This is due to the fact that in pri

This is due to the fact that in principle, the rigid
Marcel Isandro R. de Oliveira et al
188 / Vol. XXIX, No. 2, April-June 2007 ABCM
connections adopted in this strategy can lead to some disturbing
and/or spurious effects, especially when the tower critical buckling
loads are considered.
Based on an extensive parametric investigation, a modelling
strategy combining three-dimensional beam and truss finite
elements was proposed. In this methodology the main structure uses
3D beam elements, while the bracing system utilises truss finite
elements. This method models the structure as a static determined
system discarding the need for dummy bars present in the traditional
analysis. The adoption of truss finite elements in the bracing system
is explained by two main reasons: a single bolt indicating a hinged
behaviour usually makes the bracing system connections to the main
structural system. Additionally, the low flexure stiffness values,
associated with the bracing elements, imply that no significant
moments will be present or transmitted to these structural members.
The use of these two types of 3D finite elements (beam and
truss) also eliminates the spurious mechanisms found in the
traditional design strategy, disregarding the need for the previously
mentioned dummy bars. The authors believe, based on the
performed parametric investigations, (Silva et al, 2000, 2002 and
2005), that this mixed modelling strategy can produce more realistic
and trustworthy results in respect to the static and dynamic
structural analysis, as well as to the tower critical load assessment.
The proposed computational model, developed for the steel tower
static and dynamic analysis, adopted the usual mesh refinement
techniques present in finite element method simulations
implemented in the ANSYS program (ANSYS, 1998). In this
computational model, the main structure was represented by threedimensional
beam elements where flexural and torsion effects are
considered or truss elements having a uniaxial tension-only (or
compression-only) element. The prestressed cables were simulated
by spar elements, see Fig. 3.
The ANSYS beam element BEAM44 (ANSYS, 1998), Fig. 3, is
a uniaxial element with tension, compression, torsion, and bending
capabilities. The element has six degrees of freedom at each node:
translations in the nodal x, y, and z directions and rotations about
the nodal x, y, and z-axes. The ANSYS spar element LINK10
(ANSYS, 1998), Fig. 3, is a 3-D element having the unique feature
of a bilinear stiffness matrix resulting in a uniaxial tension-only (or
compression-only) element. With the tension-only option, the
stiffness is removed if the element goes into compression
(simulating a slack cable or slack chain condition). This feature is
useful for static guy-wire applications where the entire guy wire is
modelled with a single element.
Static Analysis
Table 1 present linear static analysis results for the investigated
guyed towers (50m, 70m and 90m high), according to the three
earlier mentioned structural models. Maximum values of stresses
and horizontal displacements are presented and compared.
The acting loads considered in the present analysis where selfweight
and two wind load cases. In theses cases the horizontal wind
loads were applied perpendicular and diagonal to the towers face.
The horizontal wind loads were calculated according to the
procedures described on the Brazilian code NBR 6123 (NBR 6123,
1988) and applied to the guyed tower nodes.
The prestress cable loads at the lateral anchoring foundation
region are normally defined as 10% of the cable nominal strength
capacity. It is relevant to observe that prestress cable load values
situated between limits of 8% and 15% are allowed by the Canadian
Standard (CSA S37-94, 1994). The present analysis adopted values
approximately equal to 14%. 13% and 11% for the prestress cable
loads of the towers with 50m 70m and 90m height, respectively
(Menin, 2002).
The largest differences between the maximum stress values
obtained for the simple truss model (Strategy I) and for combined
beam and truss element model (Strategy III) are 76.5% (50m high
tower), 83.1% (70m high tower) and 79.9% (90m high tower) when
compared to the values obtained from the third modelling. When a
quantitative analysis of the data was performed it was possible to
confirm that the maximum stress values were significantly modified,
for the three investigated towers, Table 1. The maximum stress
points are depicted in Fig. 4 for the mixed beam and truss element
model considering the perpendicular wind load case. The maximum
stresses, caused mainly by bending effects, were associated, in all
cases studied, to the towers base members.