Detailing For Distance

The Vertical Split Mullion Connection



Photo of Daniel Kelso

Daniel Kelso

Former Design Modeler of Enclos, Advanced Technology Studio



Quickly exceeding new facade service life expectations for tall buildings imposed by increasing environmental, economic, and social pressures have created a new need for curtainwall renewability. While modern unitized curtainwall is very durable, it currently lacks many major renewing capabilities. Various forms of retrofit have been proven successful enough, but it could be far greater anticipated and planned for within the design details of new construction. Considering standard unitized curtainwall, the method of study will be an incremental approach that targets one detail at a time for intensive research and development. The first installment of the initiative will focus on vertical split mullion connection between adjacent units. Data will consist of research on the system designs currently existing, then cross-analyzed with the existing relevant performance criteria. It will then be evaluated by various newly established design criteria for retrofit compatibility. Detailing development was done with the effort to preserve successful existing design strategies while integrating retrofit compatibility. Subsequentially the new vertical split mullion connection detail featured an invented key component, the anti-buckling toggle. After much development, a prototype was made for an initial proof of concept.


Due to increasing environmental, economic, and social pressures, there is a greatly increasing expectation for façade service life and a new necessity to use building materials far more sparingly and

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Anticipating Retrofit

The need for retrofit has become increasingly evident by the many older curtainwall buildings that have reached or are approaching their inevitable end of service life due to either

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Incremental Approach

The effort to begin solving the resultant challenges will be done in incremental steps. One great approach begins by selecting a single small scope of modern curtainwall design detailing

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Scope of Study and New Design Criteria

New Design Criteria Overview

In response to greatly increasing service life expectations, it is crucial to identify opportunities for improvement objectively. There are a couple of relatively unsolved design challenges of

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Existing Design Criteria

Performance Expectations

There is a handful of existing design functions inherent to the vertical joint between adjacent units listed below.

A tight interlocking fit to ensure proper primary air and water barrier

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Compiled data to further investigate the relationship between adjacent unit vertical split mullions will be a matrix of six common existing design detail strategies overlaid by depictions of how they

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Air and Water

Details “A”, “B”, “C”, “D”, and “E” all have coupling elements that create a tight engagement so that the primary air and water barrier can be ensured to

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Through iterative sketching, modeling, and conferring with other facade designers, a vertical mullion system with rear accessible anti-buckling toggles was created. The primary design characteristic of the system is that

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Comparative Costs

By nature most novel component design is bound to have increased upfront costs due to added design and testing time. There are also a few added construction costs for

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The Advanced Technology Studio of Enclos Corp.

Rights and Permissions

All images and photos in this paper are by the author.

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