Modern reassessment of the Citicorp Building design wind loads.

Following structural engineering practice of the 1970s, engineers designed the Citicorp Building for the action of wind in each of the structure's principal axes. One problem they faced was how to determine design values by combining simultaneous wind loads from the means and root mean squares of these loads. Given the technology available at the time, the simplifying assumption was made of decomposing the effects of a corner wind into the sum of the effects on two adjacent faces, as if these wind loads were static and perfectly correlated, which they are not. As a result of this assumption, the connections in the diagonal braces were deemed overstressed, and emergency repairs had to be performed on the just completed building. Nowadays, the Database-Assisted Design method and modern computer technology are capable of accounting for simultaneous dynamic loads properly, performing structural analyses for all time steps where measurements are available, and extrapolating to longer duration windstorms using extreme value distributions. Modern analysis thus determines design loads on a more rational basis and shows that the combinations of wind loads that caused such concern in 1978 do not need to be considered for mean recurrence intervals of practical interest.

Verification of ASCE 7-16 Pressure Coefficients and Database-Assisted Design of Purlins and Girts Accounting for Wind Directionality.

For the database-assisted design (DAD) of low-rise building purlins and girts, a method is proposed that explicitly accounts for wind directionality by using directional wind tunnel measurements, directional wind speed data, and publicly available software. The method consists of four steps: (1) assignment of wind loads induced by a unit directional wind speed on purlins and girts from pressure taps and their tributary areas; (2) development of bending moment and shear force influence coefficients for line loads on purlins and girts; (3) multiplication of loads from step 1 by influence coefficients from step 2 and estimation of the peak bending moments and shear forces thus obtained; and (4) use of nonparametric statistics to calculate peak moments and shear forces with a specified mean recurrence interval for various building orientations and accounting for wind directionality. For one example of wind effects on purlins, (1) comparison of the Envelope Method in ASCE 7-16 (taken as 100%) with the most demanding aerodynamic case from wind tunnel tests shows differences ranging between +10% and -25%; and (2) comparison of the ASCE 7-16 method accounting for the wind directionality factor Kd with directional wind loads using nonparametric statistical methods shows differences ranging between +21% and -25%. The unconservatism (+) of ASCE 7-16 is thus worse after Kd is applied. The proposed method is based on the rigorous DAD approach, accounts explicitly for the actual directional wind loading, entails no onerous computational requirements, and typically results in more economical designs while assuring risk-consistent safety.

Blown away: Citicorp Center Tower repairs revisited.

Using Database-Assisted Design and the Tokyo Polytechnic University Aerodynamic Database, we show that face winds put more structural demand than corner winds do on the Citicorp Tower in Manhattan. This 59-story building, supported by four midside columns, famously had to undergo secret emergency strengthening shortly after its completion in the 1970s to withstand corner winds.

Wind Effects on a Tall Building with Square Cross-Section and Mid-Side Base Columns: Database-Assisted Design Approach.

This paper illustrates the application of the database-assisted design (DAD) method to the wind design of high-rise buildings. The paper uses publicly available wind tunnel data and DAD procedures to compare responses to (1) corner winds and (2) face winds of a high-rise building of square cross-section supported by a central core column and four mid-side legs. The responses being considered consist of overturning moments, and of demand-to-capacity indexes (DCIs) of selected members, including multistory chevron braces. The analysis accounts for structural dynamics and second-order load-deformation effects. The results show that corner winds are less demanding than face winds, both globally (overturning moments) and locally (DCIs). The along-wind and across-wind overturning moments in the corner wind case are about 20% and 50% lower, respectively, than their counterparts in the face-wind case. The peak axial forces in the legs (peak refers to absolute value) and the peak DCIs in the mid-side mast columns (continuation of the legs) induced by corner winds are lower by 20%-30% than their counterparts due to face winds. The investigation confirms that the building code of the City of New York in effect in the early 1970s can be interpreted as meaning that the design for wind of structures with a square shape in plan may be performed by assuming the wind loads to act normal to a face of the building. The building analyzed in this paper is similar to the Citicorp Building (completed in 1977, later renamed Citigroup Center, now called 601 Lexington) and the results of the analyses presented herein suggest that a re-examination of the history of the Citicorp Building design and retrofit may be warranted.

Analysis of Wind Pressure Data on Components and Cladding of Low-Rise Buildings.

This paper presents a methodology for analyzing wind pressure data on cladding and components of low-rise buildings. The aerodynamic force acting on a specified area is obtained by summing up pressure time series measured at that area's pressure taps times their respective tributary areas. This operation is carried out for all sums of tributary areas that make up rectangles with aspect ratios not exceeding four. The peak of the resulting area-averaged time series is extrapolated to a realistic storm duration by the translation method. The envelope of peaks over all wind directions is compared with current specifications. Results for one low-rise building for one terrain condition indicate that these specifications can seriously underestimate pressures on gable roofs and walls. Comparison of the proposed methodology with an alternative method for assignment of tributary areas and area averaging is shown as well.

Recent and Current Wind Engineering Research at the National Institute of Standards and Technology.

This paper briefly reviews recent and current National Institute of Standards and Technology (NIST) research aimed at improving standard provisions and advancing structural design practice for wind loads. The research covers: (i) New wind speed maps for the conterminous United States; (ii) Risk-consistent estimation of wind load factors for use with the wind tunnel procedure; (iii) Modern peaks-over-threshold approaches to estimation of peak wind effects; (iv) User-friendly procedures for the database-assisted design of rigid and flexible structures; (v) Novel approaches to codification of pressures on cladding and components; (vi) Modern modeling of synoptic storm planetary boundary layers and its implications for super-tall building design; (vii) Computational Wind Engineering (CWE); (viii) Tornado climatology and development of tornado-resistant design methodologies; (ix) Joint climatology of wind speeds, storm surge and waves heights, and estimates of their combined effects on structures.

