An earthquake moves the ground, both vertically and horizontally, beneath a building; typically as a sequence of shock waves radiated from an epicentre at brief intervals, like ripples emanating from a pebble dropped into a pond.
All properly-built structures can hold their own weight, along with snow, rain, loads on floors and suspended loads. As they are made to support significant vertical weight, even poorly-built buildings can withstand some vertical forces caused by an earthquake. On the other hand, horizontal loads caused by earthquakes can cause massive damage, and can even collapse a building on the initial shake.
The horizontal load on a building during an earthquake is compounded if the shocks come in waves. Some bigger buildings can vibrate increasingly, with each successive swing being greater than the previous one. This sequence of waves is more prone to occur when the earthquake affects deep, soft ground.
To make buildings earthquake resistant, engineers use certain measures and design features to mostly address potential horizontal loads.
A base-isolated building is supported by a system of pads that are positioned between the main structure and the foundation. During a side-to-side displacement brought on by an earthquake, a base-isolated building largely holds its shape, as the bearing pads between the building and the foundation are deformed. They are quite elastic, the bearings don’t experience any damage.
Research on base-isolated buildings in earthquakes has revealed that the system can reduce accelerations caused by an earthquake to one-fourth the acceleration seen in comparable fixed-base buildings.
Some buildings employ shear walls, cross braces, diaphragms and a lightweight roof to sustain the forces affecting a building during an earthquake.
Shear walls are installed to stiffen the frame of a structure. Part of a building's horizontal structure, diaphragms placed on their own deck can move tension from the floor and direct force to stronger vertical structures. Cross-bracing encompasses a range of braces, beams and columns that are designed to send seismic forces back toward the ground.
Generally speaking, the roof(s) of an earthquake-resistant structures should be as light as possible. Although a flat roof is particularly vulnerable to the horizontal forces of a seismic event, an earthquake-resistant flat roof could be fabricated with galvanised steel decking and insulation boards, topped with a specially-designed cover. To be earthquake resistant, a ‘flat’ roof ought to have a slope of about 2 degrees.
Buildings inherently possess a capability to dissipate energy to some degree. However, most buildings in a seismic event cannot dissipate significant energy before they start to deform and suffer damage.
When a building includes systems with high dissipation, or "damping", capacity, it can significantly reduce the seismic energy going into it, and therefore decrease damage that might be suffered during a seismic event.
Damping systems are generally included as a part of bracing systems. Tuned dampers are usually inserted diagonally in each level of a building. Each damper is comprised of piston heads contained within a cylinder that is filled with oil. When a seismic event is taking place, force is transferred from the building into the pistons, which then pushes against the oil in the cylinder. The energy is changed into heat, dispersing the force from the earthquake.
Some tall skyscrapers dampen using a pendulum system; a massive ball suspended by steel cables and a system of hydraulics near the top of the structure. When the building starts the sway in an earthquake, the ball becomes a pendulum and travels in the reverse direction of the building, which has a stabilizing effect. Like a damping system, a pendulum system is tuned to fit and counteract the building’s prospective vibrational frequency during an earthquake.