Components of Conventional Steel Structure.
Welcome to the first segment of Part II in our Building Structure series.
In Part I of this series, we covered construction types, introduced components of building structure, and got into specifics with foundations, joists, beams.
In Part II of this series, we’re going to look at structures on the whole, reinforce some common construction terms and language around different types of structure, and talk about how buildings physically come to be, and talk about combustibility as well with structures.
So, for today, why use steel? What are Steel Structures? How are these structures assembled? What are the different parts of a steel building?
STRONG TENSION & COMPRESSION
Well, steel is often a material of choice for buildings because it has high tensile strength and compression strength. Meaning, it’s strong in both tension and compression.
For its strength, it’s relatively lightweight. This is very important when building taller buildings, because the weight of the structure itself begins to contribute more and more to the overall weight of the building. The more weight – the stronger the structure needs to be to support it. Steel fits this bill well and the development of steel in combination with the electric elevator are the two key reasons why engineers were able to create the original skyscrapers back in the late 1800s.
Well, today, at least in the United States, steel is still a common material of choice for lighter buildings and very tall buildings due to their strength and weight properties.
So when we say “steel structure”, what do we mean?
Well for one – we don’t regularly say that a building is a “steel building” versus a “concrete building”. The most common way we would describe building structure is by calling it say, a “non-combustible” or “combustible” or “wood” structure.
And non-combustible structure could include steel, but it could also include concrete, or could a combination of the two.
Combustible structures today generally means wood structure.
Some buildings, such as Type III buildings in the IBC, have combustible interiors with non-combustible exteriors.
That leads to our second way of describing building structure, and that’s by the formal Construction Type.
Now, Construction Type is defined in the Building Code. We went through this in part one of the series, right at the beginning. The building code could be the IBC or say NFPA 5000. A quick recap, Type I and II are non-combustible in and are usually concrete or steel structures. Types III, IV, and V have combustible structure on the interior, with type IV being heavy timber construction that uses those large solid wood members.
STEEL STRUCTURE DIFFERENCES
So, let’s say we’re told that we have a “non-combustible” structure, or say a Type II-B office building that’s a two-stories tall and uses steel members. Well, are all steel structures the same?
No. Practically speaking, we can categorize steel structures in two ways. One is conventional steel buildings, which we’re covering today, and the other is pre-engineered metal buildings.
Conventional steel structures, pre-engineered metal buildings. Both of these structure types primarily use steel as the way to support the weight of the building, but they are designed and built a little bit different.
We’re gonna cover again conventional steel structure in this segment.
So, conventional steel buildings are usually built with steel in the horizontal and vertical structural members.
Let’s follow the load path as if we’re standing on an upper floor.
Well, as we’re standing, our own weight is applied to the floor below us. This floor could have carpet, could have hardwood, maybe tile, whatever kind of finish.
This finished floor, just as side note, is a key point of measurement in our buildings. When you see AFF, the letters A-F-A, on a drawing, that stands for “Above Finished Floor”. So, 10 feet AFF, that's 10 feet above the finished floor. Technically, that's the top plane of our finished floor level.
Well that finished floor, whatever material it is, it works its way down to a subfloor or a floor slab. For steel structures, a floor slab is usually going to be concrete. Concrete provides some strength, it provides some isolation for noise and vibration between floor levels, and it has some fire-resistive properties.
There are many combinations of subfloor and substrate materials between a finished floor and that floor slab. If you're curious about the details on what's going on with your project, look specifically in the architectural details and architectural sections, which identify these different layers and what stacks on top of what. That's usually in the middle of the architectural set of plans, and it'll be labeled as interior sections or details.
FLOOR SLAB SUPPORTS
So, we get down to the concrete floor slab. The concrete floor sub itself is not necessarily strong enough to span large distances. Think about it. Owners and architects really don't want columns to show up every 6 feet or every 2 meters throughout every room. That'd be awful and it'd create a lot of unusable space, but that's what we would have if we only had a 3 to 4-inch concrete floor slab without any supporting structure below it. So what goes below that floor slab?
