How to Choose a Steel Grade for a Steel Structure to DBN V.2.6-198:2014: Structure Group, Strength Class, Impact Toughness

Step 1. Categories by purpose and by stress state

Under cl. 5.3.5 every member is assigned to one of three categories by purpose (the standard designates them with the Cyrillic letters А, Б, В, read here as A, B, V):

• А: failure may make the structure, or a significant part of it, unfit for service;
• Б: failure hampers normal service or causes failure of other structures not in category А;
• В: failure does not disturb the work of other structures or their members.

The second attribute is the category by stress state, that is, by the possibility of reaching a limit state through fatigue or brittle fracture:

• I: a limit state is possible from the direct action of a dynamic moving or vibratory load;
• II: only when unfavourable factors combine (dynamic or vibratory load, stress concentrators, tension zones and so on);
• III: fatigue and brittle fracture are impossible because there are no unfavourable factors.

Table A.1 of Annex A gives a ready list for typical members. There, roof trusses under static load are А II, columns of industrial buildings (main section elements) are А III, overhead line supports are А III, roof purlins are Б II, and wall girts are В III. For members not in the table the categories are assumed by analogy (note 2 to Table A.1).

Step 2. Structure group

The group depends on the sum of points Stot = S1 + S2 + S3 + S4 + S5 from Table A.2 (cl. A.1):

Threshold for each group: group 1 when Stot > 26; group 2 when 23 ≤ Stot ≤ 26; group 3 when 19 ≤ Stot ≤ 22; group 4 when Stot ≤ 18.

The effect of welded joints (S5) is counted if the welds lie where significant design tensile stresses act (σ > 0.3Ry or σ > 0.3Rwz), or if the weld strength decides the fitness of the whole structure for service. This is an extract from the DBN tables for selection purposes; before ordering metal, check the class and the group against the current text of DBN V.2.6-198:2014 and the design.

Refining the group once sections are selected

When the sections are chosen, the group is reviewed (cl. A.2) and Stot is adjusted by no more than ±4 points. One point is added for rolled thickness over 20 up to 40 mm, for edges left by guillotine cutting, for work hardening from cold forming, or for high initial (welding included) stresses not covered by the calculation. For thickness over 40 mm two points are added.

Four points are subtracted if the load is static and the compressive stress does not exceed 0.4·Ry·γc (say, where slenderness is limited by construction requirements).

Step 3. Which strength classes are permitted for the group

Table G.1 of Annex G, for steels in shapes, bars, plate and wide flats and in cold-formed sections, marks permitted classes with “+” and prohibited ones with “–”. In condensed form it reads:

An extract; check the class and the group against the current text of DBN.

Notes to Table G.1 that most often affect the choice

In groups 1 and 2 steel С235 is not permitted, except for thin rolled products (note 3, below). In group 4 it is permitted without notes.

• С235 in group 3 is not used for unheated buildings and structures used in the open air, nor for overhead line supports, open switchgear (OSG) and traction contact networks (TCN) — note a;
• С245 and С275 in group 2 for unheated structures in the open air — rolled products up to 10 mm thick only (note b);
• rolled products up to 5 mm inclusive of steel С235 are permitted for structures of all groups except overhead line supports, OSG and TCN, and products up to 8 mm inclusive of steel С245 for group 1 (note 3);
• for rolled products up to 3 mm thick there are no impact toughness requirements (note 2);
• bars (round, flat, square) are held to the same requirements as shapes of the same thickness (note 5).

Step 4. Impact toughness

Table B.1 of Annex B sets the characteristic impact toughness of rolled products at the design temperature, depending on the structure group and the characteristic strength Ryn:

KCV (subscript t is the test temperature; with no subscript, +20 °C) is the impact toughness on V-notch specimens; KCA is the toughness after strain ageing on U-notch specimens (GOST 9454).

Impact toughness values

Note 3 to Table B.1 gives these values: KCA = 29 J/cm². For Ryn < 290 N/mm² KCV = 34 J/cm² on longitudinal and 25 J/cm² on transverse specimens. For 290 ≤ Ryn ≤ 390 KCV−20 = 34 J/cm² (longitudinal) and 25 J/cm² (transverse). For Ryn > 390 KCV−40 = 25 J/cm².

The difference between classes С255 and С345 is not only in Ry. For С345 impact toughness is required already at −20 °C, and this has to be stated in the metal order.

Design temperature

The design minimum temperature (cl. 6.1.8) is the temperature of the coldest five-day period at the construction site according to DSTU-N B V.1.1-27, with a probability of 0.92 for structures in heated rooms and 0.98 for structures in unheated rooms and in the open. For supports, masts and other structures in the open, 0.98 is used.

Table B.1 fixes the test temperature (KCV−20 or KCV−40) by Ryn, not by city. The standard gives no formula for moving from the design temperature to the test temperature. Note 2 to Table B.1 refers to cl. 5.3.1, while the design temperature itself is defined by cl. 6.1.8. Where it is lower than the test temperature, the designer specifies the requirement in the technical brief.

Values from Table 2 of DSTU-N B V.1.1-27 (columns 17 and 18) for regional centres. The names are as in the standard: Dnipropetrovsk is now Dnipro, and Kirovohrad is now Kropyvnytskyi.

