Vulnerability Assessment of Selected Buildings
Designated as Shelters
Dominica

Organization of American States
General Secretariat
Unit for Sustainable Development and Environment

USAID-OAS Caribbean Disaster Mitigation Project
OAS-ECHO Project to Reduce the Vulnerabilityof School Buildings to Natural Disasters
1998


This report was prepared by Consulting Engineers Partnership Ltd.
Dominica and Barbados

TABLE OF CONTENTS

1 INTRODUCTION

1.1 The Purpose of the Project
1.2 Terms of Reference
1.3 Natural Hazards in the Caribbean
1.3.1 Earthquakes
1.3.2 Volcanic Activity
1.3.3 Tsunamis
1.3.4 Hurricanes
1.3.5 Torrential Rains
1.3.6 Storm Surge
1.3.7 Landslides

2 INVESTIGATION PROCEDURE

2.1 Briefing
2.2 Field Surveys
2.3 Field Reviews
2.4 Desk Studies

3 RESULTS

3.1 Vulnerability to Hurricanes
3.1.1 General Observations
3.1.2 Aspects of Vulnerability
3.1.3 Summary of Wind Results
3.2 Vulnerability to Torrential Rain
3.2.1 Flooding
3.2.2 Water Ingress
3.3 Vulnerability to Earthquakes
3.3.1 Multi-hazard Design
3.3.2 Differences between Designing against Earthquakes and Hurricanes
3.3.3 Aspects of Vulnerability
3.3.4 Summary of Seismic Results

4 GENERAL ISSUES

4.1 Impact of Hurricanes on Caribbean Schools
4.2 Hazards versus Disasters
4.3 Codes and Standards
4.4 The Regulatory Environment
4.5 Public Awareness
4.6 Vulnerability Surveys
4.6.1 Qualitative Assessment
4.6.2 Analytical Evaluation
4.7 Disaster Mitigation after the Event
4.8 Maintenance as a Tool for Mitigation

5 HURRICANES LUIS AND MARILYN IN DOMINICA

5.1 Damage to Shelters

Appendix I

Commentary on Vulnerability of Buildings to Water Ingress from Torrential Rain

Appendix II

Summary of Wind, Rain and Seismic Vulnerabilities of Roof and Wall Envelopes

Appendix III Survey Forms

Hurricane Vulnerability Assessment
Seismic Vulnerability Assessment - Part I
Seismic Vulnerability Assessment - Part II

Appendix IV Technical Paper

Hassan, Ahmed F. and Sozen, Mete A., Seismic Vulnerability Assessment of Low-Rise Buildings in Regions with Infrequent Earthquakes, ACI Structural Journal, January - February 1997


1  INTRODUCTION

1.1 The Purpose of the Project

Throughout the world, including the Caribbean, natural hazards cause as much damage to educational facilities as they do to buildings of less importance. This is both regrettable and avoidable. Educational facilities deserve special attention because of their roles during the active periods of storms and also as post-disaster assets.

It is traditional for schools to be used as hurricane shelters. It goes without saying, therefore, that the damage and destruction of schools would put the sheltered population at risk during severe storms. Also, the use of school buildings for temporary housing after hurricanes would not be facilitated by damage and destruction of such schools. The longer-term problem of loss of educational facilities is arguably even more severe. If the children are not at school the parents' work is often adversely affected (in part because of "baby-sitting" problems). There is also the inevitable disruption of the pupils' education.

It is often said that safe buildings may not be affordable, especially in relatively poor developing countries. This is a fallacy. Particularly with respect to hurricane resistance, safe buildings are not only technically feasible but also achievable at very modest cost. This thesis has been tested and confirmed on several occasions over the years.

The Caribbean Development Bank (CDB) proposes to assist its borrowing members in reducing the vulnerability of designated shelters to natural hazards. The initial screening of these designated properties is being carried out in some of the CDB member countries through the auspices of the Organization of American States (OAS) using CDMP[1] and ECHO [2] funding.

It is recognised that the suitability of properties for use as emergency shelters depends on several factors other than structural safety. Some of these factors are location, size, water storage and supply, sanitary facilities, kitchen facilities, standby power and telecommunications within the facility and externally. Most of these issues are already being addressed by other agencies. The focus of this study is on the physical vulnerability of the built facilities to wind forces, torrential rain and seismic forces. This initial screening will provide the relevant local agencies with some of the input required for selection of properties for the follow-on, CDB-funded projects.

1.2 Terms of Reference

The portions of the overall Terms of Reference (ToR) relevant to the subject of the present report are:

As articulated above, an important objective of this project is technology transfer and the broadening of the pool of "disaster mitigation" engineers in the region. Therefore, the active involvement of local engineers (assumed to be from the relevant public works departments) is essential.

1.3 Natural Hazards in the Caribbean

The primary natural hazards facing the islands of the Caribbean are earthquakes and hurricanes. Some of the islands are also subjected to instances of volcanic activity. There are also the related hazards of tsunamis and storm surge.

