Structural engineers usually commit to painstakingly put together fragments of materials when building, culminating into some of the most aesthetically appealing, and physically imposing skyscrapers in our cities.
But beyond what the eye sees is a meticulous marvel of structural engineering and reinforcements made to withstand heavy loads.
In areas prone to earthquakes, however, the engineering marvel is even more rigorous. “In Kenya, we barely design for earthquakes,” says Stephen Mburu, a structural engineer.
“Earthquakes are rare in the county. Only slight tremors have been felt in the past. If we were to reinforce our buildings to withstand earthquakes of significant magnitudes, then the clients would need to pay more for what is very unlikely to happen.”
While gravity loads act vertically on a building, earthquakes, like the wind, are lateral loads that act perpendicularly to the vertical axis of the building. Gravity loads include dead loads (immovable and permanent loads) and live or imposed loads.
According to the Building Code of the Republic of Kenya (2009 Edition), a load is any force to which a building is or may be subjected to and includes dead, imposed, wind and seismic loads and forces caused by dimension changes of materials.
“Earthquake loading on a building shall be calculated on the basis of the recommendations of the “Design and construction of buildings and other structures in relation to earthquakes” obtained from Kenya Building and Research Centre,” reads the document.
As such, even with earthquakes uncommon, engineers have to take into consideration the bending moments and shear forces likely to be imposed on the buildings in the event a lateral load was felt by the building. They ensure it can withstand such loads.
“When the buildings are very high, there is a likelihood the loads will be felt more. We make sure the displacements are not excessive. Both vertical and horizontal loads incur displacements. The displacements should be within serviceability limit state of deflection,” Mburu says.
The serviceability limit state is the type of design beyond which a structural system loses operationally its serviceability for the actual service load that the structure is subjected to.
National Construction Authority of Kenya (NCA) General Manager Maurice Akech says construction is guided by codes of practice and the building code. “It is not on the designer to decide whether they want to design for earthquake or not,” Akech says.
“The construction is guided by various codes of practice, which are the standard design documents that have been developed, reviewed and are approved for use in the design of various buildings. We also have the building code which is a regulation; a legal document to guide the designs.” The two guide the designers on design requirements for a structure.
The engineers usually look at the standard load requirements, and what the building will be used for, together with the environmental load such as earthquakes, winds, and floods.
The design code, he says, considers earthquake zones. Every zone has its likelihood of experiencing an earthquake and the magnitude that would be felt to determine if the effects of the earthquake would be major or just minimal.
Seismologists use the Richter scale (a quantitative measure of an earthquake’s magnitude or size) to categorise earthquakes according to waves recorded by seismographs - devices that measure earthquakes.
It is graded one to nine where one represents an earthquake of little magnitude, a micro- earthquake, which is not felt or rarely felt. Such an earthquake can, however, still be recorded by seismographs.
Magnitude three to four is considered a minor earthquake, often felt by people, but very rarely causing damage. Objects, however, can be spotted shaking.
A six to seven magnitude earthquake is strong and could cause damage to well-built structures in populated areas. Poorly designed structures could be flattened.
This earthquake could be felt in wider areas, even over 100kms from the epicentre - the point on the surface of the earth vertically above the focus or the point of origin of the earthquake.
And at magnitude nine, the earthquake will cause severe damage or collapse to all buildings and will create permanent changes in ground topography.
Akech says that during design, the engineers use the standard loads and check them against resistance to the earthquake so that if it was to occur, then the building can withstand it.
“An earthquake is more likely to affect tall buildings. For tall buildings, you can only say you have completed the building if you have checked for earthquake resistance to the magnitude on the Richter scale according to an area’s zoning,” he says.
A design for the same building but for different areas will differ due to earthquake zoning. However, like in the case of Nairobi, there is no need for designing for the worst when the worst will most likely never be experienced, he says.
“The design is about economy, efficiency and safety, and so you cannot spend too much mitigating an unlikely risk,” Akech says.
Other factors, such as the type of soil a building is planted in, determine the structural design of buildings.
This as engineers have to ensure structural strength, stability and robustness of a building.
“The architectural design may not change, but what we do not see on the inside: the amount of steel used in reinforcement, the concrete quality, the thickness of those structural elements such as beams and columns; those change as they depend on the design requirements dictated by various forces such as earthquakes,” says Akech.
Akech notes that since the wind resistance and earthquake are both lateral forces, engineers build to mitigate whichever of the two is more likely to occur. In essence, by dealing with one, they have catered for the other factor as well.
Civil + Structural Engineer media, a magazine and web news covering civil and structural engineering news, stories, updates, and education, lists an appropriate foundation, use of seismic dampers, a good drainage mechanism, structural reinforcement and use of materials with adequate ductility as five keys to designing earthquake-resistant buildings.