Tony Gibbs
 

1 Introduction

Although earthquake-resistant design was not unknown in the Commonwealth Caribbean prior to 1967, there was little conscious attention to the subject by the majority of engineers in the Eastern Caribbean, in particular. The destruction and loss of life in neighbouring Venezuela in 1967 acted as a stimulus for investigation into the earthquake hazard in the Commonwealth Caribbean and for studies into the modern techniques of earthquake-resistant design.

Engineers in the Caribbean islands could more easily relate to an event on the continental Caribbean coast than one in a distant community. In spite of the language barrier, similarities of construction materials and methods meant that there were valid and relevant lessons to be learnt from the damage caused in Venezuela.

The author happened to be in the Venezuelan Pavilion at Expo 67 (the Montreal World Fair) when the earthquake was announced. And, as this paper is being prepared, news is coming through of the current (July 1997) event in Cariaco. These coincidences have the effect of making these destructive events somewhat personal to the author.

2 Seismicity

The tectonic setting of the Caribbean is illustrated in Figure 1. It shows the approximate Caribbean plate boundaries. As can be seen, all of the Commonwealth Caribbean countries, with the exceptions of Bahamas and Guyana, lie close to these boundaries. The Caribbean Plate is moving eastward with respect to the adjacent North American and South American Plates at a rate of approximately 20 millimetres per year. A moderate level of inter-plate activity is generated along these boundaries.

Along the northern margin, including areas in the vicinities of Jamaica and the Virgin Islands, moderate earthquakes of shallow depth are generated. Near the plate boundaries there are also intra-plate earthquakes. In the northern Caribbean these intra-plate earthquakes are caused by internal deformation in a slab of the North American Plate. Concentrations of these earthquakes occur at depths of up to 200 kilometres.

Seismic events in the Eastern Caribbean are principally associated with a subduction zone at the junction of the Caribbean Plate and the North American Plate. The North American Plate dips from east to west beneath the Caribbean Plate along a north-south line just east of the main island arc. This leads to a moderate level of inter-plate seismicity. Superimposed on this is a pattern of intra-plate activity. There is a concentration of such activity in the Leeward Islands where the subduction of the Barracuda Rise imposes additional stresses on both the "subducted" North American Plate and the overriding Caribbean Plate. These earthquakes are generally shallow. In the region north-west of Trinidad there is another concentration of earthquake activity where the strike of the plate boundary changes direction. These earthquakes are of intermediate depth. The El Pilar fault, which has caused so much damage in Cumaná and Carúpano, extends eastward from Venezuela to the Northern Range in Trinidad.

Over the past forty-four years a considerable amount of research has been carried out on the seismicity of the Caribbean by the Seismic Research Unit (SRU) of the University of the West Indies (UWI). In these activities there has been collaboration with seismologists in Venezuela and other Latin American countries. In the late 1960s the hazard mapping work of Gunther Fiedler at the Observatory in Caracas was of great interest to the Caribbean islands, since their proximity to Venezuela allowed tentative approaches to the extrapolation of Fiedler's results. In some cases Fiedler's work actually spilled over into the island areas. The author was in Caracas for the 1st Anniversary of the Caracas earthquake and visited Dr Fiedler at that time. That contact proved to be helpful to Caribbean engineers in the years that followed.

The engineering community has been requesting more and more assistance from the SRU in interpreting the fundamental research and developing "code" values for seismic forces for use in structural design. The most recently published work in this field is that of SRU's former head, Dr John Shepherd, now at Lancaster University (England).

The Geophysical Commission of PAIGH is the executing agency for a major project (funded by IDRC) for preparing Seismic Hazard Maps for Latin America and the Caribbean. The project is headed by Dr J G Tanner. Dr John B Shepherd participated in this project as the Caribbean specialist. The final report and mapping from this project will soon be issued. In the meanwhile some of the information is available. Reproduced as Figure 2 is a regional iso-acceleration map for the Eastern Caribbean.

Several earthquakes have caused severe damage throughout the Caribbean archipelago in post-Columbian historic times. The seventeenth century earthquake in Jamaica and the nineteenth century earthquake in Guadeloupe are particularly well known. The researcher, Dorel, has constructed iso-seismal maps of several events of the nineteenth and twentieth centuries. Dr José Grases of Venezuela has been instrumental in bringing these maps to the attention of engineers in the Commonwealth Caribbean.

