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

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4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS The strut and tie mechanisms to be activated in the slab depend on the joint flexibility. The numerical analyses carried out under sagging bending moment have evidenced the presence of high gradients of compression stresses in front of the column flanges. It is impossible to appreciate the formation of the idealized Mechanism 2, proposed in Section 4.5.1. In fact, in the portion of the concrete slab around the column, the numerical model evidences tensile stresses that do not permit the presence of an idealized concrete strut. This means that, under sagging bending moment, the resistance of the composite connection depends only on the resistance of the proposed Mechanism 1. The experimental loss of resistance of the joint is due to the high level of stresses that the concrete slab should transmit from the composite section of the beam to the column. (a) (b) Figure 4.43. Evolution of the compression stress distribution in the concrete slab due to: (a) activation only of Mechanism 1; (b) Activation of Mechanisms 1 and 2 On the basis of the observations that have been brought to light, it is possible to design some constructional details that permit the formation of the Mechanism 2 in 147
4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS the concrete slab, with an increase of the global joint resistance and ductility. The numerical model aims to calibrate the difference, in term of global response and local stress distribution, between the experimental specimen and the improved model. In fact, to better evidence the formation of Mechanism 2, the surfaces of the concrete slab and of the concrete portion of the column have been merged by means of constraint relation introduced in the numerical model. As depicted in Figure 4.43, the improved model clearly shows a uniform distribution of the compression stresses in the concrete slab with the increase of the imposed top displacement; in detail, it is possible to evidence that for an imposed displacement equal to 50 mm Mechanism 1 results completely activated in the frontal zone of the column flange. Successively, Mechanism 2, formed by the idealized concrete struts balanced by the transversal reinforcing bars, is put in action up to a top displacement equal to 120 mm, in which both the mechanisms develop the maximum permitted strength of the concrete. The reported results evidence that: 148 • Mechanism 1 doesn’t activate at the same time of Mechanism 2. This means that Mechanism 1 results more rigid than Mechanism 2; • only through constructional and designed details it is possible to activate Mechanism 2 in the transfer of forces from the concrete slab to the column; • the compression struts in the concrete slab of Mechanism 2 don’t have, as assumed, an inclination of 45 degrees; Applied force [kN] 120 80 40 0 Mechanism 1 + 2 Mechanism 1 Mechanism 2 0 40 80 120 Displacement [mm] Figure 4.44. Monotonic applied force vs. top-displacement of an exterior joint under sagging bending: experimental response and numerical response for different activations of Mechanisms 1 and 2. Both mechanisms cause a stiffening and strengthening of the exterior joint as illustrated in Figure 4.44. Finally, it is important to underline that the constraint
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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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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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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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