NEESR: Near-Collapse Performance of Existing Reinforced Concrete Structures

Presented by Justin Murray

Graduate StudentDepartment 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 Structures built prior

to the mid-1970s are particularly susceptible to shear-axial column failure

• Current practice in seismic rehabilitation (ASCE 41) dictates that a column reaching capacity (failure) constitutes collapse of the entire structure

• A single element failure does not necessarily lead to “structural collapse”

• Loads may be redistributed away from damaged area through different mechanisms including– Vierendeel Action– Catenary Action

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 Concrete• 10 Story Building

• Stiff Spandrel Frames• One-Way Slabs

• Designed using Ultimate Strength 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

Hybrid Simulation

• Physical Test Specimens located at University of Illinois (UIUC) MUST-SIM 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

• Transformation matrix a can remove rigid body motion from the displacement vector, where

v = a u• a is formed based on the

orientation of the element with respect to the global coordinate system

• Angles defining this orientation are shown in the figure to the right

For the sake of space, only the first few components of the a matrix are shown below:

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This approach enables testing of any beam-column element (not just fixed-base) with a single Loading and Boundary Condition Box (LBCB) available at the UIUC MUST-SIM facility.

Building Pushover Analysis

• Three Shear Capacity Models are used to determine columns that are likely to fail– ACI-318 (2011)– ASCE 41-06 (2007)– Sasani (2007)

• All are different, but have common factors:– gross cross section area– axial force– concrete strength– transverse reinforcement– aspect ratio and ductility (ASCE/Sasani)

Column Shear Failure (From Pushover)

• Pushover results of two critical columns are shown below (Shear vs. Top Building Displacement)– Plotted against the three failure models (see legend)– Axial force plotted on right axis

• ACI (Green) predicts close failures• ASCE and Sasani clearly show column A2 as the critical column• 2nd floor columns are critical due to being shorter than 1st 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. CMMI-1135005, which is greatly appreciated.

QUESTIONS?