assessment and retrofitting of old reinforced concrete...
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Assessment and Retrofitting of Old Reinforced Concrete Buildings
with an Open Ground Story
ByThemistocles A. Antonopoulos and Stavros A. Anagnostopoulos
Department of Civil EngineeringUniversity of Patras, Greece
Tongii University, Shanghai, China, October 2017
INTERNATIONAL WORKSHOP
PERFORMANCE BASED SEISMIC DESIGN OF STRUCTURESResilience, Robustness
OUTLINE• Motivation• Objectives• Methodology• Buildings descriptions and strengthening
solutions• Nonlinear modeling and earthquake input• Assessment before and after strengthening• Concluding remarks
OLD GREEK BUILDINGS ON PILOTIS
MOTIVATION: WHY SUCH BUILDINGS??
• Old Reinforced Concrete buildings on pilotis:
– classic case of weak first story
– constitute the most vulnerable R.C. building category as indicated by their numerous collapses in strong quakes.
– Their seismic capacity is unacceptably low mainly due to:
• Lack of infill walls • Old code shortcomings• Lack of knowledge on capacity design, ductility etc.• Earthquake threat forgotten for decades• Inadequate construction practices
• The only category that can be strengthened in large numbers through State introduced incentives because:
– costs are much lower, compared to full strengthening – the building can remain operational during intervention
OLD CODES NEW CODES
COMPARISON OF R.C. SECTIONS: OLD AND NEW CODES
ALSO: The old codes led to smaller sections
OLD/NEW STRENGTH RATIOS BENDING : 1/3 – 1/2 SHEAR : 1/4 – 1/15
Α Β
K1 (40/40) 2,12 4,92 6,21
K2 (70/40) 2,69 6,48 8,75
T (200/25) (200/20) 2,65 10,95 21,75
NEW GREEK BUILDINGS ON PILOTIS
OBJECTIVESTo examine effective retrofitting solution for strengthening only the open ground story (and possibly the foundation ) in order to:
• Remove the inherent weakness without shifting the problem to the stories above
• Make such buildings at least as strong as those without a weak ground story
CLASSIC CASE OF PERFORMANCE BASED RE-DESIGN
Desired performance level:
Same with other non-pilotis buildings which have infills and designed under the old codes. Substantially lower seismic strength than modern buildings
TYPES OF STRENGTHENING
STRENGTHENING USING COLUMN JACKETS
Existing footing
STRENGTHENING USING STEEL BRACING
SELECTED STRENGTHENING SOLUTION
Typical detail of X-Bracing for strengthening the ground story
Headedstud
Post-installedanchor
Brace
4.50
Mortarjoint
Steel rim
Existing RC frame
New tie beamExisting footing
METHODOLOGY & STRENGTHENING METHOD
• 3 and 5 story symmetric and eccentric buildings, designed according to the old Greek Codes, are selected
• The buildings are strengthened by means of suitable X-bracing restricted to the open ground story
• Original and Braced buildings are evaluated according to the provisions of Eurocode 8 – Part 3 using 3D Non-linear dynamic time history analyses
• Bracing properties are modified and analyses are repeated until a target optimum interstory drift profile along the building height is achieved
BUILDING DETAILS
•Designed according to the Old Greek Codes of 1959 and 1954
• Concrete quality / Steel grade, B160 / St. I (smooth steel bars)
• Seismic Zone I, Soil Class A (rock)
• Design base shear 4% of the total G+P gravity load
LAYOUT OF 5 AND 3-STORY SYMMETRIC BUILDINGS
C1
4.50 6.00 6.00 4.50
4.50
15.0
06.
004.
50
C2 C3 C4 C5
C6 C7 C8 C9 C10
C11 C12 C13 C14 C15
C16 C17 C18 C19 C20
CM=CR
X
Y
(X-) or (Ë-) DIAGONALBRACING
B1 B2 B3 B4
B5 B6 B7 B8
B9 B10 B11 B12
B13 B14 B15 B16B1
7B1
8B1
9
B20
B21
B22
B23
B24
B25
B26
B27
B28
B29
B30
B31
LAYOUT OF 5 AND 3-STORY ECCENTRIC BUILDINGS
X C2 C3 C4 C5
C6 C7 C8 C9 C10
C11 C12 C13 C14 C15
C16 C17 C18 C19 C20(WALL)
4.5021.00
6.00 6.00 4.50
CM
CR
STIF
F SI
DE
FLEX
IBLE
SID
E
åx=15%
å y=1
9%
Y
F
S
4.50
15.0
06.
