Getting Jamaica earthquake ready
IN THE earlier part of the 20th century, the engineering considerations related to earthquakes evolved into being a requirement for the design of structures.
In our time, say since the mid-1950s, the criteria for loads and design evolved in many ways.
The earliest and most basic approach was to apply a static lateral force, equivalent to a percentage of the mass of the structure, based on approximate seismic zoning.
The strength design was then done with these forces combined with gravity loads with no consideration for the dynamic effects of seismic events (reversals, fatigue, resonance, soil elasticity, and general dynamics).
As building sizes and heights increased over subsequent years, the cost for the ‘over strength’ became significant, so the next round of refinement made in the mid-’60s introduced factors for importance, soil types, building flexibility, and their natural frequencies and sharper definitions of seismic zones.
The equivalent lateral loads were now distributed based on the overturning effect of the weight of the upper floors, and proportionately applied. Also, the distinction between absolute strength and durability was developed, and the need for detailing began to emerge to protect the vertical supporting members, and to limit the occurrence of failure in less critical components.
During the 1970s to the mid’80s, the swing to more optimal design came about, and the focus was on ductility, preservation of columns and connections, with the introduction of dynamic analysis, plastic failure mode analysis, and more. This modification defines the distribution and effects of the forces caused by earthquakes.
The less severe cosmetic damage is a result of the brittleness of the finishes used in buildings, so more flexible drywalls and cement boards have led to a reduction in cracks and dislocation in modern buildings.
From the late 1980s onward, the zone, location, and building type factors became more refined.
Seven classes of rock to soil to soft clay were defined, with associated acceleration amplification factors.
Geographic seismic zones were fine-tuned based on more historic and seismographic data, proximity to faults and possible epicentres, with specific ‘design’ ground accelerations.
Upward of 60 different seismic-force-resisting systems were defined for all types of structures, with assigned factors based on ductility performance and compulsory detailing.
SET LIMITS
Limits were set for allowing the use of simple static lateral loading as against the need for the more exact seismic dynamic analysis.
Specific detailing for durability and ductility in reinforced concrete, structural steel, wood, light gage and wall board construction and even ceiling tiles have been codified, all to allow the structures to flex repeatedly during a seismic event without collapse.
In Jamaica, our building code made the first real mention of earthquake loadings in the 1983 draft, where it referred to the SEAOC recommendations. This was not convenient as that document was not widely available.
However, the UBC (USA) and CSA (Canada) codes had very similar provisions, and the ACI 318 structural concrete code expanded on the reinforced concrete ductility detailing.
The ASCE 7 (design loads) then came in as the essential guide for all structural loadings. Not only did it stipulate the design requirements for earthquake loading, but it also included provisions for gravity, wind, snow, rain, flood, and impact loading.
Jamaica, as part of the Caribbean community, has had the option to be guided by the CUBIC (Caribbean Unified Building Code) for earthquake design, but our community of engineers generally default to the codes that formed the basis for their education curriculum, which are usually from North America or the United Kingdom.
The projects driven by foreign investment have imposed the need for familiarisation with design codes and standards from around the world, with the need to recognise the equivalences and differences.
We have found that the net effect on design outcomes have insignificant differences due to specific regional codes, as the design philosophies with respect to loads and load factors, analysis, detailing, and material specifications are not dissimilar.
For years, the general simplicity of many of our buildings, as well as the unawareness of our public of real seismic peril, has sustained informal design practices, and the technical institutions continue to treat seismic design as a ‘postgraduate’ privilege.
As financiers, mortgage, and insurance companies became more cautious about the structural capabilities of construction projects and properties, there was a gradual move to adopt the International Building Code (IBC) as the basis for our building codes.
MAINSTREAM PRACTICES
This essentially brought proper design practices into the mainstream, and contributed to the mitigation of catastrophic losses and economic consequences as has occurred in the region.
In a series of visits to Port-auPrince, Haiti, the consequence of years of accommodation for the ‘trying man’ by way of relaxed standards and enforcement became apparent.
The loss of schools and commercial buildings was devastating. Squatter communities, with incrementally built and expanded homes were largely destroyed, and most of the high boundary walls caused significant death among street vendors and subsequent obstructions to rescue.
The most recent advancement in the provision of earthquake requirements for engineering projects in Jamaica is the passing of the new Building Act.
This legislation provides an updated and comprehensive framework that will ensure more sustainable development and enhance health and safety in the construction sector.
More specifically to earthquake-resistant structures, this legislation will enforce the inclusion of shear walls, specially detailed members and connections, diaphragms and collector members, and foundation sizes that are suited to good earthquake performance.
This does not necessarily translate to more and expensive materials, but principally requires the use of proper designs and construction supervision.
The elaborate cantilevers and excessive heavy concrete embellishments will generally be discouraged, and the positioning of door and window openings will require consideration to balance the strength with the distribution of the weight of the structures.
Peter Jervis has 38 years of experience as a civil engineer. Send questions and comments to, editorial@gleanerjm.com or jie@cwjamaica.com. You may also leave your comments for the JIE's Technical Committee at our Facebook page: Jamaica Institution of Engineers -JIE.