Last time we talked about the Charpy test. This time let’s take it further.
STEP BY STEP…
The Eurocode defines the fracture energy to be assumed for a tested steel specimen. It is 27 J, and at this energy the steel is tested at different temperatures until it cracks under dynamic load. So we already have two variables: the energy needed to break the specimen and the design temperature. There is a third one as well – thickness. Fracture toughness decreases as material thickness increases. In other words, a thick plate is less resistant to cracking than a thin one. We will go through the details of this next time… For now – take our word for it.
The design temperature for a building depends on the location of the steel elements – whether they are inside or outside. For interiors, unless specified otherwise, about −5 °C can be assumed. For external elements, the detailed criteria are given in the NA to EN 1991-1-5. For Poland the NA gives them as a map (see the picture above). For Wrocław 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 steel grade (S235/S275/S355…), the minimum required fracture energy (typically 27 J), the stress level in the element under the design load – 0.75fy / 0.5fy / 0.25fy – and the design temperature.
So let’s see what maximum thickness may be used “outside” for S355JR in Wrocław, Poland, for a column loaded to 0.75fy – that is, very heavily utilised:
It is only 15 mm, so with S355JR the largest possible column is HEB200. For comparison – with S355J2 the maximum thickness is 40 mm (i.e. HEM320).
That is a big difference!
The Polish NA to EN 1993-1-10 gives no other way of assigning a steel grade. That is not the case for BS EN 1993-1-10 and SCI Publication P419, which allow steel grades to be assigned differently when the load on the structure is quasi-static (that is, most of the load comes from dead loads rather than live or wind loads):
SCI P419:
Under the BS EN national annex the permissible thickness for S355JR would be as much as 54 mm instead of 15 mm. That is a valuable gain. For S355J2 it can be as much as 200 mm! It all comes down to dynamic loads and brittle fracture – quasi-static loads make far smaller demands.
We will talk about brittle fracture soon.