Strength
Yield strength
Yield strength is the most common property that the designer will need as it is the basis used for most of the rules given in design codes . In European Standards for structural carbon steels (including weathering steel ), the primary designation relates to the yield strength, e.g. S355 steel is a structural steel with a specified minimum yield strength of 355 N/mm².
The product standards also specify the permitted range of values for the ultimate tensile strength (UTS). The minimum UTS is relevant to some aspects of design.
Hot rolled steels
For hot rolled carbon steels, the number quoted in the designation is the value of yield strength for material up to 16 mm thick. Designers should note that yield strength reduces with increasing plate or section thickness (thinner material is worked more than thick material and working increases the strength). For the two most common grades of steel used in UK, the specified minimum yield strengths and the minimum tensile strength are shown in table below for steels to BS EN 10025-2 .
Minimum yield and tensile strength for common steel grades
Grade |
Yield strength (N/mm2) for nominal thickness t (mm) |
Tensile strength (N/mm2) for nominal thickness t (mm) |
t ≤ 16 |
16 < t ≤ 40 |
40 < t ≤ 63 |
63 < t ≤ 80 |
3 < t ≤ 100 |
100 < t ≤ 150 |
S275 |
275 |
265 |
255 |
245 |
410 |
400 |
S355 |
355 |
345 |
335 |
325 |
470 |
450 |
The UK National Annex to BS EN 1993-1-1 allows the minimum yield value for the particular thickness to be used as the nominal (characteristic) yield strength fy and the minimum tensile strength fu to be used as the nominal (characteristic) ultimate strength.
Similar values are given for other grades in other parts of BS EN 10025 and for hollow sections to BS EN 10210-1.
Cold formed steels
There is a wide range of steel grades for strip steels suitable for cold forming. Minimum values of yield strength and tensile strength are specified in the relevant product standard BS EN 10346.
BS EN 1993-1-3 tabulates values of basic yield strength fyb and ultimate tensile strength fu that are to be used as characteristic values in design.
Stainless steels
Grades of stainless steel are designated by a numerical ‘steel number’ (such as 1.4401 for a typical austenitic steel) rather than the ‘S’ designation system for carbon steels. The stress-strain relationship does not have the clear distinction of a yield point and stainless steel ‘yield’ strengths for stainless steel are generally quoted in terms of a proof strength defined for a particular offset permanent strain (conventionally the 0.2% strain).
The strengths of commonly used structural stainless steels range from 170 to 450 N/mm². Austenitic steels have a lower yield strength than commonly used carbon steels; duplex steels have a higher yield strength than common carbon steels. For both austenitic and duplex stainless steels, the ratio of ultimate strength to yield strength is greater than for carbon steels.
BS EN 1993-1-4 tabulates nominal (characteristic) values of yield strength fy and ultimate minimum tensile strength fu for steels to BS EN 10088-1 for use in design.
Toughness
It is in the nature of all materials to contain some imperfections. In steel these imperfections take the form of very small cracks. If the steel is insufficiently tough, the ‘crack’ can propagate rapidly, without plastic deformation and result in a ‘brittle fracture’. The risk of brittle fracture increases with thickness, tensile stress, stress raisers and at colder temperatures. The toughness of steel and its ability to resist brittle fracture are dependent on a number of factors that should be considered at the specification stage. A convenient measure of toughness is the Charpy V-notch impact test – see image on the right. This test measures the impact energy required to break a small notched specimen, at a specified temperature, by a single impact blow from a pendulum.
The various product standards specify minimum values of impact energy for different sub-grades of each strength grade. For non-alloy structural steels the designations of the subgrades are JR, J0, J2 and K2. For fine grain steels and quenched and tempered steels (which are generally tougher, with higher impact energy) different designations are used. A summary of the toughness designations is given in the table below.
Specified minimum impact energy for carbon steel sub-grades
Standard |
Subgrade |
Impact strength |
Test temperature |
BS EN 10025-2
BS EN 10025-5
BS EN 10210-1 |
JR |
27J |
20oC |
J0 |
27J |
0oC |
J2 |
27J |
-20oC |
K2 |
40J |
-20oC |
BS EN 10025-3 |
N |
40J |
-20oc |
NL |
27J |
-50oc |
BS EN 10025-4 |
M |
40J |
-20oc |
ML |
27J |
-50oc |
BS EN 10025-6 |
Q |
30J |
-20oc |
QL |
30J |
-40oc |
QL1 |
30J |
-60oc |
For thin gauge steels for cold forming, no impact energy requirements are specified for material less than 6 mm thick.
The selection of an appropriate sub-grade, to provide adequate toughness in design situations is given in BS EN 1993‑1‑10 and its associated UK NA. The rules relate the exposure temperature, stress level etc, to a ‘limiting thickness’ for each sub-grade of steel. PD 6695-1-10 contains useful look-up tables and guidance on selection of an appropriate sub-grade is given in ED007.
These design rules were developed for structures subject to fatigue such as bridges and crane supporting structures, and it is acknowledged that their use for buildings where fatigue plays a minor role is extremely safe-sided.
SCI publication P419 presents modified steel thickness limits which may be used in buildings where fatigue is not a design consideration. These new limits have been derived using exactly the same approach behind the Eurocode design rules, but crucially reduce the crack growth due to fatigue. The word “reduce” is used, since to assume no growth at all would be to eliminate the effect of fatigue altogether. Some fatigue (20,000 cycles) is allowed for based on indicative guidance from a DIN Standard.
The term “quasi-static” would cover such structures – in reality that there may be some limited cycling of load, but that would not normally be considered – the design approach is to consider all loads as static. The key to the new approach is the formula to express the crack growth under 20,000 cycles. Experts at the University of Aachen (who were involved with the development of the Eurocode) provided this all-important expression.
