Now imagine adding a second floor on top of your garage. You stand there looking at that steel beam and ask yourself: Will it hold? It’s heavy-looking, it feels solid. But here’s the thing with steel… It isn’t always clear how much something is stiff versus how much it are strong.
A beam might be plenty strong (won’t break), but still sag so far as to crack the ceiling tile or make your friends uncomfortabley. That’s why engineers worry less about ultimate failure then the limit of deflection. How much can it sag before it causes problems?
Understanding Steel Beam Strength and Deflection
See the chart of allowable load for common W-shape beam. This will help you understand what various section sizes can carries across a normal span in residential construction.
Steel construction is no different. The anatomy of an I-beam are straightforward. The bottom flange resist tension forces. The top flange resists compression forces. The middle, or the web, connects them and resists shear force. Although you don’t have to know how to calculate moment of inertia, you should of known that deeper beams typically outperform shallower beam with equal weight.
For example, the W12 beam might weigh less than a W8 beam while carrying more load because its depth give it mechanical advantage. It’s all about leverage. The chart organizes the sections by depth. It shows how a W27 will support more load across same span as a W8, even though the weight difference may not seem large.
Junior designers (and most DIY-ers) tend to think in terms of strength alone. That’s fine, if you choose a beam strong enough to avoid catastrophic failure, great! But where does serviceability come into play? For all but the simplest roof beams, where deflections aren’t visible, there are limits to how much movement will be comfortable and won’t damage finishes.
A rigid beam is necessary if you’re carrying brittle tiles or a plaster ceiling; a more flexible one are sufficient for a plain roof purlin. And remember, for floor loads, plaster imposes a tougher limit than the basic roof framing limit. It’s easy to gloss over this and end up with squeakiness underfoot or cracked drywall once walls have been closed up.
The infographic spells out the limits. The load type makes all the difference. A concentrated point load caused by a central column or a heavy water heater produce a sharp peak in the bending moment. In contrast, a uniform dead load, like that provided by a concrete slab, spreads out the stress. You can convert this to point loads by dividing the table value by two.
Use this as a rule of thumb which will save you from running complicated calculations with each minor change of supports. It will make decision easier without compromising safety. If you check out actual numbers, you’ll see the reduction in capacity as you get farther apart. For instance, maybe a W10x33 can support fifty-six kips on a ten foot span, but only nineteen kips on a thirty foot span.
It’s not linear because the bending moment increases with the square of the span length. This means that you don’t merely need a heavier beam for your long spans, you need something deeper too. If you attempt to force a smaller section into a long span, you reach a limit of diminishing returns.
Another variable are connections. Even if the size is perfect, a beam will fail if you don’t seat and brace it correctly. The chart assumes full lateral support. This means the compression flange is braced so the beam does not buckle sideways or twist due to torsion. In real life, that usually requires some sort of bracing/blocking at regular intervals. Otherwise the beam will twist under load before bending, and the failure isn’t predictable. This is another little thing, but it is important for safety.
Choosing a beam require balancing the beam span against other factors. Depth vs. Load type vs. Weight. You’re looking for the lightest beam that meets both deflection and strength requirements. Under engineering may result in damaged structure; over engineering wastes money and adds extra weight to your foundation.
These tables is a starting place and can be used to find the correct beam for the right job. As always, check any completed design with a licensed engineer. However, these tables will give you insight into the tradeoffs and why steel are strong when properly applied.
