Basic wind speed map under high emission extreme scenario and probable structural loading

Abstract

Wind load is one of the most important environmental forces on structures. Due to the complex interaction with the atmospheric boundary layer, statistical methods are used to represent the wind interaction as a combination of an average wind speed and a gust speed. The average wind speed is generally provided in the building codes as basic wind speed maps with an exceedance probability of 0.02 or a return period of 50 year. Such maps are usually prepared based on the historic records of the wind speeds using appropriate extreme value distribution. This method inherently does not consider for the future climate scenario. According to International Panel on Climate Change (IPCC) report, climate change due to anthropogenic carbon emission is a scientifically accepted phenomenon and warming of the ocean and the atmosphere leads to more intense and frequent extreme wind events like nor'westers and cyclones. Rapid urbanization also increases the demand for high-rise building, where wind force is a limiting factor due to its increase with the height following a power law. As the structures are designed for a 50-year design period, it is imperative to know if the design wind speeds are likely to change in future climate. The development of basic wind speed map from daily wind speed projection of climate model is a challenging task. To facilitate this, first the basic wind speed from the observed climate record has been prepared. Compared to this map, the BNBC 2006 and 2017 map is found to overestimate the speeds by about 4\%, leading to about 8\% overestimate of wind pressure. From the same record, a log-linear relationship has been developed between the daily mean wind speed and daily maximum wind speed. Between least-square fitting and objective estimation, the second one is found to perform better for parameter estimation. Before applying this model to estimate the gust speed, bias correction was needed to apply over the model data to account for the underestimation of calm and extreme daily winds. Three atmospheric model dataset has been tested in this study -- ERA-Interim Analysis and Forecast dataset, and WFDEI dataset. Among these three dataset ERA-Interim forecast product is found more skillful at capturing the extreme wind climatology. Similar method is considered applicable for climate model also. An ensemble of 13 Global Climate Model (GCM) dataset following the Representative Concentration Pathway (RCP) 8.5 or extreme scenario is considered in this study. The basic wind speed maps calculated from climate model show a general underestimation compared to the atmospheric models. Despite this limitation, climate models were able to capture the geospatial pattern of the extreme wind climatology. Compared to 1971--2000 baseline period, an overall 10 to 15\% increase in 50-year return period wind speed is projected during the mid of the century and about 15 to 20\% increase at the end of the century. Considering the overestimation in the BNBC 2006 as well as BNBC 2017 basic wind speed map, the structures built today may experience increased extreme wind speed by about 6 to 10\% by the end of the 50-year design period. This findings add to already rising demand for incorporation of climate change factor in engineering practice.