Target Reliability Indices for Existing Structures Based on Economical and Sustainable Optimisation
Authors
Skov, Victor ; Nørgaard, Katja Elisabeth ; Sørensen, Frederik Dam
Term
4. term
Education
Publication year
2025
Abstract
This thesis examines how to set appropriate target reliability indices for existing structures by balancing life safety, economic cost, and environmental impact. It distinguishes between upgrade scenarios, where new elements are added, and no-intervention decisions, where the structure is left as is. For upgrades, several objective functions are developed and evaluated: a cost-based, an emission-based, and a combined approach that assigns a monetary value to emissions; one formulation is extended into a Life Quality Index framework. For no-intervention, decision strategies are compared by equating the costs of intervening and not intervening to determine acceptable target reliability levels, with the Life Quality Index adapted accordingly. The approach is applied to timber joist systems and concrete beams, and matching partial safety factors are calibrated using two methods. Results indicate that emission-based optimization generally yields lower target reliability indices than cost-based optimization. When both costs and emissions are included in an economic framework, the monetary value of emissions must be dramatically higher than currently proposed values before emissions influence the optimal reliability level. For upgrades, using the target reliability indices prescribed for new structures is recommended because the cost difference is marginal. Conversely, no-intervention decisions are governed by human safety criteria that determine whether an upgrade is warranted from a socio-economic perspective.
Denne afhandling undersøger, hvordan man fastsætter passende målte pålidelighedsindekser for eksisterende konstruktioner ved at afveje personsikkerhed, økonomiske omkostninger og miljøpåvirkning. Der skelnes mellem opgraderingssituationer, hvor nye elementer tilføjes, og no-intervention, hvor man lader konstruktionen være uændret. For opgraderinger udvikles og vurderes flere målfunktioner: en omkostningsbaseret, en emissionsbaseret og en kombineret, hvor emissioner tildeles en monetær værdi; en formulering udvides til en Life Quality Index-tilgang. For no-intervention sammenlignes beslutningsstrategier ved at ligestille omkostninger ved indgreb og ved at undlade indgreb for herigennem at bestemme acceptable mål for pålidelighed; LQI tilpasses tilsvarende. Metoden anvendes på bjælkelag i træ og betonbjælker, og tilsvarende partialkoefficienter kalibreres med to metoder. Resultaterne viser, at emissionsbaseret optimering typisk giver lavere målte pålidelighedsindekser end rent omkostningsbaseret optimering. Når både omkostninger og emissioner indgår i en økonomisk optimering, skal den monetære værdisætning af emissioner være markant højere end de nuværende foreslåede værdier, før emissioner påvirker det optimale pålidelighedsniveau. For opgraderinger anbefales det derfor at anvende målte pålidelighedsindekser for nye konstruktioner, da meromkostningen er marginal. Omvendt styres no-intervention af personsikkerhedskriterier, som afgør, om en opgradering er samfundsøkonomisk nødvendig.
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