Seismic Design Of Bridges

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25 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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1

Seismic Design

of Bridges



Lucero E. Mesa, P.E.

2


AASHTO
-

Division IA


Draft Specifications, 1996


SCDOT 2001 Seismic Design Specifications


Comparison Between LRFD & SCDOT Specs.


SCDOT Seismic Hazard Maps


Training and Implementation


Conclusions

SCDOT Seismic Design Of Bridges
Overview

3


USGS 1988 Seismic Hazard Maps


Force based design


Soil Classification I
-
IV


No explicit Performance Criteria


Classification based only on acceleration
coefficient

AASHTO Div IA

4

CHARLESTON, SOUTH CAROLINA

August 31, 1886 (Intensity IX
-
X)

5

Earthquake of August 31, 1886

Charleston, South Carolina

Magnitude=7.3M, Intensity = X

6

Sandblow
in Charleston
7



1996 USGS Seismic Hazard Maps



Difference in spectral acceleration
between South Carolina and California



Normal Bridges : 2/3 of the 2% in 50
yr. Event




Essential Bridges: Two
-
Level Analysis



Draft Specifications

8




Force based specifications


N (seat width)


Soil classification: I


IV


Draft Specifications Version of
1999


Draft Specifications

9


Maybank Bridge over the Stono
River


Carolina Bays Parkway


Broad and Chechessee River
Bridges


New Cooper River Bridge


Bobby Jones Expressway

Site Specific Studies

10


SC
-
38 over I
-
95
-

Dillon County



Maybank Highway Bridge over the
Stono River
-

Charleston County

SEISMIC DESIGN TRIAL

EXAMPLES

11

SC
-
38 over I
-
95

Description of Project



Conventional bridge structure



Two 106.5 ft. spans with a composite
reinforced concrete deck, supported by 13
steel plate girders and integral abutments



The abutments and the interior bents rest
on deep foundations


12

Original Seismic Design



SCDOT version of Div
-
IA
AASHTO (Draft)


2/3 of 2% in 50 yr


1996 USGS maps used


PGA of 0.15g, low potential
for liquefaction


Response Spectrum
Analysis

Trial Design Example



Proposed

LRFD Seismic
Guidelines


MCE

3% PE in 75 yr.


Expected Earthquake


50%
PE in 75 yr.


2000 USGS maps


PGA of 0.33g, at MCE,
further evaluation for
liquefaction is needed.


Response Spectrum
Analysis


SC
-
38 over I
-
95

13

Maybank Highway Bridge

over the Stono River


14


Highest Hazard
Lowest Hazard

Highest Hazard
Lowest Hazard




Seismicity of South
Carolina 1977 to 1996

1977 to 1997


15




118 spans



1
-
62 flat slab deck supported by PCP



63
-
104 /33
-
meter girder spans and 2 columns
per bent supported by shafts.



The main span over the river channel consists of
a 3 span steel girder frame w/ 70 meter center
span.



105
-
118 flat slab deck supported by PCP

Maybank Highway over Stono River

Description of project

16

Original Seismic Design


SCDOT version of AASHTO
Div. I
-
A (Draft)


Site Specific Seismic Hazard


Bridge classified as essential


Project specific seismic
performance criteria



Two level Analysis
:


FEE


10% in 50 yr. event


SEE
-

2% in 50 yr. event


Trial Design Example


Proposed LRFD Guidelines
-
2002




Two Level Analysis:


Expected Earthquake
-

50%
in 75 yr.


MCE


3% in 75 yr.


Maybank Highway over Stono River


17

Table C
-
1. LRFD Spectral
Accelerations and Site Coefficients


Earthquake

Spectral
Accelerations

Site Coefficients


S
S

S
1

S
DS

S
D1

F
a

F
v

Maximum Considered

1.43

0.407

1.43

0.651

1.00

1.60

Expected

0.0503

0.0104

0.0503

0.0167

1.00

1.60


SEE - Compare LRFD to Original Design Curve
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Period, T (sec)
Spectral Acceleration, Sa (g)
LRFD Curve
Site Specific Original Curve
SCDOT Curve, soil type II
SCDOT Curve, soil type III
* The cumulative mass participation for
mode shapes at periods indicated and
higher, is approximately 70%.
* Transverse
* Longitudinal

18


Original Seismic Design



Soil Classification: Type
II



Trial Design Example



Stiff Marl classified as
Site Class D

Maybank Highway over Stono River


19



The SCDOT
'
s new specifications adopted the
NCHRP soil site classification and the Design
Spectra described on LRFD 3.4.1



If this structure were designed using the new SCDOT
Seismic Design Specifications, October 2001, the
demand forces would be closer if not the same to
those found using the
Proposed LRFD Guideline
-
2002
.


20

Cooper River Bridge

Charleston Co.


