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Technical 26 NSC September 15 The increase in resistance is more pronounced at low to medium slenderness. At high slenderness, the improvement is modest, as can be seen in Figure 2. Two examples are shown below to illustrate the values shown in Figure 2. Clause 6.3.1 of BS EN 1993-1-1 should be consulted. Example 1: 305 UKC 118, 6 m length Ncr = 2EI L2 2 × 210000 × 9060 × 104 = × 10-3 = 5216 kN 60002 In S355, = Aƒy Ncr 150 × 102 × 345 5216 × 103 = = 0.99 Note the use of 345 N/mm2 as tf > 16 mm In the minor axis, curve c is to be used, so α = 0.49 = 0.5 1 + ( – 0.2 ) + 2 = 0 .5 1 + 0.49 ( 0.99 – 0.2) + 0.992 = 1.184 1 = = = 0.545 1.184 + 1.1842 – 0.992 1 + 2 – 2 Nb,Rd = 0.545 × 150 × 102 × 345 × 10-3 = 2820 kN In S460, = Aƒy Ncr 150 × 102 × 440 5216 × 103 = = 1.125 Note the use of 440 N/mm2 as tf > 16 mm; design grades in S460 do not follow the usual 10 N/mm2 steps. In the minor axis, curve a is to be used, so α = 0.21 = 0.5 1 + ( – 0.2) + 2 = 0.5 1 + 0.21 ( 1.125 – 0.2) + 1.1252 = 1.23 1 = + 2 – 2 1 = = 0.579 1.23 + 1.232 – 1.1252 Nb,Rd = 0.579 × 150 × 102 × 440 × 10-3 = 3821 kN The resistance of the S460 column is 1.35 that of the S355 column. This corresponds to a slenderness of 1.0 in Figure 2. Example 2: 305 UKC 118, 12 m length Ncr = 2EI L2 2 × 210000 × 9060 × 104 = × 10-3 = 1304 kN 120002 In S355, = 150 × 102 × 345 1304 × 103 = 1.99 = 0.5 1 + 0.49 ( 1.99 – 0.2 ) + 1.992 = 2.919 = 1 2.919 + 2.9192 – 1.992 = 0.198 Nb,Rd = 0.198 × 150 × 102 × 345 × 10-3 = 1025 kN In S460, = 150 × 102 × 440 1304 × 103 = 2.25 . In the minor axis, curve a is to be used, so α = 0.21 = 0.5 1 + 0.21 ( 2.25 – 0.2) + 2.252 = 3.247 1 = 3.247 + 3.2472 – 2.252 = 0.179 Nb,Rd = 0.179 × 150 × 102 × 440 × 10-3 = 1181 kN At this higher slenderness, the improvement in resistance is 15%, illustrating the diminishing advantage shown in Figure 2 at higher slenderness. Lateral torsional buckling The relationship between steel strengths and lateral torsional buckling resistance is more complicated than flexural buckling, because of the influence of the shape of the bending moment diagram. As the steel strength increases, the slenderness increases, but the effect is modified by the ƒ factor found in clause 6.3.2.3(2) of BS EN 1993-1-1. The same LTB curves are used for all steel grades. Until the end of the plateau at a slenderness of 0.4, clearly the full increase in strength can be utilised. As slenderness increases, and buckling behaviour becomes more significant, the advantage of the increased strength diminishes. Where to consider high strength steel… The advantages of higher strength steels are lighter weights for similar resistance, so applications where light weight, or where smaller cross sections are required, are situations where higher strength steel may be advantageous. Higher strength steels are used in long span bridges where minimising self weight is important. Reduced self weight can also be a significant benefit in long span roof structures such as stadia and aircraft hangers. …and where not. Reduced section sizes mean reduced second moment of area, so any situation where deflection is dominating the design or where fatigue is critical will not benefit from higher strength. Similarly, decreased stiffness may increase the vibration response. Where might steel strengths be in another 20 years? Perhaps there is no definitive answer, but it seems likely based on the last two decades that higher strengths will be in more common use. When revised, the Eurocodes will bring some higher strength steels into the ‘general rules’ indicating their increased use. At present, SCI is co-ordinating a pan-European RFCS project HILONG, looking at new technologies to enable a greater proportion of the strength of higher strength steels to be exploited in long span truss structures. The structural performance of innovative cross-sections with greater resistance to local buckling, such as U shapes and polygonal shapes, is also being studied. 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0 0.5 1 1.5 2 2.5 3 Slenderness Reduction Factor S460 compared to S355 S355 to S460 Figure 2 Comparative flexural resistance between S355 and S460 – minor axis


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