model for anchored reinforcing bars under seismic excitations

106 Pages·2002·4.39 MB·English

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RIEPORTNO.
UCB/IEERC-93/08 EARTHQUAKE ENGINEERING RESEARCH CENTER
DECEMBER 1993
MODEL FOR ANCHORED
REINFORCING BARS
UNDER SEISMIC EXCITATIONS
by
GIORGIO MONTI
ENRICO SPACONE
FILIP C. FILIPPOU
Reportto the National Science Foundation
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-COLLEGE OF ENGINEERING
UNIVERSITY OF CALIFORNIA AT BERKELEY
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DISCLAIMER
Any opinions, findings, and conclusions or
recommendations expressed in this publication
are those of the authors and do not necessarily
reflect the views of the National Science
Foundation or the Earthquake Engineering
Research Center, University of California at
Berkeley.
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t. PB95-1921B3
MODEL FOR ANCHORED REINFORCING BARS
UNDER SEISMIC EXCITATIONS
by
Giorgio Monti
Visiting Scholar
Dipartimento di Ingegneria Strutturalee Geotecnica
University ofRome, Italy
Enrico Spacone
Doctoral Student
and
Filip C. Filippou
Associate Professor
Department ofCivil Engineering
AReportonResearchConducted
under GrantECE-8657525
from theNationalScienceFoundation
Report No. UCBIEERC-93/08
Earthquake Engineering Research Center
College ofEngineering
University ofCalifornia, Berkeley
\
\ December 1993
I
~\

ABSTRACT
This study presents a finite element model for reinforcing bars anchored in concrete
and subjected to severe cyclic excitations. The solution to the problem of stress transfer
between reinforcing steel and concrete is based on the flexibility method. In this case the
governing differential equations are solved by force interpolation functions that strictly
satisfy equilibrium along the anchored reinforcing bar. This solution method results in a very
robust and stable nonlinear algorithm, particularly, for systems that exhibit severe stiffness
and strength deterioration, as is the case for anchored reinforcing bars.
In the systematic derivation ofthe proposed solution method the model is viewed as a
simple mechanical system that is made up of two components in parallel. The first
component is the reinforcing bar and the second is the interface between reinforcing steel and
surrounding concrete. The nonlinear hysteretic behavior of the model derives entirely from
the nonlinear constitutive behavior of these two components. The hysteretic behavior of the
reinforcing bar is described by a cyclic steel stress-strain relation, while the hysteretic
behavior of the interface derives from a cyclic bond stress-slip relation that includes a
damage parameter for representing the progressive deterioration ofbond.
In formulating the finite element solution of the governing differential equations of
the stress transfer problem, two different methods are discussed and compared. In the first
method the governing differential equations are solved by approximating the displacement
field with interpolation functions. The selection of appropriate displacement shape functions
that are compatible with the node displacements is straightforward. In the second method the
governing differential equations are solved by approximating the stress field with
interpolation functions. In this case the selection of appropriate interpolation functions is not
straightforward. In a parallel system the total stress field results from the superposition ofthe
component stress fields. In this case the force interpolation functions in the components of
the proposed model must satisfy the requirement that the steel force distribution be in strict
equilibrium with the bond force distribution along the element. The proposed model is based
on the second method, since this offers significant numerical advantages overthe first.
The integration of the flexibility based finite element model in a conventional
stiffness based finite element program faces several challenges that are addressed in this
study with a new iterative algorithm. This algorithm is characterized by robust and stable
numerical behavior even under conditions of significant strength and stiffness loss of the
anchored reinforcing bar.
The study concludes with correlation studies between analytical and experimental
results and several parametric studies. The former are intended to establish the validity ofthe
proposed model, while the latter serve the purpose of identifying the significance of key
parameters on the local and global response of anchored reinforcing bars and for providing
some guidance for their design in regions ofhigh seismic risk.
ii
ACKNOWLEDGMENTS
This study was supported by Grant No. ECE-8657525 from the National Science
Foundation with Drs. A.J. Eggenberger. H. Lagorio and V.P. Singh as program directors. The
opinions expressed in this report are those of the authors and do not reflect the views of the
sponsoring agency.
The first author would like to thank Professors Paolo E. Pinto and Camillo Nuti ofthe
University of Rome, La Sapienza for their support and encouragement during the course of
this study.
iii

TABLE OF CONTENTS
ABSTRACT 1
ACKNOWLEDGMENTS iii
TABLE OF'CONTENTS v
LIST OFFIGURES vii
LIST OFTABLES ix
CHAPTER 1 INTRODUCTION ~ : 1
1.1 General 1
1.2 Review ofPrevious Studies 2
1.3 Objectives and Scope 4
CHAPTER 2 FINITE ELEMENT FORMULATION 5
2.1 General 5
2.2 Governing Differential Equations ofthe Problem ' ~ 7
2.3 Finite Element Approximation ~ 9
2.4 Stiffness Formulation 11
2.5 Flexibility Formulation 14
2.5.1 Approximation ofStress Field 15
2.5.2 Element State Determination 18
2.5.3 Determination ofElement Stiffness Matrix 19
2.5.4 Determination ofResidual Displacements 21
2.5.5 Selection ofInterpolation Functions and ExplicitForms
ofElement Stiffness Matrix and Displacement Residuals 22
2.6 Numerical Integration 23
2.7 Summary ofElement State Determination Algorithm 24
CHAPTER3 ANALYTICAL STUDIES UNDER MONOTONIC LOADING 29
3.I General 29
v
3.2 Correlation Studies with Experimental Results 29
3.3 Assessment ofModel Accuracy and Convergence Characteristics 32
3.4 Parametric Studies 36
3.4.1 Effect ofSteel Hardening on Spread ofYielding 36
3.4.2 Effect ofYield Strength, Anchorage Length and Hardening Ratio .
Under Monotonic Pull-Out .40
3.4.3 Effect ofYield Strength, Anchorage Length and Hardening Ratio
Under Monotonic Push-Pull 44
CHAPTER 4 ANALYTICAL STUDIES UNDER CYCLIC LOADING 49
4.1 General 49
4.2 Correlation Studies 49
4.2.1 BarSpecimen B85 ofLin and Hawkins (1982) : 50
4.2.2 Straight Anchored Bar ofViwathanatepa et al. (1979) 56
4.3 Parameter Studies under Cyclic Push-Pull Loading Conditions 59
4.3.1 Reinforcing Bar with Anchorage Length of 15 BarDiameters 60
4.3.2 Reinforcing Bar with Anchorage Length of25 BarDiameters 60
4.3.3 Reinforcing Barwith Anchorage Length of35 BarDiameters 65
4.3.4 Bond Damage Distribution Along Anchorage., 65
CHAPTER 5 CONCLUSIONS 69
REFERENCES 71
APPENDIX A MATERIAL MODELS 73
A.] Steel Stress-Strain Relation 73
A.2 Bond Stress-Slip Model · : 76
A.2.1 General Model Description 77
A.2.2 BondStress-Slip Relation in Confined Concrete 80
A.2.3 Bond Stress-Slip Relation in Hook Anchorages 81
vi