Different Components of Building Structure?

MeyerFire University | FX108.14

By Joe Meyer, PE

< PREVIOUS

NEXT >

RESOURCES

FX151 SERIES

RESOURCES

NOTES

• Notes Page [PDF]

FX151 SERIES

1. Different Construction Types in the IBC?
2. What are different types of structure?
3. Components of a building structure?
4. Different types of foundations?
5. Different types of joists?
6. Components of a joist?
7. Components of a beam?
8. How do beams affect sprinklers and detectors?

---

TRANSCRIPT

What are the different components of a buildings structure?

COMPONENTS OF STRUCTURES

Now, in the last video we introduced the different types of building structures that exist in mainstream construction. In this video, we're going to talk about the different components that go into a building structure. How do we identify them? What are they called? From there, we'll be able to talk about different materials and pieces that we see constructed in our buildings today.

TERMINOLOGY

So, let's start with some structural terminology. 

In the first diagram here, we identify the different parts of the building structure. Let's start with the floor level and work our way down to the bottom of the structure as the load travels. So, in this first diagram, we have a floor which is a concrete slab. We have a joist, we have a girder, a column, and a foundation. 

 In our second example here, we have a floor again, but this time we're going into beams, then a girder bearing walls, and then a foundation. Now, before we dive into what each of these terms mean, let's talk about the differences between a joist, and a beam, and a girder. 

Now when we're talking sprinkler shop, we like to call everything that is horizontal either a beam or a joist. Structural speaking, that's technically not accurate. For most purposes, works just fine, but let's look at the structure behind it and what the definitions mean. 

BEAM

So, let's start with a beam. 

What is a beam? A beam is a horizontal structural member that takes the weight of a floor or roof and transfers it horizontally. It's taking a vertical load, transferring it horizontally. A beam can frame into a bearing wall, a girder, or a column. A beam is usually considered to be a solid member, whether that's dimensional lumber or maybe a steel W shape. When supporting the same load from a floor for example, beams would usually be less in quantity than joists. 

JOIST

A joist is one type of beam. A joist is the structural member that takes the load from a floor or roof and transfers it horizontally. The joist is bearing the direct weight from the floor or roof.  

A joist will then frame into a building girder or a bearing wall. The load is transferred from the floor or the roof to the joist, and then to the girder. Joists will have short spacing, meaning they’re group closely together, which is usually somewhere between one and three feet apart, and there'll be numerous in quantity. Normally, joists are not isolated by themselves. There’ll be at least a handful of them to spread out that load. 

Joists are usually smaller and slimmer and more in number than the use of beams. Joists can be solid. They can be engineered but still solid, or they could be trusses where they have open webs. 

In sprinkler design, it's important to understand what a joist is because it impacts where we route pipe and also where we're gonna get sprinkler obstructions on the sprinkler itself. 

RAFTERS

So still talking about beams. Sometimes the term rafter is used. Technically, a rafter is a slope structural member and is a type of beam. It usually runs down the slope of a roof. Just like a joist, rafters are numerous and are supported at their bottom by a bearing wall or a girder. 

In common construction lingo, rafters are mostly associated with wood construction and attic trusses. Framers, for example, or roofers, they're very familiar with the term rafter and rafter is that single member that's running down the slope supporting a roof. 

PURLIN

So again, on the topic of beams, what is a purlin? Purlins crop up all the time in pre-engineered metal buildings. Pre-engineered buildings are the less expensive metal structures that are readily sized and fabricated on an economy level. Purlins are the members that run between rafters and provide some stiffness for the roof, as well as an area to attach the roof. Purlins are significantly weaker than a joist or a beam. They are also prone to twisting, so when we're hanging from purlins, we typically are more concerned about our structural attachment up at the top. 

They are also more prone to twisting. So when we're hanging from purlins, we typically are more concerned about how we attach to them, what their thickness is, and what they're capable of taking on. 

GIRDER

What is a girder? A girder is a larger horizontal member that picks up loads from beams. Now the beam could be a joist, or it could be a rafter. Either way, a girder is collecting those loads and transferring them to a column. Girders are usually significantly larger than a beam or a joist because they're collecting loads from all of those beams or joist. 

They're often thicker and deeper than a beam. Girders in many cases are solid deep steel members. For sprinkler design, girders can present significant challenges for routing pipe because their depth is usually lower than all of the other structure in the area. And unless we coordinate it, we're typically not running through them. 

