Examples of Unobstructed Construction: Standard Mill Construction
In this segment, we’re continuing in our series on Unobstructed Construction – here touching on Standard Mill Construction.
So Standard Mill Construction is a type of heavy timber construction that was popular with the rise of factories and mills back in the 1800s. It’s the original method of heavy timber construction. Original in terms of modern industrialization.
Now today, we use the term “Heavy Timber” somewhat casually to mean really any wood structure with large structure members. Standard Mill Construction uses significantly larger wood columns and beams than what we have as 4 inches beams.
So in our code path, NFPA 13 tells us that Standard Mill Construction is heavy timber construction that’s defined in NFPA 220. NFPA 220 talks about minimum thicknesses to qualify as heavy-timber construction, and that’s when columns must have a nominal thickness not less than 8 inches, beams at least 6 inches wide and 10 inches deep, and a few other things. That would be about 200 mm thickness for columns, 150 mm width and 250 mm height for beams.
That’s a significant departure from light wood frame construction that’s common in Type V construction we see here in the US residential market.
So Standard Mill involves heavy timber, but what else? We don’t have much in the way of hard line black and white definition to run with here, but there is an outside source that give us an idea of what Standard Mill Construction is, and what it’s counterpart that was developed later on as Semi-Mill Construction is.
STANDARD MILL CONSTRUCTION
So according to a 1916 pamphlet of Fire Prevention and Protection: A Compilation of Insurance Regulations, we get some education on what Standard Mill Construction is and what Semi-Mill Construction is.
Standard Mill Construction is the original method for heavy timber construction. It has solid wood beams that frame directly into columns. They don’t sit on top of girders, they just frame right into the column.
When you go back and actually look at the details for how these beams frame into the columns using iron brackets. Well, they're sometimes very ornate and overall just really neat, but effectively, standard mill construction has solid wood beams that create beam pockets.
Now each beam frames into a column like we said, so our column spacing is generally pretty small in Standard Mill Construction. According to the pamphlet, column spacing would usually be around 8 to 10 feet apart. That’s roughly 2.4 to 3 meters apart between columns. So, not a lot. We've got a high frequency of columns.
What makes Semi-Mill Construction different? Well, Semi-Mill Construction was developed at a later date as a compromise to get those columns spaced further apart and allow for more uninterrupted floor space.
Instead of each beam running directly to a column, beams framed into a girder, and that girder is then supported by columns.
If you remember in our structural series we talked about beams and girders. Well, beams pick up the load directly from a floor or roof above. Girders collect the load from multiple beams.
Here, in Semi-Mill Construction, these large girders collect the weight from the beams and then distribute that weight from framing into columns. This allows columns to be spaced much further apart. Going back to our pamphlet, the suggestion is that columns would be spaced about 14 to 16-feet apart, or 4.3 to 4.9 meters apart.
That’s good for the building owner or the factory operator, because they can get more uninterrupted floor space. Room for equipment, flexibility can change the layout of the space, all of that. That’s all that. But that changes the ceiling construction a little bit when we go from standard to semi-mill. In semi-mill, these beams can rest on-top of girders. This opens up some pathways for heat and smoke to spread out and move away from the source.
HOW IT AFFECTS SPRINKLERS
What does this change for a fire sprinkler?
Well, in Standard Mill Construction the beams are spaced 8- to 10-feet apart (or 2.4 to 3 meters apart). That’s a significant distance. Remember, the annex of NFPA 13 suggests that beam spacing over 7.5 feet (2.3 m) is generally considered to be Unobstructed Construction. Meaning that spacing is so wide that it's not significantly affecting the movement of heat or water from a sprinkler.
In Semi-Mill Construction, because those beams tie into girders, there can be multiple intermediate beams in each bay. These are smaller beams and they’re more frequently spaced. They're shorter spacing. The spacing ends up less than 7.5 feet apart. So, in Semi-Mill, our beam pockets are much smaller than in Standard Mill Construction. Standard Mill ends up as Unobstructed, while Semi-Mill is considered Obstructed. Those more frequent beams, with smaller spacing, will have a greater impact on heat flow and water distribution from a sprinkler. Standard goes to unobstructed. Semi mill goes to obstructed.
WHY DOES IT MATTER
That’s great, but why does it matter?
Standard Mill, in NFPA 13, is considered Unobstructed Construction. That means we’re going to need our sprinklers to be closer or up higher to the deck above. That also has ramifications for sprinkler spacing.
And Semi-Mill, where those beams can sit on top of girders and the columns are spaced out wider, it’s an example of Obstructed Construction in NFPA 13. That gives a little more leeway in locating sprinklers further down from the deck and it also affects sprinkler spacing.
Where do we find all this? It’s back in the annex of NFPA 13 under the definitions of Obstructed and Unobstructed Construction.
So, in summary – what is Standard Mill Construction?
Well, it’s a method of construction used with the advent of mills and factories from the 1800s that uses large wood structural members where beams tie directly into columns with closer column spacing.
It’s different than Semi-Mill Construction because we have tighter columns spacing, but we effectively have larger beam pockets up at the roof or the ceiling instead of smaller beam pockets because of the way the beams are spaced.
Next in our series, we’re gonna get into Truss Construction before wrapping it all up.
I’m Joe Meyer, this is MeyerFire University.
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