WIND EFFECTS ON THE ROOF OF LOW-RISE STRUCTURES

Abstract

Severe windstorms, including tropical cyclones, cause significant damage globally, especially in coastal and inland regions of India. Roofs of low-rise buildings are particularly vulnerable, and wind loads on roofs have been a key area of research due to increasing public concern about windstorm damage. Roof geometry plays a critical role in wind pressure distribution but is often overlooked, resulting in conflicting and inadequate documentation in most Standards and Codes of Practice. A review of existing literature highlights gaps in understanding wind pressures on roof projections, eaves, canopies, and open verandahs, emphasizing the need for detailed studies on diverse roof forms. Wind loading data is primarily derived from wind tunnel tests on scaled models, but recent comparisons with full-scale studies underline the importance of accurate flow simulation for better insights. Additionally, wind standards of various nations have inadequate information to address the effects of nearby buildings on wind pressure patterns and magnitudes. Research in such interference scenarios is limited, with wind tunnel tests and CFD simulations being the main tools for analysis. This study aims to use CFD simulations to investigate wind effects on low rise buildings with different roof designs, both in isolation and in the presence of neighbouring structures. This study examines the wind flow patterns around low-rise buildings with various roof shapes through numerical simulations. Using Computational Fluid Dynamics (CFD), the research employs Reynolds-averaged Navier-Stokes simulations (RANS) and large eddy simulations (LES). The study uses scaled-down (1:50) models, and the results are compared with established codes to validate the simulations. With the rapid advancements in CFD, it has gained increasing acceptance in recent decades. The study also identifies critical flow regions around differently shaped buildings. These areas, which experience notably high positive and suction pressures, are more susceptible to partial structural failures compared to other sections, highlighting the need for careful consideration by wind engineers. In the present study, low-rise buildings with four types of roof forms are considered namely dome roof, cylindrical roof, mono-slope roof and hip roof. A Dome roof building is considered to have a square plan. Buildings with other three roof forms are considered to have a rectangular plan. For interference conditions, there are three different configurations of interference considered i.e., rectangular pattern, T pattern and Z pattern in which the six isolated models of vii low-rise buildings are arranged in the mentioned pattern with different spacing configurations i.e., 0, 0.5B, B, 1.5B and 2B where B is the width of building. The building models are considered to be situated in sub-urban terrain with well-scattered objects having heights between 1.5 m to 10 m, defined as terrain condition 2 in IS 875 (Part-3): 2015. The wind pressure distribution and pressure coefficient (Cpe) on all the roofs are measured at different angles of wind incidences, namely 0° to 180° at 15° wind intervals. Depending upon the symmetry of the roof, the dome roof is only tested for 0°, and 45° wind angles, cylindrical and hip roofs are tested for 0° to 90°, and the mono-slope roof is tested for 0° to 180° wind incidence angles. Similarly, for interfering conditions, the buildings arranged in different patterns (rectangular, T and Z) with variable spacing configurations are subjected to various angles of wind incidences at an interval of 15°. The pressure contours, pressure coefficient (Cpe), interference factor (IF) and interference difference (ID) are evaluated and presented for different interfering conditions. The results from the analysis showed that the overall effects of wind on the roof surface are suction in nature, which fluctuates due to the change in wind incidence angles. For cylindrical roofs, the small roof portion near the windward edge is subjected to the positive wind-induced pressure, and the rest of the roof portion is under suction, while the dome, mono-slope and hip roof are under only suction. The effect of shielding plays a vital role in reducing the wind load on the interfering roof when the spacing between buildings and angles of wind incidence angles change. The reduction in suction for dome roof is 88.46%, 87.41% on cylindrical roof, 85.27% on mono-slope roof and 80.18% on hip roof. Also, it is observed from the CFD simulation that the rectangular pattern of arrangement with variable spacing is more beneficial and stable than that of T and Z patterns in reducing the wind-induced pressure on the different roofs of low-rise buildings. The results presented in the thesis can be used in future for the revision of codal recommendations about wind loads on low-rise buildings with different roof forms. These can also be used by the designers while designing the roofs of similar low-rise buildings.