S355JR or S355J2 part 2
What steel should I choose – S355 JR or J2? Part 2
Last time we talked about Charpy Test. This time let’s go ….
STEP BY STEP…
#Eurocode defines what energy test should be provided for a tested piece of steel bars. This is 27J and for this energy steel is tested for different temperatures until it cracks under dynamic load. So we have two variables already: the amount of energy to brake bar and the design temperature. But there is also the third one – thickness. Fracture toughness decreases as material thickness increases. In other words – a thicker plate is less resistant to cracking than a thin one. We will go through the details of this phenomenon next time… For now – believe me ?
Design temperature for a building depends on the location of steel elements – whether they are inside or outside. For inside areas, if not specified otherwise, the temperature of about -5*C can be specified. For outside – specific criteria can be found in NA to EN 1991-1-5. For Poland it is shown in form of map in NA – as per picture above with map of Poland. For Wroclaw (Poland) it is -32*C.
OK, there you have it – input data collected. What next?
Table 2.1 from EN 1993-1-10 specifies maximum permissible values of element thickness in [mm] taking into account #steelgrade (S235/S275/S355…), the minimum required energy from braking (typically 27J), level of #stress (i.e. how big stress is in element, when design load is applied) – 0,75fy/0,5fy/0,25fy and #design temperature.
So let’s see what maximum allowable thickness can be used “outside” for steel S355JR in city of Wroclaw in Poland for column which is loaded with 0,75 fy (so very heavy loaded):
It is only 15mm, so from steel S355JR the maximum column size is only HEB200. For comparison – if we go for S355J2, the maximum thickness is 40mm (i.e. HEM320).
That is a big difference!
Polish NA to EN 1993-1-10 does not give other options to assign steel grade. That is not the case for i.e. BS-EN 1993-1-10 and SCI Publication P419, which allows to assign steel grades differently, when load on structure is quasi-static (so most of the load is from dead loads and not from live loads / wind loads):
For NA BS-EN this minimum thickness would be even 54mm for S355JR (instead of 15mm) . This may be considered as valued upgrade. For S355J2 it can be even 200mm! It is all about dynamic loads and brittle fracture – quasi static loads have much smaller demands.
We will talk about brittle fracture soon.