1. Seismic Design of Bridges
Lucero E. Mesa, P.E.

2. SCDOT Seismic Design Of Bridges Overview
AASHTO - Division IA
Draft Specifications, 1996
SCDOT 2001 Seismic Design Specifications
Comparison Between LRFD & SCDOT Specs.
SCDOT Seismic Hazard Maps
Training and Implementation
Conclusions

3. AASHTO Div IA USGS 1988 Seismic Hazard Maps Force based design
Soil Classification I-IV
No explicit Performance Criteria
Classification based only on acceleration coefficient

4. CHARLESTON, SOUTH CAROLINA August 31, 1886 (Intensity IX-X)

5. Earthquake of August 31, 1886 Charleston, South Carolina Magnitude=7
Earthquake of August 31, Charleston, South Carolina Magnitude=7.3M, Intensity = X
These photographs are evidence of the destruction caused by the 1886 Charleston Earthquake.
There is also evidence of sand blows from different events from Mirtle Beach (border with NC) to Bluffton on the border with Georgia.

7. Draft Specifications 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
The 1996 Draft Specifications adopted the 1996 USGS maps because South Carolina recognized that the “Low Seismicity” label was inappropriate and misleading.
The Caltrans Bridge Design Criteria was used as a reference,
The difference between the FEE and SEE events in SC is large, while in California the FEE and SEE events are closer
The SC 10% in 50 yr. Event is very small compared to the 2% in 50 yr. event.
On the Draft, the counties were assigned with the largest accel coeff. value from the seismic hazard contours for the 2% in 50 yr. event and a table was developed based on 2/3 of those values for the normal and essential bridges.
For some special bridges defined by the Department a Two Level Analysis was required and a seismic scope of services was developed based on the performance criteria

8. Draft Specifications Force based specifications N (seat width)
Soil classification: I – IV
Draft Specifications Version of 1999
The Draft was a force based specification.
The seat widths equations were those of the AASHTO Div IA.
The same with the soil classification,
The use of seismic restrainers became mandatory at the expansion joints.
The last version of the Draft Specs was done in February of 1999, when the ATC -32 document was available to us.
We were concerned with the mixed application of different specifications.

9. Site Specific Studies Maybank Bridge over the Stono River
Carolina Bays Parkway
Broad and Chechessee River Bridges
New Cooper River Bridge
Bobby Jones Expressway

10. SEISMIC DESIGN TRIAL EXAMPLES
SC-38 over I-95 - Dillon County
Maybank Highway Bridge over the Stono River - Charleston County
The SC Department of Transportation participated in developing two seismic design trial examples which compared the actual design with the LRFD-Guidelines.
The porpoise of the comparison was to learn how the new specifications could affect the design of the bridge projects.
I will be very brief with this part of my presentation, since Lee will be presenting on this issue a little bit later.
The Department hired a consultant to develop the trial examples for the:
SC-38 over I-95 in Dillon County
and the Maybank Highway Bridge over the Stono River in Charleston

11. SC-38 over I-95 Conventional bridge structure
Description of Project
Conventional bridge structure
Two 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
At the Bridge Design office we tried to choose two bridge projects that could give us a good view of how the LRFD would affect them.
Performance level/ Original Design: Conventional bridge. Extensive damage will be allowed but not collapse. Traffic on I-95 has to be kept.
Performance Level/LRFD: The performance level is “Life Safety for both earthquakes, the MCE and the Expected earthquake

12. SC-38 over I-95 Original Seismic Design Trial Design Example
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

13. Maybank Highway Bridge over the Stono River

14. The Maybank Highway is located is Charleston.
The SC map shows the seismicity of SC from 77 to present. Actually the last 2 tremors of magnitude 4.2 are not included in this map.

15. Maybank Highway over Stono River Description of project
118 spans
1-62 flat slab deck supported by PCP
/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.
flat slab deck supported by PCP
This bridge consists of 118 spans of various structural types,
The flat slab section is typically of 4 span continuous
Spans 63 –104, 33- meter prestressed bulb-tee girders, and three span continuous
105 to 118 similar flat slabs as other approach.

