Upload
mohan-naik
View
228
Download
0
Embed Size (px)
Citation preview
8/12/2019 Structures Sstability Under Earthquake
1/24
STRUCTURES SUBJECTED TOEARTHQUAKE AND THEIER
VULNERABILITY
1
8/12/2019 Structures Sstability Under Earthquake
2/24
INTRODUCTION
Indian buildings built over past two decades areseismically deficient because of lack ofawareness regarding seismic resisting measures .
Also seismic design is not normally practiced inmost of the buildings being built in India.Therefore, seismic vulnerability estimation is
2
- .
The difficulties faced in seismic vulnerabilityestimation of a building are there is no reliableinformation/database available for existing
building stock, construction practices, in-situstrength of material and components of thebuilding
8/12/2019 Structures Sstability Under Earthquake
3/24
The aim of evaluation is to assess the seismiccapacity of earth quake vulnerable buildings orearthquake damaged buildings for the future use.
The evaluation may also prove helpful for degreeof intervention required in seismically deficientstructures
3
The methodologies available so far for theevaluation of existing buildings can be broadlydivided into two categories
qualitative methods analytical methods
8/12/2019 Structures Sstability Under Earthquake
4/24
Methods forseismic evaluation
Qualitative
methods
Analyticalmethods
4
Conditionassessment
Visualinspection
Non destructivetesting
C/D methodPush over
analysis
Inelastic timehistory method
8/12/2019 Structures Sstability Under Earthquake
5/24
Qualitative methods are based on the backgroundinformation available like architectural & structural dwg.
Condition Assessment
means the collection of information about the structureand its past performance characteristics to similar type of
I. Qualitative methods
5
s ruc ure ur ng pas ear qua eData collection
Collection of data is an important portion for the seismicevaluation of any existing building. The information
required for the evaluated building can be divided asfollows.
8/12/2019 Structures Sstability Under Earthquake
6/24
Building data
Architectural, structural and construction drawings
Vulnerability parameters:
no. of stories, year of construction and total floor area
Seismicity of the site.
Construction data
6
Identification of gravity load resisting system Identification of lateral load resisting system
Maintenance, addition, alteration, or modification in
structure Field surveys of the structures existing condition
8/12/2019 Structures Sstability Under Earthquake
7/24
Structural data
Structural concept: vertical and horizontal irregularities,torsional eccentricity, pounding, short column and others
Detailing concept: ductile detailing, special confinement
reinforcement
Pounding- column distress, possibly local collapse
-
7
effects and concentration of damage at the re entrantcorners
Unsymmetrical buildings in elevation- abrupt change in
lateral resistance Vertical strength discontinuities
Short columns
8/12/2019 Structures Sstability Under Earthquake
8/24
Large tie spacing in columns lack of confinement of core shear failures.
Insufficient column lengths concrete to spall.
Locations of inadequate splices- brittle shear failure
Insufficient column strength for full moment hingecapacity brittle shear failure.
Lack of continuous beam reinforcement- hinge formation
8
.
Inadequate reinforcing of beam column joints or locationof beam bar splices at columns joint failures.
Improper bent up of longitudinal reinforcing in beams as
shear reinforcement shear failure during load reversal Foundation dowels that are insufficient to develop the
capacity of the column steel above local column distress.
8/12/2019 Structures Sstability Under Earthquake
9/24
Seismic evaluation data The criteria of evaluation of building will depend on
materials, strength and ductility of structural componentsand detailing of reinforcement.
Materials evaluation
Structural detailing
9
, ,
Limitation of sections
Limitation of min. & max. flexural reinforcement
Restriction of splices
Development length requirements Shear reinforcement requirement
Special confining requirements
Column steel dowelled into the foundation
8/12/2019 Structures Sstability Under Earthquake
10/24
Field evaluation /Visual inspection method
This is very quick way of assessing the building
vulnerability based on visual screening
Visual inspection is the most widely used form ofnon destructive evaluation.
10
survey in which an experienced screener visually
examines a building to identify features that affect
the seismic performance of the building, such asthe building type, seismicity, soil conditions and
irregularities
8/12/2019 Structures Sstability Under Earthquake
11/24
The procedure for visual inspection are described
in the following steps Perform a walk through visual inspection to
become familiar with the structure
11
Gather background documents and information onthe design , construction, maintenance, and
operation of the structure
8/12/2019 Structures Sstability Under Earthquake
12/24
Identify the location of vertical structural elements columns or
walls Sketch the elevation with sufficient details dimensions, openings,
observed damage such as cracks, spalling, and exposed reinforcingbar, width of cracks
After performing a detailed visual inspection, the investigator mustbe able to distinguish between recent damage and pre existingdamage , ensuring that the observed damage may or may noit prove
12
.
