Keywords: structural engineering, semi-rigid connections, steel structures, parametric analysis, finite element modelling, dynamic response, dynamic structural modelling.

This investigation is concerned with the use of semi-rigid connections in low-rise residential steel buildings. Since a simple finite element with the necessary non- linear rotation stiffness requirements to simulate a semi-rigid connection was not found in the ANSYS finite element program, [1] a simple mechanical spring model was conceived, and calibrated. The finite element model enabled the determination of the structural system dynamical response considering the actual beam to column behaviour. The analysis considered parameters like: beam-to-column connection stiffness and resistance, structural system (steel or composite) and lateral frame stability. The semi-rigid connection used in this investigation was the second connection tested by Carvalho [2].

The most economic structure for the investigated Usiminas, [3], residential building was the non-sway composite solution, [4]. The bracing systems enabled the structural design to be focused on the cross section strength not on the beam deflections, also avoiding excessive lateral displacements.

This paper presents an initial contribution to the evaluation of the beam to column connection stiffness over the steel portal frame dynamical response. The first step of this investigation concerned the analysis of the models frame natural frequencies and associate vibration modes.

Figure 72.1a depicts the fundamental frequency variation for the investigated sway portal frames with fixed supports in terms of the adopted initial stiffness . These graphs upper and lower bounds, related to rigid and hinged connections, are represented by initial stiffness values close to infinite (assumed as to avoid convergence errors) and zero respectively. A direct inspection on this figure indicates the highly non-linear behaviour associated with the sway steel portal frame fundamental frequency curves. The non-sway steel portal frames presented a similar dynamical response, Figure 72.1b.

It can also be observed that the sway frames, Figure 72.1a, presented a substantially lower fundamental frequency when compared to their equivalent non-sway frames, Figure 72.1b. This affirmative demonstrates the coherence of the adopted model since the adopted bracing systems are very efficient, significantly improving the global structural stiffness.

As expected the frames with semi-rigid connections, commonly found in practical design situations, possessed natural frequencies present in an intermediate region between the upper and lower boundaries, i.e.: the rigid and pinned cases. The results indicated that throughout the present analysis as the connection initial stiffness is increased, the natural vibration modes, specially the modes associated with higher frequencies, presented substantially distinct modal components leading to a abrupt transition on the higher modes. This situation occurred in all the investigated models, for a residential steel structure, including the non-sway portal frame solutions.

Qualitative comparisons of the steel portal frames vibration mode configurations, based on the influence of the connection stiffness, pointed out to serious discrepancies, specially on the higher modes. This fact corroborates the idea that structural designer should bear in mind the real need for taking into account the semi-rigid behaviour associated to the great majority of commonly used structural steel connections.

1

ANSYS, "Basic Analysis Procedures Guide", 3rd Edition, Release 5.5, 1998.

2

L.C.V. Carvalho, S.A.L. Andrade, P.C.G.S. Vellasco, "Experimental Analysis of Bolted Semi-Rigid Connections", J. of C. S. Res, 46, 1-3, 1998.

3

USIMINAS, "Usiteto - Solução Usiminas Para Habitação Popular", 2000.

4

O.F. Brito Jr, P.C.G. da S. Vellasco, S.A.L. de Andrade, J.G.S. da Silva, L.R.O. de Lima, "A Parametric Study of Steel and Composite Semi-Rigid Portal Frames", Proceedings of The 6th Int. Conf. on Comp. Struct. Technology, B.H.V. Topping (Editor), Civil-Comp Press, Stirling, 2002. doi:10.4203/ccp.75.96

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