Seismic Design
Criteria
-

Seismic
Panel


Synthetic TH


PGA
-

0.65g


Sa 1.85 at T=0.2
sec


Sa 0.65 at T=1 sec


Liquefaction

21

22

US17 COOPER RIVER BRIDGES
2500-YR SEE for Main Piers
(Vertical Spectra)
0
0.5
1
1.5
2
2.5
0
1
2
3
4
5
Period, sec
Spectral
Acceleration, g
Cooper River Bridge


2500 Yr
-

SEE for Main Piers

23


New Specifications


South Carolina Seismic
Hazard Maps

Need for:

24

25


The new SCDOT specifications
establish design and construction
provisions for bridges in South
Carolina to minimize their
susceptibility to damage from large
earthquakes.

SCDOT Seismic Design Specifications

October 2001

26

PURPOSE & PHILOSOPHY
(1.1)


SCDOT Seismic Design Specifications
replace AASHTO Division I
-
A SCDOT Draft


Principles used for the development


Small to moderate earthquakes, FEE, resisted
within the essentially elastic range.


State
-
of
-
Practice ground motion intensities are
used.


Large earthquakes, SEE, should not cause
collapse.


Four Seismic Performance Categories (SPC)
are defined to cover the variation in seismic
hazard of very small to high within the State
of South Carolina.

27


New Design Level Earthquakes


New Performance Objectives


New Soil Factors


Displacement Based Design


Expanded Design Criteria for Bridges

New Concepts and
Enhancements

30


New USGS
Probabilistic Seismic
Hazard Maps


New Design Level
Earthquakes


New Performance
Objectives


A706 Reinf. Steel


New Soil Factors


Displacement Based
Design


Caltrans (SDC) new
provisions included


SCDOT Seismic Design Specifications

Background (1.2)

31


New Provisions meet current code
objectives for large earthquakes.


Life Safety


Serviceability


Design Levels


Single Level


2% / 50 years


Normal Bridges


Essential Bridges


Two Level : 2% / 50 years and 10% / 50
years


Critical Bridges

Upgraded Seismic Design Requirement

(1.3)

32

SCDOT Seismic Design Specifications

Seismic Performance Criteria

III

II


I

33

SCDOT Seismic Design Specifications

October 2001

34

VALUES OF F
a

AS A FUNCTION OF
SITE CLASS AND MAPPED SHORT
-
PERIOD SPECTRAL RESPONSE
ACCELERATION S
S

(TABLE 3.3.3A)

Site

Class


Design Spectral Acceleration at Short Periods


S
S


0
.
25


S
S
=
0
.
50


S
S
=
0
.
75


S
S
=
1
.
00


S
S

1
.
25


A


0
.
8


0
.
8


0
.
8


0
.
8


0
.
8


B


1
.
0


1
.
0


1
.
0


1
.
0


1
.
0


C


1
.
2


1
.
2


1
.
1


1
.
0


1
.
0


D


1
.
6


1
.
4


1
.
2


1
.
1


1
.
0


E


2
.
5


1
.
7


1
.
2


0
.
9


a


F


a


a


a


a


a


35

SCDOT – Pilot Workshop
Imbsen & Associates, Inc. –
I
I
A
1-
6
Increasing
performance
Increasing earthquake
hazard
Recent
Technology
b
c
d
e
f
i
h
g
Collapse
Preventi on
Limited
Damage
Essentiall y
Elastic
2% in 50 Yrs.
2/3 (2% in 50 Yrs.)
10% in 50 Yrs.
Proposed Design or Retrofit Objective
a
f, h, i
a, b, c, d,
e, g
Secondary
System
Primary
System
Design or
Retrofit
Objective
36

SCDOT Seismic Design Specifications

October 2001

37

DESIGN SPECTRA FOR SITE
CLASS A, B, C, D AND E, 5%
DAMPING (3.4.5E)

S
s
=1.00g, SEE(2%/50years)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
1
2
3
4
SD_4A
SD_4B
SD_4C
SD_4D
SD_4E
Periods T (sec)
Site Class
A
B
C
D
E
S
DI
-
SEE


38

APPLICABILITY (3.1)


New Bridges


Bridge Types


Slab


Beam Girder


Box Girder


Spans less than 500 feet


Minimum Requirements


Additional Provisions are needed to achieve
higher performance for essential or critical
bridges


39

DESIGN PHILOSOPHY AND
STRATEGIES


Specifications can be used in conjunction
with rehabilitation, widening, or retrofit


SPC B demands are compared implicitly
against capacities


Criteria is focused on member/component
deformability as well as global ductility


Inherent member capacities are used to resist
higher earthquake intensities


Using this approach required performance
levels can be achieved in the Eastern US

40

Design Approaches

(4.7.1)

May require
closure or
removal

Not
warranted


May be higher

Significant
Plastic Action

May require
closure of
limited usage

May be Used

Limited

Moderate
Plastic Action

Not required to
Maintain

May be Used

Limited

Minimal
Plastic Action

Reparability

Protection
Systems

Ductility
Demand

Design
Approach

2
D


4
D


41


Plastic Hinge Region L
pr
(4.7.7)


Plastic Hinge Length (4.7.7)


Seat Width SPC A and B, C, D

(4.8.2)


Detailing Restrainers

(4.9.3)


Butt Welded Hoops


Superstructrure Shear Keys

(4.10)





Other New Concepts and
Improvements

42

Seismic Design

of Bridges



Lucero E. Mesa, P.E.

Thanks