Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES therefore, the composite connection and concrete column design strength should not be less than forces developed by plastic hinging of the steel beam adjacent to the connection. Connection design strength is obtained reducing the nominal strength by a resistance factor φ. Since a large data base does not exists for statistical analysis of joint strength, the resistance factor is determined by calibration with relative factor of safety in the allowable stress specification for steel structures (AISC Specification, 1997). It practically corresponds to the inverse of the partial safety factor γM0 used in European practice (where φ=0.9 e γM0=1.1). In the presented formula the resistance factor φ proposed by the AISC Specification (1997) is not directly take into account. The value of 0.7, that multiplies the formula, is based on the following assumption: • The connections should have a greater reliability index (or factor of safety) than structural members. Allowable stress values reflect a factor of safety for connection that is 1.3 times greater than that for structural members; • When the column axial force is not negligible, the shear resistance of the column web should be computed, by adopting the Hencky-Von Mises yield criterion, by means of the following relationship: 1 σ 2 f Sd , col − ⋅ ( 5.21 ) yd , cw f yd , wc where σSd,col is the average normal stress in the panel zone. In the current Draft of EC3 (2000), the influence of the normal stress σSd,col due the column axial force, is approximately accounted for by means of a reduction coefficient equal to 0.9. In particular, the codified value of the reduction factor is on the safe side up to a column axial load equal to 45% of the column squash load. The use of the above equations, i.e. the use of a first yield criterion for evaluating the panel zone shear resistance, should be preferred when the effect of the panel zone deformation on frame stability is not considered in the structural analysis (Liew and Chen, 1994) or when is desired to limit the panel zone behaviour to the elastic range (Mazzolani and Piluso, 1996). Thus, it is simple to obtain that 0.7 = 0.9 / 1.3. For the calculation of the shear area in the web panel zone the minimum value of two different contributions is take into account. Shear area Av proposed by EC3 that may be taken as: ( ) Av = As , tot − 2⋅ bcf ⋅ tcf + tcw + 2⋅ r ⋅ tcf ( 5.22 ) 199
5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES Shear area calculated on the basis of the length jh, used to regulate the effective length of the web panel and the distance between vertical resultant forces coupled in the compression strut. It is proposed to use a fixed value of jh = 0.8ds based on comparison with test data. Moreover, experimental tests have demonstrated that the panel zone is able to provide a significant post-yield resistance (Krawinkler, 1978). When the panel zone has uniformly reached yielding, an additional increase in shear strength ∆Vj,wps can be attributed to the resistance in flexion of the elements surrounding the panel. This resistance can be approximated by springs at the four corners whose stiffness is that corresponding to rotations of the column flanges concentrated at each corner. When the boundaries of the panel zone are assumed to be rigid, the post- elastic stiffness of the joint, attributable to the four springs, is computed as: ∆ V = min 200 j, wps 4⋅ M 4⋅ 1 b t f 4 b t f = = d d d 2 2 Pl, cf , Rd cf cf ym, d , cf cf cf ym, d , cf s s s 2⋅ M + 2⋅ M b t f + b t f = d 2⋅ d 2 2 Pl, cf , Rd Pl, st, Rd cf cf ym, d , cf p sp ym, d , s s s ( 5.23 ) Where the steel column web is encased in concrete the design shear resistance of the panel may be increased with the contribution of the inner concrete strut Vccs, that is the design shear resistance of the concrete encasement to the web panel. The concrete compression strut, shown in Figure 5.15b, is similar to the mechanism used to model shear in a reinforced concrete connection. In composite connections, the concrete compression strut could be mobilized in resisting the connection shear either due to the presence of the horizontal stiffener plates welded to the column or due to the friction and the flexural forces acting in the steel column flange. In case of presence of the stiffener plates, the location and width of these determine how effectively the concrete strut is mobilized. The shear resistance and the moment resistance for the concrete compression strut are calculated as follows: 1 1 0.85⋅ f V = ⋅ν ⋅ f ' ⋅ A ⋅ senθ = ⋅ν ⋅ ⋅ A ⋅ senθ ( 5.24) ck j, ccs 1.3 cd c 1.3 γ c c ( ) M = V ⋅ ⋅ d ( 5.25 ) Rd , j, ccs j, ccs 0.8 s
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Dipartimento di Ingegneria Meccanic
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UNIVERSITÀ DEGLI STUDI DI TRENTO D
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Ai miei genitori
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INDEX 1 INTRODUCTION...............
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INDEX iii 4.10.3 Results of the PsD
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1.1 Introduction 1 1 INTRODUCTION I
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1. INTRODUCTION imposes precise con
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1. INTRODUCTION joints whilst 2D mo
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7 2 DUCTILITY AND SEISMIC RESPONSE
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2. DUCTILITY AND SEISMIC RESPONSE O
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3. SEISMIC BEHAVIOUR OF BOLTED END
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6. SUMMARY, CONCLUSIONS AND FUTURE
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Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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