Performance-based Seismic Design in 2014 Canadian

Bridge Code

5th Tongji-UBC Symposium on Earthquake Engineering

Sharlie Huffman, P.Eng.

PERFORMANCE-BASED SEISMIC DESIGN

Performance-based design provides a consistent outcome from differing tectonic environments

allows designers the flexibility of choosing materials, design options and construction methodologies to more accurately reflect the seismic environment and configuration of the designed structure

describes the performance: Clear, easily understood terms, explicity demonstrated Does not restrict methodology Currently needs some prescriptive boundaries

provides owners, regulatory agencies, designers and the public to have a consistent expectation of the structure performance during designated levels of seismic events.

provides owners and regulatory agencies flexibility to risk-manage expenditure and performance

Enables vital retrofits to suit individual circumstances rather than a prescriptive bar.

May require more sophisticated analysis to understand or demonstrate the required performance.

PBD or FBD

While all seismic design in all categories can use performance-based design, Table 4.11 specifies where PBD is required. Where Table 4.11 shows FBD is permitted, the Authority having jurisdiction may require PBD instead.

Regular bridge requirements

SINGLE SPAN BRIDGE EXEMPTION

S6-06

Only a few bridge types when single span required detailed seismic analysis.

At one time, single span bridges were generally small and regular and with a few exceptions such as trusses and arches.

Currently single span bridges can be girder, truss, arch, cable-stay, spiral etc.

S6-14

Only a few bridge types when single span are exempted from detailed seismic analysis.

PERFORMANCE LEVELS Implied Performance Levels S6-06 Commentary

50 year Prob. Life Line Emergency Other

  Service Damage Service Damage Service Damage

10% Immediate All   Immediate 

Emergency     Repairable

5% Immediate Emergency     Repairable No Collapse  

S6-06 does not provide descriptions of the service and damage states. Going to other Codes that also use such terms provides significant variation in definition.

PERFORMANCE-BASED DESIGN Performance Levels S6-14

S6-14 provides descriptions of the service and damage criteria in primarily operational terms with some prescriptive criteria. All criteria must be met – not only the prescriptive

50 yr Prob. Life Line Major Route Other

  Service Damage Service Damage Service Damage

10% Immediate None Immediate Minimal Service Limited Repairable

5% Immediate Minimal Service Limited Repairable Service 

Disruption Extensive

2% Service Limited Repairable Service 

Disruption Extensive Life Safety Probable Replacement

PERFORMANCE-BASED DESIGN

Cost drivers for structural performance:

The human cost – deaths, injuries, social dislocation, economic loss

Structural cost – cost of repair/replacement due to direct damage and the impact on the overall capacities to respond to multiple demands – regulatory, owners/agencies, funding, professionals, constructors, suppliers, transporters.

Downtime – impact locally and regionally on loss of the asset in terms of direct financial impact (tolls, taxes), emergency response, social and economic recovery impacts .

These drivers have been incorporated into the performance criteria for the respective importance categories.

PERFORMANCE-BASED DESIGN

In use in BC for seismic design for over a decade

Initially a combined prescriptive and performance-based approach for seismic retrofits

Subsequently implemented for major projects in BC

Sea-to –Sky Highway 2010

Pitt River Bridge 2011

Port Mann/Highway 1 project 2009-2015

South Fraser Perimeter Road 2012-2014

RETROFITS S6-06 essentially required retrofits to new bridge design level.

This was problematic for agencies with inventory of bridges that were not well designed for seismic but were otherwise in acceptable structural condition. It was not always possible to either achieve new design level with the old structure or to do so economically.

Many jurisdictions designated reduced levels of capacity for retrofits.

RETROFITS Recognizes the potential limitations on retrofit of older structures.

Addresses the public expectation of post retrofit capacity

Places the performance levels within the Owner/Regulatory Authorities control.

Utilizes PBD which enables Owners and Authorities to proceed with vital retrofits at reduced levels to suit individual circumstances.

The PBD approach also provides the ability for increased capacity through the selection of only some of the criteria from a higher category where that is technically feasible and would provide an improved cost-benefit.

PBD provides an easily communicated performance expectation for discussions with owners or public to occur.

RETROFITSSample PBD Grid for Retrofits – to be determined by owner/Regulatory Authority

50 year Probability

Lifeline Bridges Major-Route Bridges Other Bridges

  Service Damage Service Damage Service Damage

10%Immediate Minimal

Service Limited

RepairableService

Disruption *Extensive *

 

5% Service Limited*

Repairable*Service

Disruption*

Extensive* Life SafetyProbable

replacement 

2%Service

DisruptionExtensive Life Safety

Probable replacement

__ __ 

* Optional performance levels

Challenges to PBD

Explicit demonstration

Optimizing effort

Explicit Demonstration One of the main challenges to the PBD requirement is the explicit

demonstration that the performance criteria have been met.

Analysis requirements in codes tend to be based on structure importance rather than structure configuration, complexity and response.

Using non-linear dynamic analysis and full FEM today’s software will enable designer to confirm member properties and displacements sufficient to confirm performance compliance.

The minimum requirements of code are just that – minimum – but do all bridges require that level of analysis?

Can partial or simplified models be effective?

Optimizing effort

Designers use “safest”, most extensive methods – excess effort

Designers look for exemptions to minimize effort – insufficient effort

Hit and miss – over time methods and results get optimized/corrected

University research and studies (that are reproduced) to provide optimization

It is important that meeting the performance criteria is understood and demonstrated by the designer – and structure specific

Research

Universities have an important role to play in moving codes forward, to facilitate implementation of seismic PBD, to contribute to ongoing design quality and future code updates. There are many opportunities for research, such as:

Matching analysis levels to structure types and performance levels

Effective modelling

Damage testing to confirm damage levels relative to inputs

Testing consistency within the levels of performance criteria

Algorithms for measuring and predicting damage (SHM)

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THANK YOU