There are two basic types of seismic waves, and they travel at different speeds through earth. The faster p waves and the slower s waves.

Primary or push waves or P waves

Primary Waves

These are longitudinal in nature like sound waves. The velocity of P waves is highest about 5.4 km/s and depends on the density of the rock and resistance to compression. P waves can pass through liquids also.

Secondary or shake waves or S waves

Secondary Waves

These are transverse in nature like light waves. The velocity of S waves is about 3.3 km/s. The velocity of S waves depends upon density of the rock and resistance to distortion. The S waves cannot pass through liquids.

Surface waves

When the P-waves and S-waves travel on the surface, they are known as surface waves

L waves and Rayleig Waves

These are also transverse in nature like S waves. The velocity of S waves is about 3.0 km/s. L waves are formed due to dashing of P and S waves against the solid crust of the earth. These are the waves, which we feel in the form of earthquake. These waves are responsible for the destruction of the life and property. Height of L waves is about 30cm and distance between two successive crests is about 10 m. and increase in their amplitude beyond only 1/16 of an inch is capable of causing lot of destruction. Due to high velocity of these waves, the civil engineering structures vibrate and a typical sound due to passage of energy is heard.

Displacement of Building according to their Height & Stiffness

Ductility is the ability to undergo distortion or deformation without resulting in complete breakage or failure.  To see how ductility can improve a building’s performance during an earthquake, see the above figure.  In response to the ground motion, the rod bends but does not break.  (of course, metals in general are more ductile than materials such as stone, brick and concrete)  The ductility of a structure is in fact one of the most important factors affecting its earthquake performance.  One of the primary tasks of an engineer designing a building to be earthquake resistant is to ensure that the building will possess enough ductility to withstand the size and types of earthquakes it is likely to experience during its lifetime.

Inertial Forces in a Structure

This is much like the situation that you are faced with when the bus you are standing in suddenly starts, your feet move with the bus, but your upper body tends to stay back making you fall backwards!

This tendency to continue to remain in the previous position is known as inertia. In the building since the walls or columns are flexible, the motion of roof is different from that of ground.

Consider a building, whose roof is supported on columns. Coming back to the analogy of yourself on the bus; when the bus suddenly starts, you are thrown backwards as if someone has applied a force on the upper body. Similarly, when the ground moves, even the building is thrown backwards, and the roof experiences a force, called inertia force. If the roof has the mass M and experiences an acceleration a, then from Newton’s second law of motion, the inertia force F1 is mass M times acceleration a, and its direction is opposite to that of the acceleration.

“More mass means higher inertia force”

Horizontal and Vertical Shaking

All structures are primarily designed to carry the gravity loads, i.e. they are designed for a force equal to the mass M (this includes mass due to own weight and imposed loads) times the acceleration due to gravity g acting in vertical downward direction (- Z). The downward force Mg is called the gravity load. The vertical acceleration during ground shaking either adds or subtracts from the acceleration due to gravity. Since factors of safety are used in the design of structures to resist the gravity load, usually most structures tend to be adequate against vertical shaking.

However, horizontal shaking along X and Y directions (both + and – directions of each) remains a concern. Structures designed for gravity loads, in general, may not be able to safely sustain the effects of horizontal earthquake shaking.

Flow of Inertia Forces to Foundation

Under horizontal shaking of ground, horizontal inertia forces are generated at a level of the mass of the structure (usually situated at the floor levels). These lateral inertia forces are transferred by the floor slab to the walls or the columns, to the foundations, and finally to the soil system underneath. So, each of this structural elements (floor slabs, walls, columns, and foundations) and the connections between them must be designed to safely transfer these inertia forces through them.

Walls or columns are the most critical elements in transferring the inertia forces. But, in traditional construction, floor slabs and beams receive more care and attention during design and construction, than walls and columns. Walls are relatively thin and often made of brittle material like masonry.

“If we have a poor configuration to start with, all the engineer can do is to provide a band-aid – improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much”.

Size of Buildings

Size of Buildings

In tall buildings with large weight-to-base size ratio the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be carried by columns and walls.

Horizontal Layout of Buildings

Buildings with simple geometry in plan perform well during strong earthquakes. Buildings with re-entrant corners, like U, V, H and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts by using a separation joint at the junction.

Vertical Layout of Buildings

Vertical Layout of Buildings

Earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path, any deviation or discontinuity in this load transfer path results in poor performance of building. Buildings with vertical setbacks cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey tend to damage or collapse which is initiated in that storey. Buildings on sloppy ground have unequal height columns along the slope, which causes twisting and damage in shorter columns that hang or float on beams have discontinuity in load transfer. Buildings in which RC walls do not go all the way to the ground but stop at upper levels get severely damaged

Adjacency of Buildings

Adjacency of Buildings

When two buildings are close to each other, they may pound on each other during strong shaking. When building heights do not match the roof of the shorter building may pound at the mid- height of the column of the taller one; this can be very dangerous.