Near-Collapse Performance of Existing Reinforced Concrete

Report
NEESR: Near-Collapse Performance of Existing
Reinforced Concrete Structures
Presented by Justin Murray
Graduate Student
Department of Civil and Environmental Engineering
Northeastern University
Students: Justin Murray1, Adam Mueller2 Faculty: Mehrdad Sasani1, Xiaoyun Shao2
1
Northeastern University, Boston, MA
2
Western Michigan University, Kalamazoo, MI
Shear-Axial Failure Risk
Concrete
built
• • Loads
mayStructures
be redistributed
priorfrom
to thedamaged
mid-1970s
are
away
area
particularly susceptible to
through
different
mechanisms
shear-axial
column
failure
including
• Current practice in seismic
(ASCE 41)
–rehabilitation
Vierendeel Action
dictates that a column
capacity
–reaching
Catenary
Action(failure)
constitutes collapse of the
entire structure
• A single element failure does
not necessarily lead to
“structural collapse”
Project Goal
• To show, through the use of Hybrid Simulation,
that element failure does not necessarily lead to
structural failure
– Examining the 3 most critical column elements
experimentally, up through element failures
• Upon completion of experimentation, develop
new methods for system level near-collapse
evaluation of existing structures under seismic
ground motions
Hybrid Simulation
• A series of hybrid simulations with full-scale column specimens at the
UIUC MUST-SIM facility looks to observe shear-axial column failure, and
it’s effect on the remaining portion of the structure
Representative Building Plan
• Reinforced
Designed using
Concrete
Strength
• Ultimate
10 Story Building
Design (USD)
– First appeared in
design codes in
1956 publication of
ACI
• Design methods
used are in
accordance with
ACI 318-63
Exterior Column
Interior Column
• Stiff Spandrel Frames
• One-Way Slabs
Hybrid Simulation
• Physical Test Specimens located at
University of Illinois (UIUC) MUSTSIM facility
– Utilizing MUST-SIM’s 3 “Load and
Boundary Condition Boxes” (LBCBs)
• Computer model constructed within
OpenSees (2011) at Northeastern
University
• Software development and control
of LBCBs will be managed by the
UIUC MUST-SIM team
• Analyzed under uniaxial and tri-axial
ground motions
Conversion from 12 Degrees of
Freedom to 6 Deformations
• MUST-SIM LBCB’s are
fixed to the top of
columns
• Base of test column fixed
to strongfloor
• Actual building columns
(upper floors) have
displacements/rotations
at top and bottom
– Must condense
displacements by
removing rigid body
motions
Matrix Transformation
For the sake of space, only the first few components of the a matrix are shown below:
• Transformation matrix a can
remove rigid body motion
from the displacement
vector, where
v=au
• a is formed based on the
.
.
.
.
.
.
.
.
.
orientation of the element
with
respect
totesting
the global
This
approach
enables
of any beam-column element (not just fixed-base) with a
single
Loading andsystem
Boundary Condition Box (LBCB) available at the UIUC MUST-SIM
coordinate
facility.
• Angles defining this
orientation are shown in the
figure to the right
Building Pushover Analysis
• Three
All areShear
different,
Capacity
but have
Models
common
are used
factors:
to
determine
that are likely to fail
– gross crosscolumns
section area
– ACI-318
(2011)
axial force
– ASCE
41-06
(2007)
concrete
strength
– Sasani
(2007)
transverse
reinforcement
– aspect ratio and ductility (ASCE/Sasani)
Column Shear Failure (From Pushover)
•
•
ACI
(Green)results
predicts
failures columns are shown
Pushover
ofclose
two critical
ASCE
Sasani
show column
A2 as the critical
belowand
(Shear
vs.clearly
Top Building
Displacement)
column
–nd Plotted against the three failure models (see legend) st
• 2 floor columns are critical due to being shorter than 1
– Axial force plotted on right axis
floor
Column Shear Failure (From Pushover)
• Available “Limit State” material in Opensees (Elwood’s
model) is modified
– Can model failure based on other failure criteria
– Sasani’s model showed higher shear strength in column A2
than ASCE and ACI
– New material model uses Sasani’s failure criteria (equation
shown below)
Current studies and Future Work
• Research is now centered on how the building
behaves after this column loses strength
• Current focus is on redistribution of the lateral
and gravity loads to the neighboring columns.
– Also after failure, shear demand on columns directly
above or below a failed column are of particular
interest.
• Experiments at UIUC MUST-SIM facility
– Current schedule shown above (based on availability)
Modeling Element Failure and
System Collapse Criteria
• The mechanism and conditions for collapse of buildings
representative of existing vulnerable RC frame structures
under triaxial seismic ground motion will be identified and
used to develop criteria, methods, and procedures for
seismic collapse evaluation of RC frame structures.
• We will develop capacity models for shear-axial-torsion
interaction failure of columns affected by triaxial seismic
actions and implementation of analytical tools in computer
program OpenSees;
• We will also develop methods and procedures to evaluate
near-collapse performance of existing RC frames systems,
which considers system level response of structures.
Acknowledgement
This presentation was based upon research
supported by the NSF Award No. CMMI1135005, which is greatly appreciated.
QUESTIONS?

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