The level of seismicity in most of the Caribbean is considered to be moderate to severe. It is certainly sufficiently important not to be ignored. The maximum historical intensities (Modified Mercalli Scale) of earthquakes in the Eastern Caribbean, as reported by Dr John Shepherd when he was head of the SRU, were:

Antigua, St Kitts-Nevis, Dominica IX

Montserrat, St Lucia, Grenada, Trinidad VIII

St Vincent, Barbados VII

Some of these figures are one degree lower than those in Dr Robson's 1964 catalogue. (Dr Robson was the first head of SRU.)

3 Earthquake-Resistant Engineering

In previous generations there was little conscious engineered attention to earthquake-resistant design in the Eastern Caribbean. Much more attention had been paid to designing against hurricane-force winds. This led to the general practice of assuming that buildings satisfactory for hurricanes would also perform satisfactorily against the other lateral forces such as earthquakes. This, of course, was a serious misconception. One of the major differences between designing against earthquakes and hurricanes has to do with performance expectations and, therefore, detailing. Conventional earthquake-resistant design uses forces which are much smaller than those which could occur in the expected event. Therefore the structure will yield (go beyond its elastic range). Detailing for toughness and ductility is of paramount importance. With hurricane-resistant design it is expected that the structure will remain elastic when impacted by the expected event.

Subsequent to the Caracas earthquake of 1967 reports became available in English describing the damage and diagnosing the principal causes. The illustrated report of the American Institute of Steel Construction was particularly instructive. That report also contained translations of the existing and proposed standards in Venezuela for earthquake-resistant design. At that time the Commonwealth (Eastern) Caribbean had no formal standards for dealing with earthquakes. The proposed Venezuelan standards were therefore very valuable (at least as a starting point) for those engineers wishing to approach the subject in a more rational and orderly manner.

Over the past three decades there has been a gradual improvement in the engineered approaches to earthquake-resistant design. In the mid-sixties avant garde engineers were beginning to apply simple formulae relating the lateral earthquake force to the mass of the structure. By the end of the 1960s a few engineers were beginning to use the techniques, similar to those in California at the time, leading to a triangular distribution of lateral forces for multi-storey structures. The acknowledgements of the influence of different structural systems came soon after.

Of particular interest following the Caracas earthquake of 1967 were the need for dynamic analyses of large or complex structures and the problems posed by hammering of adjacent structures because of inadequate separations. The first attempts at dynamic analyses of more complex structures were made early in the 1970s. The first major building in the Eastern Caribbean to be subjected to "full" dynamic analysis and to be designed and detailed as a ductile moment-resisting space frame in reinforced concrete was the 15-storey Holiday Inn in Port-of-Spain, Trinidad. That building also incorporated a separation joint of 190mm calculated in accordance with the post-1967 Venezuelan standards. The construction of this building was completed early in 1974. This year (1997) a minor earthquake in Trinidad emphasised the need for architectural detailing to recognise structural separations in buildings.

The main thrust over the past 30 years has been the broadening of earthquake-resistant design techniques to a wider audience and to a greater range of projects. The advent of low-cost personal computers and software has facilitated and encouraged the wider application of three-dimensional analyses and dynamic analyses to structures. Further contact with Venezuelan engineers and architects during these years served to broaden and deepen the regional knowledge of seismicity and of earthquake-resistant design practices. The involvement of Dr José Grases with programmes of the Pan American Health Organisation in the Caribbean islands has assisted meaningfully in the cross-fertilisation process. Such collaboration and sharing of skills can only be beneficial to the overall thrust to reduce the vulnerability of countries of the Association of Caribbean States to the earthquake hazard. At present, however, there are still many significant structures which are not subjected to conscious earthquake-resistant design techniques.

4 Government Attitudes

Currently there are no laws or regulations requiring any structures in the majority of Commonwealth Caribbean states to be designed and built to be resistant to any specified level of seismic activity. Some government agencies adopt an ad hoc approach to the issue based principally on the personalities involved in any particular project.

In most cases the administrators subconsciously assume that their engineers would do what is right without being told. In other cases, the administrators would adopt the approach of not objecting to earthquake-resistant design provided it did not interfere with their other aims for the project. The most important of these aims is low capital cost. Although this is understandable, there is much misunderstanding about the cost of providing earthquake resistance (and the potential cost of having structures which do not possess earthquake resistance).

Many government capital projects are funded by international lending agencies - the Caribbean Development Bank, the World Bank, the Inter-American Development Bank, the European Investment Bank, etc. Typically the funding agency refuses to impose earthquake-resistant design criteria on a project. The funding agency leaves it up to the government who leaves it up to the engineer. Funding agencies and clients need to be consciously involved in decisions relating to the performance expectations of capital projects.

