Michael Kasperski

The L.R.C. (load-response-correlation) - method a general method of estimating unfavourable wind load distributions for linear and non-linear structural behaviour

Abstract In current design practice, for quasi-static structures, wind load effects often are defined by the equivalent steady gust model or by a pseudo-steady approach, neglecting the influence of the correlation of the fluctuating pressures over the whole structure. The application of these load patterns may result in an unconservative design when the minima of the wind loads on parts of the structure are more critical for a response considered, depending on its influence function. This problem particularly occurs when the wind load has to be combined with e.g. dead load. Then, for the design an extreme load pattern which causes the maximum interactive response has to be defined. These load patterns may be obtained from wind tunnel tests by a conditional sampling technique. A more efficient approach, the L.R.C.-Method, is presented in this paper enabling systematic studies e.g. on the influence of the static system and on the influence of geometrical non-linearities. In an example of practical application it is shown that the extreme load patterns obtained by the L.R.C.-Method are producing accurate linear peak responses and excellent approximate values for non-linear responses. So to the designing engineer as well as to the wind engineer, an effective tool is presented to describe realistically wind load effects for linear and weakly non-linear structures with quasi-static behaviour.

Specification of the design wind load based on wind tunnel experiments

Abstract Once a high-quality wind tunnel experiment with no major flaws on the experimental side has been performed and the data have been processed/translated appropriately to the design-decisive variable, the final step is to estimate the appropriate load or load effect coefficient for the specification of the design wind load. Four questions arise: what is the appropriate length of a single run in the wind tunnel, what is the appropriate fractile of the observed extreme coefficients, how many independent experiments are required to estimate this fractile and what should be the target confidence interval. The paper tries to give some answers to these questions and discusses the findings in regard to some wind load codes.

BEATRICE joint project: Wind action on low-rise buildings: Part 1. Basic information and first results

Abstract The BEATRICE-network offered the unique opportunity for an extended comparative wind tunnel study on low-rise buildings. Five models in six different boundary layers, i.e. at six different institutes of the network — representing more or less strictly two terrain categories — have been tested. The partner institutes involved in this comparative study are AIB, BRE, CSTB, DMI, NLR and TNO (in alphabetic order). To answer the question to what extent a change of the boundary layer characteristics is able to influence the results in terms of local pressures, global forces and responses of the structure, large sample sizes of 240 individual events have been recorded each corresponding to approx. 10 min in full scale. The paper summarises the basic ideas used in the analysis and presents some first results and conclusions.

Design wind loads for low-rise buildings: A critical review of wind load specifications for industrial buildings☆

Abstract From extensive wind tunnel tests on low-rise buildings with flat roofs, two wind load distributions have been identified to be decisive for the design of the steel frames. Based on these results, the Eurocode 1 introduces a novel alternative wind load distribution with a positive roof pressure. In many design-practical cases, most of the codes reviewed do not sufficiently cover this wind action and its effects, leading to a possible under-design of the cross-sections of up to 35%. The recent trend to lighter construction and non-linear design needs further investigations of possible resonant effects. First results indicate that dynamic effects may have some additional influence on the design.

Some features of modeling spectral characteristics of flow in boundary layer wind tunnels

The present study concerns with the spectral description of the atmospheric boundary layers (ABLs) for appropriate modeling for studying the wind effects on structures. By fitting a functional spectral form to the measured wind velocity data obtained in wind tunnel (WT) as well as in nature, it is shown that the Fichtl-McVehil form of spectrum fits better. The estimation of spectral parameters like turbulence intensities, integral length scales and small scales of wind velocity and their influence on the geometric scale (or time scale) are discussed. It is concluded that proper estimation of the spectral parameters of the simulated ABLs and their variation along the height of WT help in comparing the results obtained from different sources (WT tests) for identifying the influences of various flow/body parameters on the wind-induced effects and for formulating improved modeling of wind-structure interactions.

On the correlation of dynamic wind loads and structural response of natural-draught cooling towers

Abstract Cooling towers are extremely sensitive to wind loads, static and fluctuating, and thorough knowledge of the load and the structural response is a demand of reliability and economy. With increasing height of cooling towers and decreasing shell curvature, bending effects due to loads with a non-uniform distribution, such as turbulent wind pressures, become important for the safety of the structure. The treatment of wind-induced random vibrations with the spectral analysis needs a three-dimensional description of the flow, especially for investigations of bending effects. For many design situations of the cooling tower shell it is profitable to take a look at the correlation structure of the responses.

Aerodynamics of low-rise buildings and codification

Translating the results obtained in a wind tunnel experiment to load specifications for a Code of Practice needs an intensive cooperation between aerodynamicists and structural engineers. Generally, a pure aerodynamic analysis of only the wind action could not be a sufficient basis for a load specification. Requiring both, a safe and economic design, a study of the wind effects becomes necessary taking a close look to the design. In this context, the coincidence of extreme pressures becomes most important. The paper discusses some typical codes of practice and their models of coincident pressures. The quality of these codes is reviewed by comparing their design to results obtained in a wind tunnel experiment. Most alarming, one has to state for many design-practical cases a considerable underestimation of the design-decisive effects. The new Eurocode removes this defect by introducing an alternative load distribution presenting a positive pressure on the roof.

A Refined Model for Human Induced Loads on Stairs

The tendency in modern architecture to lightweight and slender design of pedestrian structures is leading to an increased susceptibility to vibrations. Correspondingly, the evaluation of serviceability of pedestrian structures against vibrations due to human induced loads has gained more and more importance. Studies investigating corresponding action effects, usually focus on locomotion forms performed on a plane surface, e.g. jumping, running, walking. Information on the loads induced by ascending or descending stairs is scarce. In [1] a load model is presented which is based on a perfect repetitive single-foot load pattern leading to integer harmonic load contributions. Differences in the locomotion parameters for the left and the right foot are not considered leading to a loss of important information.

Optimised design of a low-rise industrial building for wind loads

The design of low-rise structures for wind loads is normally based on quasi-steady wind loads, obtained from design codes which usually overestimate the peak structural load effects, such as bending moments, axial forces and the base reactions. The work described in this paper utilised existing statistical wind tunnel data obtained for 5° roof pitch, low-rise buildings and performed frame designs optimised for the wind data.

Simulation of the Dynamic Characteristics of the Coupled System Structure

The human body forms a complex dynamic system with more than one natural frequency and provides considerable damping capacities. Therefore, the coupled system of a structure and the occupants can hardly be described with its basic dynamic characteristics considering only the mass of the occupants. Depending on the natural frequency of the empty structure, the human body dominantly acts like a mass, a mass and a damper or a damper. If the natural frequency of the empty stand increases 3 Hz, the human body induces considerable damping into the coupled system. In the limit, the dynamic characteristics can be described based on the average values of the characteristics of the human body.