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Components of Light Frame Construction
INTRO In this series so far we’ve talked about Conventional Steel Structure and Pre-Engineered Metal Buildings. Well, we also have light-frame construction. These could use either steel studs or wood studs. We’re going to talk about this approach, and the different components of light frame structure here in this module today. DEFINITION So, what is light frame construction? Well, it’s a method of construction that uses many smaller and more closely-spaced structural members to support a building. In conventional steel structures, we often see beams and joists frame into girders, and then down to steel columns before reaching the foundation. In pre-engineered metal buildings, we see roof decks send weight down to purlins, then the main frame rafters, then the main frame columns and finally down to the foundations. Well, in light frame construction, we don’t have point loads at columns; rather, we spread that weight out along load-bearing walls. These walls consist of many studs, each absorbing a smaller portion of that weight from above. Light frame construction uses smaller dimensional lumber or uses steel studs. It tends to be a more economical approach to assemble a structure. BALLOON FRAMING HISTORY Now, historically, wood buildings were constructed using heavy timber members, or timber framing. We talked about that historically as Standard Mill and Semi-Mill Construction. Roughly in the 1830s, balloon framing became popular, which used lightweight and many, more-slim wood members called studs rather than fewer and heavier timber posts. So, despite the economy that was gained with that balloon framing, one major drawback to it was that there were uninterrupted wall cavities that extended multiple floors up to the roof. There were air gaps basically from the lowest level up through the roof. This vertical open path posed a major fire risk. As we know, fire spreads much quicker vertically than horizontally since convective and radiative heat transfer work together to pre-heat the area right above the flame and helps spread fire up vertically very quickly. Well by the 1930s, balloon framing began to give way to platform framing because of this fire risk. In platform framing, each story is framed individually, so a floor/ceiling assembly can be framed before the walls on an upper level will even begin. Balloon framing had those wall studs that went from the lowest level all the way to the highest. On platform framing, you frame each individual story at a time. For wood, this can be called light frame wood construction. From a fire perspective, this helps us break up those vertical cavities and delay fire spread between floors. STEEL STUD USEAGE Light frame construction could also consist of steel studs. Just like we talked about with purlins, steel studs are cold-formed steel that’s dipped in a galvanized layer to protect it from corrosion. So this term, when we talk about steel studs or light frame steel construction, we could also describe it as light gauge steel construction, light gauge metal, light frame steel, steel framing, or a host of other combinations. At the root of it, we're talking about light frame construction using steel studs. At the root of the name is the same method as wood, only this time using metal framing members. Light gauge steel is formed in a C-Channel shape and cut with holes for passing wire or cable through; and they also tend to have a higher dimensional tolerance, or maybe I should say smaller. Factory conditions and tolerances for creating steel studs is much more narrow. They're much more consistent through the fabrication process than rough cutting lumber. The stud walls themselves can carry loads, like wood, although typically they’re not as strong as solid steel columns, open-web steel joists or solid steel beams. Again, this is light frame construction. SLOPED ROOF COMPONENTS So all that said, what are the components of light frame construction? Well let’s start with wood light frame construction. So, our snow, wind or rain load that comes from above will transfer onto the roof. We'll have some force coming down onto the roof. In light frame construction, if we have a sloped roof, well, we'll have the roofing material that could be like an asphalt layer. That's gonna sit on top of an underlayment layer, which is then on top of roof sheathing. There's a few different terms for roof sheathing. We'll just refer to it as roof sheathing for this sake. That roof sheathing sits on top of rafters. FLAT ROOF COMPONENTS & INSULATION If we have a sloped roof, we’re usually looking at an attic truss. Attic trusses have three main components. That’s the rafter, which is the sloped member that’s along the roofline. It runs from the top, which is the peak, down to the eave, at the outside of the building, the lower point. The bottom of the attic truss is the bottom chord. You’ll remember from earlier in our structural series we talked a lot about top and bottom chords, well we still use that term here when that member’s horizontal and level the bottom chord along the bottom of the attic truss. Finally, in-between the bottom chord and our rafter, we have web members. In light frame wood construction, these web members are typically going to be solid wood with metal “gusset” plates, g-u-s-s-e-t, gusset plates that attach the web members to the rafter at the top and chord at the bottom. Attic trusses can also be out of plane with the rafter and the chord and nailed to the side of the rafter and the chord. But this seems to be an older technique and it's really not common anymore, at least in the US. I don't know, personally, if there's some code provision change that happened with that, but most of the attic trusses that I see or any trusses that I see have that web member that's in the same plane as the rafter above and the chord below. If we have a flat roof in light frame construction, then we’re using roof joists to support the roof layers. Instead of an attic truss that has a peak, it’s more likely gonna be open web joists that would support a flat roof. Now, while we say the term “flat roof”, there's still some slope to that final roof layer. A roof needs to have that slope so we can get the water to run and actually drain. A true flat roof, if it were actually horizontal, that would trap water. It adds low to our building. We have potential snow loads that don't melt correctly. We have water intrusion issues. There's a lot of issues. We always want some slope to the roof. Now, a roof can have insulation on the top of the structure, which would be called built up insulation. That's gonna be thicker on one end than the other and that creates a small slope for water. We could also angle the roof joists so that the topside of the