# On time invariant probabilistic modelling of duration of load effects for timber Sven Thelandersson Structural Engineering Lund University.

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On time invariant probabilistic modelling of duration of load effects for timber Sven Thelandersson Structural Engineering Lund University

DOL effects in the probabilistic model code The following limit state function is proposed by Larsen g = 1-  (S(t),R,p) where  is the damage, S(t) is the load history, R is the resistance and p is a vector with model parameters. The design condition is that g>0 during the whole lifetime of the structure This makes problem time variant.

Problem Design of timber structures almost always involves DOL effects Probabilistic design will therefore imply time variant analysis in most cases An alternative approach allowing for time invariant reliability based design of timber structures is needed A method for this is proposed here

Basic input Random load model describing the load history S(t) Damage model describing the effect of a given S(t) on the resistance Life time T of the structure

Load model For a load combination with n loads where S i (t) is the history of random load i which can be modelled e.g. according to JCSS model code

Damage model A general form for the damage model is where R o is the initial (”short term”) resistance Any of the damage models available in literature can be used

Proposed method For a given life time T: Perform Monte Carlo simulation to generate N random load sequences S(t) during the period T Determine for each simulated load sequence

For each simulated load sequence S i (t) Assume a value for the initial resistance R o Calculate the damage  (t) and in particular the damage  (T) at the end of the prescribed life time Adjust R o until  (T) = 1 This value of R o is denoted R oDOL R oDOL is the required initial resistance to survive load sequence S i (t)

Duration of load effect The initial resistance R oST required to survive load sequence S i (t) if no DOL effect were present is R oST =S max < R oDOL A random duration of load factor  may be defined as  = R oST / R oDOL

Example: 100 % snow load- Foschi´s damage model - T= 50 years Relative strength Required resistance Short term With DOL P

Time invariant description of DOL The strength of timber segment j in board i can be determined by with notations according to Larsen.

The properties of the random variable  will depend on: Type of load Combination of loads Type of damage model Assumed life time of the structure …….? Simulations for relevant cases can be performed and the results can be included in the model code

Conclusions A time invariant description of DOL- effects should be included in the model code as an alternative A methodology to do this has been proposed Further work is needed before a complete set of data can be presented