Download Seismic Evaluation and Retrofit of Existing building in Islamabad

International Journal of
Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 02, No. 04, October 2013
Seismic Evaluation and Retrofit of Existing building in Islamabad
ZEESHAN ALAM, ABBAS HAIDER, A. Q. BHATTI
1
School of Civil Engineering, University of Lahore, Islamabad, Pakistan
2
NUST Military College of Engineering, Risalpur, Pakistan
3
National University of Sciences and Technology, Islamabad, Pakistan
Email: [email protected], [email protected], [email protected]
Abstract: One of the remedies for earthquake hazards reduction is seismic retrofitting. But without using
intervention technique which is based on seismic evaluation of the building structure, it may not be possible to
improve the seismic performance of the building structures by the use of retrofitting. In this paper the simulation
of a building is performed by using nonlinear static “push over” analysis. The said building is 4storey RC
building. In this work the real site based demand spectra is compared with the demand spectra of “ATC 40
procedure”. The simulation performed is then compared for the sake of evaluation with or without retrofitted
structure. For various safety margins the plastic hinges are plotted.
Keywords: Seismic Evaluation, Nonlinear Static Analysis, SAP2000, Existing Structures, ATC 40 Response
Spectrum Capacity
1. Introduction:
Earthquake is one of the prominent phenomenon
observed and experienced by the mankind since the
evaluation of human life on this planet. This
phenomenon has caused an extreme level of
catastrophe and devastation to the social
infrastructure and ruined the shining lives into
graveyards.
Therefore, the earthquake vulnerability of building
structures has remained a key area for the researchers
in order to minimize the hazards of earthquake as
much as possible. Country like Pakistan is under the
threat of moderate and high level of earthquakes.
October 8, 2005 earthquake in Pakistan has brought
enormous damages and challenges for the researchers
to develop measure to reduce the hazards.
The said earthquake has also highlighted the
importance of redevelopment and improvement of
“Building Code of Pakistan 1986” Though an effort
has been made in the form of “Building Code of
Pakistan 2007” which is derived mainly by the aid of
“UBC 97” and “ACI 318-05”
BCP 2007 addresses the earthquake hazards in a
better way and helps to reduce the losses during
earthquakes. It is important to note that BCP2007,
focus on the design provisions for new buildings
(BCP 2007) but to seismically evaluate and re
strengthen the existing developed buildings this code
does not help.
One of the possible ways to re strengthen the existing
buildings is seismic retrofitting but for retrofitting the
building should be seismically evaluated. If the
building is not seismically evaluated in that case the
application of retrofitting is of no use.
ASCE 31-03 is one of the recently updated guidelines
at USA for “Seismic Evaluation of Existing
Buildings”. This guideline is the assembly of three
tiers aiming to enhance analytical detail of existing
buildings & also decrease conservativeness, for
safety. Before Evaluation of tier 1, it is important to
carry a thorough study of available documents and
details & degree of desired performance i.e. “Life
safety or immediate occupancy”, the seismic Zone
“Low, moderate and high” and the type of building is
identified.
In Tier1 Evaluation, the configurational details &
strengths are checked. In this phase the potential
deficiencies are noted &observed.
In Tier2 Evaluation, the investigation is further
preceded regarding the deficiencies noted in Tier 1. It
can be done by static linear analysis or dynamic
linear analysis. If further investigation is Tier 3
Evaluation is preceded.
In Tier 3 Evaluation phase, we conduct detailed nonlinear static analysis (Pushover Analysis) or detailed
nonlinear dynamic analysis (Time History Analyses)
In terms of simplicity the preferred method for
performance based seismic Evaluation is inelastic
static or Pushover Analysis (Antoniou,s and pinho ,
R. ,2004 vol 8 , no . 5, pp 643 – 661 & Gupta,B. and
Kunnath,S.K). This paper comprises the static
nonlinear Pushover Analysis, with the aid of
SAP2000.
1.1 Problem Statement:
From the Oct 8, 2005 earthquake, we came to the
conclusion that the existing buildings are vulnerable
to damage & are earthquake sensitive. Keeping this
sensitive issue in mind, the Pakistan Building Code
was revised, in order to facilitate the ductility of new
building. But the problem regarding previously
existing building is still alive because these buildings
are non-ductile. To minimize the prevailing risk of
these non-ductile buildings, Retrofitting is the
proposed technique. Beside the usefulness of
retrofitting, it is difficult to create economic
resources to retrofit all existing buildings.
IJASGE 020406 Copyright © 2013 BASHA RESEARCH CENTRE. All rights reserved
ZEESHAN ALAM, ABBAS HAIDER, A. Q. BHATTI
The Rawalpindi city being in Seismic Zone 2B is
under the seismic risk regarding the buildings
constructed before 2007. Therefore, it is important to
seismically evaluate the existing buildings
Figure 2: Braced Model
1.2 Model Geometric Description:
Figure 1: Unbraced Model
Table 1: Geometric Configuration of Model
Member
Beam – B1
Coloum – C1
Coloum – C2
Slab
Bracing
Size (in)
12 x 18
12 x 16
9 x 16
Thickness = 6
W 12 x 30
2. Analysis Methodology:
The building analysis would be performed by using
any one of the procedure which is described above
but the limitations below should also be kept in mind.
