Earthquake resistant low rise building

Low Rise Earthquake Resistant Buildings

Anutosh Chaudhuri 11AR10004

Avijit Singh Dogra 11AR10006

Keerthana Rao Balasu 11AR10013

Harika Nelli 11AR10022

Rakesh Samaddar 11AR10031

Rahul Rathore 11AR10029

Goals of Earthquake Resistant Design

It is economically not feasible for ordinary buildings to be designed as absolute earthquake proof. However, the goals for EQRD are shown below.

Serviceability level Earthquake•Frequent and minor earthquakes•Building should not be damaged and continue to remain in service•Expected ten times during the life of building

Damageability level Earthquake•Occasional moderate earthquakes •No structural damage is expected•Non structural damage should not lead to any loss of life•Expected once or twice during the life of building

Safety level Earthquake•Rare major earthquakes •Building should not collapse•Non structural & structural damage should not lead to any loss of life.

Earthquake types

Building Shape

In symmetrical building plan the distance between Centre of Mass [CoM] & Stiffness Centre / Centre of Resistance [CoR] is less compared to asymmetrical plans.

Irregular Building configuration results in twisting (torsion or rocking) when subjected to seismic forces.

Direction of ground motion

Inertia Force due to Earthquake act through

CoM

CoM

CoR

Equal & opposite parallel forces produce couple

Internal resistive Force due to Structural Element configuration act through CoR

Asymmetry

Geometrical Asymmetry

Stiffness Asymmetry

Mass Asymmetry

Plan of building

Elevation of building

Building Joint

Geometrical Asymmetry - Plan

Asymmetrical Plans

Asymmetrical buildings undergo large amount of torsion and hence extreme corners are subjected to heavy force.

Avoid asymmetrical buildings like: I, L,U, and T shape buildings.

Symmetrical Plans

As CoM & CoR coincide in plan twisting will not occur due to earthquake. Building will need to resist the horizontal inertia force only.

Symmetrical plans like Rectangular, Square, Polygonal or Circular are favourable.

Geometrical Asymmetry - Elevation

In vertical directions, sudden change in stiffness or mass creates instability. It attracts more forces and hence undergo large deflections. Excessive deflection induces large bending moment and shear force in the structural members.

Stress concentration zone

Gradual change in lateral stiffness and building floor mass in vertical direction can be provided

Geometrical Asymmetry – Building Joint

Typical problem occurs in the junction areas as two neighbourhood block strikes each other and try to separate out in a periodic motion

During earthquake three blocks undergo twist in three different orientations

SolutionBuilding blocks can be separated by seismic Gaps. The individual building blocks now vibrate in plan separately. The Stress concentration in block joints can be avoided.

Stiffness Asymmetry

Mass Asymmetry

Difference in CoM & CoR will invite Torsion Couple, which produce instability

Building Foundations

Problems Differential Settlement Liquefaction

Solutions Tie-Beam

Raft Foundation Base Isolation

Differential Settlement

•The resisting inertia force in the super structurecauses uplift and compression in the flat foundation at different ends.

•The tilting of foundation due to uplift force initiate the failure of foundation of building

Liquefaction

Three main prerequisites for liquefaction : 1.A layer of relatively loose sand or silt.

2.A water table high enough to submerge a layer of loose soil.

3.An intensity of ground shaking sufficient to increase the water pressure between soil particles to cause the soil-water mixture to liquefy.

Liquefaction

•The pressures generated during large earthquakes with many cycles of shaking can cause the liquefied sand and excess water to force its way to the ground surface from several meters below the ground.

•The phenomenon may incorporate both flow of already liquefied sand from a layer below ground and a quicksand effect.

Solution

Isolated Foundation

•Individual footings should be interconnected with tie-beams or a structural slab to prevent any relative horizontal movement occurring during earthquake shaking.

Raft Foundation •As the raft has a common base and it equally and uniformly distribute the superstructure load to the sub soil.

