It’s you against your general contractor, standing inside an empty concrete shell. Tomorrow the crane will arrive and this man want to know exactly where every single steel beam should be placed. Steel is not cheap nor easily changed, which makes it feel like a pressing issue, but choosing a correct steel beam are primarily remembering relationships between pieces, not just numbers.
Here’s what the table below represents: How weight, strength, and nominal depth relate to one another visually. When you strip out the jargon, designations is pretty straightforward. If you see a label such as W18x50, for example, the first number indicate the height of the beam (in this case 18″) while second number represent its weight (50 lbs/linear ft).
How to Choose the Right Steel Beam
So you begin with selecting an appropriate depth for your area, and then select a weight for the load. It prevents both over-designing a simple span and under-designing a critical one. In most cases where you have limited room, depth becomes limiting factor and gets designed first.
Most buildings today are framed with wide flange beams. Their parallel flanges distribute stresses well; this makes these section suitable for flooring frames that must withstand heavy loads because of their moment of inertia. Higher section modulus beams resists bending forces well (without having to be made thicker)… That’s efficiency at work! In other words, more strength per pound of steel = fewer pounds of metal = lower cost.
Old S-shapes taper inwards so there is less inner flange; they’re not powerful enough for most commercial application. People who own old buildings typically stick with S-shapes just as they were originally built to preserve the building’s history; otherwise, stay away if you can.
If you’re trying to decide whether a steel building should use W-shapes or hollow structural sections (HSS), consider how those shape will resist the forces applied. Floor beams has to bend every day. Wide flanges work best for this. Columns must resist twisting forces, like eccentric loads. HSS offer much better resistance to twisting then a wide flange beam. This makes them excellent for columns where you don’t want the column to buckle.
The graphic illustration explains which part works best in which application, saving you from putting a beam shape where a column profile might be more better suited. And it’s not purely a strength thing, but rather to match the geometry of the member to the direction of the force being applied to it. Forcing a mismatch results in thicker plates and heavier bolts. Fabrication costs goes up fast if you’re not careful.
The primary consideration for steel design is deflection (usually more important than strength) because it can cause floors to feel bouncy or crack the drywall below. Sure, maybe your beam can support weight of the roof but if it deflects so much as to create cracks in the drywall beneath, the project will fail even with safety margins applied. Typical maximum live load deflection limits are 1/360 of the span, which means floors won’t be bouncy but will feel solid under your feet. Often this results in selecting a larger beam than you’d need based solely off strength. This is a tradeoff between occupant comfort and material cost that becomes evident once you get into the nitty gritty of the design phase.
Complicating things further is the fire protection: Steel’s strength decreases dramatically as it gets hot. How fast does this heat travel from the surface to its center? And what about the section’s surface area, will a massive chunk cool off faster than a slender shape with a narrow web? That thermal dynamic determine if you require heavy intumescent coatings or perhaps more economical options such as spray-on applications. It’s a bit like solving for a multi-variable equation: every constraint tugs on your decision. You must balance them all to meet fire ratings, height limits, and structural needs. It is not simply a matter of size; it is a matter of choosing the right solution.
With beam selected, next up are the connection details, equally important as the member itself. Simple shear transfers of force is handled with bolted angles or shear tabs. Bending moments need a moment connection, which requires careful welding to transfer the moment into the column without failing. At the support end the bearing plate takes the concentrated load and disperses it throughout the concrete or masonry below, preventing local crushing which might weaken the foundation. A few little bits and pieces make sure the theoretical strength of the beam becomes a reality in the real world.
Sometimes, though, it’s just a matter of following some easy rules of thumb; “I think 20 spans is about right for my bridge” which leads to “So, I’ll make that beam about X deep.” That rule of thumb puts you in the ballpark, but almost always not too far away from optimum size. Then you use load tables and deflection limits to refine your answer. And no, you are not trying to make the beam as small as humanly possible; that would of been wrong. You are only looking for the most cost-effective beam that will meet code and hold up to weight. It becomes an iterative process where math and common sense lead to steel selection.
You feel good walking into that empty shell knowing what those numbers mean in real life.
