Methods applied by NASA provide useful insights on how to implement the most cost effective mitigation program for reducing critical asset risk exposures to hurricanes.
In September 1999, Hurricane Floyd roared over the Bahamas as a strong category 4 hurricane with 145 mph sustained winds, aimed directly at Kennedy Space Center (KSC) in Florida. Luckily, Floyd slowed and turned north, causing only minor damage at KSC. However, the very real possibility of a direct hit raised serious concerns within NASA, particularly in light of the realisation that most of the critical ground facilities supporting the Space Shuttle and International Space Station programs were designed in the 1960s and 1970s to much lower wind speeds than could have been experienced had Floyd continued on its original course. NASA realised that its original most critical assets were in danger of catastrophic loss, and took steps to evaluate and mitigate the situation, using the services of EQE International and its wholly owned subsidiary company, EQECAT.
Similar to NASA, other property owners and insurers with a low tolerance for damage in hurricanes are struggling with ways to address these concerns. The methods applied by NASA can provide useful insights into how to implement the most cost-effective mitigation program for reducing critical asset risk exposures to hurricanes.
A key step in any assessment is determining the likelihood of a particular category of hurricane, typhoon or cyclone striking a given location. Although sometimes based on historical analysis, EQE used the preferred approach of conducting a probabilistic analysis for NASA. This was done using EQECAT's program USWIND, which is a sophisticated CAT modelling tool that can be used for performing hurricane risk assessments throughout the US and the Caribbean. The simulation performed by the program takes into account various hurricane features such as the forward velocity, eye wall diameter and central barometric pressure. After executing the simulation, wind speed estimates and storm surge profiles from the simulated storms affecting the location of interest were then used to create exceedance probability and return period curves.
The next step was to assess the structural integrity of all critical structures to determine the consequences of a hurricane strike in terms of wind loads and flooding due to storm surge. This effort began with a comparison of past wind criteria used to design each building to current wind standards. This comparison provided initial insights regarding potential building vulnerabilities to extreme winds associated with a hurricane, typhoon or cyclone.
Codes and standards
At this stage it is worth detailing a few important points regarding building codes and standards. First, codes and standards have changed considerably over the past 40 years as the wind engineering community has improved its understanding of wind effects on structures. Up until the early 1970s, the focus in the US was primarily on overall capacity of the structural frame to resist lateral wind loads. However, during the 1970s attention began to shift towards estimating the high, local wind pressures that occur at select areas of a building (for example, the corner of a roof). By the early 1980s, codes and standards had adopted provisions that enabled designers to account for these highly localised pressures. Throughout the history of code and standard development, the focus has always been – as it should be – on protection of life. However, little or no consideration has been given to property or business interruption losses. Moreover, codes and standards for wind (and other types of loads) were always meant to represent a minimum standard. Indeed, a building can sustain considerable damage resulting in months of repair and lost function but if it was able to remain sufficiently intact in a manner that enabled it to protect its occupants during a storm, then it has met the code's intention.
Given these understandings regarding codes and standards, buildings constructed prior to the mid-1990s – obviously the majority of all structures worldwide – have some degree of vulnerability to extreme winds, probably greater than what owners and insurers would otherwise believe. To be sure, most structures have a certain degree of reserve strength due to design factors of safety, which means they may well withstand winds in excess of their design. However, this reserve strength can provide a false sense of comfort, depending on when the structure was built.
For example, in general, a building designed for a 100 mph sustained wind in 1968 is far more likely to suffer damage than a building designed for the same 100 mph sustained wind in 1998. Moreover, wind loads on most buildings generally increase with the square of velocity. Hence, all other things being equal, a building designed for a 100 mph wind with a reserve strength ratio of 1.4 would reach its ultimate capacity if exposed to a 120 mph wind.
After comparing past and present design criteria for wind, building inspections and drawing reviews were conducted to determine the actual physical state of a structure. The two primary areas of attention were the main structural frame, responsible for transferring wind loads from the exterior cladding to the foundation, and the exterior cladding (wall and roof). In each case, the focus is on estimating the ultimate load-carrying capacity of the building frame or component. Other concerns include the surrounding terrain, which affects the wind speed profile, and the presence of nearby sources of wind-borne debris.
Structural engineers are increasingly taking advantage of recent advancements in the development of computational models that estimate surface pressures caused by wind and flood levels due to storm surge. For example, for the NASA project, EQE created an advanced fluid flow model to estimate surface pressures due to hurricane force winds on the KSC structures (see above). Another computational model was used to map flood levels caused by storm surge acting across the Cape Canaveral coastline (see previous page). Such tools provide realistic and site-specific data to more accurately capture the loads and resulting building performance in hurricanes. This was especially important for KSC, which has unique facilities and one-of-a-kind structures, some of which are national historic landmarks.
Once the site survey was completed and vulnerabilities identified, a series of mitigation steps were developed to reduce the damaging effects of wind and storm surge. These steps were progressive and listed typically as a function of storm intensity (for example, a weak, moderate and strong category 3, 4 or 5 hurricane). The cost for each mitigation step was estimated, including an estimate of the resulting reduction in losses (both structure and asset) associated with that particular form of mitigation.
With the mitigation steps in hand, the final step was to select the mitigations consistent with selection criteria established by NASA. A cost benefit analysis was performed, taking into account the costs to upgrade facilities and the residual risk. The general idea in these circumstances is that the ‘total cost' to upgrade the facility equals the sum of the ‘initial cost' to upgrade the facility plus the risk-weighted ‘future cost' that may occur in terms of damage caused by a storm. The ‘optimum' design approach can be found by combining these two values (initial cost and future cost) to define the minimum total cost design.
The damage costs include estimates of damage to assets inside the facilities, which is often the majority of the total critical asset value. Hence, though a facility might sustain a small percentage of damage for weaker storms, the consequences in terms of asset exposure could still be serious. For this reason, the estimated damage costs for weaker, but more likely, storms are sometimes higher than would normally be expected, if the value of assets inside the facilities is high.
NASA has used a systematic and rational risk-based approach to evaluate and mitigate its hurricane risk at the Kennedy Space Center complex. Using this process, other property owners and managers may be surprised to find that their exposure are much greater than they had first thought, particularly when viewed in terms of the expected future life of their property. However, they can make cost-benefit decisions to address their critical assets, and prioritise mitigation measures, rather than relying on reaction to imminent threats, be it by sandbagging, boarding up windows, and the suchlike As the old adage goes, the time to repair a leaky roof is when it's sunny, not when it's raining.