Torrential rains can also, by themselves, be considered natural hazards since they often occur without the concurrent occurrence of hurricanes and sometimes result in disastrous flooding of low-lying areas. Likewise, some landslides are naturally occurring, isolated events.

1.3.1 Earthquakes

All of the Commonwealth Caribbean countries, with the exceptions of Bahamas and Guyana, lie close to the known tectonic plate 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. The earthquakes there 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.

Over the past forty-five 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). 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 recent published work in this field is that of SRU's former head, Dr John Shepherd, now at Lancaster University (England).

The Pan-American Institute of Geography and History (PAIGH) is based in Mexico City. The Geophysical Commission of PAIGH is the executing agency for a major project (funded by the International Development Research Centre, Ottawa, Canada) for preparing Seismic Hazard Maps for Latin America and the Caribbean and 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 indicate that Dominica lies between the iso-acceleration lines representing 250 gals [4] and 300 gals. These are peak ground accelerations with a probability of non-exceedance of 90% in 50 years. The Caribbean Uniform Building Code (CUBiC), which is incorporated in the Dominica Building Code [5], assigns a zone factor (Z) of 0.75 to Dominica. Both of these references indicate a serious earthquake hazard for Dominica.

1.3.2 Volcanic Activity

Several of the islands of the Eastern Caribbean are volcanic in origin. The volcanoes there are considered to be either active or dormant. Dominica exhibits volcanic activity. This, however, is outside the scope of this report.

1.3.3 Tsunamis

A tsunami (or seismic sea wave or tidal wave) is a series of ocean waves generated by any large-scale, short-duration disturbance of the free surface of the ocean. The majority are related to tectonic displacements associated with earthquakes at plate boundaries. However, tsunamis can also be generated by erupting volcanoes, landslides or underwater explosions. In the open ocean, tsunamis may have wavelengths of up to several hundred miles but heights of less than 1 meter. Because this ratio is so large, tsunamis can go undetected until they approach shallow waters along a coast. Their height as they crash upon the shore mostly depends on the geometry of the submarine topography offshore, but they can be as high as 30 meters.

A tsunami travels at an average velocity of 500 to 600 kilometres per hour rising to a maximum of 800 km/h. Therefore within one hour of a major occurrence at Kick 'em Jenny (just north of Grenada), many of the islands of the Eastern Caribbean (including Dominica) will be affected. This, however, is outside the scope of this report.

1.3.4 Hurricanes

Much is not known about the storms which occurred in the Caribbean in the years before the advent of Columbus. But, of course, the European did not bring hurricanes to the Caribbean. Indeed the very name is derived from the Mayan storm god Hunraken and the Arawak word hurican, which meant the devil wind. The greatest of all recorded hurricanes occurred from 10th to 18th October 1780. Nearly 20,000 people perished as the storm hit virtually every island from Tobago in the south-east through the Windward and Leeward Islands and across to Hispaniola and Cuba. In the last 60 years in the Caribbean another 20,000 people have lost their lives because of hurricanes.

The Caribbean lies in the North Atlantic Ocean, one of the six main tropical areas of the earth where hurricanes may develop every year. Within the 111 years between 1886 and 1996, approximately 1000 tropical storms have been recorded in the North Atlantic. About half of these attained hurricane strength.

The destructive potential of a hurricane is significant due to high wind speeds and torrential rains that produce flooding and occasional storm surges with heights of several feet above normal sea level.

The pattern in recent times has been a reduction of deaths and injuries (because of better warning systems and other preparedness activities) and an increase in property damage (because of commercially-driven unsuitable building practices and locations).

The Caribbean Uniform Building Code (CUBiC), which is incorporated in the Dominica Building Code, assigns a reference pressure of 0.85 kPa [6] to Dominica. This is equivalent to a 10-minute sustained wind speed of 37.5 m/s [7] (83.8 mph) or an equivalent 3-second gust speed of 56.4 m/s (126 mph). These figures relate to a 50-year return period.

1.3.5 Torrential Rains

Although hurricanes are often accompanied by heavy rains, severe rainfall events resulting in flooding are also, and frequently, associated with troughs and tropical depressions. The risk of flooding is therefore not restricted to, nor more likely to occur, during hurricane events.

Drainage systems and structures in the Caribbean are generally designed for rainfall events having return periods of 20 years. This means that such systems are likely to become overloaded and cause some degree of flooding when rainstorms are experienced with return periods greater than 20 years. No guidance on this is provided in CUBiC. However, consideration should be given to moving this common standard to a longer return period. Intensity-duration-frequency relationships for rainfall events are required for rational engineering analysis. These are available from the Caribbean Meteorological Institute.

The damage caused by flooding depends on the type and elevation of facilities in the location. The results of flooding may range from the inconvenience of temporarily submerged driveways to the loss of equipment and finishes inside flooded buildings and consequential disruption of the functions.

