Cladding for Tall Structures

Cladding for Tall Structures

The concept of high-rise buildings was introduced in the United States in the 1880s due to the increased land prices and density of population. The term originally applied to high-rise buildings with about 10-20 stories. By the 20th century, the term was used for high-rise buildings of unusual height, with more than 40 or 50 stories. High-rise buildings were built with steelstructural frames and glass exterior sheathing and by the mid-20th century, it became a typical feature of the architectural landscape in most countries.

In the late 20th and 21st centuries, the focus shifted to energy conservation. Sealed windows that would earlier facilitate forced air-circulation, for instance, in mid-rise buildings were replaced with operable windows and glass walls that were tinted to reflect the suns rays. Although originally designed for commercial purposes, many high-rises are now designed for offices, residential, retail, andhotelspaces.

In India, any structure that is above 15 meters in height qualifies as a tall building. However, many experts define ahigh-risebuildingas one that is at least 75 feet (23 m)tall or has about 7 stories.

Things to consider while designing and constructing tall structures in the current times

• Ecological considerations such as -
  • Ability to support its weight, withstand lateral forces imposed by the sway, wind and seismic loads, heat island effect, thermal load, Solar Reflective Index (SRI) for different faces of the building as per Indian Green Building Council(IGBC).
  • Tall buildings are constantly exposed to climatic extremities. Cladding in such buildings involves detailed structural calculations revolving around the wind load requirement. Wrong calculations may lead to weak installations, thereby running a risk of damage and injury.
  • Outdoor lighting should be designed as per the ASHRAE standards. Floor area ratio for tall buildings to be considered as per the industry norms.
  • Water conservation, recycling and treatment along with renewable energy should also be given priority.
  • Due to its height and a huge number of occupants, the tall buildings need to have life-safety and fire-safety systems in place.
• Physical challenges such as scaffolding limitation, accessibility and logistics of construction materials and increased risk of fire and systems (water, accessories, HVAC, faade).
• Designing energy-efficient tall structures is crucial as high-rise buildings consume massive amounts of energy and require a high operational cost.
• The construction methodology adopted for tall building structures have to be region-specific and as per the Government by-laws.

Materials and faade systems to maximize the efficiency

• For high-rise buildings, the load of the structure is larger than the load from live components including people, furniture etc. Hence, more structural materials are used in the lower levels than at the higher levels. So, tall structures are essentially designed as lightweight structures. It is imperative to use specific grades of concrete like M25, M30 or M40, specific grades of reinforced steel bars, high-performance concrete (HPC), high-strength steel, pre-stressed concrete, composite construction with steel, and lightweight masonry for walls and partitions.
• Curtain glazing is a versatile exterior cladding option with glass as the primary material. It is designed to resist wind and earthquake loads, prevent air leakage and excessive heat loss. A Double-glazed unit for the curtain glazing system creates one of the most efficient heat insulators and energy-efficient systems together with adding aesthetic value.
• The double-skin facade has increasingly become popular in the last two decades due to its contribution to energy consumption and sustainable development.A double-skin faade is preferred for high-rise structures as it decreases the U-value of the faade; permanently enables the shading system to prevent cooling loads and high wind loads; enhances ventilation through controlled openings in the external skin.
• Faade materials should be sustainable, lightweight and robust, with high resistance to fire. Fire-resistant cladding materials like Exterior Grade Laminates can retard fire and stop its propagation. Planks or weatherboards made from fibre cement or steel, concrete, Greenguard certified HPL panels for green buildings, PVC or aluminium, sheets and panel materials, double hardened resin and reconstituted timber products are a few other fire-retardant cladding materials.
• For the earthquake resistance, the materials should be lighter to make the structure resistant to lateral loads and reduce the load on the building and its foundation. Fibre cement, PVC or aluminium are a few useful materials. While choosing the materials, it is essential to prioritize the geographical vulnerabilities. For seismic zones, brittle or stiff cladding materials should be avoided. For instance, although concrete is an excellent fire-retardant, it is not a stable material for earthquake-prone areas. For such places, steel-reinforced concrete is a preferred option.
• Genuine Compact Laminates can be customized for each building for different wind loads, dead loads, thermal loads and other requirements and protect the building from extreme climates.
• In a tropical country like India, buildings witness soaring temperatures and are continuously exposed to harsh weather extremities. Environmental acids, smog, sand and silica deposits/particles affect their properties, and they run a higher risk of damage. As a result, the buildings need regular maintenance. Exterior Grade Laminates are a great option for such areas as they need minimal maintenance, are sustainable, eco-friendly, and enhance thermal efficiency and energy renewability.
• Also, the faade system installed using the rear ventilated principle can be used in combination with other materials in high rise structures. These systems are designed to withstand extreme conditions in tall buildings. This cladding system improves the thermal efficiency of the building and reduces thermal bridges to avoid condensation of water vapour, thus improving the thermal comfort of the occupants in the building. It is noteworthy that such systems can help reduce the power consumption of AC and can make buildings sustainable. During monsoons, the system ensures that the rainwater drains away and never reaches the wall.
• The cladding system in tall buildings must be able to accommodate building movements and they should have hard surface for high impact resistance. Additionally, they should also have anti-graffiti properties as the smog & silica particles in the air may cause visual defects.
• As per (IGBC), commercial buildings consume more energy than highways and roads, and hence there is an increasing need for sustainable and energy-efficient buildings. The green faade in a high-rise structure aim to create more green spaces. Gardens in offices when equipped with openable windows support natural ventilation. Faade planting creates a microclimate, that reduces heat gain from solar radiation as compared to metal-or mineral-clad surfaces. Additionally, wind-resistant plants (with thicker and sturdier foliage) can be planted.

The design and decoration of high-rise buildings have evolved over time. Exterior cladding plays a key role in maintaining structural stability. Innovation is another aspect that drives the current cladding market. Emphasis is now on the aspect of sustainable cladding systems and how they can support green buildings.

References