Posted on behalf of a Hilti customer.
What is the difference between recommended, design, characteristic and ultimate load?
Hello Fethi
Thank you for your question
The following discussion refers to resistance and load (actions). As in modern codes the safety concept of partial safety factors is used for better handling uncertainties associated with BOTH, actions (load) and resistance. According to this concept it shall be shown that the value of the design actions Sd does not exceed the value of the design resistance Rd, see below
The design resistance Rd or design load Sd is calculated from characteristic resistance Rk or characteristic load Sk, while the characteristic values are derived from average resistance Ru,m or average load Su,m. The following example is dealing with the anchor resistance.
Let’s assume you have two products and you perform n=10 pullout tests. The average ultimate load Nu,m [kN] would be the average failure load measured in you tests. Based on the Figure below you would get an average ultimate load of Nu,m,product A= 60 kN and Nu,m,product B= 75 kN. Based on this results Product B would maybe be assessed as “better performing” compared to Product B.
The characteristic values NRk takes account of the scatter of the values prescribing the probability of not being violated by unfavorable values. Characteristic resistance or be more precise the 5%-fractile value provides a 90% probability that only 5% of the ultimate values are below this characteristic value. Based on the standard deviation and the number of tests for Product A we would get a characteristic value of NRk,product A ~NRk, product B ~52 kN, as Product A scatters less compared to product B.
Finally, the design resistance NRd is calculated by dividing the characteristic value NRk by the partial safety factor for material strength gMc (concrete cone failure), gMp (pullout failure) and splitting failure gM,sp., while for anchor technology this factor take account of the installation safety of an anchor system by means of robustness. Also, partials safety factor for steel failure is given, this factor depends on the fyk/fuk ratio. All partial safety factors are given in the relevant ETA. The partial safety factor for concrete related failure modes in general ranges from 1.5 (for systems with high installation safety) to 2.1 systems with low but still acceptable installation safety. Following our example, we would get or both product a design resistance of NRk,product A ~NRk, product B ~52 kN/1.5 = 34.6 kN if for both product would have the same installation safety.
To derive the recommended loads Nrec we have to divide the design resistance by a mix of the partial safety factor for dead loads (1.35) and live (variable) loads 1.5.