Dorian reconstruction recommendations and the Bahamas Building Code
In recent years, The Bahamas has experienced a number of major hurricanes, including Hurricane Matthew in September 2016, with maximum sustained winds of 165 miles per hour (mph) and Hurricane Dorian in September 2019, with maximum sustained winds of 185 mph and reported gusts of over 200 mph. Both were Category 5 hurricanes. The impact from these hurricanes varied from minor to significant/catastrophic. There was damage to homes, public and commercial structures and infrastructure including roads, electrical and potable water systems, bridges, docks, communication towers as well as the natural environment.
Following the passage of Hurricane Matthew, it was generally agreed among professionals of the built environment that the wind damage to the housing infrastructure was due to the lack of enforcement of the Bahamas Building Code (BBC). This lack of oversight was found predominantly in locations outside of New Providence and Freeport, where there were no resident buildings control officials and where buildings and structures, many of which were substandard, were being erected without regulations.
Fast forward to the passage of Hurricane Dorian in September 2019, the strongest Hurricane ever recorded in The Bahamas. Following a preliminary assessment of the damages to the housing sector, it was reinforced (no pun intended) that enforcement of the Bahamas Building Code was also an issue in Abaco and Grand Bahama, in particular, areas outside of Freeport. Also, because this hurricane had sustained winds above the minimum stated in the present BBC (150 mph) there was a clarion call to revise the BBC.
When the Bahamas Building Code was first introduced in 1971, it was based on the South Florida Building Code. This came about because of the massive damage inflicted by hurricanes over the years and the general lack of regulation seen in the construction industry at that time. A favorite story recounted by a former Buildings Control Officer John Steer was that during the assessment of the damage after a hurricane, it would be discovered that flattened soda cans were used as truss plates. The Bahamas Building Code has since been revised only twice, first in 1987 and then in 2003, more than sixteen years ago.
Building codes are based on scientific documents, research, environmental conditions and cultural practices of communities to ensure that habitable spaces used for work and living are healthy and safe. Many jurisdictions update codes on a regular basis to reflect technological advances and changes in our environment (think climate change).
Although the BBC references many other codes like the Canadian Electrical Code and American Society of Civil Engineers, the length of time since its last revision is considered excessive and therefore revision to this code should be considered as a necessity.
Revisions to the BBC, with linkages to even more codes, will allow us to address issues of sustainability and energy efficiency to ensure the future orderly development of our country.
During the passage of Dorian, significant damage was caused by flooding due to storm surge which peaked at over 20 feet in extreme cases. Flooding, whether due to rainfall or storm surge is addressed in section 302.1 (a) of the Bahamas Building Code: “The level of the lowest floor should be at least 12” above any known flood level or 18” above crown on adjacent road whichever is the greater.” If just this statement had been complied with, we would have seen a significant decrease in damages due to hurricanes.
Although the science is fairly complicated, flooding and storm surge can be predicted with a fair degree of accuracy on any given island in The Bahamas using computer generated models that have been established worldwide. We should also have collected a fair amount of historical data that can be used to improve the accuracy of these computer models. Most environmental engineering firms with specialization in coastal protection and modelling can provide accurate forecasting to help with our future planning and development. We must not repeat the mistakes that we made in the past by ignoring recommendations from these experts.
Mitigating solutions against 15 to 20 feet flooding due to storm surge could include not rebuilding on low-lying land that can be affected by flooding or elevating the building structure above known levels of flooding as mandated by the BBC. These options are untenable to average Bahamians who have already made investments in these low lying areas and are financially struggling to make ends meet. With expected sea level rise, coupled with daily inundation of these marginal areas where the majority of the population reside, this storm surge flooding presents a special urgent challenge to our people and future governments.
Traditional construction of masonry structures utilizing tie columns and beams as mandated by our building code provides for a very robust structure, has served us well and is more than adequate for the wind loads as stated in the code and also wind loads that we have experienced to date. I have not seen any evidence, neither photographs nor reports that show significant wind damage to masonry structures built according to the current Bahamas Building Code.
Roof structures generally are more vulnerable to wind damage and are typically constructed using timber framing enclosed with asphalt shingles and plywood. Roof spans less than thirty feet can be hand framed by an experienced builder with final inspection by a buildings control officer, a professional structural engineer or an experienced professional architect, without issues and will be more than adequate. Roof spans greater than 30 feet should be pre-fabricated by a truss manufacturer with wind loads not less than 200 mph. This increase in wind speed will result in only a marginal increase in the truss plates and member sizes and therefore should result in only a minor cost increase.
For anchorage of small span roofs less than 30 feet, the use of 16 gage straps embedded in concrete on wall plates as specified by the BBC is adequate. The anchorage for all other roof spans should be designed to correspond to the uplift that is generated by the design software or the detail calculations by a professional structural engineer. Typical wall plates anchored with anchor bolts may not be adequate in many cases.
