Roof systems play a crucial role in protecting buildings from the elements, with one of the primary challenges being the accumulation of water. Excessive water accumulation, or ponding, can lead to structural instability, compromising the integrity of the building. Designing roof systems to effectively manage rain loads and resist ponding is essential for ensuring the safety and longevity of structures.
There are principles when it comes to the designing of roof systems; mainly to avoid or resist water accumulation when focusing on roofs constructed with structural steel, open web steel joists, and joist girders.
Rain loads and ponding effects
Rain loads refer to the vertical force exerted by rainfall on a structure’s roof. The American Institute of Steel Construction (AISC) provides guidelines for calculating rain loads based on factors such as geographical location, roof slope, and rainfall intensity. By accurately assessing rain loads, designers can determine the necessary structural capacity to withstand the imposed forces.
Ponding occurs when water accumulates on a roof surface due to inadequate drainage or structural sagging. The weight of the accumulated water can exert significant downward pressure, leading to deflection and potential structural failure. Understanding the factors contributing to ponding, such as roof slope, drainage system effectiveness, and material properties, is crucial for designing resilient roof systems.
Figures courtesy AISC
Design considerations for steel roof systems
Structural steel, open web steel joists, and joist girders offer robust solutions for roof construction, but they require careful consideration to mitigate water accumulation and instability. The Steel Joist Institute (SJI) provides valuable resources for designing steel roof systems, including recommendations for joist spacing, bridging requirements, and roof slope limitations.
To design for ponding on roofs constructed with steel, several key factors should be addressed:
Roof slope
Increasing the slope of the roof helps facilitate water drainage and reduces the likelihood of ponding. A minimum slope is recommended to ensure adequate water runoff, as specified by industry standards.
Drainage systems
Effective drainage systems, including gutters, downspouts, and scuppers, are essential for directing water away from the roof surface. Regular maintenance and inspection of these systems are necessary to prevent clogs and ensure
proper functionality.
Structural analysis
Structural engineers must conduct thorough analyses to assess the capacity of steel components to withstand the additional loads imposed by water accumulation. Factors such as deflection limits and material strength properties should be considered in the design process.
Ponding design criteria
Adhering to established criteria for ponding design is a factor for designers. These criteria specify maximum allowable ponding depths and durations to prevent structural damage and ensure occupant safety.
Designing roof systems to avoid or resist water accumulation is critical for maintaining structural stability and prolonging the lifespan of buildings. By incorporating principles outlined by organizations such as the AISC and SJI, design professionals can develop robust solutions for managing rain loads and mitigating ponding.
Through careful planning, adherence to industry standards, and ongoing maintenance, buildings can withstand the challenges posed by water accumulation and remain safe and functional for years to come.
Expert advice
The AISC and SJI recently published a comprehensive reference volume for rain loads and ponding design, Design Guide 40, Rain Loads and Ponding.1 The guide provides guidance for the design of roof systems to avoid or resist water accumulation and any resulting instability. It includes an in-depth review of rain loads and ponding effects to help design professionals design for ponding on roofs constructed with structural steel, open web steel joists, and joist girders.
The guide also contains several recommended methods of analysis that can be used to consider the effects of ponding, methods of design accounting for ponding effects, and a presentation of the SJI Roof Bay Analysis Tool, along with design examples that illustrate concepts.
According to Mark D. Denavit, PhD, P.E., associate professor civil and environmental engineering University of Tennessee, Knoxville, who co-authored the guide along with James M. Fisher, PhD, P.E., Dist. M.ASCE, a few factors compelled them to write the guide. “The primary motivation for the guide was new provisions in ponding ASCE 7, the standard that defines loads for structural engineers. The new provisions are good in that they require a more realistic evaluation of ponding, but they also require analyses that many engineers are unfamiliar with. We wanted to help engineers get it right and
do so efficiently.”
Talking points
While the comprehensive guide covers a lot, Denavit says there are points the authors hope readers walk away with.
“One of the main points is that ponding is a real load, which was not obvious from the old ASCE 7 provisions. Another is that ponding isn’t going to control the design of every roof and evaluating ponding under the new provisions doesn’t have to be hard.” Denavit adds demands on the roof can actually go down under the new provisions, and the SJI Roof Bay Analysis Tool makes ponding analysis easy.” He also stresses along with instability, issues such as collapse, or partial collapse of the roof are factors. “This is obviously a safety issue, but the damage is very costly too.”
Denavit says one common misconception is if you have a one-quarter on 12 slope then ponding is never a problem. As well to keep in mind, “Designing roof systems to avoid water accumulation and instability is multi-disciplinary problem. Architects, structural engineers, and plumbing engineering all have a part to play. Good design team coordination, including understanding each other’s concerns, helps the process be successful.”
Notes
1 aisc.org/Design-Guide-40-Rain-Loads-and-Ponding