shingles are ripped off, due to poor quality or inadequate fixings.
Loose objects and debris from damaged buildings are picked up and thrown around, creating potentially damaging and terrifying airborne missiles of all shapes and sizes which can be hurled around at all heights. This presents an additional threat to the integrity of the building envelope—the most vulnerable part being the window and door openings. If a window breaks or a door is blown open, the envelope is breached, and wind enters the building, causing a sudden and dramatic increase in internal pressure, effectively doubling the force acting to lift the roof and push the walls outward. The structural design of the building needs to allow for this possibility, with special care given to the connection detail between the roof and the walls.
In the past, Caribbean residents would stick tape across their glass, in a desperate but fairly futile effort to minimise potential damage. Today, most new building owners choose to use thickly laminated impact-resistant glass which, if used in properly designed and well installed windows and doors, will stay in place even if badly cracked. Wooden shutters have been a distinguishing feature of Caribbean buildings since colonial times but are becoming far less common, with the introduction of impact-resistant glass. Another recent alternative to wooden shutters are heavy-duty fabric sheet systems which are custom-made to cover openings and to fix around balconies and decks using stainless steel fasteners; these systems can be fairly easy to install and allow light through, which is a real benefit, in contrast to being shut up in a house with solid opaque shutters.
A commonly held belief is that some windows should be left open during the storm, to allow the internal pressure to equalize with the external, to prevent pressure buildup inside the building. This theory seems to have been discredited, and now we’re encouraged to ensure that all openings are tightly shut. This, however, only increases the need to ensure that things are totally secure and that a sudden failure won’t occur. The other component of the storm, the torrential rain, will be lashing against the building from all directions and will find its way in through even the tiniest of cracks, so it pays to keep things sealed up as much as possible to minimise water damage.
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Thankfully, the quality of construction in the BVI is generally very good. Architects and structural engineers design for the worst case scenario, and contractors are experienced in ensuring that buildings are as robust as possible. Hurricane Earl pounded us with Category 4 strength, but damage to buildings was relatively light. Unfortunately, however, there is still much room for improvement in the management of storm water; last year, far more damage was done by rain than by wind, and we saw some serious flooding, mudslides and sedimentation, wrecked roads and failed retaining walls. It seems that we’re good at ensuring the quality of our structures, but we need to understand that all of our construction and development has a combined affect on drainage patterns, and the land around our buildings is as prone to damage and disruption as the buildings themselves. But that’s the subject of another article…
