Experimental and computational determination of wind loads on netted/sheeted scaffolds

• Main Author: Irtaza, H.
• Published: Oxford Brookes University 2009
• Subjects: 624.18
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543807

This thesis describes an investigation into the wind loading on access scaffolds erected around a cubical building, clad by impermeable sheeting or permeable debris netting. The subject was investigated experimentally by tests in a wind-tunnel and theoretically using computational fluid dynamics techniques. The results were verified from the wind tunnel tests and computational analyses on the Silsoe Experimental Building (SEB) using data from the full-scale tests made in 1993-94 at Silsoe, U.K. The lower portion of the Atmospheric Boundary Layer exhibits different flow properties to the upper elevations. A procedure is presented for modelling the atmospheric surface layer flow properties in a boundary-layer wind-tunnel at useful model scales. The full-scale data available from the cubical 6m x 6m x 6m SEB was used to validate the results presented in this study. A model scale of 1:30 were used both for experiments in a wind-tunnel and in the computational analyses undertaken in the study. Pressure data obtained from the wind-tunnel experiments on the SEB model were compared to full-scale data with good agreement. These data were also compared with various computational fluid dynamics techniques available commercially and the conclusions drawn on the use of the different techniques. The wind-tunnel simulations on an SEB model and on a sheet/elevated sheet clad scaffold models were undertaken based on a duplication of the turbulence intensities and small-scale turbulence of the incidence flow. It is very difficult to achieve equality of Reynolds number in the wind-tunnel as it is very difficult to achieve exactly the same integral scales of turbulence. Two different types of terrain and inflow boundary conditions were simulated in the wind-tunnel for the models and results are reported here. Large suctions (separation of flows) occur near the leading edges and roof corners. The modelling of these phenomena in the wind-tunnel remains a problem. Because of the limited space near the corners and leading edges, it is difficult to make reliable measurements by introducing probes in these areas. This difficulty can be overcome by modelling the flow with Large Eddy Simulation (LES) numerical techniques. However, the disparity between the large and small scales, especially under extreme wind conditions, makes it extremely difficult to resolve the entire range of dynamic scales. The pressure force on bare pole access scaffolds are further influenced by the presence of the building façade which induces a shielding effect. A 2-D model of bare pole scaffolds surrounding the SEB using CFD techniques was successfully achieved whereas a 3-D model could not be produced because of the limitations of the meshing-software GAMBIT available to the author. Cladding increases the wind loads on scaffold structures above the pressure force on bare pole access scaffolds. To determine the wind forces on net/sheet clad scaffolds the Silsoe Experimental Building was used as a base model and simulated scaffolds erected around it. Although, sheeting/netting exhibits aero-elastic behaviour under wind load, an assumption was made to treat the cladding (sheeting/netting) surrounding the scaffold as being made of static solid thin plates. Models were tested in a wind-tunnel and the same assumptions were used in the computational fluid dynamics analyses. For the sheet clad scaffolds, two models were made, one with sheeting touching the ground and the other with an elevated sheet surrounding the building. These models were tested in a wind-tunnel to determine the pressure coefficients on the outer and inner faces of the sheeting. The permeability of the two types of net were successfully obtained from wind-tunnel tests. The simulated data from the wind-tunnel tests were used as input for different computational techniques with good agreement. A new procedure was developed to extend the computational model to net clad scaffolds (both elevated and touching the ground) with the netting simulated as porous media. The author presents new results of the pressure coefficients on sheeted scaffolds obtained using CFD and wind-tunnel techniques and also CFD results on netted scaffold structures. This thesis is the result of research undertaken to assess various methods available for the numerical simulation of turbulent fluid flow using the Fluent Software Package and to see their applicability in computational wind engineering. Investigations have concentrated on analysing the accuracy and numerical stability of a number of different turbulence models including both widely available models and state of the art techniques. Furthermore, Large Eddy Simulations using the dynamic kinetic energy sub-grid-scale model have been completed on some models, in order to account for the four dimensional nature of turbulent flow and to show the best correlation between wind-tunnel, full-scale and sheeted scaffolds. The author has detailed and tested all the above techniques and gives recommendations on the appropriate turbulence model to be used for successful computational wind engineering. Finally the author has given recommendations on the wind pressures to be used in analysing the scaffold structures.