Dependence of wind load on air density for highrise buildings

https://doi.org/10.1016/j.jweia.2021.104558Get rights and content

Highlights

  • Examined dependence of air density on latitude, altitude and season.

  • Compared measured results with provisions in wind load code.

  • Proposed a profile model of air density in typhoon-prone area.

  • Explored air-density dependence of wind effects on high-rise buildings.

Abstract

Accurate determination of wind load is of great importance for the wind-resistant design of building structures. Despite the fact that air density varies systematically with altitude, and somewhat less so with barometric pressure, air temperature, and latitude, there is a lack of studies on the dependence of wind load on air density especially for highrise buildings. This article presents a special study on this topic. The dependence of air density on latitude, season and altitude is investigated first, based on meteorological records at several national stations in China. The height-dependence of air density in the inner region of a tropical cyclone (TC) is highlighted, since there is a fast development of highrise buildings in TC-prone areas and TC wind load dominates the design wind loads for such slender wind-sensitive structures. A data-driven model for the height-dependence of TC air density is established. It is shown that the measured TC air density is reduced by 8%–10% compared with the values recommended in the wind load codes. The dependence of wind load and wind-induced structural response on air density for a supertall building is then analyzed through a combined usage of the proposed TC air density model and wind tunnel testing. Results suggest that the wind load and wind-induced response of the building obtained by considering the height-dependence of air density can be decreased by as much as ~12% compared with those without considering such effects. Thus, more economic wind-resistant designs for highrise buildings may be achieved by taking into account the height-dependence of air density.

Introduction

The development of high-rise buildings has seen a rapid increase in recent decades. As tall buildings are getting higher and more flexible, they become increasingly more sensitive to wind action. Actually, wind load has become one of the main controlling loads for the structural design of highrise buildings, especially in tropical cyclone (TC) prone areas (Li et al., 2019; Tanaka et al., 2012). Accurate determination of wind load is of great importance for the wind-resistant design of such slender civil structures. While underestimation of wind load usually corresponds to non-conservative designs for highrise or supertall buildings and potentially increases safety risk, overestimation of wind load generally corresponds to conservative and therefore uneconomical designs of these buildings.

Unfortunately, it remains a challenge to estimate wind load on highrise buildings accurately. It is well acknowledged that wind load can be typically expressed as a form of the product of three parameters: air density, the square of wind speed and a dimensionless coefficient which is closely related to the aerodynamic configuration of the building. Previous studies have shown that there are large uncertainties involved in the estimation of design wind speed (Stathopoulos and Alrawashdeh, 2020). Meanwhile, traditional techniques for estimating the aerodynamic force on highrise buildings may suffer from certain limitations, such as inaccurate reproduction of approaching wind field, insufficient simulation of aeroelastic effects as well as issues related to Reynolds number similarity (usually due to adoption of a too small model scale), etc. (Kim et al., 2010; Irwin, 2009; Bowen 2003). These limitations tend to result in uncertainties when estimating the wind load coefficient via wind tunnel testing. Accordingly, many studies have been conducted to refine the estimation of design wind speed and wind load coefficient during the past years.

Despite the great efforts that have been made, there is a lack of investigation on the dependence of wind load on air density. In many existing studies, air density is simply assumed as a constant. However, it has long been recognized that air density varies directly with temperature, pressure and humidity, or geographically with regions in cyclone-prone areas or areas dominated by very cold temperatures and the ground elevations above sea level. Taking China for example, the extreme wind climate in northern China is dominated by strong monsoon wind in winter, while the one in southeastern China is dominated by tropical cyclones that usually occur in summer. Apparently, the air densities in the above two cases can vary significantly, and adoption of a constant air density value tends to result in inaccurate determination of wind load for the wind-resistant design of building structures.

It is noted that provisions are provided in some wind load codes/standards, e.g., ASCE 7–16 (2016) in the U.S. and GB 50009 (2012) in China, to account for the height-dependence of air density. In ASCE 7–16, changes in air density are allowed based only on the ground elevation above sea level at the building site. However, users are allowed to neglect this variation and a constant air density value is commonly utilized for engineering practices (Stathopoulos and Alrawashdeh, 2020).

Given the significant role of air density in accurate determination of wind load on building structures (especially on highrise buildings) and the fact that few studies have been conducted on this topic, this paper presents a special research on the dependence of wind load on air density for highrise buildings. Basically, there are three types of air density effects which can be significant and be treated variously. (1) Air density decreases with elevation above mean sea level, and related effects are significant for coastal locations and very tall buildings. (2) Air density is decreased at ground level compared to the standard air density, when the studied site is located well above mean sea level. Related effects are significant for all buildings when they are located at high ground elevations (>200 ​m). (3) Air density may decrease due to climatology. Such effects are significant for all buildings, especially for highrise buildings located in either very cold climates or TC-prone areas. It is stressed that the primary motivation of this work is to highlight the significance of considering the site/season-specific and height-varying features of air density when analyzing wind effects on building structures. The remainder of this article is organized as follows. Section 2 focuses on the field results of air density at ground levels associated with different sites and seasons, as well as those in the inner region of TCs. Section 3 examines the dependence of wind load and wind-induced structural response on air density for a supertall building at a TC-prone area in China. Main conclusions are summarized in Section 4.

Section snippets

General model

The density of humid atmosphere ρ(z) at any altitude z can be expressed as follows:ρ(z)=Pd(z)RdT(z)+Pv(z)RvT(z)=Pd(z)Md+Pv(z)MvRT(z)with:P(z)=Pd(z)+Pv(z)where T is the atmospheric temperature (unit: K); P(z) is the atmospheric pressure (unit: Pa), while Pd and Pv are partial pressures, which respectively denote the dry-air pressure and the pressure of water vapor; Rd=287.06Jkg1K1 and Rv=461.50Jkg1K1 are the specific gas constants for dry air and water vapor; Md=0.028964kgmol1 and Mv

Experimental setup

Wind load and wind-induced structural response are two typical wind effects for highrise buildings. This section examines the dependence of such wind effects on air density for a supertall building through wind tunnel testing.

A 1:600 downscaled rigid model was fabricated to reproduce the prototype supertall building which is cuboid in shape, with the height and width equal to 910 ​mm and 100 ​mm, as shown in Fig. 4(a). A total of 392 pressure taps were fabricated on the model. Wind direction β,

Concluding remarks

The study examined the effects of air density on wind load and wind-induced structural response for highrise buildings. The dependence of air density on latitude and season was first examined and demonstrated through case studies at five sites in China. The maximum value (i.e., 1.38 ​kg ​m−3) of air density at near ground levels was found to differ from the minimum (i.e., 1.15 ​kg ​m−3) by a factor of 20%. The measurements of air density also differed from the recommended value of 1.25 ​kg ​m−3

CRediT authorship contribution statement

Y.C. He: Conceptualization, Writing - original draft, Methodology, Funding acquisition, Supervision. H.B. Lin: Writing - original draft, Data curation. J.Y. Fu: Conceptualization, Writing - review & editing, Funding acquisition, Resources. P.W. Chan: Investigation, Data curation. Q.X. Zheng: Writing - review & editing, Validation. T. Deng: Conceptualization, Writing - review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to gratefully acknowledge the support from the National Natural Science Foundation of China through three projects (Grant No.: 51925802, 51878194, and 51808153).

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