16. Maybank Highway over Stono River
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 % 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.
This project was considered by the Department as a lifeline and was classified as essential according to the draft specifications.
ATC-32 was used as a reference and also the
Caltrans Seismic Design Criteria, 1999.
The Two-Level analysis was required and a Push-over analysis to check capacity was required in the scope of work.
If this project were designed under LRFD, still the site specific study will be required.
The Expected LRFD earthquake is quite smaller than the FEE.
The 100%-30% vs. the 100%-40% is not severe.
For this structure little to no impact was seen from the increase

17. This graphic shows the comparison between the LRFD and the original design curve.

18. Maybank Highway over Stono River
Original Seismic Design
Soil Classification: Type II
Trial Design Example
Stiff Marl classified as Site Class D
The most significant change between the two designs is the soil classification to determine the site coefficient.
For the SCDOT criteria the stiff marl (dense stiff clay like materia over 200 ft. in depth) classifies as Type II with a site coefficient of 1.2.
For the LRFD Guidelines, the same soil classifies as Site D, based on the additional shear wave velocity, with a site coefficient of 1.6 for a S1=0.4g
This is a 33% increase

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

20. Cooper River Bridge Charleston Co.
Seismic Design Criteria- Seismic Panel
Synthetic TH
PGA g
Sa 1.85 at T=0.2 sec
Sa at T=1 sec
Liquefaction

22. Cooper River Bridge 2500 Yr - SEE for Main Piers
The Cooper River Bridge spectral accelerations are higher than the Maybank project.
For 0.2 sec the CRB spectral accel. is 1.85g
Maybank is 1.14g
Both projects are in Charleston.
It seems that the sediment thickness plays an important role in defining those values.
Both bridges are a few miles apart.

23. Need for: New Specifications South Carolina Seismic Hazard Maps
After using the draft specifications for a few years and developing scope of services for seismic design including the new requirements for seismic performance, the need for the new specifications was obvious.
Also, the differences between values of ground motions according to the different maps created the need for the SC seismic hazard maps.

24. The new specifications were developed by Imbsen and Associates in the year 2001.
Dr. Roy Imbsen took the 1996 draft specifications and kept the same format we had.This format was the AASHTO Specifications format.

25. SCDOT Seismic Design Specifications
October 2001
The new SCDOT specifications establish design and construction provisions for bridges in South Carolina to minimize their susceptibility to damage from large earthquakes.
The primary function of these specifications is to provide minimum standards for use in bridge design to maintain public safety in the extreme earthquake likely to occur within the state of South Carolina.
They are intended to safeguard against major failure and lose of life, to minimize damage, maintain functions, or to provide for easy repair.
Some of the new methodologies that have been incorporated into the specifications for the SDOT have been also included in the Recommended LRFD Guidelines and the Caltrans seismic design criteria (SDC, July of 1999)

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 Concepts and Enhancements
New Design Level Earthquakes
New Performance Objectives
New Soil Factors
Displacement Based Design
Expanded Design Criteria for Bridges

28. SCDOT Seismic Design Specifications
October 2001
Small to Moderate Earthquakes
Essentially Elastic
No Significant Damage
Functional Evaluation Earthquake
(FEE) or 10% in 50 yr. event
The philosophy used for the development of the new provisions are those that
small to moderate EQ should be resisted within the essentially elastic range of the structural components without producing significant damage.
The Functional Evaluation EQ or the 10% in 50 yr. event is adopted to represent ground motions produced by small to moderate earthquakes.

29. SCDOT Seismic Design Specifications
October 2001
Large Earthquakes
Life Safety
No Collapse
Serviceability
Detectable and Accessible Damage
Safety Evaluation Earthquake
(SEE) or 2% in 50 yr. event
Also, the philosophy used for the development of the new provisions are those that
a large EQ should not cause collapse of the structure.
Where possible, damage that does occur should be readily detectable and accessible for inspection and repair unless prohibited by the structural configuration.
The Safety Evaluation EQ or the 2% in 50 yr. event is adopted to represent ground motions produced by large earthquakes.

30. SCDOT Seismic Design Specifications
Background (1.2)
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
The new probabilistic seismic hazard maps developed by the USGS have been used in the specifications, in conjunction with the NCHRP Site Classification.
According to the spectral acceleration for T=1 sec and the importance category of the bridge, the bridge will be classified for seismic performance category A, B, C or D.
The performance levels are establish by the post-earthquake usage of the bridges.
The A706 steel has been adopted by the SCDOT and all bridges designed after February, 2003 have to comply with this requirement. Special provisions have been written on this respect. This have been taken from Caltrans SDC and also the butt welded hoops. Butt welded hoops are mandatory for columns and columns on standard bridges shall be round.
We are working on specifications for ultimate splices in column reinforcing. We are also in the process of changing the standard specifications in this regard.

