How Earthquakes affect Reinforced Concrete Buildings

Architect — Tue, 15 Jun 2010 00:10:55 +0000

A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls), and supported by foundations that rest on ground. The system comprising of RC frame. The RC frame participates in resting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake induced inertia forces primarily develop at the floor levels. These forces travel downwards – through slabs and beams to columns and walls, and then to foundations from where they are dispersed to ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storey experience higher earthquake- induced forces and are therefore designed to be stronger than those in storey above.

Floor Bends with he Beam but moves all columns at that level together

Role of Floor Slabs and Masonry

Floor slabs are horizontal plate like elements, which facilitate functional use of buildings. Usually, beams and slabs at one storey level are cast together. In residential multi-story buildings, thickness of slabs is only about 110-150mm. when beams bend in the vertical direction during earthquakes, these thin slabs bend along with them (fig2a). And, when beams move with columns in the horizontal direction, the slab usually forces the beams to move together with it. In most buildings, the geometric distortion of slab is negligible in the horizontal plane; this behavior is known as the rigid diaphragm action .

After columns and floors in a RC building are cast and the concrete hardens, vertical spaces between columns and floors are usually filled-in with masonry walls to demarcate a floor into functional spaces (rooms). Normally, these masonry walls, also called infill walls, are not connected to surrounding RC columns and beams. When columns receive horizontal forces at floor levels, they try to move in horizontal direction, but masonry walls tend to resist this movement. Due to their heavy weight and thickness, these walls attract rather large horizontal forces. However, since masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus masonry walls is enhanced by mortars of good strength, making proper masonry courses, and proper packing of gaps between RC frame and masonry infill walls.

Effects of Horizontal Earthquake Vibrations

Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus, under strong earthquake shaking, the beam ends can develop tension on either of the top and bottom faces. Since concrete cannot carry this tension, steel bars are required on both faces of beams to resist reversals of bending moment.

Strength Hierarchy

For a building to remain safe during earthquake shaking, columns should be stronger than beams, and foundations should be stronger than columns. If columns are made weaker, they suffer severe local damage, at the top and bottom of a particular storey.

Energy Dissipation Devices for Earthquake Resistant Building Design

Architect — Tue, 15 Jun 2010 00:03:47 +0000

Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing Seismic Dampers in place of structural elements, such as diagonal braces. These dampers act like the hydraulic shock absorbers in cars – much of the sudden jerks are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building.

Energy Dissipation Devices

Commonly used Seismic Dampers

Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement),
Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other),
Yielding Dampers (energy is absorbed by metallic components that yield).
Viscoelastic Dampers (energy is absorbed by utilizing the controlled shearing of solids).

Thus by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building.

How it Works?

How Dampers Work

The construction of a fluid damper is shown in (fig). It consists of a stainless steel piston with bronze orifice head. It is filled with silicone oil. The piston head utilizes specially shaped passages which alter the flow of the damper fluid and thus alter the resistance characteristics of the damper. Fluid dampers may be designed to behave as a pure energy dissipater or a spring or as a combination of the two.

A fluid viscous damper resembles the common shock absorber such as those found in automobiles. The piston transmits energy entering the system to the fluid in the damper, causing it to move within the damper. The movement of the fluid within the damper fluid absorbs this kinetic energy by converting it into heat. In automobiles, this means that a shock received at the wheel is damped before it reaches the passengers compartment. In buildings this can mean that the building columns protected by dampers will undergo considerably less horizontal movement and damage during an earthquake.

Fluid Viscous Dampers

New Breed of Energy Dissipation Devices

The innovative methods for control of seismic vibrations such as frictional and other types of damping devices are important integral part of seismic isolation systems as they severe as a barrier against the penetration of seismic energy into the structure. In this concept, the dampers suppress the response of the isolated building relative to its base.

The novel friction damper device consists of three steel plates rotating against each other in opposite directions. The steel plates are separated by two shims of friction pad material producing friction with steel plates.

When an external force excites a frame structure the girder starts to displace horizontally due to this force. The damper will follow the motion and the central plate because of the tensile forces in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite way. The damper dissipates energy by means of friction between the sliding surfaces.

The latest Friction-ViscoElastic Damper Device (F-VEDD) combines the advantages of pure frictional and viscoelastic mechanisms of energy dissipation. This new product consists of friction pads and viscoelastic polymer pads separated by steel plates. A prestressed bolt in combination with disk springs and hardened washers is used for maintaining the required clamping force on the interfaces as in original FDD concept.

Design Earthquake Resistant Structures » RCC Buildings