P. Harikrishna

Full scale measurements of the structural response of a 50 m guyed mast under wind loading

Abstract Guyed masts are used for wireless communication, meteorological measurements, and recently, even for power transmission. The behaviour of the mast is non-linear due to its slenderness and compliant ‘guy-support’ system. The guys also exhibit non-linear behaviour especially at low values of pretension due to possible multimodal excitations and dynamic response to wind turbulence. This paper presents the results of measured wind characteristics and associated dynamic response of a 50 m tall guyed mast located on the east coast of India in ambient wind conditions. The measured root mean square values of displacements have been compared with a patch load method suggested by Davenport and Spalding [4] . The adequacy of current design practice is reviewed, in the light of the full scale experimental observations.

Wind, terrain and structural damping characteristics under tropical cyclone conditions

A large number of buildings and structures, including some well-engineered structures, have been reported to be damaged during tropical cyclones. This stresses the need to study the various characteristics of tropical cyclone winds. A full-scale field experiment on a 52 m tall steel lattice tower has been undertaken to study the wind, terrain and structural characteristics under normal and tropical cyclone wind conditions. Data collected from the instrumented tower during tropical cyclones in June and December 1996 were analyzed and compared with the characteristics obtained during normal wind conditions. The measured wind and terrain characteristics have been compared with the results reported in the literature. The measured structural characteristics, such as the fundamental frequency and damping ratio of the structure, have been compared with the analytical values and those reported in the literature.

Experimental Determination of Wind-Induced Response on a Model of Natural Draught Cooling Tower

The detailed and well-publicized documented paper by Bamu and Zingoni in 2005 on cooling-tower collapses accentuates the effect of wind on the performance of cooling towers. As it is well established that wind-tunnel tests on tower models result in a more realistic assessment of wind-induced stresses, wind-tunnel tests are conducted on aero-elastic model of an isolated cooling tower and the cooling tower with surrounding structures. The details of the experimental programme, measurements, analysis, and results for both the isolated and interference cases are presented in this paper. Based on the analysis of experimental data, ring-resonance frequencies, and mode shapes of the model, meridional (Nϕ) and hoop (Nθ) stress resultants and interference factor due to the presence of surrounding buildings are evaluated. The values of Nϕ, Nθ, ring-resonance frequencies, and mode shapes for isolated model are further compared with the values obtained based on finite element analysis.

Stochastic gust response of microwave lattice towers

Abstract Microwave lattice towers of heights up to about 100 m are constructed in large numbers for the requirement of an increasing network of electronic communication systems. Wind load, which is basically dynamic in nature, is the main design force for these free-standing lattice towers. The dynamic response of wind sensitive structures subjected to a random wind force can be computed using stochastic analysis. In this paper, a spectral approach for computation of the along-wind response and the gust response factor of microwave lattice towers is presented. A computer program has been developed for the calculation of the gust response factor, the along-wind response of lattice towers and wind characteristics using this spectral method. The gust response factor computed for a microwave lattice tower of 101 m height by this method is compared with the values calculated using the formulae recommended in the Indian, the Australian, the British and the ASCE Standards.

Alpha-Stable Distribution for Prediction of Negative Peak Wind Pressures on Roofs of Low-Rise Building

In this paper, an attempt is made to study the applicability of alpha-stable distribution for modeling the negative peak wind pressures on low-rise building roofs. The required wind pressure data is obtained from the aerodynamic database of Tokyo Polytechnic University. The generality and flexibility offered by alpha-stable distribution makes it a candidate distribution as a single model for predicting the extreme values of negative peak wind pressure coefficients at different regions on the building roof. The results suggest that for the prediction of extreme negative wind pressure coefficients, alpha-stable distribution is a better candidate distribution than the Gumbel minimum.

