Adjusting Structural Timber Properties for Design Use

Strength properties are time dependent. The load that timber can sustain without failure decreases with time. If the short-term ultimate load in the 5-minute static bending test is taken as the reference point, then wood will fail, on average, at about 66% of that load after 1 year, at 62% after 10 years and at 56% after about 27 to 200 years depending on the curve used to fit the data there is a dearth of long term experimental evidence ( 10 yr). 

Logarithmic plot o1 duration of stress to failure, In seconds (X) 

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