Flooding has been the cause of many of the deaths and of much property damage as well. Clearly, location is critical when it comes to flood risk. Low-lying lands, river banks and lands adjacent to gullies are to be avoided if possible. If not, deliberate drainage measures must be taken. Usually this is a municipal responsibility, at least in terms of overall control, since what happens to one property can easily be affected by a neighbour's actions.

The design of external works for shelter premises would include consideration of local flooding. It would not be practical to study the overall catchment area for the design of each particular site. However, some general appraisal of the adjacent areas should be carried out as input for the design of on-site drainage structures.

1.3.6 Storm Surge

Storm surge is associated with hurricanes and consists of unusual volumes of water flowing onto shorelines. Storm surge has been responsible for much of the damage caused by hurricanes, especially in large, low-lying coastal settlements.

Storm surge is a complex phenomenon which behaves quite differently from one shoreline to another. The several main components governing their behaviour are:

Astronomical Tide:

water levels due to tidal variation;

Initial Water Level:

elevated basin-wide water levels caused by larger storms;

Pressure Deficit:

elevated water levels caused by low pressure systems;

Inland Runoff:

raised water levels in rivers and sea outfalls due to prolonged rainfall;

Current Surge:

ocean currents caused by high winds leading to the "piling up" of shallow waters;

Wave Setup:

water accumulating from continuous trains of waves on breaking on the shoreline; and

Wave Action & Runup:

effect of actual waves superimposed on the above factors.

The increase in coastal settlement has put much of our economic investment at risk from sea damage. Future rises in sea level can only make this condition more acute. Storm surge caused by hurricanes causes the most dramatic damage. Waves cause damage without accompanying surge but they are also superimposed on storm surge.

As well as causing flooding and damage to coastal structures, storm surge may also precipitate flooding further inland through the blockage of the out-falls of drainage systems.

This is outside of the scope of the present exercise.

1.3.7 Landslides

Most landslides in Caribbean islands cannot be regarded as natural hazards since they are brought about by inappropriate farming practices although triggered by natural events. Another man-induced class of landslides is road construction in mountainous areas. Often the cut embankments are too steep for stability. Often the down-slope fills are unstable.

There are, however, a few situations where nature itself produces the events. This phenomenon is outside the scope of this report.


2 INVESTIGATION PROCEDURE

2.1 Briefing

On March 23 1998, Engineers Cecil Harris CEP (Dom.) and Vivian Trotter (MCW&H) met to discuss and review the survey forms and the theory behind the seismic vulnerability survey. The information requirements for the forms were straightforward. The forms reviewed were:

2.2 Field Surveys

The twenty schools surveyed are all designated as emergency shelters. The list was developed by a committee consisting of Mr. Samuel L. Carrette, Local Government Commissioner; Mr. Vivian Trotter (a structural engineer from the Ministry of Communications, Works & Housing) and representatives from the Ministry of Education, Ministry of Health, Ministry of Community Development and The National Disaster Co-ordinator. The list of schools is shown at the end of this chapter.

Mr. Trotter commenced his surveys in April 1998 and completed them at the end of May 1998. Mr. Gibbs kept in touch with progress through Mr. Harris at the CEP (Dom.) office throughout this period.

2.3 Field Reviews

Following the submission of the field surveys a joint field review exercise was carried out between Messrs. Trotter and Harris at the Pichelin, Goodwill and Coulibistrie schools (seven buildings). This exercise basically confirmed the information on the Hurricane and Seismic field survey forms and clarified the surveyor approach to answering the various questions on the forms.

Final field reviews were undertaken in August 1998 by Cecil Harris and Tony Gibbs.

2.4 Desk Studies

To assist with the determination of vulnerability to hurricanes (and in accordance with the ToR) the software package WIND-RITE was utilised. WIND-RITE is a software system developed by the Wind Engineering Research Center at Texas Tech University, Lubbock, TX in conjunction with the Insurance Institute for Property Loss Reduction. This program is used to establish a relative grade for the wind resistance of buildings by assigning various vulnerability weights to: site environment, roof envelope, wall envelope, structural framing, and other secondary considerations.

A relative grade between 1 and 10 is assigned to a building during the evaluation process. A relative grade of 1 indicates that the building is highly resistant to wind-induced damage (less damage), whereas a building which receives a relative grade of 10 is highly susceptible to wind-induced damage (extensive damage). For non-engineered construction, a score of five would be considered very satisfactory.

To assist with the determination of vulnerability to earthquakes (and in accordance with the ToR) the methodology of Ahmed F. Hassan [8] was used. This methodology relies mainly on readily accessible data for an existing building such as the dimensions and arrangement of its structural elements and the floor area. Masonry walls, concrete columns and steel columns are given progressively higher weightings in determining their contributions to seismic resistance. The end result is a comparative assessment of seismic vulnerability in the form of a relative vulnerability number (RVN). The lower the RVN the more vulnerable the building is to seismic forces. A copy of Hassan’s paper is provided in Appendix IV.

