Curtain-walling for curved facades | Features | Building

Liam Cummins, who has years of experience working with main contractors, has joined the cladding specialist. He sits down with Dave Rogers to discuss issues such as unsustainable profit margins, working under impressive CEOs, and the enduring appeal of London as a business hub.

You can find more information on our website, so be sure to explore.

Above: Aaron Blom presenting during GlassBuild America.

Editor's Note: The information provided here is based on the educational session 'Curtain Walls | Design Considerations, Tolerances, and Testing,' presented by Blom at GlassBuild America, which took place from October 18-20 in Las Vegas. You can find more news about the event online.

A curtain wall system fundamentally consists of insulating glass combined with extruded aluminum framing. This can either be installed piece by piece (using stock lengths) or through pre-assembled and pre-glazed framing and glass panels (unitized). Hybrid options are available, integrating various system selections, fabrication, and installation techniques that can create specific design aesthetics while enhancing both fabrication and installation efficiency.

Regardless of the chosen installation method, curtain wall systems must address five key design considerations: structural integrity, movement capability, weathertightness, energy efficiency, and sound control.

Structural Integrity

For all types of fenestration, wind load is a crucial structural aspect for curtain wall systems. Greater frame deflection from design wind loads places increased stress on the assembly, raising the risk of system failure or glass breakage. The engineering guidelines for maximum vertical framing deflection are L/175 for spans up to 13 feet 6 inches and L/240 + ⅛-inch for spans exceeding that, where L is the vertical span length. Other factors affecting structural performance include live load, dead load, seismic considerations, and inter-story drift.

Provision for Movement

When designing a curtain wall system, it is vital to account for various movements such as thermal expansion and contraction, wind load displacement, and structural shifts. Effective design must ensure that movement is accommodated to minimize stress on glass, framing, and anchors, without excessively reducing the frame's capacity to hold the glass.

Weathertightness

Weathertightness is the capacity to prevent both air and water leaks.

Water Penetration: To prevent water leakage in a curtain wall system, two methods can be employed. One involves using an "internal drainage" method that incorporates weep holes in the pressure plate and face cap. Alternatively, a "pressure equalization" design minimizes the potential negative pressure that could draw water into the system from outside.

Air Infiltration: Although less critical than water penetration, excessive air infiltration can still impact the energy efficiency of the building envelope. Industry standards limit air infiltration to 0.3 liters per second per square meter of fixed wall area (0.06 cubic feet per minute per square foot) when tested under a pressure difference of 1.57 pound-force per square foot.

Learn more about Zhuzao.

Energy Efficiency

Curtain wall systems must comply with specified maximum thermal transmittance or U-Factor (W/m2·K or Btu/hr·ft2·°F) as mandated by applicable building codes based on the project's geographical location and specific thermal performance requirements. Improved thermal efficiency can be achieved through various means, like utilizing thermally broken framing, high-performance insulating glass, and proper insulation in large spandrel areas.

Sound Control

Standard methods for assessing sound attenuation in glass and window systems are Sound Transmission Class (STC) and Outdoor/Indoor Transmission Class (OITC). Utilizing laminated insulating glass, increasing airspace in the insulating glass, and preventing air infiltration typically enhance sound insulation—critical considerations near airports and bustling urban centers.

Performance and Testing Requirements

Given the diverse forces acting on a building at specific sites, physical testing for air infiltration, water penetration, and structural performance (including frame deflection limits) remains one of the most reliable methods for evaluating performance in real-world conditions. These assessments can be performed on standalone mock-ups or completed installations, also referred to as in-place mock-ups.

If you wish to delve deeper, please visit our Curved Curtain Wall section.