Database-assisted Design Research Articles

Wind Effects on Low-Rise Metal Buildings: Database-Assisted Design versus ASCE 7-05 Standard Estimates

Peak bending moments are compared for a set of steel portal frames of industrial buildings in an open terrain calculated using database-assisted design (DAD) techniques and ASCE 7-05 Standard plots. The comparisons indicate that, depending on the building dimensions, the peak bending moments at the knee based on DAD techniques are generally larger by 10–30% than their counterparts based on the ASCE 7-05 plots. (In one case with a relatively steep roof slope of 26.6° the discrepancies exceed 70%.) For the buildings considered, the discrepancies increase with increasing roof slope and with increasing eave height.

Derivation of equivalent gust effect factors for wind loading on low-rise buildings through Database-Assisted-Design approach

The method for the simulation of extreme wind loading on low-rise buildings, currently adopted by most civil engineering Standards, is based on the concept of “gust effect factor” (GEF). The GEF is a dimensionless coefficient that quantifies the effect of turbulence on wind pressure distribution and enables the assessment of maximum external loading and internal forces in the structure from the analysis of the mean wind pressure scenario. The GEF is employed because a direct quantification of unsteady pressures, usually requiring detailed wind tunnel investigation, is sometimes uneconomical for a “simplified” low-rise building. This study analyzes different methods for the assessment of wind loading, utilizing the GEF. These include the approach employed by the Standard of the American Society of Civil Engineers (ASCE-7), the guidelines of the Seventh Edition of the Massachusetts Building Code (780-CMR) and the Database-Assisted-Design (DAD) approach. The DAD, developed by the National Institute for Standard and Technology, is a rational approach for the structural design against extreme winds, which employs a large database of wind tunnel test results. A set of simulated loading scenarios on rectangular low-rise gabled-roof buildings with various floor-plan dimensions and heights was employed. Wind loading analyses were restricted to enclosed buildings and primarily based on external pressures. The performance of the main wind force resisting system was analyzed. Two-dimensional lateral frames were considered. Peak values of the bending moments at critical locations were evaluated through the DAD procedure, and transformed into “equivalent gust effect factors” (EGEF) for comparison with ASCE-7 and 780-CMR. It was found that ASCE-7 always predicts an increment in the GEF with the height of the building, while the DAD estimations suggest that, for a large building, the EGEF should decrease with the height. Moreover, ASCE-7 can under-predict the magnitude of the gust effects is some cases. Finally, a relative increment in the EGEF was recorded between a large and a small floor-plan building, which is partially incompatible with the equivalent reduction of loading indicated by 780-CMR for “one and two family dwellings and townhouses”.

Wind-induced tall building response: a time-domain approach

Estimates of wind-induced wind effects on tall buildings are based largely on 1980s technology. Such estimates can vary significantly depending upon the wind engineering laboratory producing them. We describe an efficient database-assisted design (DAD) procedure allowing the realistic estimation of wind-induced internal forces with any mean recurrence interval in any individual member. The procedure makes use of (a) time series of directional aerodynamic pressures recorded simultaneously at typically hundreds of ports on the building surface, (b) directional wind climatological data, (c) micrometeorological modeling of ratios between wind speeds in open exposure and mean wind speeds at the top of the building, (d) a physically and probabilistically realistic aerodynamic/climatological interfacing model, and (e) modern computational resources for calculating internal forces and demand-to-capacity ratios for each member being designed. The procedure is applicable to tall buildings not susceptible to aeroelastic effects, and with sufficiently large dimensions to allow placement of the requisite pressure measurement tubes. The paper then addresses the issue of accounting explicitly for uncertainties in the factors that determine wind effects. Unlike for routine structures, for which simplifications inherent in standard provisions are acceptable, for tall buildings these uncertainties need to be considered with care, since over-simplified reliability estimates could defeat the purpose of ad-hoc wind tunnel tests.

New Nonlinear Roof Sheathing Fastener Model for Use in Finite-Element Wind Load Applications

Performance-based design is being developed and implemented in seismic and fire resistant design. The field of structural wind engineering has introduced several techniques that may eventually lead to performance-based wind engineering for buildings such as database assisted design. For wood-frame buildings, one deficiency is the lack of accurate load/response models with the ability to go beyond first failure leaving only force-based approaches which protect life safety but only infer certain levels of performance. This paper introduces a new nonlinear roof sheathing fastener model that is a function of not only the uplift pressure acting on the roof sheathing, but the effective moment arm (not just the moment) acting on the edge nails. A targeted set of experimental tests was conducted to determine the effect of the load eccentricity, a new stiffness matrix for a nail element formulated, and illustrative examples presented for a roof sheathing assembly analyzed using the finite element procedure. The model is shown to result in a significantly reduced capacity from current state-of-the-art finite element analyses which assume only nail withdrawal. It is anticipated that this more accurate nail model can aid in the development of performance-based wind engineering for wood-frame buildings.

