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