Influence of Wind Tunnel Test Duration on Wind Load Factors.

A method is presented for calculating the uncertainty associated with the estimation of peak pressure coefficients from wind tunnel test records of various lengths and how this uncertainty influences design wind effects. The proposed method is applicable to any type of structure and any type of civil engineering aerodynamic testing facility, including large-scale facilities. As an example of the application of the method, an investigation is presented of time series belonging to five categories of pressure coefficients implicit in Chapter 27 of the ASCE 7-10 Standard. The results of the investigation show that, for typical civil engineering wind tunnels, estimated design wind effects based on tests with durations as low as 10 s, corresponding to prototype durations of less than 6 min, are larger than their counterparts based on tests with 100 s duration by only approximately 5%. The proposed method provides useful indications on minimum lengths of pressure records to be measured in wind tunnels.

Wind-Pressure Coefficients on Low-Rise Building Enclosures Using Modern Wind-Tunnel Data and Voronoi Diagrams.

External pressure coefficients specified in the ASCE 7-10 Standard, used to determine design wind pressures for the components and cladding of buildings, are developed from wind tunnel test data that date back 30-50 years. In recent decades, advances in pressure measurement and computer technology have made it possible to obtain simultaneous pressure records, with high sampling rates, at many more wind tunnel pressure taps than was the case in the past. This paper proposes a method to calculate external pressure coefficients using aerodynamic wind tunnel databases such as Tokyo Polytechnic University's large, publicly available database. Voronoi diagrams are used to assign tributary areas to irregularly spaced pressure taps. User-defined grids of various sizes and shapes are placed at various offsets over the building surface to perform area-averaging of the pressure time series. Considering all wind directions for which measurements are obtained in the wind tunnel, the peak pressures are determined assuming a Gumbel distribution, and are extrapolated to a standard storm duration. The external peak pressure coefficients are then plotted as functions of their corresponding area for various zones of the building enclosure to produce plots similar to the ASCE 7-10 specifications on components and cladding. Results for three gable buildings analyzed in the paper show that the current ASCE 7-10 specifications can severely underestimate the external pressure coefficients for components and cladding of low-rise buildings.

Estimating peaks of stationary random processes: a peaks-over-threshold approach.

Estimating properties of the distribution of the peak of a stationary process from a single finite realization is a problem that arises in a variety of science and engineering applications. Further, it is often the case that the realization is of length T while the distribution of the peak is sought for a different length of time, T 1 > T. Current methods for estimating peaks of time series have drawbacks that motivated the development of a new procedure, based on the peaks-over-threshold method, an advantage of which is that it often results in an increased size of the relevant extreme value data set compared with epochal procedures. For further comparison, the translation approach depends upon the estimate of the marginal distribution of a non-Gaussian time series, which is typically difficult to perform reliably. The epochal procedure for estimating peaks combined with Best Linear Unbiased Estimates (BLUE) of the Gumbel parameters was found to depend, in some cases very significantly, upon the number of partitions being used. The proposed procedure is based on a Poisson process model for the thresholded pressure coefficient y, with threshold u. The estimated peak depends upon the choice of the threshold. However, unlike for the choice of the number n of partitions for the epochal procedure, a criterion is available that allows the analyst to make an optimal choice (according to a chosen metric) of the threshold value. Two versions of the proposed new procedure have been developed. One version, denoted by FpotMax, includes estimation of a tail length parameter with a similar interpretation to the generalized extreme value distribution tail length parameter. The second version, denoted by GpotMax, assumes that the tail length parameter vanishes, resulting in a tail of the Gumbel distribution type.

Wind Load Factors for Use in the Wind Tunnel Procedure.

A 2004 Skidmore Owings and Merrill report (in Simiu E. (2011) Design of Buildings for Wind, Appendix 5, Wiley, Hoboken, NJ) notes that the ASCE 7 Standard (American Society of Civil Engineers (2002) ASCE 7-02, Reston, Va) is incomplete insofar as it provides no guidance on wind load factors appropriate for use with the Standard's wind tunnel procedure. The purpose of this paper is to contribute to such guidance. Based on a classical definition of wind load factors as functions of uncertainties in the micrometeorological, wind climatological, aerodynamics and structural dynamics elements that determine wind loads, the paper presents a simple, straightforward approach that allows practitioners to use appropriate wind load factors applicable when those uncertainties are either the same as or different from those assumed in the development of the ASCE 7 Standard. Illustrations of the approach are presented for a variety of cases of practical interest. In estimating design wind loads, the various uncertainties should not be accounted for in isolation, for example by specifying peak pressure coefficients with percentage points higher than those corresponding to their expected values. Rather, to achieve risk-consistent designs, the uncertainties should be accounted for collectively, in terms of their joint effect on the design wind loading. The design wind effect is equal to the estimated expectation of the peak wind effect times a load factor that, in most cases, is not significantly different from the load factor explicitly or implicitly specified in the ASCE 7 Standard. Notably, the load factor is not affected significantly by errors associated with interpolations required in typical Database Assisted Design applications. However, if the available wind speed records are several times shorter than, say, 20 to 30 years, the wind load factors increase by amounts of the order of 15 %.