BS EN10025 S275 /S355 /S420 /S460

BS EN 10025 has recently been revised (2004), and subsumes standards such as the 1993 versions of BS EN 10025, BS EN 10113, BS EN 10137 and BS EN 10155. Furthermore, designers may be more familiar with the material grades of old standards such as BS 4360, so comparisons are useful. However, it must be stressed that the new steels are not simply the old steels with new names - there are some differences in the production processes and properties. Cold formed hollow sections to BS EN 10219 are now available, but are not yet accepted in the design code, BS 5400: Part 3. Use of such sections would require a ‘Departure from Standard’; the particular consideration would then be the toughness of the material in the corners of rectangular and square sections.

Yield strength

The yield strength is probably the most significant property that the designer will need to use or specify. The achievement of a suitable strength whilst maintaining other properties has been the driving force behind the development of modern steel making and rolling processes. In the European Standards for structural steels, the primary designation relates to the yield strength, e.g. S355 steel is a structural steel with a nominal yield strength of 355 N/mm

The number quoted in the designation is the value of yield strength for material up to 16 mm thick (or 12 mm for steel to BS 7668). Designers should note that yield strength reduces with increasing plate or section thickness. An example for common bridge steels to BS EN 10025 is given below.

Steel Grade	Minimum yield strength (N/mm2), Nominal thickness in mm
Steel Grade	< 16	> 16 < 40	> 40 < 63	> 63 < 80	> 80 < 100	> 100 < 150
S275	275	265	255	245	235	225
S355	355	345	335	325	315	295
S460	460	440	430	410	400	380

The strength grades covered by the BS EN standards include; S235, S275, S355, S420 and S460. ( BS 7668 covers one strength grade, S345.) Yield strengths above 460 N/mm are available to BS EN 10025: Part 6, but BS 5400 or other design codes do not yet cover the use of these strengths. Grade S235 steel is rarely used in bridge steelwork. Steels of 355 N/mm

yield strength are predominantly used in bridge applications in the UK because the cost-to-strength ratio of this material is lower than for other grades. However, some of the higher strength grades offer other advantages and may be seen more frequently in the future. However, note that the use of higher strength steels confers no benefits in applications which are fatigue critical, or in which instability of very slender members is the overriding design consideration. S460 steels are also less readily available.

Again this is not usually a problem for steels complying with the material standards, except for perhaps S460 steels & thick plates, as indicated with “grey” areas in the following table. In such cases, further specification of the ductility requirements of thedesigner will be needed.

			Elongation for thickness range (mm)
			3-40	41-63	64-100
BS EN 10025: Part 2
S275	Long	23		22	21	19
S275	Trans	21		20	19	19
S355	Long	22		21	20	18
S355	Trans	20		19	18	18
BS EN 10025: Part 3
S275	N/NL	24		24	23	23
S355	N/NL	22		22	21	21
S420	N/NL	19		19	18	18
S460	N/NL	17		17	17	17
BS EN 10025: Part 4
S275	M/ML	24		24	24	24
S355	M/ML	22		22	22	22
S420	M/ML	19		19	19	19
S460	M/ML	17		17	17	17
BS EN 10025: Part 5
S355	Long	22		21	20	18
S355	Trans	20		19	18	18

Notch Toughness

The nature of steel material is that it always contains some imperfections, albeit of very small size. When subject to tensile stress these imperfections (similar to very small cracks) tend to open. If the steel is insufficiently tough, the ‘crack’ propagates rapidly, without plastic deformation, and failure may result. This is called ‘brittle fracture’, and is of particular concern because of the sudden nature of failure. The toughness of the steel, and its ability to resist this behaviour, decreases as the temperature decreases. In addition, the toughness required, at any given temperature, increases with the thickness of the material.

A convenient measure of toughness is the Charpy V-notch impact test (hence the use of the term “notch toughness” in BS 5400: Part 3). This test measures the impact energy required to break a small, notched specimen by a single impact blow from a pendulum. The test is carried out with the specimen at a specified (low) temperature. In the C and the required minimum value is typically 27J. Other temperatures and energy values are specified for different grades. In the CEN steel standards there is not, unfortunately, a universal designation system for the notch toughness.

character alphanumeric code; there are four different codes that are used:

J0: 27J impact energy at 0℃

J2: 27J impact energy at -20℃

K2: 40J impact energy at -20℃

Source:

Tags: BS EN10025, S275, S355, S420, S460