004.
50
C1
B1 B2 B3 B4
B5 B6 B7 B8
B9 B10 B11 B12
B13 B14 B15 B16B1
7B1
8B1
9
B21
B22
B23
B24
B25
B26
B27
B28
B29
B30
B31
B20
(X-) or (Ë-) DIAGONALBRACING
Originalåx=15.0% - åy=19.0%X-Bracedåx=2.8% - åy=5.8%Ë-Bracedåx=4.2% - åy=7.7%Jacketsåx=10.0% - åy=9.3%
CR
CR
CR
+0.00
+6.00
+9.00
+12.00
+15.00
+3.00
4.50 6.00 6.00 4.50
21.00
X-DIAGONALBRACING
4.50
15.00
6.00 4.50
X-DIAGONALBRACING
Elevation of the 5-story building
X Direction Y Direction
NONLINEAR MODELING AND EARTHQUAKE INPUT
• Modeling and analyses using Ruaumoko 3D• Prismatic frame elements for beams, columns, walls• Effective stiffnesses equal to secant stiffness at yield
according to EC8 based on mean material strengths• One-component plastic hinge models following Takeda
hysteresis rule• Non-linear springs for cross-bracing members and infills• Time history representation of seismic action with 7-pairs
of code compatible artificial accelerograms
PSa Sd
Results (Non Linear Analyses): SYMMETRIC BUILDINGS(average response values, comparison of original vs. braced buildings)
3-story building 5-story building
Results (Non Linear Analyses): ECCENTRIC BUILDINGS(average response values, comparison of original vs. braced buildings)
3-story building 5-story building
Results (Non Linear Analyses): SYMMETRIC BUILDINGS(average response values, comparison of original vs. braced buildings)
3-story building: Member Demand / Capacity ratios
5-story building: Member Demand / Capacity ratios
Results (Non Linear Analyses): ECCENTRIC BUILDINGS(average response values, comparison of original vs. braced buildings)
3-story building: Member Demand / Capacity ratios
5-story building: Member Demand / Capacity ratios
CONCLUDING REMARKS
• Properly determined cross steel bracing in selected bays of the ground story of old Reinforced Concrete buildings on pilotis, can greatly improve their seismic behavior and remove the inherent weakness due to the weak open ground story
• This solution reduces the vulnerability of such buildings, at least to the level of the buildings with full infill walls in their ground story, without creating problems in the stories above
• Undesirable coupled torsional-soft story responses of non-symmetric buildings can be effectively reduced with appropriately selected bracing locations
• We believe that ground story strengthening of pilotis buildings is the only feasible large scale intervention in the existing building stock for seismic risk reduction
THANK YOUFOR YOUR ATTENTION
NATURAL PERIODS AND EFFECTIVE MODAL MASSESBefore and after strengthening (original, braced)
Building Mode T(sec) Mx*(%) My
* (%)
3stOriginal
1 0.766 - 94.0
2 0.746 91.0 -
3 0.694 4.0 -
3stBraced
1 0.541 - 78.0
2 0.522 77.0 -
3 0.404 2.0 -
5stOriginal
1 1.030 - 87.0
2 0.995 85.0 -
3 0.866 3.0 -
5stBraced
1 0.904 - 79.0
2 0.874 78.0 -
3 0.697 2.0 -
Building Mode T(sec) Mx* (%) My
* (%)
3stOriginal
1 0.726 34.0 31.0
2 0.602 41.0 47.0
3 0.493 15.0 10.0
3stBraced
1 0.550 11.0 69.0
2 0.519 71.0 11.0
3 0.423 - 1.0
5stOriginal
1 0.978 32.0 37.0
2 0.884 39.0 41.0
3 0.732 12.0 4.0
5stBraced
1 0.884 12.0 66.0
2 0.848 64.0 13.0
3 0.693 4.0 -
SYMMETRIC BUILDINGS ECCENTRIC BUILDINGS