For places not in the table, take the data of the nearest place with similar conditions (cl. 1.2 of DSTU-N B V.1.1-27).

Thick plate

Under cl. 6.1.7, plate 25 mm thick and more for members working through the thickness, and rolled products 40 mm thick and more in welded joints, must meet requirements by group. Group 1: soundness class 0 to GOST 22727 and quality group not lower than Z35 to GOST 28870. Group 2: soundness class not above 1 and Z25. Groups 3 and 4: soundness class not above 2 and Z15.

Step 5. Steel grade by standard and strength class

Table G.5 matches the strength class (С245, С345…) with the grade under GOST 380, GOST 19281 and DSTU EN 10025. Below are those most common in structural fabrication:

An extract; check designations and thicknesses against the current text of DBN.

Reading Table G.5

One grade belongs to different classes depending on thickness. Steel 09Г2С is С345 (up to 10 mm), С325 (over 10 up to 20 mm) or С295 (up to 32 mm), and the design strengths differ in each case (step 7).

Steel Ст3пс5 corresponds to class С245, which is not permitted in structure group 1 (the exception is rolled products up to 8 mm inclusive, note 3 to Table G.1). Ст3сп5 (class С255) is permitted in groups 1–3.

For welded structures, carbon and low-alloy steels must be supplied with the weldability guarantee “св” (note 1 to Table G.5). Tubes have their own block of Table G.1 and Table G.3 (GOST 10705, 10706, 8731), and castings Tables G.6 and G.7; this article does not cover them.

Step 6. Chemical composition and weldability

Table B.2 sets the limit on carbon, phosphorus and sulphur (mass fraction, %, max) and the recommended carbon equivalent Ce:

If the structure will be galvanised

For items that are hot-dip galvanised after fabrication, the steel composition also affects the coating: the appearance and thickness of the zinc layer depend on the silicon and phosphorus content. The composition of the metal for such structures is therefore worth agreeing with whoever applies the coating. More in the checklist for preparing items for galvanising.

Step 7. Design strength

The design strengths Ry in tension, compression and bending are taken from Table G.2 by strength class and thickness (for shapes, by flange thickness). Here is an extract for the steels used most often in fabrication. The section check with the chosen steel is shown in the articles on beam, column and truss design.

For cold-formed sections the design strengths of the plate they are made from are used (cl. 7.3). Ry of plate falls as thickness grows, so going from 10 mm to 12 mm can change the design strength.

Examples

All four members are taken for consequence class СС2 (S1 = 0), with no sections checked. “Present” for S4 and S5 means that the member carries tensile stresses and the welds lie in zones of significant tension.

In the examples S4 = 7 (tensile stresses present) and S5 = 6 (welds in tension zones) are taken as the worst case. If a member works in compression only (S4 = 2) and the compressive stress is not above 0.4·Ry·γc, the group can be lowered by 4 points under cl. A.2; these conditions cannot coexist with tension. The final group is set by the designer.

What to show on KM drawings and in the metal order

Under cl. 5.1.3 of DBN the working documentation (KM drawings) states: the category of structures and their members by purpose and stress state (Annex A); the consequence class and the adopted coefficient γn; the service life from the technical brief; the steels and additional requirements for them; the types, methods and sections of welds; the strength class and accuracy class of bolts; the degree of aggressive environmental action and the corrosion protection system. On the form of such drawings, see the rules for KM and KMD drawings.

If the rolled product works in tension through the thickness, the order states the soundness class to GOST 22727 and, if needed, the classification by reduction of area to GOST 28870 (cl. 5.1.4). The rest of what belongs in the order:

• the strength class (С345) and the grade by standard (09Г2С GOST 19281), together;
• impact toughness requirements (KCV−20, longitudinal or transverse specimens) if they differ from the basic ones;
• “св”, the weldability guarantee, for welded structures;
• the thickness: the class in Table G.5 and Ry in Table G.2 depend on it;
• the category of the structure and the group (per Table A.2), so the fabricator can see why this class was chosen.

Common mistakes

• choosing steel by Ry alone, although the class, the impact toughness and the structure group must agree;
• using С235 or Ст3кп where it is prohibited: in groups 1 and 2, and in group 3 for overhead line supports, OSG and TCN and for unheated structures in the open (Table G.1, note a);
• leaving the impact test temperature out of the order: KCV−20 for С345 is part of the steel requirement;
• ignoring thickness, on which both the steel class and Ry depend (Tables G.2 and G.5);
• forgetting the group after the sections are selected and after allowing for thickness over 20 mm (cl. A.2);
• ordering steel for a welded structure without the weldability guarantee.

What we fabricate

The main steel grades from which POLYGONAL fabricates steel structures are Ст3, С245, С345 and 09Г2С. The grade for your item is chosen from the load, the service temperature and the project requirements. If the project already states the class and grade, we fabricate to your drawings; the properties of grades are collected in the “Steel grades for metal structures” section.

Standards used for this article

• DBN V.2.6-198:2014 “Steel structures. Design standards” (cl. 5.1.3, 5.1.4, 5.3.5, 6.1.7, 6.1.8, 7.3, Annexes A, B, G: Tables A.1, A.2, B.1, B.2, G.1, G.2, G.5);
• DSTU-N B V.1.1-27:2010 “Construction climatology” (Table 2: temperature of the coldest five-day period; in force from 01.11.2011).