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How Curtain Walls Impact The Structural Design of a Building

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How Curtain Walls Impact The Structural Design of a Building

Curtain walls are an important structural component that face wind loads as their primary stress. In some instances, wind loads have a significant impact on the structural design of curtain walls. This can involve conducting bluff body aerodynamics analysis, adhering to relevant codes, carefully determining material geometry and composition, and considering production and placement restrictions.

Floor slabs play a crucial role in the design of curtain walls by providing suitable anchor points for the curtain wall system. These anchor points enable the curtain walls to be arranged as vertical members that span continuously from one floor to another.

Design of Curtain Walls for Wind Loads

The design of curtain walls for wind loads involves several key aspects that have been discussed:
• Determining the order for the structural design of the curtain wall and establishing the load path.
• Designing the appropriate thickness of glass for the curtain wall.
• Selecting the mechanisms used to attach the glass panes to the frame.
• Incorporating structural glass elements into the design.
• Considering the importance of scale in the design process.
• Taking manufacturing and placement factors into account.
• Following a calculation procedure to determine the design wind load.
• Considering wind load combinations.
• Addressing safety requirements for glazing.
• Developing analytical models and paying attention to detailing, especially related to connections, anchors, and fasteners.

Order for Curtain Wall Structural Design and Load Path

Wind loads are initially applied to the glazing panels of curtain walls and then transmitted to the perimeter supports. From there, the framing members receive the wind loads either directly or indirectly from the perimeter supports. Typically, the framing members are arranged in a perpendicular manner, consisting of beam and column mullions that extend vertically from floor to floor, effectively transferring the wind load to the building structure.

The horizontal elements positioned between the vertical mullions are considered secondary members. They play a role in transferring a portion of the dead and wind loads to the vertical elements.

In cases where openings such as large mechanical ventilation systems or operable doors and windows are present, the load transfer path can vary. This can lead to the generation of interior pressure, which may or may not align with the external pressure. As a result, it is crucial to consider this possibility during the design of the curtain wall to ensure its structural integrity.

Design of Curtain Wall Glass Thickness

The design of curtain wall glass thickness is influenced by several key factors. These factors include the imposed loads, span value, and potential shading temperature effects, which can impact the stress capacity design.

The strength of flat glasses is significantly affected by the heat treatment applied during the glass production process. Industry requirements specify stress limits for three types of glasses: annealed, heat-strengthened, and fully tempered.

While wind loads are typically the controlling factor, situations may arise where blast loads or debris impacts need to be considered.

Architects establish performance specifications for curtain wall glass, which may include limitations on deflection and other factors that can affect glass design. It is essential to consider resident comfort, interference with adjacent materials, and potential accumulated movements when specifying the deflection at the center of the glass.

For structural analysis, the boundary conditions of glass panels are typically assumed to have simply supported edges, such as in dry gasket pocket glazing and silicone glazing. Additionally, lateral pressure on glass panels is considered to be evenly distributed across the entire panel surface.

In the United States, the most commonly used glass thickness for high-performance commercial projects is reported to be 0.25 inch. Additionally, a 1 inch insulated glass unit consists of 0.25 inch outer lite glass, a 0.5 inch air or gas-filled void, and 0.25 inch inner lite glass.

ASTM E1300 provides a detailed procedure for sizing glass after obtaining both the geometry and design loads.

Structural Silicone Glazing

The structural silicone glazing technique is employed to attach glass tiles to the framing perimeter. It involves using a rubber adhesive as a component to secure the glass unit against wind forces. Unlike other methods, there is no mechanical restraint around the edge of the glass unit to ensure its integrity. Therefore, the expertise of a structural engineer is necessary, combining it with hyperelastic rubber theory to achieve the required integrity.

Additionally, it is important to design the glass panel as a diaphragm to withstand in-plane wind forces, as these forces have the potential to occur. This ensures that the glass panel can effectively resist and distribute the wind forces acting on it.

Point Supported Glass

Various methods have been developed to secure and restrain glass panels in curtain wall systems. Initially, small rectangular clamps known as patch fittings were commonly used for this purpose. These fittings help hold the glass panels in place.

