PSI - Issue 64

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André Weber et al. / Procedia Structural Integrity 64 (2024) 74–80 Weber A. / Structural Integrity Procedia 00 (2019) 000 – 000

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With more live load more safety is added. This is not needed for our materials, as the resistance for live load is higher than for dead load because of the shorter duration. Other guidelines as ACI 440 or CSA S 807 neglect the live load for creep rupture check. The experience has shown, if the f fk100a strength is determined directly by tests, that in most cases not the ultimate limit design but the serviceability design is limiting the use of the material. There is an ongoing discussion which part of the variable load shall be taken into account for sustained stresses. For example, a live load being present for 10% of the service life can be seen as representative for a 25% reduced load for the full time and loads being present between less than 1% of the time can be seen as representative for a 45% reduced load for the full time as seen in Weber (2014). 2.3. Advanced live load design safety concept So something similar to a k mod factor which is known in different other Eurocodes for reduced loading time can be defined. It shall be stated that this is not a reduction factor for the short term strength, but an increasing factor for the long term strength for shorter durations or even better a reduction factor on the action side of the design. To simplify the use of the factors the probability and duration as well as load can be evaluated together for the different cases already defined in Eurocode 0. The different cases defined here are the permanent load combination, the frequent load combination and the characteristic load combination. With this as background the different live load combinations can be estimated for their effect on the resistance against sustained load. While the characteristic load combination  0 is relatively unlikely and can be estimated as less than 1% of the time, there is the more likely frequent load combination which is identified with the combination factor  1 . The estimation of the duration of this action combination is 10%. The permanent load combination is identified by the load combination factor  2 with an estimated duration of 100% of the service life. The range of values for the load combination factors for different categories is quite wide. For the relevant rating in a special application category the combination value, the rating according the last paragraph and the safety factor for the action is multiplied and the maximum of the three effects is taken as relevant. Max (  0 x (1-0,45) x 1,5;  1 x (1-0,25) x 1,5 ;  2 x 1 x 1,5) The calculated rating is done here for all categories. The rating is in the most cases without storage areas between 0,56 and 0,9. This is much less than 1,5 which has been assumed in the FprEN1992-1 or abZ 1.6-238. Table 1 different sensitivities for sustained load in different building categories

The representative load for creep rupture testing can be represented by the sum of the full dead load and the reduced live load. For the accidental part of actions no representation through sustained load testing is needed.