Wind Load Combinations Research Articles

Collapse Time Analysis of Multi-Story Structural Steel Buildings

Until recently, the progression and time of collapse of structures has not been a primary objective of the structural engineering profession. However, such events are likely to be of major interest in high rise building situations where a fire-initiated failure takes place, or where a controlled demolition is to be implemented. The paper attempts to shed light on this problem and to develop velocity profiles during a collapse event, also potentially important, especially in built-up areas. Because of the complex nature of both the structural system and the process of collapse, analysis methods need to be simplistic, which are yet realistic and are easily understood by designers and code writers of the nature and cause of a building collapse. The objective of this paper is to shed light on this problem by employing the most basic equations of Newton’s laws of motion. A formulation of the problem of a building frame of N stories, subjected only to gravity loading is postulated that involves an analysis employing a generic one-dimensional discrete model of progressive collapse. The supporting elements within its nth story are suddenly degraded by whatever cause, with the velocity and time steps calculated. Several design scenarios are presented for the columns with prescribed energy dissipation properties ascertained at any level, thus allowing the motion to be determined in accordance with the known equations of energy and momentum. An example 10-story structure is proposed to illustrate the method, in which the column designs, utilizing square hollow sections and W-shapes, are governed by a combination of dead, live and wind loads. For the 6 designs postulated involving tubular columns alone, and with sequential removal of columns in each of the 10 stories, it was found that progressive collapse was arrested in 90% for such hypothetical scenarios. For the other 10%, global collapse times exceeded that of free-fall by 57 to 228%.

Long-term fatigue analysis of multi-planar tubular joints for jacket-type offshore wind turbine in time domain

Long-term fatigue analysis of welded multi-planar tubular joints for a fixed jacket offshore wind turbine designed for a North Sea site in a water depth of 70 m is performed. The dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy (Gao et al., 2010). Hot-spot stresses at failure-critical locations of each reference brace for 4 different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from axial, in-plane and out of plane action, the effects of planar and non-planar braces are also considered. Both a 2-parameter Weibull function and generalized gamma function are used to fit the long-term statistical distribution of hot-spot stress ranges by a combination of time domain simulation for representative environmental conditions in operational conditions of the wind turbine. A joint probabilistic model of mean wind speed U w , significant wave height H s and spectral peak period T p in the northern North Sea is used to obtain the occurrence frequencies of representative environmental conditions (Johannessen, 2002). In order to identify the contributions to fatigue damage from wind loads, wave loads and the interaction effect of wind and wave loads, 3 different load cases are analyzed: wind loads only; wave loads only; a combination of wind and wave loads. The representative environmental condition corresponding to the maximum contribution to fatigue damage is identified. Characteristic fatigue damage of the selected joints for different models is predicted and compared. The effect of brace thickness on the characteristic fatigue damage of the selected joints is also analyzed by a sensitivity study. The conclusions obtained in this paper can be used as the reference for the design of future fixed jacket offshore wind turbines in North Sea.

Integrated structural design methodology for agricultural protecting structures covered with nets

The use of agricultural nets is increasing rapidly in a similar manner to the increase in greenhouse plastic covering materials. The expansion of nets in agriculture is expected to have a strong impact on the economy by improving the cost effectiveness of low-input agriculture. A methodology is proposed to support the design of new optimised net-supporting structural systems, fully exploiting the advantages offered by novel agricultural nets. The first step in the design procedure concerns the determination of loads and load combinations. The determination of wind loads, other actions and load combinations are briefly presented, based on a critical analysis and synthesis of available research results on the aerodynamic and airflow characteristics of nets and the special characteristics of the net-structural systems and on appropriate adaptation of relevant standards for design actions, when available. The design criteria on loads and load combinations are combined with the design criteria governing the analysis and design of nets and the functional design requirements. All design criteria are integrated into a methodology for the structural design of net-supporting structural systems. The proposed design methodology is expected to allow for the design of low cost optimised structures.

