Keywords: steel structures, spatial structures, steel tower design, structural steel design and behaviour,.

The growing development and expansion of the Brazilian telecommunication systems, as well as the natural requirements for expanding the Brazilian electrical power transmission systems were the main reasons for the increasingly demands for the production of steel transmission and telecommunication towers. This truss type steel structures have long been largely utilized in Brazil to support cellular and microwave transmission antennas or to enable electrical power transmission lines to be built interconnecting the vast Brazilian territory.

However, structural collapses, mainly associated to the wind action, are not uncommon to this structural solution. A typical example of this failure was found in the Itaipu electrical power line between the cities of Foz do Iguaçu and Ivaiporã, in the Paraná State, Brazil. This incident occurred on November, 2nd, 1987, where ten (10) steel towers collapsed due to wind forces that reached, at that time, 130km/h (36m/s), [1]. Several other failures associated to microwave transmission antennas towers occurred and are listed in reference [1].

Despite these facts most of the traditional methods used for structural analysis of steel telecommunication and transmission towers tend to assume simple truss behaviour. In this structural solution all the steel element connections are considered as simple or hinged. In reality these connections are not simple but semi-rigid, fact that prevents the premature failure of the structure.

On the other hand, the most commonly used tower geometries, when the truss solution is adopted, possess structural mechanisms that compromise the assumed structural behaviour. The traditional methodology of steel towers structural analysis as simple truss elements has lead to the use of dummy bars. These bars, possessing substantially smaller axial stiffness values when compared to the structure's main bars, are employed to prevent the occurrence of structural mechanism enabling the use of standard finite element programs.

Previous investigations, [2,3] on an existing telecommunication steel tower showed that maximum stresses and displacements for the structural modelling based on the two investigated methodologies (simple truss element and combined beam and truss element modelling), lead to similar results. However, the simple truss method substantially increases the amount of work to model the structure and generates a potential error source if the rigidities and/or number of spurious bars were not properly considered. This fact conducted to the use of a methodology in which the main structure uses spatial beam finite elements while the bracing system utilises spatial truss finite elements.

Figure 1: 100m High Steel Tower. This paper proposes a new structural analysis modelling strategy, based on a less conservative solution, for the steel tower design considering all the actual structural forces and moments. Further comparisons of the two above mentioned design methods with a third method based on the use of spatial beam finite elements to model the main structure and the bracing system for an existing 100m high steel telecommunication tower, are described.

The author's intention was to further investigate if the previous difference in results, [2,3] was also valid for slender telecommunication towers. The comparison is based on a parametric analysis of the tower geometry in order to obtain the structure's response in terms of displacements, bending moments, stresses, natural frequencies and buckling loads associated to the tower ultimate and serviceability limit states.

1

C.F.C. Júnior,"Análise Numárica e Experimental do Efeito Dinâmico do Vento em Torres Metálicas Treliçadas para Telecomunicações", PhD Thesis, Universidade de São Paulo, USP, In Portuguese, Brazil, 2000.

2

M.N. Policani, J.G.S. da Silva, L.F. Estrella Júnior, P.C.G. da S. Vellasco, S.A.L. de Andrade, "Structural Assessment of Steel Telecommunication Towers", Int. Conf. on Steel Structures of the 2000's, Istanbul, 2000.

3

J.G.S. da Silva, P.C.G. da S. Vellasco, S.A.L. de Andrade, M.I.R. de Oliveira, "An Evaluation of Structural Steel Design Systems for Transmission and Telecommunication Towers", International IASS Symposium, Warsaw, 2002.

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