31. Upgraded Seismic Design Requirement
(1.3)
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

32. SCDOT Seismic Design Specifications Seismic Performance Criteria
III II I
The performance levels are expressed in terms of service levels and damage levels.
THE SERVICE LEVELS ARE:
Immediate - Full access to normal trafic is available almost immediately following the EQ
Recoverable - Limited period of closure to Public. Open to emergency vehicles
Impaired - Extended closure to Public. Open to Emergency Vehicles, Possible replacement needed
DAMAGE LEVELS
Minimum damage - No, collapse, essentially elastic performance
Repairable damage - No collapse, Concrete cracking, spalling of concrete cover, and minor yielding of structural steel will occur. However the extent of damage should be sufficiently limited that the structure can be restored essentially to its pre-EQ condition without replacement of reinforcing or structural members, ductulity demands, less that 2.
Damage can be repaired with a minimum risk of losing functionality.
Significant Damage - Although there is a minimum risk of collapse, permanent offsets may occur in elements other that the foundations. Damage consisting of concrete cracking, reinforcing yielding, mayor spalling of concrete, and deformations in minor bridge components may require closure to repair. Partial or complete demolition and replacement may be required in some cases.

33. SCDOT Seismic Design Specifications
October 2001
The performance levels are expressed in terms of service levels and damage levels.
THE SERVICE LEVELS ARE:
Immediate - Full access to normal trafic is available almost immediately following the EQ
Recoverable - Limited period of closure to Public. Open to emergency vehicles
Impaired - Extended closure to Public. Open to Emergency Vehicles, Possible replacement needed
DAMAGE LEVELS
Minimum damage - No, collapse, essentially elastic performance
Repairable damage - No collapse, Concrete cracking, spalling of concrete cover, and minor yielding of structural steel will occur. However the extent of damage should be sufficiently limited that the structure can be restored essentially to its pre-EQ condition without replacement of reinforcing or structural members, ductulity demands, less that 2.
For seismic performance category A, no design is needed
For SPC B very small effort is also required in design…..
For SPC C and D more complexity in analysis and design is requred. Pushover analysis is required to check capacity.
Joint shear also has to be checked.

34. Design Spectral Acceleration at Short Periods
VALUES OF Fa AS A FUNCTION OF SITE CLASS AND MAPPED SHORT-PERIOD SPECTRAL RESPONSE ACCELERATION SS (TABLE 3.3.3A)
Site Class
Design Spectral Acceleration at Short Periods
SS 0.25
SS=0.50
SS=0.75
SS=1.00
SS1.25 A
0.8 B
1.0 C
1.2
1.1 D
1.6
1.4 E
2.5
1.7
0.9 a F
Insert maps

35. Insert maps

36. SCDOT Seismic Design Specifications
October 2001
This is the same design response spectrum as fig on the LRFD Guidelines.

37. DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E)
SDI-SEE

38. APPLICABILITY (3.1) New Bridges Bridge Types Spans less than 500 feet
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
We have included provisions for vertical acceleration.
Bridges under seismic performance Category D, shall have at least 25% of the longitudinal top and bottom mild reinforcement continuous over the length of the bridge superstructure.
The continuous steel reinforcement shall be spliced with “service load” couplers capable of achieving a minimum of 80 ksi strength capacity
A case-by-case determination on the effect of vertical ground motions is required for essential and critical bridges.

40. Design Approaches (4.7.1) Reparability Protection Systems
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
Minimal Plastic Action
Reparability
Protection Systems
Ductility Demand
Design Approach

41. Other New Concepts and Improvements
Plastic Hinge Region Lpr (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)
The restrainers are required at all expansion joints for the extra protection, this is the belt and suspenders approach.
We have developed the Department special provisions and details based on Caltrans practice.
The cable yield indicators are required on the restrainers.
Restrainers shall be detailed to allow for easy inspection and replacement
Restrainer layout shall be symmetrical about the centerline of the superstructure
Restrainer systems shall incorporate an adequate gap for expansion
Yield indicators shall be used on cable restrainers to facilitate post earthquake investigation
Shear keys are typically designed to fuse at the SEE hazard level and to stay elastic at the FEE hazard level.
For slender bents shear keys on top of the bent cap can still function elastically at the SEE hazard level

42. Seismic Design of Bridges
Thanks
Seismic Design of Bridges
Lucero E. Mesa, P.E.