Facade and Roof Cladding Pressure Studies

Introduction

Being able to reliably predict external wind pressures acting on a building facade is important in order to design cladding systems for high-rise and medium-rise buildings. An accurate estimate of the design wind loads on facade panels can also be given by using cladding pressure studies. Investigation of this type supersedes estimates from wind loading standards.

  

A Cost Saver

Studies by structural engineers have found that there is a cost to benefit ratio of 1:30 in carrying out a cladding pressure study for high-rise development and this finding just happens to confirm what other recent feasibility studies by Windtech have found. Wind tunnel investigations of of external pressures enable the designer to account for localised extreme pressures peculiar to the subject development which cannot always be adequately accounted for in the wind loading codes. This is in addition to rationalising the amount of cladding material being used. Using the area averaging technique, we can test for wind pressure for bay loads on long spanning structural members and for membrane structures.

Methodology

Wind pressures from the surface of the scale model to pressure transducers via PVC tubing connected to pressure scanning switches are conveyed using pressure taps. An advanced multi-channel pressure management system and digital filtering is used by Windtech to ensure no bias is introduced by the pressure taps. The upcrossing technique, a reliable statistical method, is used to statistically analyse peak pressures, and results are presented in pressure zone and contour diagrams for peak net external pressures. Windtech use a fully computerised data acquisition system and with their strong research background in this area, can offer an accurate and efficient service for all our clients. This, is in addition to a 500Hz low pass filter being used.

Internal Pressures

There is the issue of large internal pressures that comes with the trend to have high-rise buildings with operable facade which can affect not only the net facade pressures but also the internal partitions and floor slabs. The simplest and most accurate method for determining these loads is to measure these differential pressures directly from the wind tunnel method. A state of the art technique has been developed by Windtech that allows them to determine these loads, along with a risk analysis approach that guarantees the resulting loads are at a comparable level of risk to the surface pressures, which are designed based on working stress loads. This effectively does two things; it reduces costs by eliminating over-design of the partitions, floor slabs and the facade, and provides a reliable prediciton.

Verification against full-scale data

A comparative study with full-scale measurements based on a standard experimental building on Texas, USA, has shown the accuracy of our pressure measurement system. No other published data shows better agreement with the full-scale data than our wind tunnel results which demonstrates its effectiveness.