Projects and Features
How to Specify Steel: Design leads to specification
Designers and specifiers of structural steelwork need to ensure that their requirements are made clear to steelwork contractors. This is a guide to the relevant European steel product standards. Roger Pope reports.
As the scheme design develops, decisions are made about the locations of structural members, and the loads they have to sustain. In some cases there may be architectural constraints or preferences that might lead to a Universal Column (UC) being used as a beam or hollow sections to be preferred to open sections. By the time the scheme design is complete and ready for the detail design to commence it should be possible to identify the size, shape and design grade for every main member.
The design grades commonly used in the UK are S275 (mild steel grade 43 of old) and S355 (high yield steel grade 50). In continental Europe S235 is nearly always preferred to S275. The grade designation represents the nominal yield stress for “S” = structural steels, as most design rules are parameterised in terms of the yield stress.
The range of shapes (“sections” as we call them; “profiles” as they are termed on the continent) is very wide. Hollow sections are standardised across Europe in metric sizes. Grades, section dimensions and tolerances are given in EN 10210 for hot-finished hollow sections (CHS, SHS and RHS), and in EN 10219 for cold-formed hollow section. The shapes, grades and capacities of hot-finished and coldformed hollow sections with the same nominal dimensions are not directly equivalent and care needs to be taken in substituting one for the other.
For open sections only the grades are standardised across Europe. The UK and Ireland generally use sections rolled to BS 4 which has metric sizes that are conversions of imperial sizes (36” becomes 914mm), whereas on the continent sizes are properly metric. We use the terms UB (universal beam) and UC, whereas on the continent columns are Hsections (HEA etc) and beams are I-sections (IPE etc).
UK specialists claim that comparison of the aspect ratios and other shape characteristics of, say, UB sections compared with IPE profiles will reveal that the UB range is more extensive and more structurally efficient. However, the efficiency of a shape does depend on its design grade and the development of code rules (e.g. for buckling of thin flange outstands). Theoretically shapes might have changed over the years as code rules have, but this has not occurred as there is considerable manufacturing and construction advantage in continuing to use shapes that match previous production.
In addition, the ambitious designer can design a bespoke shape or select one from the ranges offered by manufacturers of welded profiles. It has been common for plate girders or heavy columns to be made from plates, but in the UK there has been relatively little take-up for beam sizes below 1000mm or columns below 350mm that overlap with the hot-rolled section range. In contrast, the market for welded profiles is more developed on the continent with French manufacturers offering sizes as small as 150mm deep. One feature that welded profiles (or compound sections) facilitate is the production of shapes that are asymmetric in dimensions or which have differing grades in flange and web, although Corus market hotrolled ASBs as a commercial response to this. So, scheme design is complete and the size and design grade for every member has been decided. End of story?
No – purchasing of steel products requires other specification decisions to be made, and in particular the subgrade needs to be specified as without it the order is incomplete. Considering the designation S275J2 it is “J2” that is the subgrade and it represents the notch ductility of the steel and serves to ensure that the steel is sufficiently tough in resisting fracture.
However, before the choice of subgrade is explored, it should be noted that specifying the grade S275 automatically ensures more than just a guaranteed minimum nominal yield strength of 275 MPa. By reference to EN 10025 it also ensures ductility and weldability.
Steel product standards
In 2003, the steel product standards for the whole range of weldable structural steels used onshore are being amalgamated into one standard – EN 10025. This will bring together the following standards in Parts 2 to 6 respectively of the new EN 10025:
- EN 10025 Non-alloy steels that contain less than 1.65% Mn, 0.30% Ni, 0.30% Cr etc (in grades S235, S275, S355 and the rarely specified S450)
- EN 10113-2 Normalized or normalized rolled fine grain alloy steels (grades S275N, S355N, S420N and S460N being additionally designated as, say, S275NL if the grade has improved low temperature notch ductility)
- EN 10113-3 Thermomechanical rolled fine grain steels (grades S275M, S355M, S420M and S460M also with ML possibility as for N/NL)
- EN 10155 Weather-resisting steels with improved atmospheric corrosion resistance (grades S235W, S355W and S355WP if the grade has higher phosphorus content)
- EN 10137-2 Quenched and tempered steels (grades S460Q, S500Q, S550Q, S620Q, S690Q, S890Q and S960Q again with improved low temperature performance possibilities designated as L and L1, eg S690QL1)
For each grade strength (both yield and tensile), ductility (as elongation at normal ambient temperature) and weldability (as carbon equivalent value based on chemical composition) are given for differing thicknesses. Taking material nominally 50mm thick in S355NL from EN 10025-3 as an example:
- Minimum yield strength is 335 MPa (note that as thickness increases the value drops from the 355 value associated with the grade designation for the thinnest sections, and also that for higher strength steels that have no yield plateau what is termed “yield” is defined by EN 10025-1 as the 0.2% proof stress which is taken as the elastic limit)
- Tensile strength is in the range 470 – 630 MPa
- Minimum elongation is 22% based on fracture of a standard coupon specified in EN 10025-1 (note that this does not mean that differently shaped pieces would stretch by 22% before fracture as the final rupture is localised and much longer pieces would thus experience an overall strain at failure of much less than 22%)
- Carbon equivalent value (CEV) is 0.43
By specifying NL both the process route of manufacture (normalized or normalized rolled as opposed to thermomechanical rolled) is chosen and the notch ductility is specified in terms of the minimum impact energy in a Charpy V-notch (CVN) test. For our example the CVN impact energy value would have to meet 27J at -50ºC for a test piece taken in the longitudinal direction of rolling. For steel products manufactured to the technical
delivery conditions specified in EN 10025-2, the subgrade designations correlate with minimum CVN values as follows:
- JR requires 27J at 20ºC (ie R = room temperature)
- J0 requires 27J at 0ºC
- J2 requires 27J at -20ºC
- K2 requires 40J at -20ºC
Thus S275JR corresponds with grade 43B in former terms, and the CVN test may be taken as representative of the steel’s toughness or resistance to fracture at low temperature. The choice of which subgrade is needed is dependent on several factors which are parameterised in BS 5950-1 (or BS 5400-3) and Eurocode 3 Part 1.10 (EN 1993-1-10).
