Figure 68.1: Minor axis beam-to-column joints

Eurocode 3 (EC3) proposes a general methodology to predict the behaviour of the most common joints, the so called "component method". This method uses the relevant parameters of the joint moment-rotation curve, and is based on a mechanical model (spring model) that consists of extensional springs and rigid links - where each spring characterizes a source of deformability - the component.

Characterization of the behaviour of any joint requires the identification of its strength, stiffness and ductility. Providing that all the joint components have a previously established structural response, it is then possible to establish the relevant properties of any joint configuration, by the application of the component method to steel connections.

Force-deformation characteristics of the component column web in bending is not currently covered by (EC3). In this paper, an experimental characterization of the behaviour of this component is presented, based on a series of tests performed at the University of Coimbra with minor axis joints geometries.

The moment-rotation () curves for the whole joint and for the component column web are presented. The theoretical value of the plastic moment corresponds to generalised yielding and may be identified by the change of the slope in the curves. However, this component may support much higher moments due to the development of web plate membrane action.

Further attention is given to the stress distribution, deformation layout, and characterisation of transversal force-displacement response of the column. Stress distribution on the column web is characterised by:

In the load introduction zones (upper and bottom) the stresses in the web near the flanges are maximum in the horizontal alignment of the points of loading, and decrease as the distance from this line increases. Stresses in the web near the clamping line (flanges) are mainly in the direction spanning between the flanges. In the perpendicular direction, stresses with opposite sign appear due to Poisson's effect. First yielding of the web starts at a moment level of approximately 50% of the plastic moment. For the level of moment observed for a joint rotation of about 20 mrad (assumed as the rotation corresponding to ) the web is practically yielded in most of the instrumented points (at least in the upper and lower introduction forces) zones, and this confirms the formation of the yielding mechanism. When much larger rotations are reached, the previously compressive stresses may become tensile stresses, as the column web be

$M_pl$) the web is practically yielded in most of the instrumented points (at least in the upper and lower introduction forces) zones, and this confirms the formation of the yielding mechanism. When much larger rotations are reached, the previously compressive stresses may become tensile stresses, as the column web becomes a membrane.

Experimental results give the web transversal displacement at any point, and the force of the tensioned bolts. curves may then be plotted. However, this has the added difficulty of identifying the action line of the compressive force on the web, as it depends on the component relative stiffness and interaction.

In cyclic testing joint behaviour was governed by the column web as well, and rotation capacity remains quite important. In addition, instable behaviour of the curves leads to a very important drop in stiffness in the unloading stage just after moment reversal. This limits the energy absorption and the moment reached by the column web, eliminating most of the overstrength caused by membrane action.

As the cyclic loading process goes on, component degradation occurs in a progressive way with fatigue induced cracking of the web. Consequently a new stress distribution arises.

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