Peak normal stresses and effects of wind direction on wind load combinations for medium-rise buildings

In order to examine wind load combinations, fluctuating surface pressure measurements are made for flat roof building models with square or rectangular plans. The fluctuating pressures are integrated over all their surfaces and results are obtained of along-wind force, across-wind force, vertical force, along-wind overturning moment, across-wind over-turning moment and torsional moment on their frames. The maximum wind force component and other simultaneously observed wind force components are examined, and some interesting facts regarding the effects of wind direction are discussed. In order to directly check the wind load combination effects, the maximum normal stresses in column members of simple frame models are examined, and a wind load combination factor is proposed.

C203 低層建物に作用する風力成分と流速場

In this paper, fluctuating surface pressure measurements are made for four flat roofed building models with square or rectangular plans in order to examine wind load combinations. The fluctuating pressures are integrated over all their surfaces and results are obtained as along-wind force, crosswind force, vertical force and torsional moment their frames. The maximum wind force in consideration of the wind direction component and other simultaneously observed wind force components are examined, and some interesting facts regarding the wind force combinations are discussed on the basis of absolute value correlations, phase-plane expressions and so on. This investigation concurrently measured wind pressure applied to a wall surface and the resulting wind flow field around the building using a wind pressure model and DPIV systems. Data obtained through the measurements enabled identification of the wind flow field around the building that results from the maximum and minimum wind force applied to it.

Dynamic Wind Effects on Buildings with 3D Coupled Modes: Application of High Frequency Force Balance Measurements

Contemporary high-rise buildings with complex geometric profiles and three-dimensional (3D) coupled mode shapes often complicate the use of high frequency force balance (HFFB) technique customarily used in wind tunnel testing for uncoupled buildings. In this study, a comprehensive framework for the coupled building response analysis and the modeling of the associated equivalent static wind loads using the HFFB measurement is presented. This includes modeling of building structural systems whose mass centers at different floors may not be located on a single vertical axis. The building response is separated into the mean, background, and resonant components, which are quantified by modal analysis involving three fundamental modes in two translational and torsional directions. The equivalent static wind load is described in terms of the modal inertial loads. The proposed framework takes into account the cross correlation of wind loads acting in different primary directions and the intermodal coupling of modal responses with closely spaced frequencies. Wind load combination is revisited in the context of modeling of the equivalent static wind loads. A representative tall building with 3D coupled modes and closely spaced frequencies is utilized to demonstrate the proposed framework and to highlight the significance of cross correlation of wind loads and the intermodal coupling of modal responses on the accurate prediction of coupled building response. Additionally, delineation of the proper role of the correlation between integrated loads, modal response, and respective building response components in the evaluation of wind effects on coupled buildings is underscored.

Quasi-static wind load combinations for low- and middle-rise buildings

Fluctuating surface pressure measurements are made for four flat roofed building models with square or rectangular plans in order to examine wind load combinations. The fluctuating pressures are integrated over all their surfaces and results are obtained of along-wind force, across-wind force, vertical force, along-wind overturning moment, across-wind overturning moment and torsional moment on their frames. The maximum wind force component and other simultaneously observed wind force components are examined, and some interesting facts regarding the wind force combinations are discussed on the basis of absolute value correlations, phase-plane expressions and so on. The maximum normal stresses in the column members of simple frame models are then examined in order to directly check the wind load combination effects.

Extreme wind pressure distributions on low-rise building models

This paper first discusses the conditionally sampled actual wind pressure distributions causing maximum quasi-static wind load effects at the base of low-rise building models with square and rectangular plans. The maximum normal stresses in column members were also examined to discuss the wind load combination of the along-wind, across-wind, uplift, and three moments. Then, it examines the actual wind pressure distributions causing the maximum quasi-static stresses in structural frames. These are compared with Kasperski’s load-response-correlation formula and the quasi-steady pressure distributions.

Wind load combinations and extreme pressure distributions on low-rise buildings

The main purpose of this paper is to demonstrate the necessity of considering wind loadrncombinations even for low-rise buildings. It first discusses the overall quasi-static wind load effects andrntheir combinations to be considered in structural design of low-rise buildings. It was found that the maximumrntorsional moment closely correlates with the maximum along-wind base shear. It was also found that therninstantaneous pressure distribution causing the maximum along-wind base shear was quite similar to thatrncausing the maximum torsional moment, and that this asymmetric pressure pattern simultaneously accompaniesrnconsiderable across-wind and torsional components. Secondly, the actual wind pressure distributions causingrnmaximum quasi-static internal forces in the structural frames are conditionally sampled and their typical pressurernpatterns are presented.

