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FAÇADE ENGINEERING FOR TALL BUILDINGS

Rajan Govind- Director at BES, has >20 years of international experience within the facade industry. With Structural engineering background Rajan has good overall façade specialist experience. Rajan has worked in various markets such as South East Asia, Middle East and Australia.. He has successfully delivered several prestigious projects with integrated facade designs, involving complex 3D design, façade engineering and building physics. He is a Certified SGMM (Singapore Green Mark Manager), enable him to understand sustainable design requirements from Architectural and practical perspectives.

Known face to Indian façade industry and he attained good reputation with high quality works completed.

ABSTRACT

Building Envelope / façade is one of a key element of buildings, playing a critical role of providing weather proof protection for the building occupants. Building envelope / façades are subject to severe weather conditions and experience various natural forces throughout its lift cycle. Hence Façade engineering which deals with building envelope design and its engineering is becoming challenging in particular to tall buildings due to various aspects. This paper is aimed to present key challenges of tall building façade engineering with regards to wind pressure aspects. Keywords: Building façades, Tall Building, Wind Pressure

Rajan Govind, Director, BES Consultants Pvt Ltd

E mail: rajan@besconsultants.net

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1 IMPACT ON FAÇADE Wind pressure has several impacts as illustrated below, key design drivers such as wind pressures and building movements has direct impact on façade designs.

Image 1: Wind Impact 2 UNDERSTANDING CLADDING PRESSURES Façade pressures or generally referred as cladding pressures are different from wind pressures for building structural designs. The key differentiations are as follows.

Differential pressures

Internal pressure coefficients

Openings

Permeability of walls

Building corners

Local effects

3 ACTUAL VS ADAPTATION On a typical flat building there are various wind pressure zones that may occur, for example for a 200 metre tall building façade may experience from 1.0 kPa to as high as 2.7 kPa. However it is not practical to design façade systems to suit various wind zones of particular building. In order to have a unified / standardized designs pressure zones to be grouped. Terefore there is a general practice in façade engineering where we could see a difference between wind pressures actual Vs adopted.

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Myth: “Low wind pressures at low level, high wind pressures only at high level” General assumptions that building lower level will experience low to moderate wind pressures and pressure increases relative to the height. This is generally not true for high rise buildings, high localized pressures are experienced right at the ground or low levels on certain cases due to building geometry, funneling and other effects. This imposes major challenges in structural design of façade elements with regards to wind pressure zones. Image 2 presented with actual building pressure of a 50 sty Building at Mumbai.

Image 2: Wind pressure contour highlighted yellow represents highwind pressures 4 ADOPTING TO BUILDING BEHAVIOUR Façade systems are to be well designed such that it will adopt to the building structural behavior in particular building sway and floor movements. Furthermore inter-storey drift or building sway is one of critical aspect of façade engineering. For tall and slender building which will undergo significant magnitude of sway and inter-storey drift, facade movements are atmost important as safety in the structural design. This will require façade systems to accommodate large magnitude of sideways and vertical movement on framing joints and brackets. 5 FAÇADE SYSTEM- GENERAL ARRANGEMENT To accommodate large magnitude of movements and sway, primary requirements of tall building façade system is to have panellised system in which the building envelope is mapped with individual façade panels that forms the building skin. In the façade industry this type of system is referred as unitized Curtain wall system referring prefabricated panelized systems. This is referring pre-fabricated factory finished glazed panels installed on to the building as pre-finished glazed units. This approach has limited interface with main building and allow large movements in three axes of the building. Main structural

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elements are extruded aluminium frames, brackets and glass or cladding as in-fill material. Building sway / movement are directly accommodated within the framing system and

Occur in the plane of facades due to following key aspects. Building Sway General design guidance is to consider inter-storey drift H/500 (H = floor height)

Consider H : 4000mm (typical floor height)

Horizontal movement Δ1 = 8mm (4000/500) Thermal shortening of façade system

Horizontal movement Δ 2 = Δt x B x=1.72 mm

Where:

Δt=50 oC

B = 1500 mm

A = 23x 10-6

/ o C

Total horizontal movement = Δ=Δ1 + Δ2 = 9.72 (Say 10 mm) Other aspects such as tolerances, installation aspects are to be added in the joint movement design.

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5.2 Vertical Movement Occur due to key aspects such as floor movement, differential live loads, concrete creep, and column shortening. These are accommodated within the panel joints at floor or sill level. This effect is generally due to direct vertical loads rather than wind induced effect. Careful detailing of sill and head details of

Image 4 : Typical Façade Glazing Framing Plan details (called as Mullion), highlighted in circle showing movement allowance of a particular system

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