Tall Masts Research Articles

New Robust Linearized Seismic Analysis Method For Tall Guyed Telecommunication Masts

AbstractDesign of tall telecommunication masts is usually governed by serviceability criteria under high wind conditions, typically combined with icing in cold climates. However, there is a need for seismic design checks for guyed masts constructed in zones with moderate to high seismicity, as is routinely prescribed for buildings in modern codes. There have been few efforts towards proposing a robust simplified method of general applicability for the seismic analysis of tall masts. These structures can be represented by the simple concept of a continuous beam-column (the lattice mast) on nonlinear elastic supports (the guy cable clusters at various stay levels). In this study, the guy cables are replaced by equivalent linear lumped parameters (stiffness, mass, and viscous damping) and the effects of their interaction with the mast stiffness and inertia on the structural characteristics are discussed. The approach was tested with nine case studies of real telecommunication masts subjected to five differen...

A novel approach to evaluate the equivalent dynamic stiffness of guy clusters in telecommunication masts under ground excitation

Telecommunication structures are essential components of communication and post-disaster networks and critically important facilities require reliable earthquake-resistant design procedures in seismically active regions. Calculation of the nonlinear seismic response of tall guyed masts using detailed time-step finite element methods is far more complex than linear response spectrum analysis that is routinely used in structural engineering practice. It is recognized that detailed computational procedures are not always necessary, in particular when the goal of the analysis is to provide a global earthquake-resistant design check on a regular structure with predictable response: this is when rational simplified methods are called upon to calculate the seismic demand. However, up until now, no such method exists for tall guyed masts that can account for the dynamic cable–mast interactions which dominate seismic effects in these structures. In the study reported here, the writers explore the dynamic behavior of guy cables under harmonic and seismic support ground motion through detailed computational modeling and propose a novel frequency-dependent method to evaluate the equivalent dynamic stiffness of guy clusters. This development will contribute efficiently towards a new robust rational model of general applicability for simplified seismic analysis of tall guyed telecommunication masts. Detailed numerical simulations involving 57 guy cables from eight existing towers with varying heights of 150–607 m were used in the study. A mathematical procedure was developed to replace the nonlinear time-variant cable stiffness with an equivalent linear frequency-dependent spring/mass system, based on the response spectrum of individual guy cables and the frequency content of the seismic excitation. Comparison of the main response indicators of these equivalent models and the detailed nonlinear finite element analysis results confirmed the reasonable and consistent engineering accuracy of the proposed simplified method with considerable savings in analysis effort.

Seismic Response of Tall Guyed Masts to Asynchronous Multiple-Support and Vertical Ground Motions

The seismic behavior of tall masts was investigated with special considerations for the realistic modeling of ground motion at the supports. Three existing masts with varying heights of 213, 313, and 607 m were numerically studied under the effects of three earthquake records with all three translational ground motion components. The earthquake excitation was prescribed as a displacement-controlled motion providing the opportunity to consider the asynchronous shaking of the cable ground anchors and mast base. This effect was studied by varying the shear wave velocity of the traveling wave corresponding to different degrees of soil stiffness and observing the general trends of selected response indicators. The three towers showed sensitivity to asynchronous shaking of their ground supports. More severe structural response was obtained for softer soil conditions, and the 607 m mast showed sensitivity even for relatively stiff soils. The effects of considering or excluding the structural damping and the vertical component of ground motion were also separately investigated and both factors were found to affect the response indicators significantly.

Seismic sensitivity indicators for tall guyed telecommunication towers

Very tall towers are a fundamental component of post-disaster communication systems and their protection during a severe earthquake is of high priority. In North America, there are no simple rules to evaluate the seismic sensitivity of guyed masts, and very limited attention has been paid to the seismic behaviour of such structures to date. Since guyed towers may exhibit significant geometric nonlinearities, their detailed nonlinear seismic analysis is complex and time-consuming. In addition, climatic loads such as wind and ice are likely to govern their design in most cases. As a result, earthquake effects are often ignored or improperly evaluated. The objective of this paper is to propose some seismic sensitivity indicators for tall guyed masts, which would help tower designers decide whether seismic effects are important and whether detailed dynamic analysis of the structure is required. The indicators proposed relate to the maximum base shear and the dynamic component of the axial force in the mast and guy cable tensions. A conceptual model is also presented to explain the distribution of earthquake effects along tower elevation. The study is based on detailed nonlinear seismic analyses of eight existing guyed telecommunication towers with heights varying from 150 to 607 m.

Modeling of lightning incidence to tall structures. II. Application

For pt.I see ibid., vol.9, no.1, p.162-71 (1994). The present paper comprises an extensive computer investigation of lightning incidence to tall masts, based on a theory presented in part I, for both flat and hilly regions. The investigation covers both downward negative lightning incidence and upward flash from tall masts under negative cloud. The factors investigated include: mast height, statistical distribution of the ground electric field, as well as mountain height and topology. Predicted lightning incidence and probability of upward flash are successfully verified against extensive field observations. >

An optical displacement monitor

Although large structures, such as buildings, tall masts and bridges, often move by quite large amounts, their rates of movement are normally very low, so that they only generate low levels of acceleration. This makes the measurement of their responses (to loadings from wind, vehicular traffic, etc.) difficult when using currently employed methods, especially if there are relatively high frequency (but usually structurally insignificant) vibrations present. In this paper, details of a measurement technique designed to overcome such problems are given, together with an outline description of an instrument which uses the technique, and the results provided by the instrument from tests carried out both in the laboratory and on actual structures.

Investigation of wind-induced fatigue in tall guyed steel masts

This paper describes full scale instrumentation and analysis of tall guyed lattice masts to correlate wind speed and direction with structural stresses, particularly in welds. It outlines a method of predicting residual fatigue life from past meteorological records, where regular inspection is difficult.

Wind-induced static instability of cylindrical shells

The static collapse of circular cylindrical sheels under steady wind loading is considered with particular emphasis on shell structures typical of wind shields for antennae on tall masts. The relevant literature is briefly reviewed and the basis for theoretical buckling analyses described. Experimental results for shells symmetrically and asymetrically supported are presented and as a result of these, and the buckling analyses, simple design criteria are derived.