PERFORMANCE STUDY FOR REINFORCED CONCRETE BRIDGE PIERS CONSIDERING SEISMIC CAPACITY

AND DEMAND

Presented by: Sasithorn THAMMARAK (st109957)16th May 2011

Introduction

• It is obvious that earthquake phenomenon is possible to happen in Thailand.

• Evaluation of existing buildings and bridges is needed.• Capacity spectrum method(CSM) is considered.

BTS, Bangkok

BTS, Bangkok

Nimitz Freeway, San Francisco

Capacity Spectrum Method (CSM)

Modify spectrum according to soil-structural interactive (FA, FV)

Develop structural model Select 5% damping ground motion spectrum

Modify structural model for flexible base (soil effect)

Select static load vector

Generate global force-deformation curve (push-over curve)

Convert force-deformation curve to equivalent SDOF model (ADRS format)

Determine equation for effective damping

Determine equation for effective period

Select solution procedure (A,B or C) and calculate performance point

Convert the spectrum to ADRS format

Spectral Displacement

Spec

tral

Acc

eler

ation

(g)

ay, dy api, dpi

2.5CA

CV /T

2.5SRACA

SRVCV /T

ap, dp

Performance Point

Assumptions

• Uniform multiple spans simply supported on uniform pier columns.

• Each bent is stand-alone model, only transverse responses are investigated.

• Effects of soil are considered in terms of demand spectra.

APPLICATION I SINGLE COLUMN BENT

Model of single column bentsSix columns with similar material properties but are different in cross section.

9 meters15 meters25 meters

Area = 4.5 m2

ρs = 1.7%Ties = 4x4 DB16

Solid section Hollow section

No buckling

Material Property

- Concrete : ACI318-08Compressive strength 35 MpaEc 4700

- Reinforcement ; TIS24-2548Reinforcing steel grade SD30 SD40 SD50

Minimum tensile strength (MPa) 480 560 620Minimum yield strength (MPa) 295 390 490Elongation (%) 17 15 13

Main Rebar

Confinement

Structural Modeling

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 15000

1000000

2000000

3000000

Base

She

ar (K

N)

Roof Deflection (mm)

Application I Result

9m

15m

25m

Energy (KN-m)

4.31 1337

3.91 1053

3.81 1956

3.19 1564

3.54 2867

3.00 2246

µΔ

Sung et al. (2006) ATC-40 (1996) + Japan Road Association (2001) +Building Technology Standards (Taiwan) (1997)

PGA-displacement relationship

PGA vs. Roof Displacement

Linear relation while nonlinear analysis is applied?

Performance of a particular structure under a particular earthquake and site can be directly obtained.

Higher PGA?

0 100 200 300 400 500 600 7000.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00Case 0.1 g

Case 0.2 g

Case 0.3 g

Case 0.4 g

Case 0.5 g

Case 0.6 g

Case 0.7 g

T1

T2

Response Spectra (ADRS format) : Soil Type B (Rock)

Spe

ctra

l Acc

eler

atio

n (g

)

Spectral Displacement (mm)

Rock site response spectrum

Original acceleration spectrum Converted demand spectra (ADRS)

CALTRANS, 2006

ATC-40, SAP2000

For the single column bent, almost linear shape Hollow piers are stiffer = better performance

0 100 200 300 400 500 600 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Roof Deflection (mm)

PGA

(g)

PGA vs. Roof displacement9m 15m 25m

Part I concluding remarks

• Hollow piers provide better ductility and absorb more energy.

• The shapes of PGA vs. roof displacement curves are almost linear, though the analysis is based on the nonlinear analysis.

APPLICATION II DOUBLE-DECK BRIDGE PIER

Double-deck Bridges in Bangkok

Mwafy and Elnashai (2000)

Pushover load patterns

Pushover load patterns

• Structures with irregular geometry, higher mode effects may be critical on some structural components than the fundamental mode.

• Other load patterns (e.g. uniform) rather than 1st mode pattern (i.e. triangle) should be employed.

Sung et al. (2006)

PGA-displacement relationship

PGA vs. Roof Displacement

Linear relation while nonlinear analysis is applied.

Performance of a particular structure under a particular earthquake and site can be directly obtained.

Irregular structure?Does soil affect the result?

S_1 S_21K : 4K1K : 1K

Case studies

f’c = 35 MPafy = 490 MPafyh = 390 MPaADL@each deck = 1500 tons

Uniform Stiffness Structure

Non-Uniform Stiffness Structure

Section 3x1.25 m2

ρs = 1.27%ρh = 0.75%

Section 3x2 m2

ρs = 1.26%ρh = 0.71%

Double-deck cross sections

Loads

Triangular load pattern Uniform load pattern

Soil Condition

• Rock Soil

• Soft Soil

Model of double-deck bridge pier

Pushover curves

0 50 100 150 200 250 300 350 400 4500

2000000

4000000

6000000

8000000

10000000

12000000

14000000

16000000

18000000

20000000

Roof Deflection (mm)

Base

She

ar (K

N)

PGA vs. Roof displacement

0 50 100 150 200 250 300 350 400 4500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Roof Displacement (mm)

PGA

(g)

PGA

(g)

Regular Pier

Irregular Pier

Soft soil site

Rock site

0 50 100 150 200 250 300 350 400 4500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Roof Displacement (mm)

Rock site

Soft soil site

Irregular Pier

PGA

(g)

PGA vs. Base Shear

PGA

(g)

Regular Pier

Irregular Pier

0 5000000 10000000 15000000 20000000 250000000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Base Shear (KN)

PGA

(g)

Base Shear (KN)

PGA

(g)

Soft soil site

Regular Pier

0 5000000 10000000 15000000 20000000 250000000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Base Shear (KN)

PGA

(g)

Rock site

Soft soil site

Rock site

Irregular Pier

Base Shear

ATC-40 procedure un-conservative

Irregular Structure

Higher mode effect

Not considered by ATC-40

Further analysis for higher mode is important

Irregular Structure

Vt

Vb

Mt1

Mt2

Mb1

Mb2

Fr

F1

Shear Diagram Moment DiagramApplied loads

Member forces

Mbottom2

Mbottom1

Mtop2

Mtop1

Vbottom

Vtop

0 20000 40000 60000 80000

Triangular LoadUniform Load

-14%

3%

-13%

-16%

4%

2%

Uniform Structure

Triangular pattern governed the responses of the regular structures. Therefore ATC-40 is adequate.

KN

KN-m

Mbottom2

Mbottom1

Mtop2

Mtop1

Vbottom

Vtop

0 20000 40000 60000 80000

Triangular LoadUniform Load

-20%

17%

-19%

-21%

18%

17%

Non-Uniform Structure

For irregular structures, the responses should be computed from the more conservative results

KN

KN-m

Thesis Conclusion and Recommendations

• Hollow pier provides better performance in terms of stiffness and ductility.

• The ATC-40 structural evaluation method is appropriate for considering maximum displacement, but it is not adequate for estimating forces or base shear response for the irregular structural geometry.

• Higher mode effect is important for analyzing base shear.

Thank You