These concepts are different ways to express the performances of a fastener.
- Ultimate resistance (Fu ): it refers to the ultimate value in a test of a fastener, when the failure occurs.
- Ultimate mean resistance (Fum ): it is the mean value of ultimate resistances of a test series.
- Characteristic resistance (Fk ): it is the value with a probability of 95% to be reached in a test. It depends of the ultimate mean resistance, the scattering of the results and the number of tests performed. For its calculation it is assumed that the test results follow a Normal (Gauss) distribution. This is the value that usually appears in the ETA for each type of failure of the fastener.
- Design resistance (Fd ): it is the characteristic resistance divided by the material safety factor. This safety factor is applied to take into account unavoidable and unforeseen deviations of the material or of the installation procedure of the fastener. Safety factors g M are reported in the ETA or in the relevant design code. So:
Fd=Fk/Gm
In general, for the design of a anchorage, the following inequation must be met for each type of fastener failure:
SGf≤Fd
where S is the service load (or action) and g F the corresponding increasing safety factor. This increasing coefficient for loads is usually established in the design code; in Europe it is established in the Eurocode 0 (standard EN 1990), and it is typically taken as 1,35 for dead (permanent) loads and 1,50 for life loads. - Maximum recomended load (Rrec ): if we consider a increasing safety factor for loads gF = 1,40 (as weigthed average between dead and life loads), then we can set the check of the anchorage for each type of failure as:
S≤Rrec
so, the service load (that is, without applying an increasing safety factor (esto es, sin mayorar) should never exceed the maximum recommended load.
