Climbing Higher: Considerations for Rooftop Railings and Windscreens

By Stewart Jeske

As glass becomes increasingly popular, we see more use of structural glazing at the tops of buildings. Applications include penthouse office suites, private condominiums, restaurants, pools, gardens, and open entertainment areas. Glass is an integral part of these areas because people want a sense of awe and thrill—no material does that better.

Unique Installations

Glazing contractors and architecture-engineering (AE) design teams aren’t always versed in the unique engineering criteria of exposed glass railings and windscreens at rooftop and balcony locations. These applications require significant consideration of windload resistance.

Why are windloads greater on the rooftop? In highrise locations, the wind is squeezed between buildings and speeds up to get around and over building obstructions. At high altitudes, there is less ground obstruction to slow wind velocity. Engineers have conducted wind tunnel studies for many years because of the impact of wind and taller buildings. Structural engineers understand this, so building code requirements include special wind criteria for rooftop structures and parapet walls. Unfortunately, the building code does not delineate design wind pressure for glass railing and windscreens.

In the Details

In a recent project, the AE design team did not have a sufficient structural base to attach a windscreen shown in the contract documents. The structural base consisted of a thin 6-inch concrete curb extending 12-inches above the finished roof. The glass windscreen was about 6 feet in height. Many engineering iterations tried to get post-installed anchors from the  windscreen into the concrete curb to no avail. The structure was deficient to support high loads from the windscreen. Just as comparison, the loads on the glass windscreen for each support were about three times the design load for a cantilever diving board with a 250-pound person bouncing on the end. Thinking about the loads in this comparison drives home the need to have significant structural support to anchor windscreens and glass railings.

Guardrails are an essential consideration when discussing windscreen applications. Code-specific guard requirements are applicable if the windscreens need to prevent a fall from a height greater than 30 inches. Windscreen panels or supporting posts may also support the guardrails, designed using the building code’s load requirements. If the designer does not wish to add guardrails, the glass panels must support the required guard loads and other loading requirements.

While architects love glass railings as beautiful and ornamental features, they are also crucial to the safety of the building occupants. If not caught, mistakes could cost you a lot of money and delays, if not worse.

Stewart Jeske is the founder and president JEI Structural Engineering in Kansas City, Mo.

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