5 Engineering Attitudes

In the Commonwealth Caribbean each territory seems to have a different attitude and approach to earthquake-resistant design. Within any particular territory each firm or design group may have a different approach to the subject. Within any one design group each designer may have a different approach. Lastly, a particular engineer may adopt quite different attitudes and criteria from one project to another without any objective technical basis for so doing. Expediency is the order of the day. This lack of consistency is costly to the communities and is part of the reason for the unfavourable rating given to the region by the catastrophe insurance industry.

The engineering profession is very vulnerable to pressures from clients, architects and funding agencies. Pragmatism often encourages the engineer to turn a blind eye to important earthquake-resistant issues. Many engineers pretend that the problem does not exist in the hope that by so doing it would disappear.

There are noticeable exceptions to that attitude however. There are a few engineers who have been prepared to become unpopular by making a considerable effort to improve the rational approach to earthquake-resistant design. One of the penalties paid by such engineers is a greatly increased design effort for no additional fee. Indeed design costs, not construction costs, are the most important financial effect of introducing proper earthquake-resistant procedures to the building industry in the region.

6 Code Development

In 1968 an informal meeting of a few senior engineers from different territories in the Commonwealth Caribbean was held in Guyana. The purpose was to discuss matters of mutual interest to the profession. Out of that meeting came the Council of Caribbean Engineering Organisations (CCEO).

Two of the functions of the CCEO were the development of building codes and the co-ordination of such activities among the various constituent bodies of the CCEO. In 1969 the CCEO requested the Association of Professional Engineers of Trinidad & Tobago (APETT) to prepare a Code of Practice for Earthquake-Resistant Design on behalf of CCEO for use throughout the Commonwealth Caribbean. The authors of the Report, which was issued in 1970, were David Key (consulting engineer), Desmond Imbert (lecturer at the University of the West Indies) and John Tomblin (head of the Seismic Research Unit).

Essentially, the recommendation at that time was for designs to be carried out generally in accordance with the 1968 edition of the "Recommended Lateral Force Requirements" of the Structural Engineers Association of California (SEAOC).

Regional seminars were held in Jamaica in 1970, 1973 and 1974 with the aim of developing and finalizing the Code. In 1972 APETT had issued a draft of such a Code.

Significant changes were made to the SEAOC Code in 1975 and this led to considerable re-thinking of the position in the Caribbean.

A major conference was held in Trinidad in January 1978 devoted entirely to the seismicity of the Caribbean region and earthquake-resistant practices. Following that conference CCEO set up a committee to prepare interim guidelines for use by engineers pending the re-writing and publishing of the Code. The members of that committee were Myron Chin (UWI), Arun Buch, Alfrico Adams, Tony Gibbs and Maurice St Rose (four consulting engineers). The committee issued its report in July 1978.

A major exercise was mounted in the 1980s to prepare a total building code for the Commonwealth Caribbean. The first phase of the Caribbean Uniform Building Code project (CUBiC) was completed in 1986. That year, those sections of CUBiC dealing with structural design requirements were accepted by the Council of Health Ministers of the Caribbean Community (CARICOM). Funding for this exercise came from the United States Agency for International Development, the Caribbean development Bank, the Council of Caribbean Engineering Organisations and the Caribbean Community Secretariat. The management of the CUBiC project was undertaken by Myron Chin, Alfrico Adams, Tony Gibbs and Alwyn Wason (development consultant). Specialist consultants for the seismic code provisions were Principia Mechanica of London.

The philosophy and structure of the seismic code were not dissimilar to those of the California "Blue Book" - Recommended Lateral Force Requirements of the Structural Engineers Association of California. The zone factors recommended for the Eastern Caribbean in the CUBiC document are:

Zone 3 Z = 0.75 Antigua-Barbuda St Kitts-Nevis

Montserrat Dominica

St Lucia North-west Trinidad

Zone 3/2 Z = 0.50 St Vincent Grenada the rest of Trinidad Tobago

Zone 2 Z = 0.375 Barbados

The recent work done by Dr John Shepherd was referred to in the early part of this paper. Based on these studies, if the separate island states were to be put in zones as defined by the Caribbean Uniform Building Code (CUBiC), the listing would probably be as follows (with the CUBiC Z-factors shown in parentheses for comparison):

