Reference Criteria for Consulting Services
for Infrastructure Projects

Part III: Notes for the Consulting Engineer

Executive Summary | Introduction | Part 1: Guidelines for Owners | Part II: Guidelines for Maintenance | Part III: Notes for the Consulting Engineer | Appendix I | Appendix II

Part III: Notes for the Consulting Engineer

Part III: Notes for the Consulting Engineer


These procedural Notes are intended for use by consultants engaged in infrastructure projects. They comprise a comprehensive list of issues to be addressed in designing to counteract the effects of natural hazards. Such design work is a complex process if done properly and thoroughly.

It is therefore recommended that these Notes be issued by owners to their consultants when embarking on new projects, extensions to existing facilities, and major renovations. However, the owner must be conscious that the design criteria and the concept of the project guide all of the design work, and are therefore matters not only for the design consultants. Owners must understand that their involvement on this process is vitally important to the success of the project. It is therefore considered that these notes must also be reviewed by owners for discussion with the consultants on the principles to be used for the design of the projects.

Design Criteria

Codes of practice and specifications should be used for new construction, for alterations to existing facilities, for major maintenance and for retrofitting of existing facilities to improve the levels of safety. In some cases codes of practice are not available to address particular problems. In such cases the designer must resort to the fundamental principles of science and technology. This usually requires a higher-than-average quality of professional knowledge and commitment.

Consultants commonly use the minimum standards recommended in the of codes, usually because of commercial pressures. "Minimum standards" are usually interpreted as "maximum standards". This is not the intention of codes. In addition, many codes are for general construction and do not meet the needs and requirements of critical infrastructure projects such as hospitals. Consequently, where such critical structures are envisaged the owner must ensure that the codes and specifications used meet and provide for the enhanced performance required for such facilities.

A related problem is that of building to unnecessarily high and expensive standards. Owners (in consultation with their consultants) should therefore select, on informed and rational bases, design criteria appropriate to facilities of differing importance. Owners should also recognize and accept the need to review, on an ongoing basis, the conditions of their facilities and their standards, for standards do change. [See Annual Reviews in Part I]

Owners should also remember that codes of practice and specifications apply not only to new buildings and facilities, but also to alterations and major renovations of existing buildings and facilities. Such considerations should constitute an integral part of the annual reviews recommended above.


Basic wind speeds and reference pressures

Different codes and standards define and describe wind forces and speeds differently. Since Caribbean owners have to deal with different standards and their provisions, it is important to be able to convert from one standard to another. The major areas of concern are dealt with below.

The main parameters (characteristics or relevant factors) used in defining wind speeds are:

In the commonly used OAS/NCST/BAPE Code of Practice "Wind Loads for Structural Design"[8] the definition reads:

"The basic wind speed V is the 3-second gust speed estimated to be exceeded on the average only once in 50 years ..... at a height of 30 m above the ground in an open situation ....."

Figure 1 in Appendix I shows a map of the Caribbean region with isolines of reference velocity pressures taken from CUBiC for 50-year return periods.

Table 1 in Appendix I gives the CUBiC reference pressures (50-year return periods) along with corresponding wind velocities for different averaging periods.

Averaging periods

Figure 2 in Appendix I presents graphs which may be used to convert wind speeds of one averaging period to speeds of another averaging period. Present thinking is that the Durst curve should be used instead of the Krayer-Marshall curve for hurricanes.

Return period

The owner, in consultation with the consultant, should make conscious decisions with respect to desired levels of safety for different facilities. These decisions may then be translated into return periods. The longer the return period the greater the level of safety. Figure 3 in Appendix I presents graphs from the OAS/NCST/BAPE Code dealing with this parameter.


Much less is known about the earthquake hazard than about the wind and rainfall hazards in the Caribbean. Because of this, and because of the ongoing research in this field, there is the need for regular reviews of design criteria by the construction industry in general and consultants in particular. There may also be the justification for site-specific and project-specific studies for large or critical facilities so as to increase the likelihood that adequate safeguards are built into the structure.