There were some inherent defects in older buildings that although built to code at the time of construction, did not perform well in Hurricane Dorian. Most of these structures were built using a code that predated the BBC (2003 edition) and therefore utilized wind loads that were significantly less than the current 150 mph wind. This resulted in damage to steel framed buildings, roof structures, upper level storeys, wooden structures, facades and cladding, especially when these structures where not upgraded and or properly maintained over the years. Many large warehouses, commercial buildings and churches which are steel framed structures, fall into this category and the risk of future damage to these older buildings is significant.
Another mode of failure was the damage in prefabricated roof trusses due to the truss plates being attacked and losing its integrity/strength by the treatment in the wood itself. This problem was uncovered many years ago by the truss manufacturing associations in the United States. The solution was to use hot dipped galvanized truss plates whenever pressure treated wood was used. I suspect that some unscrupulous suppliers/fabricators ignored this special notice and supplied these unsuitable roof systems throughout The Bahamas.
From Hurricane Joaquin to Hurricane Dorian, we have seen telecommunications towers fail at an unacceptable rate. There is no reason why these structures should fail based on the hurricanes that we have experienced to date. These structures, as well as recently built steel framed buildings can be engineered with a great deal of accuracy to easily withstand the hurricane conditions required in the code. Our present code, Section 2009.1, references the American Society of Civil Engineers (ASCE) for its wind requirements under ASCE 7-88 (88 refers to the year 1988,) which stipulates minimum wind loads of 150 mph. When this requirement is compared to a later ASCE wind code, ASCE 7-10 (10 refers to the year 2010), the wind requirement is 180 mph but when the factors are applied the resulting design loads are similar to the current BBC, for example, within 10 percent. This means that our current code is comparable with the latest ASCE 7-10 wind code.
Under the ASCE code, communication towers are classified under category IV, an essential facility, similar to airports and health facilities, which mandate use of factors that will cause an increase in the wind loads for design. Therefore, these facilities should be designed with a much higher wind load capacity. If the telecommunication towers in New Providence and the remainder of The Bahamas not yet affected by hurricane-force winds from a Category 5 hurricane, were designed and built to the same standards as those that failed in Andros, Long Island, Abaco and Grand Bahama, then we can expect wide spread failures of these towers during the next Category 5 hurricane.
Small wooden structures that were well built, not subject to storm surge and suitably anchored to a competent foundation performed admirably. I noted many photographs of small wooden structures fully intact surrounded by structures that were heavily damaged or totally destroyed. Most of these structures that survived where built a long time ago using quality material and more than likely constructed by a seasoned builder who did not take short cuts, paid meticulous attention to details and took pride in doing a good job. Wooden buildings, when well maintained, are resilient to hurricanes and reconstruction using this material is more practical and sustainable and should be given high priority.
Selected hurricane shelters in The Bahamas are typically large buildings with wide span roof and large open spaces. These structures are generally churches and government buildings. These large buildings give a sense of security and comfort during hurricane conditions. It is important that each of these designated shelters are thoroughly inspected and evaluated by the relevant professionals, are above known flood levels, are not within proximity to anything that can cause damage during a hurricane like large trees and other abandoned structures. These structures should be recertified annually to ensure that occupants will be safe during the passage of a Category 5 hurricane.
Asphalt shingles can provide only limited protection during a hurricane and therefore should not be relied on to withstand any category of hurricane without damage and total destruction. Some degree of protection can be expected if the asphalt is painted with an elastomeric sealer, but there is no guarantee. Suitably pre-engineered metal roofs has had acceptable performance in hurricanes and should be considered the preferred roof finish in The Bahamas going forward.
Hurricanes over the past ten years have caused billions of dollars in damages to our communities. There were many teachable moments and the impact is likely to last for decades and will affect our future generations. Although our people have demonstrated resilience in the face of these disasters, we must also ensure that the lessons are internalized so that we can remain sustainable in the face of climate change in order to protect the future viability of our country.
All professionals and government institutions, both domestic and foreign, that were engaged in our built environment over the past 10 years must take responsibility for some aspects of the damage that occurred due to these hurricanes. It is unsettling to see the level of damage that occurred, some of which could have been avoided if we did not take short cuts, were more meticulous and or consulted professionals and the local jurisdiction for guidance and the authorities having jurisdiction were more proactive. We must redouble our efforts to improve on the product that we provide to our clients, many of whom are family members, friends and visitors and uninformed and rely on us to be professional in our undertaking. Apart from the messages of hope, or sitting back expecting to be called, we should put our money where our mouths are and start an intense campaign of public awareness, workshops, training and conferences to fill in the gaps of ignorance and slackness.
Climate change is here, hurricanes are becoming more frequent and are much more powerful. We must focus our planning and preparation for these annual events in order to survive as a nation. Category 5 hurricanes are now a fact of life and every Island and cay in The Bahamas will experience its devastating impact at some time in the near future, some places more than once. The only unknown at this time is whether we will experience its devastating impact next year or the year after.
– Hammond V. Rahming, PE, engineer
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