Safety Evaluation of Low-Rise Steel Structures under Wind Loads by Nonlinear Database-Assisted Technique

The design of a large proportion of the low-rise metal buildings in the United States is based on the ASCE 7-93 Standard and the use of allowable stress design (ASD). In this paper we examine, for a typical case, the question of whether such buildings satisfy requirements implicit in more recent versions of the ASCE Standard. We base our investigation into this question on all three wind loading conditions specified in the ASCE 7-02 Standard. In particular, we use the nonlinear database-assisted design technique to assess the degree of safety of a typical portal frame belonging to the wind-force resisting system of a low-rise industrial structure designed in accordance with ASCE 7-93 and ASD. The conclusion of our assessment is that the frame being considered satisfies all ASCE 7-02 requirements with respect to wind loading, but that its safety level is relatively low, and could be improved substantially at very low cost.

Database-Assisted Design of Low-Rise Buildings: Aerodynamic Considerations for a Practical Interpolation Scheme

Database-assisted design (DAD) is becoming a realistic possibility whereby archived wind pressure time series on building envelopes from wind tunnel experiments are directly used in structural analysis software for building design. In order for DAD to be of practical use, a data handling system is required. The main task of the system is to perform interpolation in order to obtain wind pressure time series beyond the basic configurations available in the database. The functional dependence of the pressure coefficients is reduced to geometric variables only, greatly simplifying the process of interpolation. It is shown that all interpolation schemes require estimation of the first and second order statistical moments.

Probabilistic Descriptions of Wind Effects and Wind-Load Factors for Database-Assisted Design

Database-assisted design (DAD) for wind results in structures with reduced material consumption and failure risks, i.e., in improved safety and sustainability. The paper presents a DAD-compatible probabilistic procedure for estimating: (1) Wind effects corresponding to any mean recurrence interval of the wind speeds, and (2) wind-load factors. Unlike the ASCE 7 Standard and earlier structural reliability estimates, the procedure accounts for the inherent randomness in the estimation of peaks and for relevant wind-related knowledge uncertainties. On the basis of a selection of uncertainty parameters believed to be reasonable, it is concluded that: (1) The extreme wind speed distribution type significantly affects the estimates of the load factors. Our estimates, based on the Extreme Value Type III Reverse Weibull distribution, yield load factors consistent with values specified in the ASCE 7 Standard—in contrast to estimates published in the 1980’s. (2) The variability of the ratio between wind speeds aver...

Building Orientation and Wind Effects Estimation

The methodology for estimating wind effects presented in this paper is based on the database-assisted design approach. It accounts for the effects of wind directionality, for the effects of the uncertainties in the parameters that determine wind effects, and for the effects of building orientation. The methodology yields estimates of wind effects that are far more realistic than those based on the conventional building code approach, which disregards uncertainties in those parameters, as well as the effects of wind directionality and building orientation, or accounts for these effects through the use of a blanket reduction factor. The pilot software on which the calculations presented in this paper are based is a first step toward modern computer-intensive electronic standards wherein wind loads can be calculated by using database-assisted reliability-based calculations of wind effects. We believe such standards will go a long way toward achieving significantly safer and more economical buildings in regions affected by strong winds.

Sampling Errors in the Estimation of Peak Wind-Induced Internal Forces in Low-Rise Structures

Peak values in time histories of wind effects may be obtained by using the entire information inherent in the time series of the wind effect, a method entailing the estimation of the probability distribution of the peak through the application of the classical Rice procedure extended for non-Gaussian time histories. We present estimates of the sampling errors inherent in this method. These are needed for structural reliability calculations and for decisions on the requisite length of wind tunnel pressure records, especially for database-assisted design. If based on the analysis of 1-h-long records generated by Monte Carlo simulation, typical sampling errors in the estimation of peaks of time histories corresponding to windstorms of 1-h duration are about 5%. If based on 30- or 20-min records, they are about 1.5 times or twice as large, respectively. Consideration of the sampling errors in reliability calculations entails an estimated increase in the requisite safety margins with respect to wind loading of roughly 2, 3, and 5% if 1-h, 30-min, or 20-min records are used, respectively.

Achieving safer and more economical buildings through database-assisted, reliability-based design for wind

Following such landmark aerodynamic tests as Irminger's in 1894, Flachsbart's in 1932—the first to be conducted in boundary-layer flow—and University of Western Ontario (UWO) 1970s tests, considerable progress has been achieved in low-rise building design for wind. Nevertheless, the present state of the art remains inadequate. UWO tests were conducted at low angular and spatial resolutions. Their results were then used to create drastically simplifying standard aerodynamic tables and plots designed for slide-rule era calculations and entailing errors that can exceed 50%. We review material which shows that significant improvements in main wind-load resisting system and component design can be achieved by using database-assisted design (DAD) and associated structural reliability tools, thus accounting realistically for the complexity of the wind loading as well as for the stochasticity and knowledge uncertainties affecting wind effects calculations. We illustrate DAD's capability to obtain, for the first time in a wind engineering context, realistic estimates of ultimate limit states due to local or global buckling failure. In the future other types of nonlinear behavior associated with ultimate limit states can be similarly dealt with. We note that DAD is ideally suited for use with data likely to be obtained in the future by Computational Fluid Dynamics methods. We discuss the need for assuring quality control procedures for wind tunnel testing so that inter-laboratory comparisons of test results and wind tunnel certifications can be conducted effectively. We also discuss the possibility of systematic database corrections based on full-scale test results, and the possibility of using tests in non-standard wind environments and/or on buildings with non-standard shapes.