For most projects, the guidance provided by existing standards and research papers would suffice. Some of these documents are listed below.

i) Caribbean Building Code (CUBiC)

Table 2 in Appendix I gives the CUBiC zone factors (Z) for different locations in the region. The table also shows the corresponding values for the contemporary editions of the Uniform Building Code (USA) and the Structural Engineers Association of California (SEAOC).

ii) Seismic Research Unit (UWI)

Figure 4 in Appendix I shows a map of the Eastern Caribbean region with isolines of accelerations due to earthquakes. The acceleration data are based on a three-year research programme undertaken by Professor John Shepherd, which was completed in 1994, and which counted with financial support of the Pan American Institute for Geography and History (PAIGH).

It should be noted that:

  1. BVI, Antigua & Barbuda and Montserrat would warrant a Zone 4 rating (CUBiC Z = 1.00, SEAOC 1990 Z = 0.4);
  2. the whole of Trinidad would warrant a Zone 3 rating;
  3. Dominica would warrant a Zone 3/2 rating;
  4. Grenada, St Lucia and St Vincent would warrant a Zone 2 rating.

Table 3 in Appendix I shows the same information in comparison with the CUBiC, UBC, and SEAOC factors.

More recently, in 1999, the Seismic Research Unit of the University of the West Indies has been contracted by the USAID-OAS/CDMP to produce seismic hazard maps for all Caribbean countries at scales and in a form suitable for incorporation in design standards. This exercise has led to some modifications in the isoacceleration lines in Figure 4 for the Eastern Caribbean. The results of this 1999 exercise can be seen on the Regional Seismic Hazard page of the CDMP web site.

iii) Importance factor

Earthquakes are not amenable to statistical analysis and return periods (i.e. cannot be analyzed in the same way or in the same detail) as windstorms or rain. Nevertheless the Owner, in consultation with his/her consultant, must still make conscious decisions with respect to desired levels of safety for different facilities. These decisions are then translated into importance factors in codes and standards. These factors usually vary from 1.0 to 1.5. Thus for a house the factor is likely to be "neutral" at a value of 1.0 whereas for a hospital it may be about 1.5.

Torrential Rain

Lirios' curves

Intensity-duration-frequency curves (charts picturing the relative intensity, duration and frequency of rain storms) have been developed for several territories in the region and may be available through the Caribbean Meteorological Institute in Barbados. A sample is given at Figure 5 in Appendix I.

Return period

In designing a structure certain assumptions have to be made as to the likely severity of each natural hazard the facility should be prepared for. Traditionally, quite short return periods have been selected for design rainstorms. "Design rain storms" refers to the severity of storms for which the facility is being designed. It was quite common for facilities to be designed for 1-in-20-year storms but a great deal of damage and disruption has been caused with increasing frequency by torrential rains. There is therefore the need for a reassessment of this time frame as a determining criterion in the design of facilities.

Changing conditions

Runoff and flooding can be heavily influenced by "upstream" development outside the control of owners. It is possible, therefore, for well-designed drainage systems to prove inadequate some time after they have been implemented because of greater runoff than could have been anticipated and provided for at the time of design. This typically happens when land use upstream is changed, e.g. because of urban expansion. It is therefore wise and appropriate to adopt a conservative approach to the selection of rainfall design criteria, i.e. to provide against greater rainfall effects than would normally be considered likely in the particular context.

Storm Surge and Tsunami

Storm surge

This complex phenomenon is of particular interest for coastal sites. Computer models are available for developing storm-surge and wave height scenarios for coastlines. One such model is TAOS (The Arbiter of Storms) developed by Watson Technical Consulting and tailored for the Caribbean under the USAID/OAS Caribbean Disaster Mitigation Project (CDMP). The model is installed in the Caribbean Institute for Meteorology and Hydrology (CIMH), which is using it to produce systematic wind and wave hazard maps for Caribbean member states.

Figure 6 in Appendix I shows a credible scenario from a likely eruption of the Kick 'em Jenny submarine volcano just north of Grenada.


The studies of both these hazards are highly specialized subjects and expert advice should be sought for them with respect to all developments in low-lying, coastal areas.

Footnotes (Part 3)

8. OAS: Organization of American States. NCST: National Council of Science and Technology. BAPE: Barbados Association of Professional Engineers

Executive Summary | Introduction | Part 1: Guidelines for Owners | Part II: Guidelines for Maintenance | Part III: Notes for the Consulting Engineer | Appendix I | Appendix II

CDMP home page: Project Contacts Page Last Updated: 20 April 2001