Wind Load Statistics for Probability-Based Structural Design

ASCE Standard 7, “Minimum Design Loads for Buildings and Other Structures,” has contained provisions for load combinations and load factors suitable for load and resistance factor design since its 1982 edition. Research in wind engineering in the intervening years has raised questions regarding the wind load factor 1.3 and load combinations in which the wind load appears in ASCE 7-95. This paper presents revised statistical models of wind load parameters based on more recent research and a Delphi, and reassesses the wind load combinations in ASCE Standard 7 probabilistically. The current approach to specifying wind loads in ASCE 7 does not lead to uniform reliability in inland and hurricane-prone regions of the country. It is recommended that the factor accounting for wind directionality effects should be separated from the load factor and presented in a separate table in the wind load section, that the wind load factor should be increased from 1.3 to approximately 1.5 or 1.6 to achieve reliability consis...

A refined method for calculating wind load combinations on open-framed structures

Literature on open-framed structures has shown that the maximum wind load normal to a set of frames occurs not when the wind is normal to the frames, but rather at some angle to the normal, typically between 20° and 40°. This means that when one set of frames experiences its maximum load, the orthogonal set of frames simultaneously experiences a significant wind load. Currently used procedures for determining wind loads on open-frame structures either ignore this significant load combination or use simplified load combinations. This paper presents an empirical method to determine the maximum normal wind load on a set of frames of an open-frame structure, plus the wind angle at which it occurs and the corresponding load on the orthogonal frame set. It is applicable to open-frame structures rectangular in plan with 3–10 frames in each direction and solidity ratios between 0.1 and 0.5.

Reliability based optimal design of reinforced concrete frames

Reliability based optimal design (RBOD) of structures aims at minimization of structural cost constrained by a specified allowable failure probability. A methodology has been developed for RBOD of reinforced concrete frames which considers the failure of structure, both at the component and system level simultaneously, under two different live load and wind load combinations. System failure probability is evaluated based on the generated stochastically dominant failure modes which consist of collapse mechanism failure modes and rotational failure modes (arising due to critical sections reaching their permissible inelastic rotation capacities). Optimal values of the moment capacities of critical sections are obtained based on the minimization of the total structural cost of the frame using the cost functions for member sections as a function of their moment capacities, which are derived by optimizing the member sections. It is observed that designs obtained using RBOD procedure give cost reduction for the chosen target reliability level, equal to that inherent reliability in the designs obtained as per the present Indian standard specifications.

Design Criteria for Offshore Structures Under Combined Wind and Wave Loading

Offshore codes do not give sufficient guidance regarding design criteria for loads resulting from combinations of stochastic environmental processes such as wind and waves. To assist design engineers in defining such criteria, a suite of methods that use environmental data to calculate the probability distributions of load effects resulting from combination of stochastic loads were investigated. An approach has been developed for using the results to calculate structure-specific and generalized load combination criteria. Extensive application of this approach in connection with Environment Canada’s wind and wave data bases for the Canadian East Coast region formed the basis for some interesting conclusions regarding the process of estimating combined extreme loads on offshore structures. It was found that data based on actual measurements of wave height and wind speed are preferable to hindcast data, since the latter have artificially high correlations that lead to overly conservative results. External analyses are most reliable when 20 or more years of data are used with analysis methods based on distribution tails. Reasonably good results can be achieved with 10 yr of data. Methods based on the point-in-time data and using mathematically convenient assumptions regarding distribution types and process characteristics can lead to large errors if the assumptions made are not substantiated by appropriate data. Load combination solutions are highly dependent on the geographic location and data base. Therefore, a separate analysis should be carried out for the structure and location being considered if possible. Wind and wave load combination solutions are sensitive to correlations and assumed distribution types; closed-form solutions for independent and Gaussian correlated processes can lead to significant errors. If site-specific analyses are not practical, companion action factors of 0.65, 0.60, and 0.55 for return periods of 20, 100, and 1000 yr, may be used for wind and wave loading on slender offshore structures in the Canadian East coast region. For wide structures in the same region, the suggested companion factors for the same return periods are 0.75, 0.70, and 0.65.