Limitations of Visual inspection method
Applicable for surface damage that can be visualized No identification of inner damage- health monitoring of building,
change of frequency and mode shapes
8/12/2019 Structures Sstability Under Earthquake
13/24
8/12/2019 Structures Sstability Under Earthquake
14/24
8/12/2019 Structures Sstability Under Earthquake
15/24
Ultrasonic pulse velocity
It is used for determining the elastic constants(modulus of elasticity and Poissons ratio) and thedensity by conducting tests at various points on the
structure. Impact echo
15
discontinuities within the thickness of a wall.
Penetrating radar
It is used to detect the location of reinforcing bars,cracks, voids and other material discontinuities,verify thickness of concrete.
8/12/2019 Structures Sstability Under Earthquake
16/24
II Analytical methodsCapacity/demand (C/D) method
The forces and displacements resulting from an elastic
analysis for design earth quake are called demand.
16
members to resist these forces and displacements.
A (C/D) ratio less than one indicates member failure and
thus needs retrofitting.
The main difficulty in using this method is that there is no
relationship between member and structure ductility factor
8/12/2019 Structures Sstability Under Earthquake
17/24
Push over analysis
The push over analysis of a static non linearanalysis under permanent vertical loads andgradually increasing lateral loads.
The equivalent static lateral loads approximatelyrepresent earth quake induced forces.
17
in a structure is obtained by the analysis that wouldindicate any premature failure.
The analysis is carried out upto failure, thus itenables determination of collapse load andductility capacity.
This type of analysis enables weakness in the
8/12/2019 Structures Sstability Under Earthquake
18/24
Inelastic time-history analysis
A seismically deficient building will be subjected toinelastic action during design earthquake motion.
The inelastic time history analysis of the building under
strong ground motion brings out the regions of weaknessand ductility demand in the structure.
This is the most rational method available for assessin
18
building performance. There are computer programs available to perform this
type of analysis.
This methodology is used to ascertain deficiency and postelastic response under strong ground shaking.
8/12/2019 Structures Sstability Under Earthquake
19/24
Seismic evaluation of RCC columns
Possible damages which are frequently observed
after the earth quakes are mainly due to lack of confinement,
19
,
insufficient splices length,
inadequate splicing at the same section ,
hook configurations, poor concrete quality etc.
8/12/2019 Structures Sstability Under Earthquake
20/24
Seismic evaluation of RCC beams
There is little evidence that the buildings have
collapsed due to beam failure.
Reasons for the possible damages are due to
20
ac o ong tu na compress ve re n orcement, infrequent transverse reinforcement,
bad anchorage of the bottom reinforcement into the
support,
bottom steel termination at face of column.
8/12/2019 Structures Sstability Under Earthquake
21/24
Seismic evaluation of RC beam column joints
Beam column joints are critical element in frame
structures and are subjected to high shear and bondslip deformations under earthquake loading
21
are due to
inadequate reinforcement in beam column joint,
absence of confinement reinforcement,
inappropriate location of bar splices in column etc.
8/12/2019 Structures Sstability Under Earthquake
22/24
Seismic evaluation of RC slabs
Generally slab on beams performed well during
earth quakes and are not dangerous but cracks inslab creates serious aesthetics and functional
22
Damage to slab often occurs due to
irregularities such as large openings at
concentration of earth quake forces, close to widely spaced shear walls.
8/12/2019 Structures Sstability Under Earthquake
23/24
SUMMARY
There are many buildings that have primary
structural system, which do not meet the currentseismic requirements and suffer extensive damageduring the earthquake.
23
The methodology can be implemented to estimateseismic vulnerability of different types of existingbuilding stock in Indian Cities and further extended
to investigate the impact of mitigation measures onthe consequences of an earthquake.
8/12/2019 Structures Sstability Under Earthquake
24/24
Based on the vulnerability analysis, risk in theform of casualties and economic losses can further
be estimated after collecting wardwise
demographic and census information for a city. Further, the vulnerability studies demands special
attention with reference to monumental buildin s
24
lifelines like rail/road, water supply, electricsupply, sewage, communication, dams, hospitals &
schools, vulnerable industries.
Also there is a need to identify safe zones/
domains/ structures and secure routes to work as a
relief centers and relief dispersion on incidence of