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Hurricane Loading

A Practical Approach  –  We can learn a lot from others’ experiences during hurricanes. In particular professionals who are there at the scene and can give us the benefit of knowledge and hindsight, hopefully for prevention rather than comparison.

In September 2009, the Turks and Caicos Islands was hit by a hurricane after a long absence, or to be more precise, they had a tropical storm, a Category 1 hurricane and a Category 4 hurricane in one week.

There had long been a belief on the islands that the Turks and Caicos never get hit by hurricanes because they are protected from the south by the Dominican Republic, so they are therefore safe. This belief was held by a lot of the long-term ex-pats who had not seen a storm on the islands for their duration there or had only seen the passing winds from distant storms. One enlightened gentleman even went so far as to tell me that islands with internal salt ponds did not get hit by hurricanes because of the water in the island. Hmmmm.

The tropical storm that hit the Turks and Caicos Islands actually stopped a lot of people from getting injured during the following hurricanes, as it awoke people to the fact that hurricanes could hit the islands and, as a result, everyone was much better prepared for Hurricane Ike.

 

A series of visits after the storms to Grand Turk and Salt Cay, which are the two islands that got the worst of it, revealed plenty of roofs off buildings and debris everywhere. There were cases where the whole roof had blown off some of the buildings and a closer inspection revealed that this occurred to roofs without strapping and roofs where hurricane clips had been used. I had also noted on the Grand Turk visit that in some cases the strapping had also failed in tension. I did not, however, notice any damage to buildings where the rafters had been built into the roof ring beams. To understand the need for roof strapping, the forces involved need to be understood. As any wind passes over a roof, a force is developed perpendicular to the line of the rafters on the roof. This is much in the same way as lift is developed in an aircraft wing. To resist the uplift forces, the roof must be strapped down to a weight that exceeds the uplift of the wind. Obviously the higher the wind speed the higher the uplift forces, and the stronger the strapping needs to be to hold the roof down. Engineers, therefore, try to provide a continuous load path between the roof and the foundations. It is no good strapping a rafter to a timber beam at eaves level if that beam isn’t also strapped down.

Block wall and concrete construction provides a good, safe way to build a house as it is intrinsically heavy (and thus resists uplift) and also provides good lateral resistance. We had noted that after the storms, the only walls that had blown over during the hurricanes were the walls that had not been completed and had not been capped by a beam (to tie the wall together).

The construction process normally involves the design stage and the construction stage. When you decide that you do want to build a house, it is normal to have an architect and engineer employed to translate your wishes into a practical property. The engineer’s responsibility in this process is to give the details for the materials and the strength of the structural items. They will tell your contractor the amount of steel required in a slab and the size of the beams and columns.

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As an engineer, I am always trying to impress on people the need not only for good design work but also that as an industry we are responsible to build for durability. This is fundamentally true when strapping down a roof. Galvanized hurricane straps may be strong enough on day one of their use but are extremely likely to corrode, especially when exposed, and the corrosion of the straps inevitably leads to a loss of strength and subsequent failure of the strap. Strapping and their fixings should also be of the same composition. If two differing materials are placed together, then bi-metallic corrosion can occur.

It is important that nothing is introduced into the building at the design or construction stage that can be deleterious to the construction.  An example of this is construction using beach sand in concrete. Examples of this can be seen around the islands when you see the cracking in the concrete and the exposed (and corroded) reinforcement visible.  Iron oxide (rust) occupies about seven times the volume of the steel that it replaces, and the expansive forces associated with the corrosion cause localized tensile forces in the concrete resulting in cracking in the concrete and then further exposure of the steel and an accelerated degradation process.

 

Once the design is done, and the approvals have been obtained, you need to have a contractor that you can work with. It then normally falls on the engineer to ensure that the design requirements given via the drawings and specifications are met on site. This process should be undertaken with sufficient checks to ensure that the materials are strong enough to take the loads expected during the life of the building. Concrete is available in various strengths, and it is important that the concrete specified on a project is not only strong enough for the use but also provides the necessary protection for the embedded reinforcement. Concrete is very strong in compression but comparatively weak in tension. As a result in areas of a slab, beam or structure where direct tension, or tension due to bending, occurs then the engineer will design reinforcing steel (which is strong in tension) to control the forces that develop.  The concrete (which is naturally alkaline) also helps to protect the steel from the aggressive atmosphere on the islands. The result is what is referred to as the cover of the reinforcement. The (usually) inert concrete is placed between the reinforcement and the atmosphere, and provides an inert barrier to prevent the corrosion occurring to the steel. 

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