A relatively thin floor slab, even with something like reinforced concrete, doesn’t have a whole lot of strength from bending forces. So, if you were to imagine a big, long concrete slab, that's 30 feet wide. Well, if you put a lot of weight in the middle, it's gonna pull down almost like a mattress cover. It's gonna deflect and go lower and lower towards that center. It just doesn't have a lot of bending strength.
So, because of that, we support the underside of a floor slab with beams or joists. Now, we introduced joists and beams earlier in Part I and went into some detail on those, so see Part I on this series if you haven’t already. We generally refer to solid horizontal structural members as beams, and we generally refer to open-web horizontal members as joists. Now beams can be called “solid beams”, they can be called “I-beams”, “w-shapes”, and I’m sure a whole lot of other names. Joists can be referred to as “open-web joists”, “bar joists”, “k-series joists”, “trusses”, and handful of other names too.
Either way, a beam or joist is what’s gonna take the weight from the live load that’s on top of the floor slab, and the weight of the floor slab itself, and the weight of the beam or joists and transfer that all horizontally. It’s moving those weights, those forces, and concentrating them out towards the ends of the beam or joist.
For steel buildings, again, these are steel joists or steel beams that will transfer these loads.
BEAMS AND JOIST FRAMES
So from here, these beams or joists are gonna frame either into a girder, directly into a column, or frame into a bearing wall.
For conventional steel structures, we’re usually gonna see these beams or joists frame into a girder. Now remember a girder is a type of beam that collects loads from other beams or joists. Girders are usually deeper and stronger than a typical beam.
Instead of a girder, we could also see a bearing wall. Metal studs do provide some strength, so they can bear weight. But more commonly though, if we’re in a steel structure building, we’re probably going to want large open spaces with more flexibility in the future on where walls need to be. We wanna keep large open spaces. We want the structure to take the load, not a bunch of interior walls. To accomplish that kind of flexibility, we're gonna use girders that pick up the loads from beams and joists instead of having big bearing walls.
Now, from here, the girder will frame into a column. A column is a vertical structural member that is taking all those loads and running them down to a single point.
For steel structures, these columns are often in the shape of a capital letter “I”, or more properly identified as a steel “w-shape”. W is the designation that a, you know, I-beam shape has. These columns could stand on their own, they could be fireproofed with spray fireproofing around all sides, or they could even be enclosed with a gypsum enclosure that’s around all sides to that beam. It really depends on the look that’s trying to be achieved on the inside of the building, and whether the columns are required to have a fire resistance rating. The fire resistance rating for a column really depends on the building's construction type. That's again, going back to that construction type that we find in the building code.
At the base of the column, or at the base of a load bearing wall, that’s where we’re gonna have the footing. Even in a steel building, this weight transfer is now gonna go back to concrete for the footing. We discussed this in a lot more depth in Part I, but the footings take those vertical forces and distribute those forces out into the soil beneath a building. They bear weight on the soil, hence the term “bearing strength” for different types of soils.
So, in summary, what are the key components of steel structure?
Well, we have floor plates or floor slabs. These accept the “live” loads from the building and are supported themselves by steel joists or beams.
The joists or beams then transfer both that live load, the weight of the floor slab, and the weight of the beam or joists over to a girder or a load-bearing wall.
And then from there, if we have girders, these are gonna take the weight and transfer it over to a column.
A column, or if we go directly into a load-bearing wall, they really serve the same purpose. They take the weight and send it down vertically. For a steel structure, that load-bearing vertical transfer could happen through a steel stud wall, which would be a load bearing wall, or what is more typical down through a steel column.
At the base of a load bearing wall or column. We finally get back to concrete as part of a footing for a building. So that's how a conventional steel structure functions. And those are some of the terms that we use around steel structures.
In the next video, we're gonna cover pre-engineered metal buildings and how and why they're different from conventional steel structures.
I’m Joe Meyer, this is MeyerFire University.
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