Components of a Beam?

MeyerFire University | FX108.18

By Joe Meyer, PE

RESOURCES

• Notes Page

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TRANSCRIPT

Today, we're continuing on our series covering components of building structure, and discussing beams in more detail. 

We have different types of beams. Beams are usually solid members such as a solid cut of timber, beams could be a built up series of pieces like a glue-laminated beam (or glulam), beams could be forged like a steel beam, or it could be poured in place like a concrete beam. 

The shape of a beam has a specific purpose. A beam typically includes flanges, which are the wide horizontal portion on the top and bottom of the beam. In-between the two flanges is the web portion of a beam. 

In the 1800s with the innovations in steel fabrication came the ability to forge specific shapes that offered better structural properties.  

For instance, creating a beam profile in the shape of the letter “I” allowed a beam to use less and less material yet still be very efficient structurally.  

The taller a web section is, the more it is able to resist the bending forces from a load above.  

Also, the wider that flanges are, the more they are better able to resist compression forces on the top flange and tension forces on the bottom flange. Essentially we use this beam profile so that we are able to use less material yet have strong structural members. 

Construction standards dictate the profiles of beam members that we use today. The most common and most familiar shape for us in building construction is the W shape. This W letter designation designates a beam that has a wide flange. 

Different steel beam profiles have different designations. The W shape that we are most familiar with has a slim web and flange profile were the web and the flanges are roughly the same depth and the inner edge of the flange is parallel to the outer edge.  

When we're looking at structural drawings the key piece that we usually want to know is the depth of a beam. This is an important piece of information so that we can route conduit or pipe around that beam, but also this helps us make decisions for sprinkler spacing, sprinkler height, and smoke detector locations. 

Let's take a look at a designation such as W14x22. The 14 in this designation indicates the depth of the beam in inches, and the 22 is the linear weight of the beam in pounds per foot. So a W 14 by 22 would be a steel W shape beam that is nominally 14 inches deep and weighs 22 pounds per foot. again here that number right after the W designation is going to tell us our nominal depth. If we're looking for precision here, a 14 by 22 is actually 13.7 inches deep, Which can be found from a Google search or if you have your steel construction manual handy in the look up tables for W shapes. 

In Canada, Mexico, the UK and Australia have similar conventions where the first number designation represents the depth of a beam.  

Other steel shapes are created to such as angles with an L designation, W T shapes, which looks like the letter T, rectangular shapes, which have that designation double angles which have the designation 2L back to back ,C shapes which has the designation 2C and a host of others again the most common is the W shape because it tends to be efficient in as a beam and as a column. 

In general, when we use the term beam on a project site we're referring to a solid member. If we have to route something through a beam, the best portion to make that happen is through the center third of the beam. This would be between a third of the way from the bottom and a third of the distance from the top. Why the center third? This area of a beam has relatively neutral forces that are not in heavy tension or heavy compression. 

That being said, we always would need approval from a structural engineer if we're going to be impacting a building structure. It's much more common to route around structure than it is to coordinate precision prefabricated holes within structure. 

We've talked quite a bit on steel, but the same premises apply two other materials. Solid concrete beams, for instance, use reinforcing steel rods at the top and the bottom of the beam to allow that beam to act in compression at the top and intention at the bottom. Without the tensile strength from the reinforcing rods along the bottom, the concrete would crack quickly and have very little to no tensile strength. These reinforcing rods at the top and bottom are there for the exact same reason that a W shape has wide flanges at the top and bottom. The top is able to be resist compression, and the bottom is able to resist tension. 

Wood tends to act fairly well in both tension and compression, So there often is no reinforcing magic on one side or the other period solid wood beams are cut using large timber trunks or they are built up using glue and smaller members. 

One last thing before we wrap up today, and that is the difference between a beam and a girder. The only difference here is how the member is used within a structural frame. A beam will accept a load directly onto that beam from a floor or roof or some piece of equipment above. A girder collects the loads from beams or joists. 

Girders will need to be significantly stronger because they are collecting the weight that comes from all of those beams or all of the joists in an area. Girders often have taller webs in order to give it more strength and stiffness. looking at just the beams that are laid out across an area it's like looking at only half the picture. We have to look at the girders that run perpendicular to the beams and be sure to check the depth of the girders as well. It's a bad day if we design or install completely horizontal sprinkler system I let the same elevation only to get out to the field and find that the girders drop down 8 inches below all of our joists or beams. 

In summary beams can be made out of any of our four principle construction materials for building structure which is wood steel and concrete. When we talk about beams we are usually talking about solid horizontal structural members. And when we're talking about girders we are usually talking about deeper and stronger horizontal structural members that collect the loads from beams and joists. steel beams specifically are identified by their profile and are given a letter designation. The most common is a W shape. 

Lastly because the beams are solid members they affect how heat rises from a fire and effect how our systems respond to that movement of heat. 

I'm Joe Meyer, this is MeyerFire University.