Another method involves using T-headed bolts that are inserted into pre-drilled holes in the glass. These bolts play a role in securing the glass panels and keeping them in position.

To maintain a seamless appearance on the outside surface, countersunk holes are made in the glass. This prevents the metal fixtures from being visible. It’s important to note that the dimensions required for countersunk holes affect the determination of the glass panel thickness.

In addition to these techniques, there are alternative options for point support collection. Spider fittings on space frames, cable networks, and tension framing can be employed instead of the traditional orthogonal grid of aluminum framing. These alternative structural frames provide different design possibilities and variations for supporting and distributing the loads on the glass panels.

Structural Glass

One commonly used type of structural glass in curtain wall systems is fin glass, which functions similar to vertical mullions. Structural glass has been employed in the construction of building frames and envelopes, such as the Apple Store building.

It is crucial to consider that glass can fail abruptly and destructively, emphasizing the need for accurate evaluations to ensure structural integrity. Comprehensive assessments must be conducted to assess the strength and stability of the glass components.

Finally, proper stability can be achieved by incorporating large safety factors, sacrificial plies (additional layers of material designed to absorb impact), and alternative load paths. These measures help enhance the structural glass system’s overall strength and resilience.

Importance of Scale

In contrast to steel and concrete construction, where dimensional measurements are often rounded to the nearest millimeter, precise and accurate sizing is crucial for various components of curtain walls. This is because curtain wall contractors aim to minimize the use of costly materials, and the intricate fit of detailed assemblies requires high levels of accuracy, ranging from 0.078 inch to a 4 inches.

Deflection is a critical factor in the design of curtain walls. Additionally, the deflection of spandrel beams located at the building’s perimeter, caused by live loads, can impact the expansion joints of the curtain wall. Understanding and considering these deflection factors is essential to ensure the proper functionality and performance of the curtain wall system.

Manufacturing and Placement Considerations
In the design of curtain walls, designers aim to create a system that efficiently withstands wind loads. However, there are additional considerations from the perspectives of producers and erectors. Producers seek to minimize the use of expensive materials, while installers prioritize ease of installation at the construction site. Therefore, it is essential to take into account the demands of both manufacturers and installers during the design process. By addressing these various requirements, a successful curtain wall product can be achieved that satisfies the needs of all stakeholders involved.

Calculation Procedure for Design Wind Load on Curtain Walls

The design wind load for curtain walls can be determined using the guidelines provided in ASCE 7-10, which outlines the minimum design loads for buildings and other structures. The computation procedures for wind loads are divided into two sections: wind loads for main wind force resisting systems and wind loads on components and cladding.

For the main wind force resisting systems, there are four different calculation procedures available. On the other hand, wind loads on components and cladding can be calculated using either the Analytical Procedures or the Wind Tunnel Procedure.

Typically, the Analytical Procedure is used to calculate the design wind loads for curtain walls. Detailed steps for wind load calculation on curtain walls can be found in ASCE 7-10, specifically in Chapter 30, Part 1 to Part 6. These guidelines provide comprehensive instructions for accurately determining the wind loads that the curtain walls will experience.

Wind Load Combinations

ASCE 7-10 also provides wind load combinations to consider in the design process. In most cases, the wind load combination specified below is applied as it creates the most critical scenarios:

Wu = 1 x Dead Load + 1 x Wind Load (Equation-1)

This combination accounts for the combined effects of the dead load and wind load on the structure. It ensures that the design adequately addresses the simultaneous impact of these loads, resulting in a structurally robust curtain wall system.

Anchor Designs and Embeds

In the design of curtain walls, designers must be familiar with various anchors and embeds to select the most suitable type for the given conditions. Examples of anchors include welded steel, bolts, and slotted channels.
Slotted channels are commonly used in high-performance curtain wall systems, offering a wide range of post-installed anchor options. This flexibility allows for accommodating missing embeds or tolerances.

Effective communication between the structural engineer and the curtain wall designer is necessary to specify anchor design assumptions and assess the impact of loads and moments imposed on the structure by the curtain walls.

It is important to provide sufficient allowance for structural movement at the anchor-wall interface. This allows the curtain wall system to accommodate any potential shifting or expansion of the structure without causing damage or compromising the overall integrity of the system.


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