progressive collapse analysis of rc multi-story building
TRANSCRIPT
IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 01, 2016 | ISSN (online): 2321-0613
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Progressive Collapse Analysis of RC Multi-Story Building B. Ragavendar1 G. Saravanan2 1PG Student 2Assistant Professor
1,2Department of Civil Engineering 1,2Adhiyamaan College of Engineering, Hosur, Krishnagiri, Tamilnadu, India
Abstract— The term “progressive collapse” is the spread of
local failure from an element to other element within the
structure ultimately causing total collapse of the build-ing. It
is caused due to abnormal loading being induced in the
building. The abnormal loading may be induced by
earthquake, bomb blasting, tsunami, aircraft impact, any
other man-made intentional or unintentional activi-ties and
other natural hazards. These loadings causes change in the
loading pattern and boundary condition of the structure, this
result in other structural elements within the structure to load
beyond their capacity and fail. Thus the failed structure is
required to seek alternate load path in order to redistribute its
load. This process continues until the structure finds the
equilibrium by element failing or by finding alternate load
path to dis-tribute the load. In this project a multi-story
building designed, using IS codes is considered for
progressive collapse analysis. Linear static analysis is carried
out by using ETABS as per GSA guidelines. Story shear and
story drift are calculated to know the potential for progressive
collapse.
Key words: Progressive collapse, Demand Capacity ratio,
linear stat-ic analysis, column removal
I. INTRODUCTION
According to GSA guidelines (2013) (Alternate Path
Analysis and Design Guidelines for Progressive Col-lapse
Resistance), the progressive collapse is defined as “an extent
of damage or collapse that is dis-proportionate to
the magnitude of initiating event” Pro-gressive collapse is
described as ‘collapse to an extent disproportionate to the
cause”, it is frequently triggered by unanticipated extreme
events. Progressive col-lapse causes irreplaceable human
loss, financial loss to the country, public exposed to high
psychological shock due to this traumatic event. The
unanticipated extreme events cause abnormal loading in the
elements of the Structure. The abnormal loading within the
structure may be caused by manmade intentional activities or
unintentional activities, and other natural hazards. The
abnormal loading cause initial damage which propa-gates in
the structure, due to incapability of structure redistribute the
load that were carried by the initially damaged element to the
adjacent element, resulting in failure/ collapse of an entire or
part of the structure.
The progressive collapse is defined in ASCE/SEI 7
(2010) as the spread of an initial local failure from element
to element, eventually resulting in the collapse of an entire
structure or is proportionately large part of it. Initially
structural engineers did not paid attention on this issue, a
number of high profile disasters made it into consideration.
A. Objective of The Study:
To evaluate the potential for progressive col-lapse of
multi-story building using the linear dynamic analysis,
nonlinear dynamic analysis and push over analysis by
column removal condition
To find the elastic behavior of building by plotting push
over curve
To compare various results such as story shear and story
drift for bare frame model and model with removal of
column in different stories.
B. Scope of The Study:
The focus of this project is to determine if a structure is
susceptible to progressive collapse. The building is de-
signed as per IS 456- 2000, IS 1893-2002, SP6. It is modeled
using ETAB software and analyzed for progressive
collapse resistance using GSA guidelines.
C. Need for The Present Study:
The difference in performance of seismic resistant and
progressive resistant is to be examine, to help the structural
engineer to recognize which priority need to be considered to
make the structure robust, de-pending on the basic properties,
such as connection strength, stiffness and ductility as well as
frame man-agement.
It is unclear that the structures with seismic re-
inforcement and sway frames designed for seismic re-gions
are having resistance against progressive collapse.
II. MODELLING FEATURES
Three dimensional finite element model of a 7 story building
is developed in ETABS 2015. Analyses for column removal
are performed using linear static analy-sis, linear dynamic
analysis, and linear static analysis technique.
III. PRELIMINARY DATA
Length X width - 30mX32m
No of story - 7
Bottom story height - 3.1m
Rest of story height - 3.4m
Beam size - 225mmX450mm
Column size - 375mmX450mm
Slab thickness - 150mm
Support condition - Fixed
IV. DESIGN DATA
Live load = 3KN/m2 on typical floor, 1.5KN/ m2 on other
floors.
Floor finish = 1.5KN/m2
Wall load on all beams =9.54KN/m
Earthquake load – the structure is designed for zone IV as per
IS 1893-2002
Type of soil – type II (medium)
Response reduction factor – 3
Importance factor – 1.5
fck = 25N/sq.mm , fy= 415N/sq.mm
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V. PROGRESSIVE COLLAPSE ANALYSIS
To evaluate the potential for progressive collapse anal-ysis
using GSA 2003 guidelines linear static, linear dy-namic, non
linear static analysis were done using ETABS 2015. GSA
recommends removing a column from the middle of the
traverse side of the building, near the middle of the
longitudinal side of building, and at the corner of the building
for the structure to be analyzed. When analyzing the structure
for progressive collapse potential, GSA (2003) recommends
a general loading factor to be used for every structural
member in the building being tested.