List of Schools Investigated

Name of School

Code Number

No. of Buildings

Giraudel Government School

Coulibistrie

Morne Jaune

Pichelin

Salybia

Marigot

Bagatelle

Goodwill

San Sauveur

Bense

Paix Bouche

Colihaut

Thibaud

Vieille Case

Woodfordhill

La Plaine

Beotica

Petite Soufriere

Atkinson

Tete Morne

DOM-GIR

DOM-COV

DOM-MJN

DOM-PCH

DOM-SAL

DOM-WSS

DOM-BAG

DOM-GDL

DOM-SSV

DOM-BEN

DOM-PBX

DOM-COL

DOM-THI

DOM-VCS

DOM-WHL

DOM-LAP

DOM-BOE

DOM-PSF

DOM-ATK

DOM-TMN

 

TOTAL

1

1

2

2

2

3

1

5

1

1

1

1

1

1

3

1

3

2

2

1

 

35


3 RESULTS

3.1 Vulnerability to Hurricanes

3.1.1 General Observations

It would not be unreasonable for the Government to require that its new, formally designed, school buildings would have the capacity to be largely undamaged by a 50-year-return-period hurricane.

The objective stated above requires simply the application of known technology by the school's architects and engineers to:

The next requirement is for faithful attention to be paid during construction to the details incorporated in the drawings, schedules and technical specifications for the project. Supervision and inspection are central to success in this area of execution.

The final requirement is for the property to be adequately maintained.

The above precepts seem simple enough. Yet schools are often damaged and indeed sometimes destroyed by hurricane events no worse than the "design" storm. That design storm cannot be less than (and could reasonably be greater than) the "50-year" storm. These failures come about because of the failure of the systems (or the inadequacy of the systems) set up to order the construction industry and to maintain public property.

3.1.2 Aspects of Vulnerability

The experience of recent hurricanes has demonstrated that most failures occur to roofs, windows and external doors. In a very few cases there is damage (or collapse) of walls. In even fewer cases there is foundation failure. The experience in Dominica provides confirmatory evidence of this pattern.

Roofs:

Windows (and other openings):

The convenience of aluminium and timber louvers for everyday use is self-evident. But it must be recognised that such windows have difficulty in keeping out torrential rain driven by strong winds. Decorative (or "breeze") blocks are practical and durable. Welded wire mesh is practical but much less durable. Both of these types of openings are clearly vulnerable to the ingress of wind-driven rain.

External Doors:

3.1.3 Summary of Wind Results

The table on the following pages summarises the conclusions of the survey with respect to wind vulnerability.

A summary showing the assessed vulnerability of the roof and wall envelopes as well as the window and door openings, is shown in Appendix II.

The WIND-RITE analysis results appear to indicate that the properties have an approximate equally high vulnerability to hurricanes. It should be noted that the WIND-RITE program will produce high vulnerabilities if the building has unprotected openings and lightweight roof sheeting. Since these vulnerable areas can be mitigated against relatively easily, it is not considered prudent to exclude a property from being considered as a shelter based on these results.

Property

Individual Building and Code Number

WINDRITE Rating

Comments

Recommendations

Atkinson DOM-ATK-01 3.68   Repair louver windows.
DOM-ATK-02 3.59   Repair louver windows.
Bagatelle DOM-BAG-01 3.83   Install wooden louvers over glass windows.
Bense DOM-BEN-01 3.39    
Boetica DOM-BOE-01 3.68   Repair leaks to roof on Bldg. #2.
DOM-BOE-02 3.75   Repair spalling concrete in columns & floor edges in both buildings.
Colihaut DOM-COL-01 3.44 Galvanised cladding is extensively rusted. Replace galvanised roof with new sheeting.
Coulibistrie DOM-COU-01 5.83    
Giraudel DOM-GIR-01 8.71 Result using "non-engineered" grade.

Impractical design to resist wind and water ingress.

Provide shutters for open mesh windows in the front of the building.

Seal skylight along stepped roof edge and install lighting to compensate.

La Plaine

DOM-LAP-01

3.79

The roof slope on buildings 2 & 3 are shallow making it susceptible to uplift forces.

 

DOM-LAP-02

4.67

 

Replace cladding on building #3 and increase number of fasteners on both buildings 2 & 3.

DOM-LAP-03

6.06

 

Morne Jaune

DOM-MJM-01

3.51

   

DOM-MJN-02

     

Pichelin

DOM-PCH-01

3.67

   

DOM-PCH-02

3.67

 

Repair damaged aluminium louver windows.

Petite Soufriere

DOM-PSF-01

3.59

   

Paix Bouche

DOM-PXB-01

3.57

 

Repair existing leaks on roof.

Install additional fasteners at eaves and ridge.

Salybia

DOM-SAL-01

3.55

   

DOM-SAL-02

3.64

   

San Sauveur

DOM-SSV-01

5.65

   
 

3.60

Improved rating when 2 feet thick wall assumed to be reinforced masonry.