•It spreads concentrated loads onto a larger area and makes the structure tolerant of minor ground subsidence.

•It mobilizes the entire weight of the building to resist inertia-induced overturning moments.

Base Isolation

Techniques for making Earthquake-Resistant Structures

Currently, there are several design philosophies in earthquake engineering, making use of experimental results, computer simulations and observations from past earthquakes to offer the required performance for the seismic threat at the site of interest. There are five broad categories of structural systems which are taken into account when designing earthquake resistant buildings.

Bearing wall systems, Building frame systems,Moment-resisting frame systems,Dual systems,Cantilever column systems

1. Bearing Wall Systems

•Structural walls located throughout the structure provides primary vertical support for the building’s weight and that of its contents as well as the building’s lateral resistance.

•Bearing wall buildings are commonly used for residential construction, warehouses, and low-rise commercial buildings of concrete, masonry, and wood construction.

•Unlike standard masonry walls which are solid, the interlocking mortar-less masonry blocks allow slight movement and lock tighter over time, aided by an innovative application of steel reinforcement. This construction technique is also better at dissipating the energy of a seismic wave than traditional masonry.

2. Bearing Frame Systems

•For buildings constructed of structural steel and concrete Lateral resistance is provided either by diagonal steel members (termed braces) that extend between the beams and columns to provide horizontal rigidity or by concrete, masonry, or timber shear walls that provide lateral resistance but do not carry the structure’s weight.

•In some building frame structures, the diagonal braces or walls form an inherent and evident part of the building design. In most buildings, the braces or walls may be hidden behind exterior cladding or interior partitions.

3. Moment-Resisting Frame

•Used for both structural steel and reinforced concrete construction.

• The horizontal beams and vertical columns provide both support for the structure’s weight and the strength and stiffness needed to resist lateral forces.

•Stiffness and strength are achieved through the use of rigid connections between the beams and columns that prevent these elements from rotating relative to one other.

•Moment-resisting frame systems are popular because they do not require braced frames or structural walls, therefore permitting large open spaces and facades with many unobstructed window openings

4. Dual Systems

•Dual systems is an economical alternative to moment-resisting frames, are commonly used for tall buildings.

•Dual system structures feature a combination of moment-resisting frames and concrete, masonry, or steel walls or steel braced frames.

•The moment-resisting frames provide vertical support for the structure’s weight and a portion of the structure’s lateral resistance while most of the lateral resistance is provided either by concrete, masonry, or steel walls or by steel braced frames..

Earthquake Building Codes in Japan

Japan is a seismically active country and has some of the most rigorous earthquake building standards in the World.

1924: Earthquake resistant construction regulations were introduced.

Cause:1923 Kanto Earthquake

Effect: Regulations for wooden beams, reinforced concrete used in construction in town areas

1950: The introduction of the Building Standards Act “kyu-taishin”

Cause: 1948 Fukui earthquake

Effect: Regualtions were no longer limited to town areas.

1971: Amendments to the Act

Shearing reinforcements and reinforced concrete foundations

1981: New Earthquake Resistant Building Standard Amendment“Shin-taishin”

Cause: 1978 Miyagi Earthquake

Effect: Buildings should not suffer during a mid-size

earthquake, and a buiding shold not collapse during l

arge earthquakes

Case Study: Shigeru Ban's Paper Tube School

Shigeru Ban along with a team of Chinese and Japanese students, built temporary but resilient schools out of plywood and recycled cardboard tubes.

a. Recycled paper tubes are molded into load-bearing columns, bent into trusses and rapidly assembled.

b. Can be made waterproof and fire resistant.

c. Various thickness and diameters of paper can be added to a structure to support more weight as necessary

d. Can build structures a few stories high.

Framework -made from paper tubes

Walls –cheap material, easy to produce

Roofs -made of plywood, and used polycarbonate as insulation.

Feasibility: Uses materials that are available anywhere in the world and is also structurally sound