Wind Load Transfer for Curtain Wall Systems

Wind load is a critical consideration in the study of curtain walls, because it is one of the primary forces that act on the facade system. Curtain wall systems form the first defense against wind loads acting on a building structure.

Curtain walls are non-load bearing systems that are attached to a building's structure, and they rely on their own strength to resist external loads such as wind, gravity, and seismic forces. Wind load can cause significant stresses on a curtain wall system, which can lead to deformation or even failure of the facade.

Designers must consider wind load in the curtain wall study to ensure that the system is strong enough to withstand the wind forces in the building's location. The design must take into account the local wind speeds, building height, shape, and orientation, as well as the size and weight of the individual components of the curtain wall.

In this article, wind load transfer in a typical curtain wall system is discussed, along with the analysis of curtain wall elements for the same.

Load path in a curtain wall

The glazing or infill panels are the first to resist wind loads in a curtain wall system. These loads are transferred to the perimeter frame (mullions and transoms) via structural sealants or mechanical clips as per the system design. See image below for the two most common methods of curtain wall perimeter support systems.

In figure below, wind loads imposed on surface 1, gets transferred to elements 2 and 3 via the perimeter supports, which can either be structural sealant or mechanical fixings as shown above.

The primary support bracket of the curtain wall is connected to the mullion, and hence the mullions are considered as the primary load transfer element in the curtain wall structural system.

The transoms are connected between the mullions and act as secondary members transferring DL and WL back to the vertical elements (mullions). The DL of the infill panel is transferred to the transom via dead load setting blocks, which is then transferred back to the mullion via the mullion transom connection.

Load transferred to element 2 from element 1 and 3, gets transferred to the building structural elements (5) via the mullion bracket (4).

Curtain Wall Free Body Diagrams

Although there are multiple softwares available for structural analysis, Free Body Diagrams are the most efficient and simplest method to represent curtain wall structural behavior. Facade elements are designed as external systems fixed back to the main structural support framework and hence, they are mostly designed to behave as simply supported systems. The principle behind this being that curtain wall systems need to be designed for thermal expansion as well as be able to accommodate movements and settlement of the main structure.

Mullions (primary frame members) and transoms (secondary frame members) are most commonly analyzed as simply supported beams with trapezoidal or triangular load patterns to suit the curtain wall panel typology it serves. Below figure shows the free body representation of wind load and dead load transfer in a typical curtain wall mullion and transom system.

Simply Supported Systems: In basic structural engineering language, a simple beam or simply supported system is one that transfers reactions, but not moments, back to the supporting structure.

Consider the above figure, of a simple beam with a pinned and roller support. This is the design principle behind a curtainwall mullion and transom, where the mullion is dead load supported on the bracket only at one end and the other end is allowed to expand (vertically). However, similar to figure above, the system is supported at both ends for wind load. The same principle applies to transoms fixed back to mullions.

Below image represents the principle of fixing for a curtain wall. The hook bracket transfers both DL and WL from the mullion to the slab bracket. In the below condition the system is top hung (i.e DL is at the top), with the bottom being engaged to the panel below for WL transfer.

As discussed in the principle of simple beam design, the panel is free to thermally expand to the bottom, and at the same time accommodate any vertical slab movements and tolerances on the system.