 

Thibaud

DOM-THI-01

3.85

   

Tete Morne

DOM-TMN-01

4.94

   

Vieille Case

DOM-VCS-01

3.13

Result using heavy steel.

Ensure that all critical areas of roof envelope, i.e. ridge, eaves and gable ends are securely fastened.

DOM-VCS-01A

6.06

Result using light steel.

DOM-VCS-02

3.13

Result using heavy steel.

DOM-VCS-02A

6.06

Result using light steel.

DOM-VCS-05

3.13

Result using heavy steel.

Woodfordhill

DOM-WFH-01

3.94

 

Fix leaks in roof and repair windows in poor condition. Add additional fastener to critical roof areas.

Marigot

DOM-WSS-01

2.93

Wind loss history shows roof susceptible to damage.

Ensure that all critical roof areas (ridge, eaves and gable ends) are adequately secured.

DOM-WSS-01A

5.05

Result using light steel.

 

DOM-WSS-02

3.33

 

Repair damaged louvers.

DOM-WSS-03

3.25

   

Goodwill

DOM-GDL-01

3.14

Open ventilated design provides no protection against wind and rain.

Repair all existing wooden louver windows.

DOM-GDL-02

3.68

   

DOM-GDL-03

2.72

   

DOM-GDL-04

3.57

Wooden extension not considered.

 

DOM-GDL-05

6.58

Wooden pre-fab building.

Ensure that building is properly secured to the foundation.

3.2 Vulnerability to Torrential Rain

Torrential rains can have a significant effect both on the building and its contents.

3.2.1 Flooding

As stated earlier, flooding of low lying areas is often a natural result of torrential rains. None of the buildings investigated are located in flood prone areas. Dominica’s steep terrain and absent coastal plane make such occurrences highly unlikely.

3.2.2 Water Ingress

One of the most uncomfortable and damaging aspects of a hurricane is the effect of water on sheltering persons and physical belongings. Being physically safe from flying debris, falling structure etc. is only one function of a hurricane shelter. Keeping dry is just as important to those sheltering both from a psychological and medical standpoint. The ingress of water can either be through the roof or wall envelopes. Of the buildings investigated only twenty three percent possessed wall and roof envelopes that would offer any resistance to water penetration. Roof and wall envelopes are discussed in more detail in Appendix I.

3.3 Vulnerability to Earthquakes

3.3.1 Multi-hazard Design

When compared with dealing with a single hazard, designing against multiple hazards is more than doubly difficult, especially when those hazards are wind and earthquake. Some favourable features of wind-resistant design are unfavourable for earthquake-resistant design and vice versa, e.g.:

Both hurricanes and earthquakes impose horizontal loads on buildings. Earthquakes also impose significant vertical loads on the overall building. The vertical loading derived from wind is usually significant on parts of a building as determined by aerodynamic considerations.

However, there are many similarities in the effective design and construction of buildings to resist hurricanes and earthquakes:

3.3.2 Differences between Designing against Earthquakes and Hurricanes

There is a basic difference in the performance expectations in the event of an earthquake as opposed to a hurricane. A building is expected to survive its "design hurricane" with virtually no damage. Even a catastrophic hurricane should only lead to repairable damage. On the other hand the "design earthquake" is expected to cause (hopefully repairable) damage, and a catastrophic earthquake is likely to lead to a situation where the building cannot be repaired and must be demolished. In such an event success is measured by the absence of deaths and serious injuries.

3.3.3 Aspects of Vulnerability

Designing schools to be safe during earthquakes is a complex process requiring the involvement of specialist structural engineers. In Dominica, as in most parts of the Caribbean, the problem is compounded because of the absence of mandatory earthquake-resistant standards (See section 4.3) and the lack of a tradition of conscious engineering attention to this subject. Indeed there is no consensus that the hazard is sufficiently serious to warrant concerted action. Thus successes in the area of earthquake-resistant construction tend to be accidental. This comes about principally when the dominant design criterion overrides the demands of earthquake-resistant design.

The main problem areas are summarised below.

3.3.4 Summary of Seismic Results

A summary showing the assessed seismic vulnerability of the individual building components viz. roof and walls, is shown in Appendix II.

The following table summarises the conclusions of the survey with respect to earthquake vulnerability.

