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.

Effects of Y2O3 Additive on Mechanical and Thermal Properties of Cordierite-Mullite Ceramics.

Cordierite is an alumina-magnesia-silica compound widely used as a thermal shock resistant material due to its high thermal shock resistance, low coefficient of thermal expansion (CTE), low dielectric constant, and good electrical insulation. However, its narrow sintering temperature range and low mechanical strength hinder its application in ceramic heaters. Although mullite shows excellent thermal and chemical stability, heat resistance, and mechanical strength, it has the disadvantages of high sintering temperatures (1600-1700 °C) and poor thermal shock resistance. In this study, a composite phase was prepared by mixing cordierite and mullite to expand the narrow sintering temperature range of cordierite and adjust its CTE to be similar to that of Si. Furthermore, Y2O3 was added to reduce the sintering temperature and to increase the mechanical strength. Therefore, the composite showed the highest density of 2.5 g/cm³ at 1380 °C when the ratio of mullite to cordierite was 20 wt%. When 11 wt% Y2O3 was added to this composition, the highest density was 2.8 g/cm³ for a sintering temperature of 1320 °C, and the mechanical strength was relatively good as 180 MPa of 3-points bending strength was comparatively good. The CTE was 2.6×10-6.K-1, which was similar to that of Si.

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.

Planetary Boundary Layer Modeling and Standard Provisions for Supertall Building Design.

According to recent results of planetary boundary layer research relevant to the design of tall buildings subjected to large-scale synoptic storm winds, for elevations of up to at least 1 km, the longitudinal mean wind speeds are monotonically increasing with height. It is shown that, for this reason, to avoid the possible unconservative design of supertall buildings significantly affected aerodynamically by neighboring buildings, an explicit derogation from the ASCE 7 standard specification of the gradient heights zg is necessary for buildings with heights greater than zg .

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 %.

Sintering and Microstructure of BaTiO3 Nano Particles Synthesized by Molten Salt Method.

In order to establish thinner dielectric layers in thick film electronic components such as MLCC (Multilayer ceramic capacitor), BaTiO3 nanoparticles have been utilized. However, studies on the synthesis of nanoparticles smaller than 20 nm, the characteristics of the BaTiO3 powder, and the powder's sintering are lacking. Therefore, this paper aims to synthesize BaTiO3 particles smaller than 20 nm by using the molten salt method and evaluate the microstructure and dielectric properties by varying the sintering temperature from 750 degrees C to 1200 degrees C. Through the molten salt method and by using KOH-KCl mixed salt, 20 nm BaTiO3 powder was synthesized at a low temperature of 150 degrees C. Sintering the pellets formed from the synthesized 20 nm BaTiO3 nano powder led to the observation of an unusual phenomenon where the particles grew to approximate sizes below 850 degrees C where densification progressed. At sintering temperatures above 950 degrees C, particles that expanded into rod shapes were observed and these particles were identified to be unreacted TiO2 based on the results of the EDX (Energy Dispersive X-ray Spectroscopy) analysis and phase analysis results.

Planetary Boundary-Layer Modelling and Tall Building Design.

Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both "neutral" flows, and "conventionally neutral" flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5°. This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers.

Effect of Ca-Al-Si-O common glass on dielectric properties of low-temperature co-fired ceramic materials with different fillers.

High-density integration in single component used for mobile communication is highly demanded with the miniaturization trend in multi-functional light-weighted mobile communication devices. Embedding passive components into multi-layered ceramic chips is also increasingly needed for high integrity. The need for high strength materials to be used in handheld devices has also increased. To this end, many attempts to join different low-temperature co-fired ceramics (LTCC) materials with different dielectric constants have been made, but failed with de-laminations or internal cracks mainly due to difference of thermal expansion coefficients. It is thought that this difference could be minimized with the use of common glass in different LTCC materials. In this study, several candidates of common glass were mixed with various fillers of LTCC to have various dielectric constants in the radio-frequency, and to minimize the mismatch in joining. Ca-Al-Si-O glass was mixed with 1.3MgO-TiO2, cordierite and CaTiO3. Mixtures were tape-cast and sintered to be compared with their micro-structures, dielectric properties and thermo-mechanical characteristics. When 1.3MgO-TiO2 with volumetric ratio of 30% was mixed with Ca-Al-Si-O glass, the measured dielectric constant was 7.9, the quality factor was 3708. With 45 volumetric percent of cordierite, the dielectric constant was 5 and the quality factor was 1052.