Further background is available in a technical article in the September 2017 issue of NSC magazine.
Stainless steels are generally much tougher than carbon steels; minimum values are specified in BS EN 10088-4. BS EN 1993-1-4 states that austenitic and duplex steels are adequately tough and not susceptible to brittle fracture for service temperatures down to -40°C.
Ductility
Ductility is a measure of the degree to which a material can strain or elongate between the onset of yield and eventual fracture under tensile loading as demonstrated in the figure below. The designer relies on ductility for a number of aspects of design, including redistribution of stress at the ultimate limit state, bolt group design, reduced risk of fatigue crack propagation and in the fabrication processes of welding, bending and straightening. The various standards for the grades of steel in the above table insist on a minimum value for ductility so the design assumptions are valid and if these are specified correctly the designer can be assured of their adequate performance.
Weldability
All structural steels are essentially weldable. However, welding involves locally melting the steel, which subsequently cools. The cooling can be quite fast because the surrounding material, e.g. the beam, offers a large ‘heat sink’ and the weld (and the heat introduced) is usually relatively small. This can lead to hardening of the ‘heat affected zone’ (HAZ) and to reduced toughness. The greater the thickness of material, the greater the reduction of toughness.
The susceptibility to embrittlement also depends on the alloying elements principally, but not exclusively, the carbon content. This susceptibility can be expressed as the ‘Carbon Equivalent Value’ (CEV), and the various product standards for carbon steels standard give expressions for determining this value.
BS EN 10025 sets mandatory limits for CEV for all structural steel products covered, and it is a simple task for those controlling welding to ensure that welding procedure specifications used are qualified for the appropriate steel grade, and CEV.
Other mechanical properties of steel
Other mechanical properties of structural steel that are important to the designer include:
- Modulus of elasticity, E = 210,000 N/mm²
- Shear modulus, G = E/[2(1 + ν)] N/mm², often taken as 81,000 N/mm²
- Poisson’s ratio, ν = 0.3
- Coefficient of thermal expansion, α = 12 x 10-6/°C (in the ambient temperature range).
Durability
A further important property is that of corrosion prevention. Although special corrosion resistant steels are available these are not normally used in building construction. The exception to this is weathering steel .
The most common means of providing corrosion protection to construction steel is by painting or galvanizing. The type and degree of coating protection required depends on the degree of exposure, location, design life, etc. In many cases, under internal dry situations no corrosion protection coatings are required other than appropriate fire protection. Detailed information on the corrosion protection of structural steel is available.
Weathering steel
Weathering steel is a high strength low alloy steel that resists corrosion by forming an adherent protective rust ‘patina’, that inhibits further corrosion. No protective coating is needed. It is extensively used in the UK for bridges and has been used externally on some buildings. It is also used for architectural features and sculptural structures such as the Angel of the North.
Stainless steel
Stainless steel is a highly corrosion-resistant material that can be used structurally, particularly where a high-quality surface finish is required. Suitable grades for exposure in typical environments are given below.
The stress-strain behaviour of stainless steels differs from that of carbon steels in a number of respects. The most important difference is in the shape of the stress-strain curve. While carbon steel typically exhibits linear elastic behaviour up to the yield stress and a plateau before strain hardening is encountered, stainless steel has a more rounded response with no well-defined yield stress. Therefore, stainless steel ‘yield’ strengths are generally defined for a particular offset permanent strain (conventionally the 0.2% strain), as indicated in the figure on the right which shows typical experimental stress-strain curves for common austenitic and duplex stainless steels. The curves shown are representative of the range of material likely to be supplied and should not be used in design.
Specified mechanical properties of common stainless steels to EN 10088-4
Description |
Grade |
Minimum 0.2% proof strength (N/mm2) |
Ultimate tensile strength (N/mm2) |
Elongation at fracture (%) |
Basic chromium-nickel austenitic steels |
1.4301 |
210 |
520 – 720 |
45 |
1.4307 |
200 |
500 – 700 |
45 |
Molybdenum-chromiumnickel austenitic steels |
1.4401 |
220 |
520 – 670 |
45 |
1.4404 |
220 |
520 – 670 |
45 |
Duplex steels |
1.4162 |
450 |
650 – 850 |
30 |
1.4462 |
460 |
640 – 840 |
25 |
The mechanical properties apply to hot rolled plate. For cold rolled and hot rolled strip, the specified strengths are 10-17% higher.
Guidelines for stainless steel selection
BS EN ISO 9223 Atmospheric Corrosion Class |
Typical outdoor environment |
Suitable stainless steel |
C1 (Very low) |
Deserts and arctic areas (very low humidity) |
1.4301/1.4307, 1.4162 |
C2 (Low) |
Arid or low pollution (rural) |
1.4301/1.4307, 1.4162 |
C3 (Medium) |
Coastal areas with low deposits of salt
Urban or industrialised areas with moderate pollution |
1.4401/1.4404, 1.4162
(1.4301/1.4307) |
C4 (High) |
Polluted urban and industrialised atmosphere
Coastal areas with moderate salt deposits
Road environments with de-icing salts |
1.4462, (1.4401/1.4404), other more highly alloyed duplexes or austenitics |
C5 (Very high) |
Severely polluted industrial atmospheres with high humidity
Marine atmospheres with high degree of salt deposits and splashes |
1.4462, other more highly alloyed duplexes or austenitics |
Materials suitable for a higher class may be used for lower classes but might not be cost effective. Materials within brackets might be considered if some moderate corrosion is acceptable. Accumulation of corrosive pollutants and chlorides will be higher in sheltered locations; hence it might be necessary to choose a recommended grade from the next higher corrosion class.
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