Property

Individual Building and Code Number

Relative Vulnerability Number

Comments

La Plaine DOM-LAP-O1

0.58

DOM-LAP-02

1.76

DOM-LAP-03

1.49

Boetica DOM-BOE-01

2.19

DOM-BOE-02

2.40

Giraudel DOM-GIR-01

3.31

Tete Morne DOM-TMN-01

1.39

Pichelin DOM-PCH-01

1.69

DOM-PCH-02

0.49

Coulibistrie DOM-COU-01

0.05

San Sauveur DOM-SSV-01

4.04

2’ thick stone masonry walls.
Atkinson DOM-ATK-01

2.00

DOM-ATK-02

1.89

Paix Bouche DOM-PBX-01

0.65

Thibaud DOM-THI-01

0.52

Bense DOM-BEN-01

0.66

Colihaut DOM-COL-01

0.62

Bagatelle DOM-BAG-01

1.21

Woodfordhill DOM-WHL-01

2.25

Vieille Case DOM-VCS-01

3.19

Steel framed buildings
DOM-VCS-02

2.09

(01, 02, 03)
DOM-VCS-03

2.83

Salybia

DOM-SAL-O1

3.27

DOM-SAL-02

2.13

Morne Jaune

DOM-MJN-01

0.55

DOM-MJN-02

2.80

Marigot

DOM-WSS-01

1.13

DOM-WSS-02

0.27

DOM-WSS-03

0.96

Petite Soufriere

DOM-PSF-01

2.60

Goodwill

DOM-GDL-01

0.74

DOM-GDL-02

0.57

DOM-GDL-03

0.57

DOM-GDL-04

0.3

DOM-GDL-05

N/A

Vulnerability method not applicable to wooden buildings.


4 GENERAL ISSUES

4.1 Impact of Hurricanes on Caribbean Schools

Hurricanes David in 1979, Allen in 1980, Hugo in 1989 and Luis in 1995 caused significant damage to schools in Dominica, St. Lucia, Montserrat and Antigua respectively. The review of the damage to most of the schools showed that roof failure was the primary cause of problems, leading in some cases to complete collapse of the walls. Most of the roofs were made of wooden rafters or trusses with 26-gauge (or thinner) galvanised sheeting. Few roofs of aluminium sheeting survived. Some roofs remained partially intact but the damage to windows, doors and internal partitions was significant. Some wooden structures were completely destroyed, sometimes by falling trees and sometimes by being blown off their foundations.

In general, therefore, the response of the school buildings to hurricane forces must be judged to have been poor. It is disturbing to note that the reconstruction of many of the schools in Dominica which suffered from the recent hurricanes have been repaired in accordance with the existing design with little or no retrofitting carried out. The construction of all the new schools however, have been based on engineered concepts which would lead to better performance during future hurricanes.

4.2 Hazards versus Disasters

Disasters are often seen as unpredictable, having to do with luck and part of the risks of everyday living. Surely we have progressed beyond the stage when superstition, mythology and fatalism were the public responses to natural hazards. Hurricanes are not natural disasters, they are natural events which sometimes lead to manmade disasters. In these days of widespread technological education, sophisticated research, reliable building materials, computer-based geographical information systems and satellite-assisted warning programmes, hurricanes in the Caribbean should not lead to disasters. The one exception to this would be vulnerable agricultural crops such as bananas.

It is now evident that disasters due to natural hazards are largely preventable and soon the public will demand deliberate actions to protect communities against hazardous events.

4.3 Codes and Standards

The development and maintenance of building standards and codes is a continuing process in many countries. The mandate of national or regional professional and governmental institutions usually includes the development of building standards and the co-ordination of such activities among its various constituent members.

Dominica now has a satisfactory building code. It is understood that the Planning Department is now using the Building Code as an official document to review development applications. The government is presently reviewing a model physical planning act done for the OECS by UNDP/UNCHS with a view to incorporating these standards and having them enacted in parliament. No deadline has been set.

4.4 The Regulatory Environment

Some government agencies adopt an ad hoc approach to standards based, principally, on the particular individuals involved in the specific projects. In most cases the administrators tacitly assume that their designers and builders would do what is right without being told. In other cases the administrators adopt the approach of the objecting to safe design and construction, provided that these attributes do not interfere with their other aims for the projects.

Many government capital works projects are funded by international lending agencies. Typically there is reluctance on the part of these agencies to impose structural design criteria on their projects. The funding agencies leave it up to the governments and the governments leave it up to their designers and builders. This laissez-faire approach leads to inconsistent performance, lack of reliability and, arguably, to higher overall life-cycle costs for the built environment.

Building codes (incorporating appropriate standards) must be adopted immediately. Sufficient technical documentation exists. The fact that such documentation has deficiencies should not be an excuse for non-implementation. Codes and standards can be improved only through usage, which leads to inevitable revisions.

The system of check consultants (bureaux de contrôle), routinely used in French territories, is proposed. Check consultants are independent of the design consultants. It is well recognised that quality assurance is more effective where checking is done independently of creating. The system mentioned above formalises the process.

4.5 Public Awareness

During the past 100 years a total of approximately 900 tropical storms and hurricanes have been recorded over the North Atlantic area. Of these, about 50 percent were hurricanes, many in the general area of the Caribbean. With such a record, it is not surprising that everyone in this region accepts that hurricanes are a fact of life. However, the frequency of direct hits by hurricanes on any one territory is low. This has led to a considerable lack of consciousness amongst Caribbean people as to the dangerous risk to their own properties. Few believe that their island would be hit.