When the desired level of nonlinearity is low then the
linear procedures could be appropriately used.
For irregular and complex buildings the approach of
nonlinear analysis is used. If one of the given
characteristics in the following is found in a building
then the linear analysis approach should not be used.
 Fundamental time period for a building T ≥
3.5 x( SD1/ SDS )
 When the horizontal dimension of a building
is more than 1.4 times the adjacent
building’s horizontal dimension.
 When the torsional irregularities exist in any
storey in a building and it occurs when the
diaphragm for a considered storey is not
flexible.
 Stiffness irregularity and vertical mass
irregularity exist in a building, it occurs
when the average drift for a particular storey
is more than the storey’s above or below by
150%.
 There is non- orthogonal system of lateral
force resistance.
Reinforcement (in2)
Top2.2, Bot 2.2 (Stirrup 0.22)
2.64
(Stirrups 0.22)
2.64
(Stirrups 0.22)
3. Static Nonlinear Analysis using FEM Software:
SAP2000 nonlinear version offers very strong and
significant characteristics for the nonlinear static
pushover analysis. Both 2D & 3D structures can be
analyzed as pushover analysis on SAP2000 nonlinear
version (Edward L Wilson)
The nonlinear behavior of the frame members are
determined by particular hinges and the structural
capacity drop occurs for the said hinges, When these
hinges takes a portion of negative slope displacement
curve, While performing the pushover analysis.
After performing analysis certain point are achieved
ranging from A to E, point A shows the unloaded
state, Point B shows yielding state of an element,
Point C represents nominal strength and co-ordinate
of point C on displacement axis shows deformation at
which significant amount of strength degradation
occurs. The part from C to D in the above figure
shows the starting failure of an element and the
strength of the element to resist lateral forces is
unreliable after point C. The portion D to E on the
curve shows that only the gravity loads are sustained
by the frame elements. After point E, The structure
has no more capacity to sustain gravity loads
(Computers and Structures Inc).
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 02, No. 04, October 2013, pp 157-160
Seismic Evaluation and Retrofit of Existing building in Islamabad
3. As the earthquake gets severe the inertia forces
distribution changes, while in pushover analyses
they are assumed to be constant.
Figure 3: Hinges for UnBraced Model
Figure 6: Pushover Curve for Braced Model
Figure 4: Hinges for Braced Model
Figure 7: ATC 40 Spectrum Capacity for Unbraced
Model
Figure 5: Pushover Curve for Unbraced Model
4. Limitations of the Adopted Procedure (Static
Nonlinear Analysis Pushover Analysis):
Though pushover analyses are quite useful, it has
following limitations.
1. Due to the higher modes of vibrations, the
indication of failure mechanism is an important
issue as the accuracy of pushover result is
affected.
2. Target displacements are very difficult to be
estimated.
Figure 8: ATC 40 Spectrum Capacity for Braced
Model
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 02, No. 04, October 2013, pp 157-160
ZEESHAN ALAM, ABBAS HAIDER, A. Q. BHATTI
Table 2: ATC 40 Spectrum Capacity Table for Unbraced Model
Table 3: ATC 40 Spectrum Capacity Table for Braced Model
5. Conclusion:
1. ASCE 31-03 gives a detailed and thorough
approach for the seismic evaluation of already
constructed buildings.
2. Tier 1 analysis method provided by ASCE 31-03
helps to quantitatively check the existing building
& it’s very easy & suitable for countries like
Pakistan.
3. In Linear Elastic Analysis ASCE 31-03, instead
of using single R factor for the entire structure,
different m factors are used depending on the
ductility of component being evaluated. This is a
more realistic approach.
4. Pushover analysis is preferably used for static
nonlinear procedures because of its simplicity.
5. Location of plastic hinges indicates the weak
zones in a building which is very helpful for re
strengthening the existing buildings.
6. Recommendations:
1. Using the guidelines of ASCE 31-03, building
code of Pakistan should be developed for existing
buildings.
2. Those buildings whose construction has been
made before BCP (2007), Tier 1 evaluation must
be carried out under the supervision of different
building authorities.
References:
[1] Antoniou,s and pinho , R. ,2004 b,
Development and verification of a
displacement-based
adaptive
pushover
procedure . Journal of earthquake
engineering vol 8 , no . 5, pp 643 – 661
[2] ATC 40, 1996, Seismic evaluation and
retrofit of concrete buildings, ATC-40,
Redwood City, CA.
[3] Gupta,B. and Kunnath,S.K. ,2000, Adaptive
spectra-based pushover procedure for
seismic evaluation of structures, Earthquake
spectra, 16(2), pp.2666-2684
[4] Applied Technology Council, ATC-40,
1996, Seismic Evaluation and Retrofit of
Concrete Buildings, Volume 1-2, Redwood
City, California
[5] ASCE/SEI 31-03, Seismic Evaluation of
Existing Buildings
[6] BCP (2007), “Seismic Provision for
Building Code of Pakistan” Ministry of
Housing and Works Government of Pakistan
Islamabad
[7] Computers and Structures Inc CSI, (1995),”
ETABS Extended Three Dimensional
Analysis of Building Systems Nonlinear
Version 9.5”, Berkeley, California USA
International Journal of Advanced Structures and Geotechnical Engineering
ISSN 2319-5347, Vol. 02, No. 04, October 2013, pp 157-160