Steel decking provides strength and support for the floors and roofs of many buildings. Understanding the properties of steel decks is a requirement for engineers, architects, and steel fabricators who regularly work with the material.
What are section properties?
Section properties tell us how strong steel decks are. There are two main ways to calculate these properties: the effective area method and the direct strength method. The effective area method is used most often. It looks at how much of the steel is actually doing work under certain conditions. The section modulus is used along with the material’s yield strength (how much weight it can hold before it stretches) to figure out the deck’s bending strength. Another important property is the moment of inertia, which helps us understand how much the deck will deflect under load instead of weight.
Effective area method and steel thickness
When figuring out section properties, the widths of flat parts of the deck are sometimes not effective along the entire width. This is part of the effective area method, which looks at how much steel is actually taking the load. Stronger steel needs less area to be effective, so the section properties often decrease as the yield strength increases. This does not necessarily mean higher yield steel decks can carry less as the higher yield strength will offset the lower section modulus for the strength calculation. The Steel Deck Institute (SDI) gives a list of thicknesses for different gages of steel. For example, 22-gage steel is 0.0295 inches thick. Additionally, it’s important to only consider the thickness of the base metal, not the coating on top. The SDI also allows the thickness to be 95% of the listed size to allow for some variation from the steel mills.
All SDI standards include a referenceable chart to help with these calculations.
How section properties affect load capacity
Effective section properties, like the moment of inertia and section modulus, help us discern how much load the steel deck can hold without it yielding or deflecting beyond acceptable limits. The grade of steel (how strong it is) affects these properties. Higher-grade steel, like 80 KSI, might have slightly smaller section properties than 50 KSI steel because it needs less area to be effective under load. But the overall strength is still good because higher-grade steel is stronger. The widths of flanges (flat parts) on the deck also matter. Wider flat sections might need stiffeners to strengthen the section and reduce the likelihood of buckling. Proper design and reinforcement make sure these sections can hold weight well.
The Engineer of Record should consider the overall strength and serviceability of the roof and floor deck instead of simply setting minimum section properties. For a more technical explanation on this, please see The SDI Technical Note No. 19 – Steel Deck Properties at Varied Yield Strengths: An Explanation for How to Evaluate the Differences.
Understanding section properties is essential for anyone involved in building with steel decks. Engineers, architects, and steel fabricators all benefit from knowing these details to ensure their projects are safe and reliable. The Steel Deck Institute is pleased to make steel deck guidelines accessible to professionals in the field who take the design of strong, durable steel decking systems seriously. Keeping this information in mind will help you make informed decisions and create structures that stand the test of time.