In the post-Columbian history of Dominica, there have been approximately 45 recorded hurricane events which have affected the island. Not all of these were direct hits by hurricane-force winds. In many cases the effects were by way of heavy seas and torrential rain. The most recent of these events were hurricanes Hugo (1989), Luis (1995) and Marilyn (1995). None of these three events produced more than minimum hurricane winds. The damage that can be expected from direct hits from such hurricanes is one or two orders of magnitude greater than what occurred in those two years. The success of school buildings during those hurricanes cannot be regarded as proof of adequacy.

4.6 Vulnerability Surveys

4.6.1 Qualitative Assessment

This is the level of assessment being carried out in the present programme. This level of evaluation does not envisage exhaustive testing of materials in place nor sophisticated computation of stresses. It does involve a careful review of all readily available data (such as drawings), an inspection of the building without destructive testing and a non-mathematical review of the data.

4.6.2 Analytical Evaluation

Facilities whose performances are deemed to be doubtful when assessed qualitatively would be subjected to an analytical evaluation. This procedure would also be used as a second stage, prior to implementation of retrofitting, in cases deemed to be self-evidently inadequate by qualitative assessment. Since analytical evaluation is a time-consuming and expensive exercise it would be appropriate to carry it out only when the funds were available for implementing the possible actions indicated by such evaluation.

It is envisaged that analytical evaluations will be undertaken for those schools selected for the CDB retrofit project.

4.7 Disaster Mitigation after the Event

In the aftermath of a disaster the focus is understandably on getting educational facilities to function again as soon as possible. Also, technical personnel and financial resources are spread very thin at such times. This combination of factors often leads to repairs being carried out in an expedient manner without adequate attention to safety issues. Indeed, post-disaster repairs often leave the buildings even more vulnerable than they were in their pre-disaster, inadequate states. There are examples of this in Dominica.

If such a scenario is to be avoided, very deliberate steps must be taken by the custodians of the educational facilities. Such actions would include:

4.8 Maintenance as a Tool for Mitigation

The physical condition of many Caribbean schools is poor. Windows and doors show lack of maintenance and repair. It is considered that a major effort should be taken to bring the condition of the buildings to the standard where a normal maintenance crew can be expected to deal with the routine maintenance requirements of the facility. It is considered, also, that the existing staff and maintenance budget are generally insufficient to provide for proper maintenance.

It is recommended that for public buildings with the heavy use of a school, the annual maintenance budget should amount to about 4% of the contemporary capital cost of the building and equipment, assuming that the facilities are in good condition to start with. For schools, it is estimated that the replacement cost is about US $150,000 per classroom. (This figure includes amounts for common and administrative areas as well as infrastructure.) The maintenance allocation should therefore be no less than US $6,000 per classroom per year.

The maintenance of a school, rather than being a one-time activity as is the construction of the school, is a continuous daily operation of the institution and is an important ingredient in the delivery of education.

A good maintenance system is also a good disaster mitigation system, as the review of damage caused by recent hurricanes and floods has shown. To some extent the damage to buildings was due to lack of sustained maintenance of critical items. Also, a well operated system of maintenance for buildings and equipment has the effect of being a very effective disaster mitigation measure in terms of cost and facility usage. It ensures the most economic way to keep the building and equipment in the best of form for normal use, given the original design and materials. It is essential that a maintenance plan be included in disaster mitigation plans.

It should be noted that the establishment of a secure and orderly system of archiving of construction drawings depicting the as-built condition of the final structure is an essential ingredient of the long-term maintenance schedule of any constructed facility. Because the archive will serve its purpose over the lifetime of the facility, the protection of its contents against the effects of time cannot be overlooked.


5 HURRICANES LUIS AND MARILYN IN DOMINICA

In September of 1995 Dominica was affected by two hurricanes namely Luis (September 4th & 5th ) and Marilyn (September 14th). These hurricanes did not produce much structural damage to buildings but there was considerable damage to coastal roads and food crops.

5.1 Damage to Shelters

Damage to shelters as a result of these two hurricanes was minimal because they were not direct hits. Three shelters Marigot, (DOM-WSS-01), San Sauveur and Paix Bouche suffered some damage in the form of a few loosened roof sheets. Vieille Case (DOM-VCS-03) suffered severe damage as a result of the impact from the toilet block roof, which was rooted off its eave beam and dumped on the roof of this building.

No detailed records of the repairs are available. The existing school maintenance system is such that, for such miscellaneous works, a general carpenter would have carried out these repairs. It is highly unlikely that any building code standards were used.


Appendix I

Commentary on Vulnerability
of Buildings to Water Ingress from Torrential Rain

Roof Envelope

The roof envelope is generally covered with corrugated steel cladding or concrete.

In the case of metal cladding, its vulnerability to water is generally dependent on:

(i) the pitted condition of the roof metal;

(ii) the adequacy of the length of laps;

(iii) the location installation and quality of the fasteners; and

(iv) the slope of the roof.

The investigation revealed that the general condition of the metal roofs reviewed in the context of parameters (i) to (iv) above is only fair.

For concrete roofs the vulnerability to water is generally dependent on:

(i) the effectiveness of the waterproofing covering (if any);

(ii) the density of the concrete;

(iii) the presence of cracks; and

(iv) the slope of the concrete.