Interpolation of pressure time series in an aerodynamic database for low buildings

Database-assisted design (DAD) is a relatively new concept whereby archived pressure time series from wind tunnel experiments are used directly in structural analysis software for the design of a structure. The US National Institute of Standards and Technology (NIST) is currently creating an aerodynamic database of wind-induced pressure time series on the envelope of various low buildings. In order for DAD to be of practical use, simple interpolation schemes must be used to cover the wide range of low building geometries a designer may desire since only relatively few geometries could be included in any database. The approach investigated herein consists of using a set of re-scaled pressure time series from an aerodynamically similar building at the same wind angle and in the same terrain. The set of reference time series are corrected by adjusting their mean and rms. pressure coefficients with artificial neural network models which have captured the variability of these statistical parameters. The use of the re-scaled time series is a simple way to get the correct spatial correlations for the interpolated building without resorting to more elaborate simulation schemes. However, interpolation must be limited to situations where the spatial correlations are similar. The methodology was applied to two test cases with reasonable success.

Closure to “Database-Assisted Design, Standardization, and Wind Direction Effects” by Antonio Rigato, Peter Chang, and Emil Simiu

Closure to “Database-Assisted Design, Standardization, and Wind Direction Effects” by Antonio Rigato, Peter Chang, and Emil Simiu

Discussion of “Database-Assisted Design, Standardization, and Wind Direction Effects” by Antonio Rigato, Peter Chang, and Emil Simiu

Discussion of “Database-Assisted Design, Standardization, and Wind Direction Effects” by Antonio Rigato, Peter Chang, and Emil Simiu

Database-assisted design for wind: basic concepts and software development

Standard provisions for wind loads on buildings have traditionally been based on summary tables and/or plots suitable for slide-rule calculations. The accuracy in the definition of wind loads inherent in such tables and plots is far lower than that inherent in current methods for stress computation. Advances in computational power now make it possible to reduce this discrepancy and achieve structural designs for wind that are significantly safer and more economical than current designs. This is true both for routine, low-rise structures and for flexible structures experiencing significant dynamic effects. In this paper, we present the concept of database-assisted design (DAD) along with a discussion of the application software Wind Load Design Environment, a user-friendly tool for designers and code writers that employs the DAD approach. The DAD approach entails the use of large databases of aerodynamic pressures, the optional use of databases of directional extreme wind speeds, and the use of structural information needed for the description of linear or nonlinear structural behavior. We present progress achieved to date, describe current efforts and future needs, and discuss the implications of DAD for reliability-based design and performance-based standards development.

Peak Non-Gaussian Wind Effects for Database-Assisted Low-Rise Building Design

Current procedures for estimating the peaks of the stochastic response of tall buildings to wind are based on the assumption that the response is Gaussian. Those procedures are therefore inapplicable to low-rise buildings, in which time histories of wind-induced internal forces are generally non-Gaussian. In this paper, an automated procedure is developed for obtaining from such time histories sample statistics of internal force peaks for low-rise building design and codification. The procedure is designed for use in software for calculating internal force time series by the database-assisted design approach. A preliminary step in the development of the procedure is the identification of the appropriate marginal probability distribution of the time series using the probability plot correlation coefficient method. The result obtained is that the gamma distribution and a normal distribution are appropriate for estimating the peaks corresponding, respectively, to the longer and shorter tail of the time series’ histograms. The distribution of the peaks is then estimated by using the standard translation processes approach. It is found that the peak distribution can be represented by the Extreme Value Type I (Gumbel) distribution. Because estimates obtained from this approach are based on the entire information contained in the time series, they are more stable than estimates based on observed peaks. The procedure can be used to establish minimum acceptable requirements with respect to the duration and sampling rate of the time series of interest, so that the software used for database-assisted design will be both efficient and accurate.

Database-Assisted Design, Standardization, and Wind Direction Effects

The writers present a simple methodology, developed for use in design assisted by electronic aerodynamic and climatological databases (for short, database-assisted design), that allows a realistic assessment of wind directionality effects. The methodology is applied to typical low-rise industrial steel frame buildings with a rectangular shape in plan, located in hurricane-prone areas, and results are compared with results obtained by the procedure specified in the ASCE 7-98 Standard, which, for buildings, consists of applying a blanket directionality reduction factor \IK\dd\N = 0.85 to wind effects obtained by disregarding directionality. The results show that, for significant numbers of buildings in hurricane-prone areas, the use of the ASCE procedure can result in the underestimation of wind effects corresponding to strength design. They also show that database-assisted design for wind loads offers the potential for significantly more risk-consistent, safer, and economical design for buildings with both known and unknown orientation.