Western Bridge Engineers Seminar, Reno, NV, Sep. 9-11, 2015 Structural Response of Bent Caps in Reinforced Concrete Box-Girder Bridges Mohamed A. Moustafa, PhD, PE University of Nevada, Reno Khalid M. Mosalam, PhD, PE University of California, Berkeley Sponsor:
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Western Bridge Engineers Seminar, Reno, NV, Sep. 9-11, 2015
Structural Response of Bent Caps in Reinforced Concrete Box-Girder
BridgesMohamed A. Moustafa, PhD, PE
University of Nevada, Reno
Khalid M. Mosalam, PhD, PEUniversity of California, Berkeley
Sponsor:
Courtesy of Hamid Saadatmanesh, Simpson Strong-Tie
Jyi Lu Brigde, Chi-Chi, Taiwan 1999 Bent caps damage in past events• e.g. San Fernando 1971, Whittier 1987,
Chi-Chi 1999
Post-event repair is not feasible • $$$ + long down time
Why accurate bent cap response important?• Modeling and analysis • Optimized design (new bridges)• Retrofit design (old bridges)
2
Bent Cap Response
Motivation
?
Integral Bent Cap BeamRC Box-girder
Bridge
Problem Statement1. What is box-girder slab contribution to bent cap?
strain-based effective slab width approach
2. How can bent cap capacity accurately estimated? slab reinforcement inclusion
3. Could column over-design migrate damage to bent cap? informed retrofit decisions
3
Stage 1: Pre-test analyses:• FE analyses of prototype bridge• FE analyses of test specimen
Stage 2: Experimental program (2 large-scale specimens):• Quasi-static testing of SP1 (as-built & repaired)• Hybrid Simulation testing of SP2 (retrofitted)
Stage 3: Post-test analyses:• FE model calibration/parametric study• Design implications
4
Methodology
420'126' 168' 126'
30'6' 6'-9"
Elevation View
Prototype BridgeAdopted from Caltrans Academy Bridge
(Typical RC box-girder bridge in CA)
Soffit Slab (inverted)
Deck Slab (inverted)
Test specimen subassemblage
10'
6' 27'
66'89'-10"
918"
814"
30'
2'-11"
5'1'
11'
Cross-Section
5
Specimen Design & Test Setup
3D schematic of Test Setup
Design acc. to AASHTO, Caltrans SDC, and ACI-318
Two Specimens tested in an inverted position at UC Berkeley Structures Laboratory
Gravity & lateral loads applied at the column top
1/4 scale
6
Column16 #6 longitudinal bars
#3 spiral at 2-1/2 in.
Cap beam
8 #5 negative reinforcement
8 #5 positive reinforcement
#3 stirrups 4 branches at 5 in. spacing
Box-girder
#3 in transverse dir. at 4 in. spacing
#3 in longitudinal dir. at 2-1/2 in. spacing
#3 single branch tie at 4 in. spacing
Summary of specimen reinforcement
2'-6"
8'-6"
1'-6"
11'-738"
1'-2"
1'-4"
3' 3'
1'-4"
2 116"
1'-6"
2'
1'-838"
8'-6"
2'
6'-6"
1'-7 516"
1'
9"
2 516"
1'
1'-5"
5"
5"
7'-6"
Ø9" hole
3'-138"
1'-2"
Elevation Side View
6.7 m2.6 m
3.5 m
22'
7
Prototype Model
Pre-test FEA: OpenSees
Summary of critical GMs causing bent cap beam failure
GM Earthquake Year Magnitude Station#1 Nahanni- Canada 1985 6.76 Site 1#2 Loma Prieta 1989 6.93 LGPC#3 Northridge-01 1994 6.69 Rinaldi#4 Kobe- Japan 1995 6.90 Takarazuka#5 Chi-Chi- Taiwan 1999 7.62 TCU068
Objectives:
• Choose GMs with most severe effect on bent cap beam
• Select final GMs for loading protocol Prelim. nonlinear time history analysis of
prototype w/ & w/o vl. excitation
> 80 ground motions (GMs) from PEER NGA database (Criteria: Magnitude > 6 & Distance to fault < 20 km)
Beam Section 2
-10 -8 -6 -4 -2 0 2
x 10-4
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
Curvature [1/inch]
Mom
ent [
kip-
inch
]
-10 -8 -6 -4 -2 0 2
x 10-4
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
Curvature [1/inch]
Mom
ent [
kip-
inch
]
w/o vertical
w/ vertical
Moment-curvature relationship
7
Objectives:
• Behavior & mode of failure• Setup design & instrumentation
Pushover Analysis
• Vertical & lateral direction• Different constant gravity load
TYPES?Slow Hybrid Simulation vs. Real-time Hybrid Simulation
20
+ Hybrid Simulation (HS)=
More convenient & feasible than shaking table tests Mass modeling Larger structures Observing the damage progression Modeling physical boundary conditions
HS Tests: Background
Shaking Table
VS.
Hybrid Model
u1m, Im
u2
WHY Hybrid Simulation?
21
Slow HS Test Components:
• Physical Substructure: specimen subassemblage
• Computational Substructure:MDOF mass & damping
How HS works?
Communication Loop?
Main components of HS system
Two Actuators(Two Experimental DOFs)
Two MTS 407 Controllers
PI Interface Software
OpenSees/OpenFresco Computational Platform
DAQ/ DSP Card
Ethernet Connection
USB Cable
Displacement
Force
Ope
nFre
sco
1st development:New setup in OpenFresco2nd development:
New PI Interface
22
1 1 1 1( )i i r i i+ + + ++ + =.. .
M U CU P U P
HS Tests: Hybrid System
Retrofit Procedure
23
HS Tests: Retrofitted SP2
HS Test In-progress
24
HS Tests: Retrofitted SP2
Retrofitted SP2 Damage after HS TestsBent cap concrete crushing in compression
25
HS Tests: Retrofitted SP2
Bent cap moment-curvature relationship
Retrofit increased overall SYSTEM capacity by ~25%
Increased demand from column + higher gravity (15%) bent cap failure
26
HS Tests: Retrofitted SP2
Force-displacement relationship (transverse)
As-built SP1 vs. Retrofitted SP2
Overall mean values for effective slab width from SP1 cyclic & SP2 HS tests
Cyclic Tests HS Tests0
20
40
60
80
100
120
Effe
ctiv
e W
idth
[inc
h]
Experimental Value (using ε
mean)
Experimental Value (using εmin
)
Average Bef f
using εmean
Average Bef f
using εmin
Bef f
using Caltrans Value
~13ts
~ 19ts
12ts
Strain distribution from transverse-only up to 200% (left) & bidirectional up to 100% (right) at Section B