Zone 4 Z = 1.00 British Virgin Islands Antigua-Barbuda (cf 0.75)

Montserrat (cf 0.75)

Zone 3 Z = 0.75 St Kitts-Nevis (cf 0.75) Trinidad (cf 0.75 and 0.50)

Zone 3/2 Z = 0.50 Anguilla Dominica (cf 0.75)

Tobago (cf 0.50)

Zone 2 Z = 0.375 St Lucia (cf 0.75) St Vincent (cf 0.50)

Barbados (cf 0.375) Grenada (cf 0.50)

It can be seen that there are several changes to the earlier 1985/86 CUBiC thinking. Clearly there is a need for continuing debate on (and research into) the seismic hazard and corresponding revisions of engineering recommendations. In the meanwhile a conservative approach is warranted.

7 The Way Forward

What is needed now is a period of intense lobbying, public awareness campaigning and education of the whole building industry.

The lobbying is required to persuade government agencies, other regulatory bodies and funding agencies to require that established seismic design criteria be incorporated into the programmes of all public and major private development projects. This is the minimum area that should be addressed.

Public awareness campaigns should be undertaken to facilitate the implementation of earthquake-resistant design regulations. An uncooperative public would make effective progress difficult, if not impossible.

The majority of engineers, architects and builders are still in need of much formal education in the field of earthquake-resistant design and construction. From 1969 to 1990 the CCEO did an admirable job in fostering continuing education programmes in conjunction with the University of the West Indies. The CCEO has been dormant since then. The UWI has continued its efforts, principally through its regular programmes. However, a new thrust of increased intensity is now required.

The recent crisis in the catastrophe insurance industry provided a window of opportunity. Catastrophe insurance (earthquakes and hurricanes, principally) had become scarce and expensive. In this environment a degree of self-insurance was an option, but not one which could be taken casually. Contingent on this approach was the need for more predictable performance of facilities. This demanded better information on the hazards, better designs and better construction. Such a window of opportunity will come again, when we must seize it.

8 Collaboration Between Venezuela and the Commonwealth Caribbean

There are many tools and strategies for reducing the vulnerability of the Caribbean so as to convert our area into a disaster-free zone. Informing all of my proposals is a recognition that natural hazards are not conscious of political and language boundaries. All of the peoples of the Caribbean must cooperate in this campaign. It is hoped that Venezuelan stalwarts such as Dr Teresa Guevara and Dr Juan Murria, who have demonstrated their interest in the wider Caribbean, would spearhead the outreach programme from the continental side. Several collaborative programmes are proposed for consideration:

8.1 Post-disaster Diagnostics

Lessons are always there to be learned when an earthquake strikes. However, in the immediate aftermath of an earthquake the diagnostic surveyor is not welcomed. He gets in the way of the relief worker. There is a need for diagnostic surveys to be organised and formalised regionally and for the results of such diagnoses to be disseminated regionally.

8.2 Reconstruction with Mitigation

Immediately after a damaging earthquake, repair and reconstruction is sometimes carried out to worse standards than were used in the original construction of the damaged facilities. Avoiding these occurrences requires preplanning and an institutionalised approach to monitoring standards during the periods of reconstruction following disasters.

8.3 Research into Natural Hazards

We know a lot about the hazards of our region but not nearly enough. The research effort required is considerable. The information being sought often overlaps our individual boundaries. Progress is hampered by duplication of effort. Some of this duplication is deliberate and has to do with competing agencies and competing researchers. This is inevitable. However, a lot of duplication happens because we do not know what our neighbours are doing. A mechanism must be established to reduce involuntary duplication.

8.4 Sharing Technology

A counterpart activity to that described above (in 8.3) is warranted on the implementation side of the problem. We can learn a lot from our neighbours. Caribbean countries possess expertise in earthquake engineering and seismic retrofitting. These considerable resources which are available in the region should be used regionally. Mechanisms should be established to facilitate the exchange of technology.

8.5 Continuing Professional Development

The skills required for designing against natural hazards need to be broadened and deepened regionally. The levels of these skills in our professionals need to be increased so that Caribbean engineers and architects are accepted as having a higher general level of proficiency in these fields than others worldwide. What we have at the moment is a minority of our professionals with outstanding experience, skills and knowledge and a majority who are found to be short of the required levels of skill and knowledge.

We need a more structured approach to the post-university formation (and testing) of our professionals. This would also serve to ease some of the tensions being experienced when negotiating reciprocal agreements in NAFTA and the other trade blocks that exist or are being contemplated.