joists has some slope to them. This isn’t terribly difficult to do with open web joists, because they can be built with a bottom chord that’s flat and horizontal and a top chord that has some slope. I guess, technically you could call that a rafter. For solid wood joists, this is possible, but it also would create a lot of waste. It’s difficult with solid wood joists on a flat roof to allow much in the way or ductwork, wiring, or pipe within that ceiling or roof space. I haven't had many projects personally that take the approach of solid wood joists that have the top edge slope or cut down. But nowadays, at least a new construction, we're usually seeing open web joists at that top roof level. One quick flag to point out while we're talking about this, especially with wood, if the entire roof ceiling cavity is filled with insulation, then running pipe up there could result in some freeze up. Pipe needs to be on the warm side of insulation. It's gotta be if it's wet, it needs to be maintained above 40 degrees Fahrenheit. Also, we can't have spray in insulation touching CPVC pipe because the chemicals have incompatibility, and that spray insulation will deteriorate the CCVC pipe. So flat roofs (or even sloped roofs for that matter), pay special attention to where the insulation is going to be and confirm whether the space that we're routing pipe is going to stay above freezing. If it’s not, consider routing pipe elsewhere or maybe even moving to a dry system. If we have combustible attic spaces or combustible concealed spaces along the roof, under an NFPA 13 system, we're probably gonna be protecting that. We need to think about, is it wet or is it dry? If it's wet, can we use CPVC and how we go about approaching, protecting that space up there. Even if that space is not protected on the roof level, we still have to think about where we route the pipe to serve sprinklers that are at the ceiling level. So, it's possible we could run pipe up the walls and “bird cage”, the top level so we're feeding pipe up the walls and using sidewalls, so we never have to route pipe up in this area anyways. That’s certainly one approach. But regardless, be mindful of what's heated and not heated and where we can and cannot route pipe based on where the insulation is up at this roof level. COMPONENTS BELOW THE ROOF STRUCTURE Back to structure, below our roof structure, we’re gonna have load-bearing walls. These load-bearing walls, in light frame construction, are built with studs, or vertical dimensional lumber or if it's steel, we're gonna have C-channels that actually bear the weight. These studs are set at specific spacing, and these walls will include the stud top and bottom plates. For light frame construction, these are often assembled right onsite but could also be pre-assembled in a factory and shipped in as well. We touched on it with balloon and platform framing a little bit earlier, but the bottom plate of a stud wall nowadays will sit on top of the subfloor layer. Wood subfloors can be OSB, which is Oriented Strand Board. They could be plywood. They could be particleboard for light frame wood construction. Now, obviously the subfloor doesn't just support the stud walls, it also takes the load from the main floor areas, the flooring materials, any dead and live loads that are inside the structure. What’s below the subfloor? Those are our joists. Here in this image, we have composite wood joists, which we talked quite a bit about earlier in the series. But there could also be open web wood joists, or even solid wood joists. The joists take the weight from the floor and transfer that load over to our load-bearing walls. The joists from an upper-level are gonna rest on the top plate(s) of a stud wall from the level below. As we work our way down, again as that load would travel, we have the studs, the bottom plate, another subfloor, another set of joists, and finally the sill plate that sits on the top of the foundation wall. The foundation wall, if you remember, is that vertical “wall” portion of the foundation and the footing is the flat “bottom” portion of the foundation. The footing then takes that load, spreads it out into the soil. Soil bears the weight. Everybody goes home happy. STEEL LIGHT FRAME CONSTRUCTION So, we talked a lot about wood, but what about steel light frame construction? Well, most of the techniques are similar here between wood and steel light frame buildings with a few exceptions. Instead of wood attic trusses, this could be built of metal C-channels. Instead of solid stud wood walls, we could use metal C-channels that we talked about earlier. However, for joists, these same C-channels that we use for studs just don’t serve as good floor joists. They may be used along ceilings, or in some limited cases maybe along roofs, but they don’t function well to support a floor. In light-frame steel construction, floors might be supported by open-web joists instead or maybe even poured concrete. At least in the US, we often see wood for light frame construction because it can be less expensive, it can be fabricated onsite, it usually serves well for residential buildings where we have relatively small rooms and shorter ceiling spans. When we get into steel structures, which are more common for commercial buildings, we generally are gonna have heavier building loads and longer ceiling spans. So, because of that, we need a more robust structure. Steel light frame construction can work for smaller and shorter buildings, like say a single-story commercial bank or coffee shop, as an example. But it’s a technique that’s just not a great fit for heavier buildings, taller buildings, or commercial spaces that are gonna need some flexibility in their layout in the future. And in other words, if we need large spans, we have heavy loads or tall buildings, well, we're not gonna see light frame construction. So, in the US at least, we don't see this approach for steel light frame construction used all that much when we get into larger commercial buildings. SUMMARY So, in summary – what are the components of light frame construction? Well, our basic principle is that we use specific walls to bear the weight of the building. These walls are built up of many smaller studs which are each responsible for a smaller portion of the building’s weight. At the roofline, we generally see trusses used to support the roof, which then bear atop stud walls. The weight of the building gets transferred down through the subfloor, floor joists, and eventually down to the sill plate, foundation wall, and the footing before releasing that load to the soil. That’s about it for the different components of light frame construction. Next in this series, we’ll touch on masonry structure. I’m Joe Meyer, this is MeyerFire University.
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