1Contradictions and Synergies
2by Tony Gibbs, BSc, DCT(Leeds), CEng, FICE, FIStructE, FASCE, FConsE, FRSA
1 Contradictions and Synergies in Designing Against Hurricanes and Earthquakes
Designing against multiple hazards is more than doubly difficult when compared with designing against a single hazard, especially when those multiple hazards are wind and earthquake. Many favourable features of wind-resistant design are unfavourable for earthquake-resistant design and vice versa.
(1) Heavy structures resist winds better. Light structures resist earthquakes better.
(2) Flexible structures attract greater wind forces. Stiff structures (generally) attract
greater earthquake forces.
Both hurricanes and earthquakes impose horizontal loads on buildings. Earthquakes also impose significant vertical loads on a building overall. 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) Symmetrical shapes are favourable
(4) Compact shapes are favourable
(5) There must be a realisation that there is a real risk that "design" forces may be
exceeded. This is particularly so in the case of earthquakes where, largely for
economic reasons, the design force is deliberately determined to be less than that
expected during the anticipated life of the building. This leads to a requirement for
redundancy in the structure and for "toughness" - the ability to absorb overloads
(6) Connections are of paramount importance. Each critical element must be firmly
1For the PAHO Leaders Course, Ocho Rios, Jamaica, 11 February 2003
2Director/Consultant, Consulting Engineers Partnership Ltd, with activities principally in the Caribbean. Eng Gibbs is based in Barbados.
connected to the adjacent elements.
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.
The accompanying table summarises the main differences between hurricanes and earthquakes as they affect structural design.
Wind Earthquake Effects
(1) Source of loading External force due to wind pressures. Applied movements from ground
(2) Type and duration of loading Wind storm of several hours' duration; Transient cyclic loads of at most a few
loads fluctuate, but predominantly in one minutes' duration; loads change direction
(3) Predictability of loads Usually good, by extrapolation from Poor; little statistical certainty of
records or by analysis of site and wind magnitude of vibrations or their effects.
(4) Influence of local soil conditions on Unimportant can be important
(5) Main factors affecting building External shape and size of building; Response governed by building dynamic
response. dynamic properties unimportant except for properties: fundamental period, damping
very slender structures. and mass.
(6) Normal design basis for maximum Elastic response required. Inelastic response permitted, but ductility
credible event. must be provided; design is for a small
fraction of the loads corresponding to
(7) Design of non-structural elements. Loading confined to external cladding. Entire building contents shaken and must
be designed appropriately.
Main Differences Between Wind and Earthquakes
From The Arup Journal
2 Quality Assurance in Design and Construction, the Human Factor
The most difficult problem to solve in the construction industry is the effective enforcement of standards. In one form or another the independent checking of design and construction is 3. widespread in many jurisdictions. The French system is one that appears to be the most successful
What sets France apart from most is the quality of their checking agencies. However, some other jurisdictions do adopt a similar approach for special facilities. In the United Kingdom all dams, tunnels and bridges are reviewed by specially-licensed, private-sector consultants. Here in the English-speaking Caribbean, the Turks & Caicos Islands have made provisions for "special inspectors" in their recently-introduced building code. A similar arrangement is being proposed for the Organisation of Eastern Caribbean States building codes.
The checking agencies in France are known as bureaux de contrôle. The bureaux de contrôle are
independent firms licensed by the state. They pay well and attract, and keep, some of the best talent.
They check designs and also make site visits during construction. Their involvement in projects is necessary if decennial (10-year) insurance cover is to be obtained by the building owner. Some lending agencies also demand the certification of bureaux de contrôle. The most remarkable
characteristic of the system that it is hard to find anyone in France who disagrees with it. In Martinique and Guadeloupe the bureaux de contrôle are seen as being generally helpful and as
having a developmental role in the construction industry. Suffice it to say that the use of bureaux
de contrôle in Martinique and other parts of France is widespread and its beneficial effect is manifest.
But how do others see the role of bureaux de contrôle? Here are two quotations from Peter Rice's
book "The Engineer Imagines":
"It is no accident of time that both the La Villette and IBM projects first appeared in
France where there exist the most intelligent and knowledgeable checking
authorities that I have come across. The large centralized controlling offices,
bureaux de contrôle, Socotec, Veritas, CEP and others each have at their head
engineers who are equal in ability to any I have encountered in the best design offices,
as Centre Pompidou amply demonstrated." - page 113
"Others not so closely involved must also be asked to review the project to question
the assumptions and demand explanations. ............ . The presence of a competent,
dedicated and sceptical checking authority is also very important in this respect." -
(Peter Rice, now deceased, was one of the outstanding structural engineers of the 20th Century.)
3See “The Impact of Bureaux de Contrôle on Damage Levels in Hurricanes” by Tony Gibbs, 1996
3 Concluding Thoughts
The extensive loss of life and property caused by hurricanes and earthquakes can be avoided by the implementation of existing technology and without great financial strain. What is required is the will to do so. Because it would require about two generations to replace the building stock in most communities, as much attention should be paid to the retrofitting of existing buildings as to the 4. At this time there are few technical constraints improved design and construction of new buildings
hindering the satisfactory design and construction of most buildings against hurricanes and earthquakes. (This is not to say that research and development should not continue.) However, there are severe cultural, socio-economic, political and bureaucratic constraints to achieving success in this field.
Education and training programmes need to have a greater emphasis placed on the specific requirements of earthquake-resistant and hurricane-resistant design. At the higher educational levels the subjects should be taught from the points of view of background studies and fundamentals.
The mere teaching of code procedures is not enough.
The lack of code enactment is a serious hindrance to progress in many territories. Of course code enactment would not be enough without enforcement. Funding agencies (loans and grants), domestic mortgage institutions and insurance companies could play pivotal roles in this regard.
4Reference:- “Vulnerability Assessment of Shelters in the Eastern Caribbean, Retrofitting -Terms of Reference
for Consultants, Standards, Global Estimates - November 1998" - Prepared for the Organization of American States under the USAID/OAS Caribbean Disaster Mitigation Project and the OAS/ECHO School Vulnerability Reduction Programme by Tony Gibbs, Consulting Engineers Partnership Ltd