This is done based on Eurocode 3, Part 1-1, BS EN 1993-1-1: 2005 (Eurocode, 2005a).
The steel structure is designed using the ULS method. So, the structural strength is based on the yield strength of structural steel fy. We describe fi rst the properties and strength of structural steel and fasteners (bolts and welds).
2.4.1 Properties and strength of structural steel and fasteners 2.4.1.1 Type of construction material
We refer to Table 3.1 of Annex A of Eurocode 3, Part 1-1, reproduced in Appendix B of this book. The structural steel is of grade S 275 (EN 10025-2): fy = 275 N/mm2 for t ≤ 40 mm, and fy = 255 N/mm2 for t ≤ 80 mm, where t is the nominal thickness of the element.
2.4.1.2 Design strength (fy)
The design strength of weldable structural steel should conform to the grades and prod- uct standards specifi ed in BS EN 100251: 2004 (British Standards Institution, 2004) (see BS EN 1993-1-8: 2005(E) (Eurocode, 2005c)). The design strength fy should be taken equal to the minimum yield strength, and the ultimate tensile strength fu should be taken as 430 N/mm2.
The above code stipulates values of the design strength of different grades of steel for various thicknesses of the product. For example, for steel grade S 275 and thickness 40 mm or less, fy = 275 N/mm2. With an increase in thickness, the value of fy decreases.
For steel grade S 355 and thickness 40 mm or less, fy = 355 N/mm2, and fy decreases as the thickness increases. For steel grade S 450 and thickness 40 mm or less, fy = 440 N/mm2, and fy again decreases as the thickness increases.
For details, refer to Table 3.1 of Annex A of Eurocode 3, Part 1-1.
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+1.1 –0.2
–0.30 –0.30
+1.1–0.2
–0.3–0.30 –0.1–0.1
–0.1–0.1 Resultant pressure coefficients (c)
35.51.5
8.5
1.5 –0.3 –0.3
–0.3 –0.3–0.3 –0.3 Internal –vepressure coefficients (suction) (b)
–0.3–0.3 30.027.012.027.0
+0.8
–0.60 +0.6+0.80
–0.60–0.60 –0.4–0.4–0.4–0.4 –0.5 External pressure coefficients (wind blowing from right) (a)
–0.50 1st bay2nd bay3rd bay4th bay Fig. 2.5.Wind pressure coeffi cients on multispan building (wind blowing from right)
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2.4.1.3 Elastic properties of steel
We refer to Clause 3.2.6, “Design values of material coeffi cients”, of Eurocode 3, Part 1-1.
The values of the elastic properties of steel are given there as follows:
Modulus of elasticity E = 210 000 N/mm2.
Shear modulus G = E/[2(1 + υ)] = 810 000 N/mm2. Poisson’s ratio ν = 0.30.
Coeffi cient of linear thermal expansion α = 12 × 10−6 per °C (in the ambient temperature range).
2.4.1.4. Bolts and welded connections, based on Eurocode 3, Part 1-8 (Eurocode, 2005c)
The specifi cations for the strengths and properties of fasteners (bolts, nuts and washers, including friction grip bolts) are given in Eurocode 3, Part 1-8, BS EN 1993-1-8: 2005.
2.4.1.5 Welding consumables
All welding consumables, including covered electrodes, wires, fi ller rods, fl uxes and shielding gases, should conform to BS EN 1993-1-8: 2005.
2.4.2 Partial factors γ M of resistance in the ultimate-limit-state concept
In the ULS concept, the structure should be designed to a limiting stage beyond which the structure becomes unfi t for its intended use. Referring to Clause 6.1 of Eurocode 3, Part 1-1, the following recommended values of partial factors γ M should be applied to the various characteristic values of resistance:
resistance of cross-sections, whatever the class is: γ M0 = 1.00;
resistance of members to instability, assessed by member checks: γ M1 = 1.00;
resistance of cross-sections in tension to fracture: γ M2 = 1.25;
resistance of joints: see BS EN 1993-1-8: 2005.
2.4.3 Ultimate limit state
Ultimate limit states relate to the safety of a structure as a whole or of part of it. In check- ing the strength of a structure or of any part of it, the specifi ed loads should generally be multiplied by the relevant partial factors. Thus, γG is the partial factor for permanent loads, and γQ is the partial factor for variable loads. The resulting factored loads should be applied in the most unfavourable combination so that the load-carrying capacity of the members sustains adequate strength without allowing any collapse.
The method of design based on the above ultimate limit states is known as the ultimate- limit-state (ULS) method.
2.4.4 Serviceability limit state
Serviceability limit states defi ne the limit beyond which the specifi ed service conditions are no longer fulfi lled. In serviceability limit states, the specifi ed loads are generally unfac-
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tored except in the case of combinations of imposed loads and wind loads, in which case 80% of the full specifi ed load is taken into account.
The method of design based on the above serviceability limit states is called the serviceability limit state (SLS) method. This method is applied in checking defl ection, vibration etc.
2.4.5 Load combinations
In structural design for load combinations using the ULS and SLS methods, the partial fac- tors γG for permanent actions and partial factors for variable actions should be taken from Table A1.2(B) of BS EN 1990: 2002(E) (see Appendix B).