Life Cycle Cost Analysis in Bridge Management Paul D. Thompson

Life cycle cost analysis in bridge managementPaul D. Thompson Purpose Benefit of preservation Value of protective systems Tradeoff between costs now and costs later Minimize costs over the long term

Life cycle cost analysis in bridge managementPaul D. Thompson Basic Ingredients Deterioration Models Cost Models Effect Models Life Cycle Activity Profiles Scope and timing tradeoffs Performance tradeoffs

Life cycle cost analysis in bridge managementPaul D. Thompson Life Cycle Activity Profiles

Life cycle cost analysis in bridge managementPaul D. Thompson Deterioration 1. There is no evidence of active corrosion, and the paint system is sound and functioning as intended to protect the metal surface. 2. There is little or no active corrosion. Surface corrosion has formed or is forming. The paint system may be chalking, peeling, curling, or showing other early evidence of paint system distress but there is no exposure of metal. 3. Surface corrosion is prevalent. There may be exposed metal, but there is no active corrosion which is causing loss of section. 4. Corrosion may be present but any section loss due to active corrosion does not yet warrant structural review of either the element or bridge. 5. Corrosion has caused section loss and is sufficient to warrant structural review to ascertain the impact on the ultimate strength and/or serviceability of either the element or the bridge. Example inspection condition states Painted steel girders

Life cycle cost analysis in bridge managementPaul D. Thompson Markovian Models From State To State These low-probability conditions are what necessitate the work

Life cycle cost analysis in bridge managementPaul D. Thompson Model Development Expert judgment – Easy and cheap – Poor agreement with real inspection results Statistical modeling – Requires ~500 inspection pairs per element/group – Spreadsheet-based modeling techniques can test and maximize accuracy Best results combine statistical modeling and judgment

Life cycle cost analysis in bridge managementPaul D. Thompson Comparison with Experts By element categoryBy element material Joints3.2Unpainted steel1.8 Railing1.6Painted steel1.9 Superstructure1.7Prestressed concrete1.7 Bearings2.2Reinforced concrete2.1 Substructure2.0Timber1.8 Movable bridge equip1.8Other material2.1 Channel1.4Decks1.9 Other elements1.4Slabs3.3 Ratio of statistical transition times to old (2000) expert judgment models Expert panel under-estimated transition times by a factor of 1.97 on average.

Life cycle cost analysis in bridge managementPaul D. Thompson Cost Models Direct cost – Labor – Materials – Equipment usage – Pay items Indirect cost – Maintenance of traffic – Mobilization – Engineering – Other (land, demolition, environmental)

Life cycle cost analysis in bridge managementPaul D. Thompson Model Development The tricks: An intermediate classification scheme adapts maintenance action codes to bridge management actions. Identify timing and affected elements by noting which elements improve in condition.

Life cycle cost analysis in bridge managementPaul D. Thompson Action Effectiveness Do-something transition probabilities Expert judgment Statistical analysis of inspections and maintenance activity Action sub-categoryState Repair deck and substrate Spot paint Clean rebar and patch Repair joint Repair/lubricate machinery Examples of the action effectiveness model (% in each state after action) New model (from activity data) vs old model (from 2000 expert panel) New modelOld Preservation actions much more effective than expert panel estimated.

Life cycle cost analysis in bridge managementPaul D. Thompson Tradeoff Analysis Example from NCHRP Report 590 Test alternative levels of work scope, vs alternative timing scenarios Which combinations give lowest life cycle cost? How much is lost by delaying work?

Life cycle cost analysis in bridge managementPaul D. Thompson Tradeoff Analysis Funding vs performance What condition levels can be maintained long term at lowest cost? How is performance affected by more/less funding? Example from one state’s draft TAM Plan

Life cycle cost analysis in bridge managementPaul D. Thompson The Fundamental LCC Tradeoff $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Condition Cost Maintenance Interval “Happy Medium” FrequentRare

Life cycle cost analysis in bridge managementPaul D. Thompson Conclusions Life cycle costing methods are quite mature and have been in use for many years Very sensitive to the quality of condition and cost data Many agencies have suitable data to develop accurate models Bridge management systems put the tool to work in project selection, priority-setting, and resource allocation

Life cycle cost analysis in bridge managementPaul D. Thompson Questions?