In this article, structural engineer, Mark Duckett provides an introduction to building guardrail failures from the perspective of a structural engineer.
Building Guardrail Failures
Form vs. Function
Guardrail systems are typically required on an edge where there is a 30” differential in height beyond that edge. While their appearance may be conventional, contemporary or artistic, they must be engineered to safely resist the loading that they will reasonably experience in their lifetime. Building codes clearly address this issue by specifying the minimum loads to which guard systems are to be designed.
The Anatomy of a Guardrail
Depending on the application, the top rail and/or handrail is located between 36” and 42” above the floor. Bottom rails (where provided) are generally constructed with less than a 2” clearance between the bottom of the rail and the floor’s surface. Intermediate, vertical rails (balusters) are typically spaced to prevent a 4” diameter sphere from fitting between them – therefore preventing a child’s head from potentially getting stuck between rails.
Guardrail systems are commonly supported/anchored at the floor. The support posts can be directly embedded into the concrete substrate or bolted to the top or side of the floor system. Expansion-type anchors and epoxy anchors are the most prevalent types of anchors used when connected to a concrete slab; wood screws and through-bolts are the obvious choices when connecting to wood members.
Top rails and handrails must be engineered for loading that can occur in any direction. One required design load is a concentrated load (single point load) of 200 pounds at any location. Another example of a code-specified load is the criteria for top rails and handrails to be designed to safely resist a uniform load of 50 pounds per foot. It is important to note that the loads are not required to be applied simultaneously; only to design for that condition which produces the worst case. To put these code specified minimum loads into our perspective, a 200 pound concentrated load is roughly equivalent to the weight of 24 gallons of milk while a 50 pound per foot uniform load is similar to the weight of 6 gallons of milk along EACH foot of the handrail/top rail.
Intermediate rails are often required to safely resist a 50 pound load applied over a 1 square foot area.
In parking garages, the guard systems are required to safely restrain a moving vehicle. An example of this requirement is for the guardrail system to be engineered to safely resist a 10,000 pound load applied 18” above the floor. 10,000 pounds is approximately equivalent to the weight of 2 full size pick-up trucks.
Understanding Guardrail Failures
Guardrail systems most often fail at their connection points, particularly the connection between the bottom of the system and the floor to which it is mounted. Some failure mechanisms can be quite complex consisting of multiple issues while other failures may be quite simple. An example of a “simple” failure would be one whose anchor bolts were installed too close to the free edge of the concrete substrate, resulting in inadequate structural support for the minimally prescribed loads. Other failure mechanisms may include poor welding, spalled and cracked concrete, faulty initial design, deteriorated wood or just inadequate maintenance.
STRUCTURAL ENGINEERING CASEWORK
Structural engineering casework takes many forms, some of our most notable casework involves the collapse of concert shells, parking garages, and buildings adjacent to construction sites; however, not all of our structural casework involves the collapse of a building. Some of our most interesting casework involves the collapse of billboards, fire escapes, and bleachers. Our structural engineers identify the causes of structural failure in cases involving professional liability, construction defect claims, and personal injury.
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Mark Duckett is uniquely qualified to understand guardrail systems and their failures drawing upon his experience of more than 30 years as a structural engineer. Mark has designed numerous guard systems comprised of steel, aluminum, concrete, masonry, wood, glazing (glass), barrier cables and other materials. In addition to engineering numerous guard systems, he has co-developed design software for the embedment of structural guardrail posts into concrete, created stair shop drawings for fabrication, designed a protocol test for the acceptance of a “questionable” in-place guardrail and has inspected hundreds of guardrail systems.
Mark is licensed as a professional engineer in multiple states and as a Special (Threshold) Inspector in Florida. Mark is a member of the American Society of Civil Engineers.