What is earthquake engineering? Key terms and concepts explained

earthquake engineering

is a field of engineering that deals with the analysis and design of structures to make buildings and civil infrastructure resistant to major earthquakes. Ultimately, this means safer communities and saving lives.

“A structure doesn’t need to be rigid and stronger than an earthquake. A well-designed structure can work smarter to make the earthquake weaker by dissipating its devastating energy and self-centre with little to no structural damage post the earthquake, achieving true resilience while saving build and repair costs and time. ”

— Dr John Jing

Key earthquake engineering terms and concepts explained

Shear wall: A shear wall is a structural member designed to resist lateral earthquake or wind loads.

Moment-resisting frame (MRF): An MRF is a structure of rigidly connected beams and columns, designed to resist lateral earthquake or wind loads.

Plastic hinge: A plastic hinge is a specific region (in a structure) designed to “fail” first to dissipate devastating energy in a stable manner to protect the rest of the building during a severe earthquake.

Coupling beam: A coupling beam is a beam that connects two shear walls to form a coupled shear wall system. It is generally designed to dissipate earthquake energy through formation of plastic hinges at the ends of the coupling beam. As a result, the earthquake forces experienced by the building are significantly limited.

Pushover: Pushover is a nonlinear static analysis method where a structure is subjected to gravity loading and a monotonic displacement-controlled earthquake load which continuously increases through elastic and inelastic behaviour until an ultimate condition (target displacement) is reached.

Time history analysis: A time history analysis is a step by step analysis of the dynamic response of a structure to real earthquake ground accelerations that vary with time.

Mode: A mode is a specific way of response of a structure subjected to earthquake excitation. It is characterized by a modal vibration frequency and a mode shape. The first mode also known as the principal mode has the lowest vibration frequency. The seismic behaviour of a building could be dominated by multiple modes. The first three mode shapes of a typical building are shown below.

Torsion: Torsion is the rotating action of a building in plan. It occurs because there is an eccentricity between the centre of rigidity (lateral resistance) and the centre of mass or the floor plate (diaphragm). Most of buildings are subject to some degree of torsion.

Diaphragm: A diaphragm usually refers to the floor plate at each level of a building. A key function of the diaphragm is to transfer lateral earthquake and wind loads to the vertical lateral force resisting elements including moment-resisting frames and shear walls, which then transfer the loads down to the foundations and ground.

Lap splice: Rebars have a limited length, so one way to joint two rebars together to form a longer piece is by overlapping. The overlapping portion is referred to as a lap splice.

Ductility: Ductility means the extent to which a structure can undergo large deformations (beyond the yield point) in a stable manner. The ductility factor is the ratio of the displacement capacity of a building to the displacement at the yield point.

Energy dissipation: Energy dissipation is a physical process in which the devastating energy from an earthquake is transformed into some other form such as heat. As a building undergoes large deformations or strains, energy is dissipated limiting the earthquake forces encountered by the building. The enclosed area under the force-displacement curve represents the amount of energy dissipated as shown below.

Ductile structure: A ductile structure is the one that can undergo large deformations beyond the yield point in a stable manner – no failing or collapse.

Elastic behaviour: An elastic behaviour means that a structure is loaded to a certain point where upon the release of the applied load, the structure regains its original shape and size. An analogy is the rubber band. When it is stretched, it becomes longer. Upon release, it goes back to the original shape and length. The applied force increases or decreases linearly with the change in length.

Inelastic behaviour: When a structure is laterally loaded past a certain point known as the “yield,” the force-displacement relationship becomes nonlinear. After this point, plastic hinges develop in frames and walls where deformations become permanent and irreversible. Within the plastic hinge regions, several things can be expected such as concrete cracking and yielding and elongation of steel reinforcement or steel members. When designed and detailed correctly, these things are good as they occur in a stable manner to dissipate devastating energy for the protection of the rest of the building and human lives.

John Jing9 September 2019