So, what's the difference between a girder and a beam? Well, a girder is picking up the load from beams. Beams don't frame into other beams, beams frame into girders. 

TRUSSES

I've heard the term truss. What is a truss? Well, a truss is a type of assembly of components that takes weight in an efficient and lightweight manner. When we have an attic truss, we might be referring to a wood truss that we see a lot in residential construction. If we have a large open space using steel construction, well then a steel truss might allow for larger open spans without columns below and have open webs so that we can route systems through the truss. 

COLUMNS & LOAD BEARING WALLS

So that makes up the majority of the horizontal members in a structure, but the weight from those horizontal members has to be transferred vertically down through a building. When we take a structural member and orient it vertically, we call that a column. Columns represent a load transfer down through a single point. 

If instead of a single point, a load is transferred downward along a wall. We would call that a load bearing wall. The wall is bearing the weight of the structure or the load from above. In home renovation, it's a big deal when a wall is load bearing or not load bearing. Every time somebody wants to take an old home and make it open concept by removing a wall, they have to first figure out if it's load bearing or not. A wall is load bearing when it's responsible for taking the weight from above and transferring it down below. A wall is considered not to be load bearing when it is only there to separate a space, but it's not responsible for taking any weight from above. 

FOUNDATION

In a commercial office building where each floor frames into joists which then frame into girders and go down into columns, well, the weight doesn't bear on each individual partition wall. If a new tenant comes in and needs to reconfigure the partitions within a space, structurally that’s not that big of a deal because each individual partition isn't bearing the weight of the floor above. 

So once our load is transferred vertically down through a building, the load has to be picked up and transferred to the earth. This portion of the building is called the foundation. There's a good handful of different ways in which we can transfer the load down into the soil. Perhaps one of the most basic ways is to have a footing, which takes a vertical load and spreads it out horizontally into the ground. If a column is bearing on a point down to a foundation, we might see a spread footing or a pad beneath that column. When we have to take a large load or big weight and transfer it at a single point, these pads have to get deeper and wider so that the weight can be transferred to the ground. Soil can only take so much pressure. We define the strength of soil as the bearing strength or a bearing capacity, which is measured as a weight divided by area. 

If soil in the area has a low bearing capacity, then our footings need to be significantly larger so that they distribute that load into smaller densities that the soil can handle. Think about silt and soft clay. They can accept significantly less weight per unit area than a very dense or a rocky soil. 

If our foundation is not taking a point load but instead that load is distributed along a wall, then we would have a continuous or wall footing. 

Depending upon the soil and how much weight is going down needing to be transferred through a building, foundations can get a whole lot more complex. Piles, mats or piers may be designed to transfer that load down to the earth. 

FOOTING

For our purposes, when we're coordinating underground pipe, we're usually concerned with avoiding the footing. We can't run a pipe horizontally through the footing itself. Otherwise, we might compromise the ability of the building to bear its own weight. We could also easily compromise the pipe. 

In my experience in talking with structural engineers, we can't run horizontally through a footing. We don't wanna compromise the pipe. We don't wanna compromise the building. Our options are either to go through the foundation wall, which has little, no impact structurally, or go beneath the footing all together.  

The tricky part we run into and it kind of a window is the frost depth. The footing has to be below the frost depth. That way the freezing temperatures in the winter don't heave the building upward, but then our pipe also has to be below that frost depth. So, if the footing's especially deep, well, we can run through the side of the foundation wall and still be below our frost depth line. But if those footings are just barely below frost depth, then we don't really have a choice. We've gotta run that underground pipe all the way under the bottom of the footing. 

While we're talking about this, we also consider we don't really want fittings to be on the bearing area of the building. We don't want our fittings to have to take the weight of the building. So, we try to locate fittings away from that zone, where the soil is transferring the way to the building into the ground. 

SUMMARY

As far as an overview for structuring components, we have floors, we have joists, beams, girders, columns, foundations, and bearing walls. As far as an overview, those are our main components. 

As far as an overview for structural components, that's about the rundown for typical parts of a building structure that we see in construction today. We'll go into more detail about how each of these components are constructed, what the rules are around them and why we pay attention to them in terms of building system design in future videos. 

I'm Joe Meyer, this is MeyerFire University.