September 2018

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As we seek to assure appropriate seismically resilient communities there is a particular hurdle to be overcome. This concerns a critical lack of awareness of what a building code provides to property owners and end-users in terms of losses. Appropriate seismically resilient communities are those that implement the latest scientific and technological advances in the design and construction of physical infrastructure in consideration of all the costs involved. Such advances are passed on to society via the latest building codes and any property owner or occupant should be keen to know of same so as to maximize safety and minimize costs. Hence there is a vital role to be played by property owners, end-users, and the population as a whole.

The following statement is invariably shocking at first and there seems to be no easy way to introduce it. When the earthquake a building is designed to resist occurs, there will always be losses in terms of casualties, damage, and functionality for a period of time (i.e. downtime).

Therefore for any town, there will always be a risk that medical and other services or resources required for recovery operations will be overwhelmed. However, it is at first believed by property owners or end-users that not only should there be no losses but that this is the engineer's job. Engineers truly wish this were possible but it is not possible due to the way nature operates. The issue is this: when a structure vibrates due to the vibration of the ground, the damage imparted to the structure depends on the sum of the forces at each instant of time during the vibration, and not just the maximum force and it is literally impossible to predict what that will be (viz. the technical term is "record-to-record variability").

So when your engineer designs your structure for a certain level of force, there is always a chance that the actual force (hence damage) will be higher. This is often observed after an earthquake; say there are 10 identical houses on a street, one will collapse and another will not have a single crack.

This implies that expected losses are to be considered by the populace who must then decide if they are acceptable to society. Therefore acceptable losses represent the basic safety, in monetary terms, that should be built into the structure. Such acceptable losses also represent a benchmark that can then be made part of local design and construction policy. This benchmark represents basic safety but, of course, a property owner or end-user can specify a different but lower level of acceptable loss and instruct the engineer to design the structure to suit. That is, the engineer is instructed on the maximum number of casualties, the maximum repair cost, and maximum downtime for the building. The owner specifying the acceptable losses, and the engineer providing a structure to suit these losses, is the core of the state-of-the-art of building design, called "performance-based design," and is the latest approach to safety under earthquakes.

At this point it is prudent to provide examples of the extent of losses associated with basic safety provided by the latest building codes applied to an earthquake prone region sufficiently similar to Trinidad and Tobago. For example, the expected number of casualties due to collapse of concrete buildings or its components is 50 to 100 persons per 1000 buildings. This is for buildings designed and built to code. If the buildings are not designed and built to code, the number of casualties is about 10 times higher. The economic cost due to building damage is about 20 to 30 percent of the replacement value of the building and roughly 15 times the cost associated with the casualties for buildings built to code.

Note that these losses are the expected losses given the latest building codes but the question remains open as to the acceptability of these and other specific levels of losses for Trinidad and Tobago. As would be expected, a higher level of safety hence lower losses requires providing more resilient buildings. Some may consider it reasonable that a more developed country should have lower acceptable losses than a less developed country for its basic safety provisions. As a first step the citizenry needs to appreciate the concept of inevitable losses even if the building is designed and built to code. Then, in order to provide a policy on basic safety in the form of acceptable losses, public consultation is needed. The aforementioned expected losses are for the region of southern California, USA. A decision was made by the Association of Professional Engineers of Trinidad and Tobago in 1978 to adopt the building codes in use in California for local application. To simplify the decision-making process, the question can be phrased in terms of what percent of this benchmark should be adopted for local application. A level of acceptable loss below 100 percent means setting a level of basic safety for Trinidad and Tobago that is higher than that for California and a level above 100 percent means a lower level of safety. Ultimately, a survey of the public can be conducted and possibly phrased as follows: “select from the following list, relative to California, what should be the Trinidad and Tobago policy on acceptable losses: 80, 90, 100, 110, 120, 130 percent”.

It is envisaged that representatives of residential districts, commercial, and governmental properties, will make the final decision. To facilitate the process, an internet-based questionnaire can be readily prepared and activated for a certain period and when that period elapses, the responses are analyzed and the results presented for discussion by these representatives, other stakeholders, and authorities responsible for policy-making. Such input will eventually be vital for the local building code development personnel, practicing engineers, disaster managers, and development economists.

Dr. Richard Clarke is a Structural Engineer in the Department of Civil and Environmental Engineering at The UWI St. Augustine Campus.