What is the maximum allowable deflection for vertical mullions of store-front framing under design load?

The maximum allowable deflection for vertical mullions of store-front framing under design load is typically specified as span divided by 175. This standard is put in place to ensure both structural integrity and aesthetic performance. The logic behind this specification lies in balancing the ability of the mullion to support loads without excessive bending while also maintaining the visual appeal of the installation. Excessive deflection can lead to problems such as difficulty in maintaining seal integrity, glass breakage, or poor performance under wind loads.

Span divided by 175 means that as the length of the mullion increases, the allowed deflection proportionally increases, which accommodates larger structures while still providing a safe and functional design. This guideline also aligns with industry standards and helps engineers and contractors make informed decisions about material selection, design parameters, and installation practices.

This approach ensures that the storefront framing remains safe, effective, and visually pleasing, ultimately contributing to the longevity and reliability of the building's exterior.