Guidelines for improving hurricane resilience of homes
by Javil Parris
Structural Engineer and Managing Director
Tropical storms and hurricanes are annual occurrences within the Caribbean region which can result in extensive damage to many homes and other infrastructure.
It is believed that, over the past decades, the destructive effects of such events have been exacerbated due to global warming and climate change. Many scientists propose that the associated warmer sea surface temperatures provide more favourable conditions for the development and intensifying of tropical cyclone activity in our region.
Therefore, it is imperative that much consideration is given to the adoption of sound construction practices which may potentially reduce the level of structural vulnerability. The following are a few guidelines for improving the overall hurricane resilience of homes:
1. When selecting a site for construction, it is best to avoid unregulated areas which may sometimes encompass steep and exposed hillsides and floodplains adjacent to rivers. Such areas may be highly susceptible to landslides and immense flooding during tropical storm and hurricane events. In addition to ensuring that there is adequate site drainage, houses should also be sufficiently elevated to reduce the risk of damage from flooding and storm-driven water.
2. Hurricane damage to houses is often caused by the extreme winds which can exceed sustained speeds of 156mph (Category 5), typically inducing strong uplift forces on roofs. The basic concept involved in ensuring that the structure can resist hurricane-force winds is quite simple. Every major component of the structural system must be sufficiently tied to each other and the foundation so that a direct structural link between the roof and the ground is maintained. Such links are, for example, often facilitated by the extension of rebar of one structural member into another (such as foundation rebar into block wall).
3. One of the areas that have been deemed most vulnerable to such uplift forces in our traditional method of construction (masonry walls with galvanized iron sheeting on timber frame) is the roof-wall-connection. Traditionally, holes have been drilled into rafters to receive horizontal rebar which would be subsequently cast into the ring beams. However, a reoccurring failure mechanism involving the longitudinal splitting of such rafters has been observed over the years and ought to be avoided. The use of hurricane straps at each rafter may provide a suitable alternative.
4. When installing roof sheeting, critical attention should be given to ensure that adequate fastening devices are used at appropriate frequencies. Generally, it has been found that screws, rather than nails, provide superior qualities in resisting uplift forces. The thickness or gauge is also another factor that must be considered when purchasing such roof sheeting.
5. The geometric configuration of the roofs may also contribute to their vulnerability to being damaged. Typically, hip roofs (4 slopes) are known for exhibiting better performance than gable roofs (2 slopes). Wind tunnel studies have also demonstrated that low-sloped roofs induce greater uplift forces than roofs of steeper slopes and therefore, should be avoided.
6. Excessive overhangs at roof eaves should also be avoided which can potentially result in roof failure in severe weather conditions. The introduction of a roof parapet can also, in principle, alleviate such effects.
7. Although costly, impact-resistant doors and windows can be installed where possible to protect against wind and windborne debris forces. Alternatively, physical protection systems such as hurricane shutters may be installed.