There were no reported leaks in any of those buildings investigated with concrete roofs.

Wall Envelope

The wall envelope is the second area of possible ingress of water from torrential rain. Water penetration is much increased when accompanied by winds which not only flattens its trajectory exposing much more of the wall surface, but also increases its penetration force. The main weak links in the wall envelope are:

(i) windows

(ii) doors

(iii) vent blocks

The windows in the shelters investigated were constructed from either wood or aluminium. Twenty-five percent of the buildings have wooden louver windows, which are highly susceptible to leakage under the best of conditions. Most of the windows have missing louvers making matters worse. Aluminium louvers tend to be more watertight, but here again, all the buildings with this type of window exhibited some damage in this area due to vandalism, forced entry and/or structural defects. Approximately one third of the buildings had aluminium louvers.

Some mention should be made here of shutters. When they form part of the wall envelope, water penetration is reduced due to the nature of the surface, and the protection it offers the openings against damage from impact from flying debris. Shutters (generally wooden) are only practical in sheltered situations, such as the "corridor side" of the buildings. About a third of the shelters have wooden shutters.

Doors were all made of wood fixed in wooden frames. The doors all appeared heavy duty but generally showed shrinkage gaps between the planks of wood. A common weak point among the doors was that they did not close against a concrete lip along the bottom edge. The space thus created between the floor and the underside of the door is a worrisome leakage point.

Vent blocks are extremely common and practical alternatives to windows on the "corridor side" of classrooms and this approach is used in a quarter of the buildings. This form of wall envelope is porous by nature and provides no protection against driving wind and rain.


Appendix II

Summary of Wind, Rain and Seismic Vulnerabilities
of Roof and Wall Envelopes

Property Name

Code Number

Overall Structural Vulnerability

Roof Vulnerability

Wall Vulnerability

Vulnerable Openings

Wind

Rain

Wind

Rain & Wind Ingress

Seismic Forces

La Plaine

DOM-LAP-01

X

X

X

DOM-LAP-02

X

X

DOM-LAP-03

X

X

X

X

Morne Jaune

DOM-MJN-01

X

X

X

X

X

DOM-MJN-02

X

X

X

Pichelin

DOM-PCH-01

X

X

X

X

DOM-PCH-02

X

X

X

X

X

Petite Soufriere

DOM-PSF-01

X

X

X

X

Paix Bouche

DOM-PBX-01

X

X

X

X

X

X

Salybia

DOM-SAL-01

X

X

X

DOM-SAL-02

X

X

X

X

San Sauveur

DOM-SSV-01

Thibaud

DOM-THI-01

X

X

X

X

Tete Morne

DOM-TMN-01

X

X

X

X

Vieille Case

DOM-VCS-01

X

X

DOM-VCS-02

X

X

DOM-VCS-03

X

X

X

Woodfordhill

DOM-WFH-01

X

X

X

Marigot

DOM-WSS-01

X

X

X

DOM-WSS-02

X

X

X

DOM-WSS-03

X

X

X

X

Goodwill

DOM-GDL-01

X

X

X

X

X

DOM-GDL-02

X

X

X

X

X

DOM-GDL-03

X

X

X

X

X

DOM-GDL-04

X

X

DOM-GDL-05

N/A

X

X

X

Giraudel

DOM-GIR-01

X

X

X

X

X

Coulibistrie

DOM-COU-01

X

X

X

Colihaut

DOM-COL-01

X

X

X

X

Boetica

DOM-BOE-01

X

X

X

DOM-BOE-02

X

X

X

X

Bense

DOM-BEN-01

X

X

X

X

Atkinson

DOM-ATK-01

X

X

X

DOM-ATK-02

X

X

X

Bagatelle

DOM-BAG-01

X

X

X

X

Notes :

  1. The symbol "X" indicates vulnerability.
  2. Walls with vent blocks are classified as vulnerable to wind and rain ingress if not provided with shutters.
  3. Vulnerable openings are those windows without shutters.
  4. The buildings classified as being structurally vulnerable (Overall Structural Vulnerability Column), are those building with a seismic vulnerability number less than the average (1.56) as determined by method developed by Ahmed F. Hassan.

REFERENCES

  1. Caribbean Disaster Mitigation Project funded by the United States Agency for International Development (USAID) and managed by the OAS
  2. European Community Humanitarian Office
  3. Insurance Institute for Property Loss Reduction
  4. 1000 gals is approximately equal to the acceleration due to gravity
  5. This is part of the OECS Building Code project funded by the United Nations Development Programme (UNDP) through the United Nations Centre for Human Settlements (UNCHS) or Habitat.
  6. kPa = kilopascals
  7. metres per second
  8. Seismic Vulnerability Assessment of Low-Rise Buildings in Regions with Infrequent Earthquakes by Ahmed F Hassan and Mete A Sozen, ACI Structural Journal, January-February 1997, pages 31 et seq