Stress Analysis is Paramount

Quoting Open Shape Storefront Systems

By Stewart Jeske, Kyle Roberts and Javier Torres-Goitia

As states continue to adopt the 2015 International Building Code, it’s important to note its differences from past editions. One major difference is the requirement for engineering to conform to the 2015 Aluminum Design Manual (ADM). The ADM is the governing manual for manufacturers and engineers in analyzing the stress of mullions and miscellaneous aluminum parts.

The complexity of stress analysis of open shape mullions (see figure 1) and components for wind is significant. Until the 2010 version of the ADM, a far simpler mathematical approach was al-lowed when determining the stress capacity. Part of the simple approach for determining bending capacity allowed the use of a shape property known as radius of gyration. The 2015 changes to the ADM eliminate the direct use of radius of gyration and require complicated and tedious math and integral calculus to arrive at solutions for stress bending capacity. These changes push designers and engineers to an alternative allowed in the 2015 ADM known as the Direct Strength Method. This requires finite strip analysis, which uses specialized software to model the mullion’s complex cross section. This approach is significantly more accurate and much less conservative than the manual alternatives.

Failure Modes

There are two types of failure modes that engineers need to account for when analyzing vertical mullions of store-front or curtainwalls: lateral-torsional buckling (figure 2) and local buckling (figure 3). Lateral-torsional buckling is covered by section F.4 of Part I of the ADM (2015 edition) and can be loosely described as a lateral twisting and dis-placement of the mullion. This failure mode depends not only on the section properties of the mullion, but also on its unbraced length. The unbraced length in a vertical mullion is the vertical spacing between horizontal mullions, which offers bracing points for the vertical mullions. The longer the vertical span between the horizontal mullions, the lower the capacity of the vertical mullion.

The local buckling failure mode in Figure 3 involves localized buckling of any individual elements when the mullion is loaded in bending. Local buckling can be described loosely as small elements of the mullion deforming under load. The capacity of open shape mullions is usually governed by this failure mode, rather than lateral-torsional buckling. Although not necessarily catastrophic, local buckling failure can cause glazing gasket ruptures or tears in the perimeter line of sealant, which can lead to permeability concerns. Vertical open shape mullions are often unsymmetrical, with complex shapes and very thin wall sections and stiffeners, as shown in figure 1.

As a result, the 2010 and 2015 ADM’s chapter B.5 provision for local buckling stress checks can become a time-consuming process for the glazing engineer who must find the capacity of each of these elements. More so, the intricate nature of the mullion’s cross section geometry includes curved elements such as screw chases, glazing gasket chases, filler snap legs and thermal break cavities that cannot be fully accounted for when using the ADM’s manual equations. The capacity of these mullions is regularly underutilized by the ADM’s manual equations and rarely replicates tested values. This makes the use of a more accurate and comprehensive approach, such as finite strip analysis as allowed by the Direct Strength Method, well justified.

Understand the Details

There are numerous software options for the Direct Strength Method, but all follow the same procedures. They involve the engineer modeling the shape manually with as much detail as practically allowed. The finite strip analysis software provides the peak stress in the mullion, taking into consideration the interaction of all cross-section elements. The math and physics of finite strip/element analysis are complex, but it’s helpful to understand that a finite strip model allows the investigation of how the stress is distributed among the various parts in the mullion’s cross section. This allows the engineer to extract the necessary parameters required by the Direct Strength Method for buck-ling analysis.

One we often hear is, “What is the required ‘I’ value for the mullion?” The problem is that “I” values (moment of inertia) are not directly related to al-lowed bending stress, but only to deflection of the member. The moment of inertia about the mullion’s strong axis, Ix (that’s typically included in manufacturer’s load tables) doesn’t play a role in determining the general flexure or bending capacity of these open shape mullions.

The belief that all engineering is considered in manufacturer load tables is a misconception. Sometimes curtainwall and storefront manufacturers publish tables that contain the general flexural capacity and deflection without considering lateral-torsional buckling or local buckling. Their charts are not designed to be a comprehensive engineering analysis, but are broad guidelines to help with estimating and system selection. This creates a challenge for glazing contractors if they place too much weight on the value of manufacturers’ charts.

Communication between the design team and the glazing engineer as early as possible in the project will ensure that the glazing systems being quoted or installed meet the requirements of the current building code. Early discussions among all parties have the potential to save extensive time delays and high change order costs. As engineers, it is much more time efficient to perform a preliminary quick check on a mullion at the bidding stage. This helps avoid costly changes after the project is already detailed. Except for building movement requirements, the challenge of open shape parts is perhaps the most common bottleneck in the delegated engineering analysis and design of storefront and curtain-wall frames.

Every project is different, with unique characteristics, challenges and opportunities. However, early communication can provide a smoother process on all projects and prevent hidden challenges, such as the evaluation of open shape mullion capacity.

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