Load=2.0(Dead Load +0.25(Live Load)) (1)
When vertical members are instantaneously
removed, GSA (2003) uses Demand-Capacity Ratios (DCR)
to analyze which structural members will exceed their load-
ing capacity and lead to progressive collapse. Using the linear
elastic static analysis, the DCR values are found using
Equation 2.
DCR= QUD/QCE (2)
Where, QUD꞊ Acting force (demand) determined in
component or connection/joint moment, axial force, shear,
and possible combined forces).
QCE꞊ Expected ultimate, un-factored capacity of the
component and/or connection/joint (moment, axial force,
shear and possible combined forces)
The allowable DCR values for primary and
secondary structural elements should be less than or equal to
2 for typical structural configurations. Structural elements
and connections that exceed DCR value is considered to be
severely damaged or collapsed.
VI. NONLINEAR INCREMENTAL DYNAMIC ANALYSIS
The destination of a building might be change during its
lifetime, from apartments to office building leading to
increased gravity loads. This assumes that the risk for
progressive collapse established in the initial phase of the
design could be changed from low to high. In this context, a
nonlinear incremental dynamic analysis is conducted in order
to establish the ultimate load bearing capacity to progressive
collapse of the building; thus, the maximum value of the
supplementary gravity load for which the structure will fail
through progressive collapse when subjected to suddenly
column removal will be identified.
Fig. 1: Bare frame model of 7- story building.
Fig. 2: Maximum story displacement of bare frame model
Progressive Collapse Analysis of RC Multi-Story Building
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Fig. 3: Story drift of bare frame model.
Fig. 4: Story shear of bare frame model.
Fig. 5: Push over curve of bare frame model.
Fig. 6: Max story displacement for corner column
removal in longer direction.
Progressive Collapse Analysis of RC Multi-Story Building
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Fig. 7: Max story drift for corner column removal in longer
direction.
Fig. 8: Story drift for corner column removal in longer
direction.
Fig. 9: Push over curve for corner column removal in longer
direction.
Fig. 10: Max story displacement for middle column
removal in longer direction.
Progressive Collapse Analysis of RC Multi-Story Building
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Fig. 11: Max story drift for middle column removal in
longer direction
Fig. 12: Story drift for middle column removal in longer
direction.
Fig. 13: Push over curve for middle column removal in
longer direction.
VII. RESULTS AND DISCUSSION
Since the DCR values of columns are less than 2 in all the
cases studied, the columns are adequate and do not need
additional reinforcement to meet GSA criteria. Columns
designed for seismic forces in all Zones have inherent ability
to resist Progressive Collapse.
1) When column was removed, among the intersecting
beams the shorter span beams tend to take the extra load
and DCR values that beams were more compared to
longer span beams.
2) For removed column C6, DCR values of B1 beams
exceed 2. Decreasing pattern of DCR values is observed
has storey increases. B5 beams below storey 7 have DCR
values more than 2 and others are less than 2.
3) For C31 column removed, DCR of 1 B25 beams and 1 -
storey B24 beams exceed 2.Including adjacent B32
beams other are well within 2.
4) For C51 column removed, DCR of 1 B40 beams and B41
beams of all stories exceed 2.Including adjacent B32
beams other are well within 2.
5) For C53 column removed, DCR of B11, B12, B6 beams
of all stories exceed 2. Adjacent B16 beams have DCR
less than 2.
VIII. CONCLUSION
The adequate reinforcement provided in extra to beams which
are unsafe can develop alternative load paths and prevent
progressive collapse due to the loss of an individual
member. This study illustrates the inherent ability of
seismically designed RC beam-column frames to resist
progressive collapse. The analysis shows that a building
Progressive Collapse Analysis of RC Multi-Story Building
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designed for zone IV is capable of generating an alternate
load path to transfer the loads if a vertical member fails, since
all the columns are having ac-ceptable DCR values. The
high moments generated on the beams are at the beam column
junction and hence design can be done to prevent
collapse.Story drift and story shear of both linear dynamic
analysis and non linear static analysis comparison were made
and found that linear dynamic analysis is greater than the
later.The potential for progressive collapse mechanism of the
above said modelling is studied.Push over curve is ob-tained
for bare frame model by displacement against base shear, and
also for column removal condition is done.
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