The BS approach considers whether conditions leading to fatigue are present, type of detail, stress level and whether strain conditions are predominantly tensile. The Eurocode approach is calibrated for both fatigue and static loading in a single table with allowance for the most likely adverse condition in terms of welded details subject to fatigue (referred to standard details in Part 1.9 of EC3) or the presence of residual defects from punching or welding for static conditions.
The advantage of the BS approach is that more latitude is allowed for beneficial details, but this in turn leads to a disadvantage that the choice of appropriate subgrade may not be possible until detailing is complete. The Eurocode approach facilitates a decision in principal on choice of subgrade when the scheme design is complete.
Although the Eurocode rules may be adjusted in the National Annex applicable to use in the UK (based on nationally calibrated safety considerations), the likelihood is that for general application the EC3 rules will be less restrictive on the use of thicker sections at low temperatures when conservative assumptions about detail types are made at design stage. So, the appropriate subgrade has been chosen appended to the design grade for every member. End of story?
Yes… BUT… there are options given in the steel product standards that might need to be specified.
EN 10025-2 lists 28 options which few specifiers use but which can be useful to consider. We will look at four – internal defects, surface condition, through-thickness properties and type of inspection document:
Internal defects: Before the advent of continuous casting steel was rolled from ingots and so-called laminations were more common whereby impurities became inadvertently rolled into the body of the product. The option still exists to verify that a product is free from such defects when tested to EN 10160 which is an ultrasonic examination conducted over a grid of points at a choice of spacing.
Surface condition: EN 10025 specifies that the surface condition of the product shall meet a certain class according to EN 10163, but there are options to vary the base specification and the method permitted for repair by the manufacturer.
Transverse testing: As the grain structure of rolled steel products is elongated in the direction of rolling, the product is stronger longitudinally than in the transverse direction. Sometimes it is important to check properties such as CVN values in the transverse direction and options exist for specifying this.
In addition, so-called Z-quality can be verified using EN 10164 to check the ductility of steel products in the “through-thickness” direction. As the specimens must be stocky by nature the ductility is not measured by elongation strain at rupture used as a measure in the longitudinal direction but by the percent reduction of area before rupture. A minimum value of 30% would be needed for Z30, and this value would indicate very ductile steel. Eurocode 3 Part 1.10 gives guidance to specifiers to establish the relatively few occasions when it might be necessary to specify through-thickness properties.
Inspection documents: What is generally termed the “test certificate” gives the purchaser all the specification information about the actual product just purchased. Yes… and No! First it needs to be understood that inspection documents to EN 10204 (as they are properly termed) may be of several types. The most common are a Type 2.2 Test Report and a Type 3.1B Inspection Certificate.
A Type 2.2 document does not give any information that is directly linked to the product itself but it states that the production process used by the manufacturer is capable of producing product that conforms to the specification – and this is verified by periodic checking of the process.
A Type 3.1B document is directly linked to the lot or cast from which the actual product came, and hence this is termed “specific inspection”. However, some properties are verified using samples taken from the finished products in the lot, and some are based on the “ladle” analysis sampled earlier in the production process. Wisely, the steelmaker will check the ladle analysis from the molten steel complies so that, if necessary, it can be adjusted before the products are cast and rolled. In BS 5950 it is a mandatory requirement that a Type 3.1B document is provided, and this should be specified on the purchaser’s order.
Rarely specifiers will require inspection that is witnessed by the purchasers themselves and is specific to their actual order rather than to the lot or cast. The option exists, therefore, to specify the Type 3.1C or Type 3.2 documents that allow this.
The choice of grade (such as S275 which designates yield strength) and choice of shape of the section/profile are readily decidable at scheme design stage. The choice of grade automatically determines ductility (elongation at normal temperatures) and weldability (as CEV).
Between scheme design and detail design the subgrade (such as J2) can be chosen. This is specified in terms of a CVN value for notch ductility and determines toughness and resistance to fracture at low temperatures. Occasionally the designer or steel purchaser may wish to specify options offered by the steel product standards such as those for internal defects, surface condition, throughthickness properties and type of inspection document.
For more information on how to specify requirements for structural steelwork see the updated Commentary on the 4th edition of the National Structural Steelwork Specification. The Commentary on the NSSS (The Grey Book – published July 2003 – price £30) and the NSSS itself (The Black Book – published May 2002 – price £20) are both available from BCSA.
Dr. Roger Pope is a technical consultant to the British Constructional Steelwork Association