Facade and Roof Cladding Pressure Studies

Introduction

In order to design cladding systems for high-rise and medium-rise buildings it is important to reliably predict the external wind pressures acting on a building facade. The results of cladding pressure studies may also be used to provide an accurate estimate of the design wind loads on façade panels. The results of this form of investigation supersede estimates from wind loading standards.

A Cost Saver

A study conducted by a firm of consulting structural engineers found that for a particular high-rise development, there was a cost to benefit ratio of 1:30 in carrying out a cladding pressure study. This figure generally agrees with other recent feasibility studies undertaken by Windtech. In addition to rationalising the amount of cladding material being used, wind tunnel investigations of external wind 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. Wind pressure tests can also be conducted for bay loads on long-spanning structural members and for membrane structures using the area averaging technique.

Methodology

Pressure taps convey wind pressures from the surface of the scale model to pressure transducers via PVC tubing connected to pressure scanning switches. Windtech utilises an advanced multi-channel pressure measurement system and also employs digital filtering to ensure no bias is introduced by the pressure taps. Peak pressures are analysed statistically using the upcrossing technique, which is a reliable statistical method. Results are presented in the form of pressure zone diagrams as well as contour diagrams for peak net external pressures. WINDTECH’s data acquisition system is fully compurterised and with a strong research background in this area, we can offer the most accurate and efficient service to our clients. In addition, a 500Hz low pass filter is used.

Internal Pressures

With the trend to have high-rise buildings with operable façade comes the issue of large internal pressures, which can affect not only the net façade pressures but also the internal partitions and floor slabs. The most accurate approach to the determination of these loads is to measure these differential pressures directly from the wind tunnel model. Windtech Consultants have developed a state of the art technique for the determination of these loads incorporating a risk analysis approach to ensure that the resulting loads are at a comparable level of risk to the surface pressures, which are designed based on working stress loads. This effectively provides a reliable prediction as well as effecting substantial savings by eliminating over-design of the partitions, floor slabs and the facade.

Verification against full-scale data

The accuracy of our pressure measurement system has been confirmed by a comparative study with full-scale measurements from a standard experimental building in Texas, USA. Our wind tunnel results show better agreement with the full-scale data than any published data.