CFD and Wind Tunnel Investigations on Rectangular Building with Corner Cuts

Aerodynamic modifications at corners are considered an effective countermeasure to minimize the wind-induced load and load effects. One of the effective corner modification measures is provision of corner cuts to reduce wind-induced along-wind loads and cross-wind vibrations of the building. However, the corner modifications are provided usually throughout the height of the building. The present study deals with the effect of providing corner cuts over limited heights from top of the building. A 1:2:5 rectangular building has been considered for the present study. A corner cut size of 7.5% has been considered. To study the effect of spread over of corner cut over the height of the building, a total number of four cases have been considered, viz. (i) building with full-height corner cut, (ii) building with half-height corner cut, (iii) building with one-third-height corner cut and (iv) building with no corner cut. The study is carried out by using both computational fluid dynamics and wind tunnel experiments. CFD studies included angles of wind incidence of 0° and 90° under open terrain condition. The wind tunnel investigations included pressure measurements on a rigid model with 1:300 geometric scale under simulated open terrain condition for angles of wind incidence of 0°, 45° and 90°. Using the measured pressures, pressure coefficients, drag and lift force coefficients have been evaluated. The effectiveness of the corner cut over different heights of the building model has been studied by comparing the evaluated aerodynamic coefficients. Further, numerical results are compared with experimental results for validation purpose.

Wind Tunnel Investigations on a Tall Building With Elliptic Cross Section

According to a statistics revealed by Council of Tall Building and Urban Habitat [1] that for every 11.5 millions of global population, there is one tall building with 200 m+ in height available globally. Tall building constructions are paving ways for rapid urbanization worldwide including India, especially during the last one decade and will be continued over next few decades. Wind loads are one of the most important loads that govern the design of tall buildings. Published data on pressure and force coefficients for 3-D building with elliptic cross section under boundary layer flows are very scanty. This paper presents the results on mean force coefficients obtained through wind tunnel pressure measurements carried out on a 3-D tall building with elliptic cross section for various angles of wind incidence under suburban terrain. It is found that the mean pressure distributions and force coefficients depend significantly upon the angle of wind incidence.

Numerical Investigations of Aerodynamic Forces on 2-D Square Lattice Tower Section Using Two-Equation Turbulence Models

Wind flow around lattice towers is a complex phenomenon involving interference effect between various leg/bracing members. Wind induced forces on such lattice towers are evaluated by considering overall effect on the grouped members and are related to solidity ratio. Most of the codal provisions on drag coefficient values for lattice towers are based on sectional model studies, carried out in wind tunnel for various solidity ratio values. In the present study, numerical studies on 2-D square lattice tower section have been carried out for different solidity ratio values using various turbulence models, viz. Realizable k–e model, two versions of RNG k-e model and SST k–ω model. The numerically evaluated mean drag coefficient values have been compared with those provided in various international codes to assess the relative performance of these turbulence models.

Wind Induced Interference Effects on a Row of Three Buildings with Irregular Plan Shape

P. Harikrishna, A. Abraham, S. Selvi Rajan, G. Ramesh Babu, S. Chitra Ganapathi, and Nagesh R. Iyer Principal Scientist, CSIR-Structural Engineering Research Centre, Chennai, TN, India, hari@serc.res.in Senior Scientist, CSIR-Structural Engineering Research Centre, Chennai, TN, India, abraham@serc.res.in Chief Scientist, CSIR-Structural Engineering Research Centre, Chennai, TN, India, sselvi@serc.res.in Principal Scientist, CSIR-Structural Engineering Research Centre, Chennai, TN, India, gramesh@serc.res.in Scientist, CSIR-Structural Engineering Research Centre, Chennai, TN, India, chitrag@serc.res.in Director, CSIR-Structural Engineering Research Centre, Chennai, TN, India, nriyer@serc.res.in

Numerical Assessment of Aerodynamic Forces on 2-D L-Angle Section Using CFD

Numerical simulations of aerodynamic forces on regular shaped bluff-bodies using CFD have been well reported in the literature as part of validation. In the present study, CFD simulations have been carried out on a 2-D Langle section using different Reynolds-Averaged Navier Stokes (RANS) based turbulence models to assess their performance in evaluating drag and lift coefficients for various angles of wind incidence. Mean drag and lift coefficients for the 2-D L-angle section have been evaluated under uniform smooth flow conditions using FLUENT 6.3 software. The performance of these turbulence models are assessed by comparing the numerically evaluated force coefficients of the 2-D L-angle section with the experimental results available in literature and also with the values provided in various international standards. SST k– model is observed to perform better in evaluating the above mentioned aerodynamic coefficients than other turbulence models, which are observed to perform better only in evaluating mean lift force coefficients than mean drag coefficients. Further, comparison of the numerically evaluated mean pressure coefficient distributions on various sides of the 2-D L-angle section have been made to discuss the relative performance and the limitations of the considered turbulence models.