Keywords: joint, mixed, model, rigid, semi-rigid, steel-concrete, structure.

In this paper two types of structural solutions are compared by means of numerical models. Using the structure erected in the Cardington Laboratory in 1993 [1] as a reference, two numerical models implementing a steel and a mixed steel-concrete structural solution have been developed. The loading considered follows the EN 1990 [2] and EN 1991-1 [3] codes according to the type of occupancy, i.e. a common UK office building. Wind loading according to the EN 1991-1-4 [4] code has been included as well. The performed analyses consisted in: 1st order elastic linear analysis, buckling eigenvalue analysis and second order linear elastic analysis. The influence of different joint modelling has been included in the study. Two cases have then been considered concerning the modelling of the moment connections: rigid and semi-rigid. The inclusion of the joint behaviour in the global structure followed the EN 1993-1.8 [5] code, using rotational springs. The software used to develop the numerical models is Sofistik Structural Desktop [6].

The analysis performed allowed the verification of a stiffer behaviour presented by the mixed solution expressed in: lower lateral deformations to horizontal actions; greater bending moments on the beam supports and smaller span bending moments; smaller beam deformations; higher buckling load factors. The comparison of the results between models with different joint modelling showed the general expected response: that lateral deformations increase due to reduction of global stiffness of the structure; beam deflections increase; distribution of bending moments on the members directly supported by springs are obviously affected; a small reduction of the buckling load factors is observed, at least for the lower modes.

The buckling eigenvalues, calculated for the first modes and for all models, required the consideration of second order effects according to the limits in the EN 1993-1-1 [7] code. However, the graphical visualization of these modes revealed that these are local modes. With the second order linear analysis performed the influence of second order effects has been checked. The results obtained showed that these effects are relevant for the original solution while the mixed solution is insignificantly affected. Consequently, one concludes that the low eigenvalues obtained for the steel-concrete solution are local buckling modes. Global modes should present much higher eigenvalues.

1

British Steel plc, Swinden Technology Centre, "The behaviour of multi-storey steel framed buildings in fire", European Joint Research Programme, South Yorkshire, United Kingdom, 1999.

2

European Committee for Standardization (CEN), "EN 1990: Eurocode: Basis of structural design", April 2002.

3

European Committee for Standardization (CEN), "EN 1991-1-1: Eurocode 1: Actions on structures - Part 1.1: General actions - Densities, self weight, imposed load for buildings", April 2002.

4

European Committee for Standardization (CEN), "EN 1991-1-4: Eurocode 1: Actions on structures - Part 1.4: General actions - Wind actions", April 2005.

5

European Committee for Standardization (CEN), "EN 1993-1-8: Eurocode 3: Design of steel structures - Part 1-8: Design of joints", May 2005.

6

SOFiSTik Aktiengesellschaft, "ASE - General static analysis of finite element structures", version 14.66, SOFiSTik AG, Oberschleissheim, 2007.

7

European Committee for Standardization (CEN), "EN 1993-1-1: Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings", May 2005.

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