"BOLABEACH" -- INDESTRUCTIBLE TO HURRICANES AND SEA-LEVEL RISE

From the outset I situated, designed and built "BolaBeach", our beach house in Great Harbour Cay, Bahamas, to withstand the strongest hurricane ever measured, with a wind of 190 MPH, and twice the worst-case sea-level rise forecast for this century due to global warming. (I'm a mechanical engineer with a Master's from Caltech*).

Flooding. Let's first look at the risk of flooding due to sea-level rise and the waves and storm surge of hurricanes. I selected one of the highest lots on the island, where the house floor is 15 feet above mean sea level, as shown on the top picture at right. The forecast for the year 2100 in the area is for a sea-level rise of 2 feet with a worst-case scenario of 6 feet -- with a floor elevation of 15 feet, the house may be safe from flooding well into the next century! During hurricanes, there is also the risk of high waves and storm surge. These depend on both the strength and direction of the hurricane, and on the degree of blockage created by the land -- high seas pushed onto the South Carolina coast have nowhere to go except up onto the land, leading to high surges, while high seas pushed onto the east coast of Great Harbour Cay (which is only 7 miles long) can go around the north and south ends of the island, leading to low surges, as shown on the bottom figure at right. Therefore, given the 15 feet elevation of the house floor, no flooding of the house is expected even if the worst-case 6-foot sea-level rise occurs in 2100 and a strong hurricane hits then.

Wind. Next, let's look at wind damage, and the design and construction techniques used. After hurricane Andrews devastated South Florida in 1992, the area adopted the most stringent building code in the country, based largely on the lessons learned from the devastation. Structures built under that Code are designed to withstand Category 4 hurricanes of up to 157 MPH winds. The design of BolaBeach 10 years later went beyond that Code, to withstand Category 5 hurricanes, where the highest velocity ever measured was 190 MPH. In compliance with the Code, all windows and exterior doors are impact resistant; the roof plywood, the roofing paper, the shingles and, most importantly, how they are attached/secured were done per the Code; all exterior walls are of concrete block with a concrete tie beam above them and poured concrete columns next to and above all windows and exterior doors, and on the corners of the building; and the roof is of a hip design rather than a gable design.

The measures that went beyond the South Florida Building Code deal principally with the attachment of the roof trusses, as shown with the aid of the figures below. Two different trusses were used on the house, with the larger ones spanning the main house (dark brown in the figure) and the smaller ones spanning the terrace (light brown in the figure). Because the large open terrace leads to a high lifting force in hurricane winds, the terrace trusses were attached to the tie beam that runs around the terrace far more solidly than the Code requires. The east (sea-facing) end of the terrace trusses were attached to the concrete tie beam with two (2) hurricane straps, where the Code requires one; see small picture below. The opposite (west) end of the terrace trusses and the east end of the main-house trusses were connected together and attached to the tie beam, not with the Code-required straps, but with massive iron connectors embedded in the concrete tie beam, as shown in gray below. These connectors were custom-built and each weighs 5 pounds.

And I didn't rely on the builder to adhere to the drawings, because I supervised the construction from start to finish. Virtually every week I flew my airplane to the island to make sure the house was built right.

But the proof is in the history, and for over 20 years the house has resisted several hurricanes, where the only damage was loss of roof shingles and the Tiki hut on the beach.

Ten years later, when the shed was built, we employed similar measures to ensure it would be hurricane resistant; see the extensive use of hurricane straps and stainless steel screws (used to attach the shed to the concrete slab) here.


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* In addition to my two engineering degrees, I have had a 40-year interest in architecture, and have designed and supervised the construction of three houses.