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FX153 SERIES
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TRANSCRIPT
Obstructed Construction: Semi-Mill Construction
INTRO Here we are continuing on our series of examples of Obstructed Construction. These are each outlined in the Annex of NFPA 13. In this video, we’re gonna be introducing Semi-Mill Construction. Really Joe? What are you smoking? What is Semi-Mill construction? The only semi I’ve heard of are Semi-Recessed sprinklers… TERM ‘SEMI’ Well, just for the record, semi-recessed sprinklers are not really a thing. A sprinkler is either recessed, or it is not. By definition, the term recess means to set back. Saying semi-recess is like saying we’re gonna partially set a sprinkler back, but only halfway. Well, we can’t really halfway a partial. We have pendent sprinklers. We have recessed sprinklers. And we have concealed sprinklers. That’s how the NFPA 13 gods handed Noah the tablets. If we were to search all of NFPA 13, there are only two places where the prefix “semi-“ comes into play. One is with semi-liquids, and the other is with Semi-Mill Construction, which, full circle, is what we’re covering today off of the.... So aside from this terrible tangent I’ve set us on, just know that some people get really upset when the incorrect term of semi-recessed sprinklers are used. It’s not my hill to die on, but just want to let you know of something I learned early-on so you don’t upset that faction like I did. SEMI-MILL DEFINITION Thanks for that Joe. Now, back to Semi-Mill Construction. It’s defined in the Annex of NFPA 13 as one of the examples of Obstructed Construction. The term Semi-Mill Construction refers to a modified standard mill construction, where greater column spacing is used and beams rest on girders. This definition doesn’t give us a whole lot of detail. However, this leads me to my next line of logic, if this is semi-mill construction, what is standard mill construction? STANDARD MILL DEFINITION Well, Standard Mill construction is listed as one of the examples of Unobstructed Construction in the next annex section of NFPA 13. Standard mill construction refers to heavy timber construction as defined in NFPA 220. HEAVY TIMBER What is NFPA 220? Well, glad you asked. It’s the Standard on Types of Building Construction. NFPA 220 does not give us a formal definition of heavy timber, but it does identify the minimum nominal sizing for different structural elements. In order to qualify as heavy-timber construction, not just a wood construction building, for floor loads, columns have to have nominal thickness not less than 8-inches. Beams have to be at least 6-inches wide and 10 inches deep. Arches no less than 8-inches wide and 10 inches deep. This is a significantly different construction method than light wood type V construction that we often see for residential structures. So, standard mill construction for us is going to mean heavy timber with specific minimum thicknesses. SEMI-MILL CONSTRUCTION The only related information I have related to this conversation for semi-mill construction dates all the way back to a pamphlet from 1916 that distinguishes Semi-Mill versus Standard Mill construction. In the Fire Prevention and Protection: A Compilation of Insurance Regulations, Semi-Mill is described as a compromise between joist construction and standard mill construction. Instead of joists only a foot or so apart, heavy timbers are used and spaced three to six feet apart. The timbers are considered beams, and they rest on top of girders. Also, unlike standard mill construction where each beam runs directly to a column, girders instead pick up the load from “intermediate” beams. So semi-mill construction allows for more open floor space. Rather than having a column spacing of every 8-10 feet like standard, in semi-mill construction, columns can be spaced 14- to 16-feet apart. So, we have greater column spacing, more open floor space, but our beams don’t tie directly to columns. They actually tie to girders which then go down to columns. STANDARD MILL CONSTRUCTION The goal of standard mill construction is to have structural properties that do a good job of enduring a fire. It’s all about stability during a fire. Thick, slow-to-weaken wood structure would char on the outside and allow time for firefighters to fight the fire before the structure collapses. Early considerations for fire in mill construction considered the loss of two inches of a column diameter due to charring during a fire. It was supposed to be incorporated that a column would lose two inches of its diameter and still support the building load from a calculation standpoint. That’s pretty substantial. Standard mill construction also has beams or joists that frame into, or on the side of, girders. These create beam pockets that we talk about later on in panel construction. Without a pocket above the beams or without the opening above the beams, the fire is slower to spread, and the exposure for each girder is only on three sides. It’s on one side, the opposite side, and the bottom. Not on the top. Because the top goes all the way up to the roof deck. This offers some level of additional protection, and also helps keep the girders from becoming major obstructions for sprinklers. SUMMARY So, thanks for the early trip back to the 20th century, Joe. Well, you’re welcome. All this differentiation here is to say that we could be under Obstructed construction if we have a heavy timber building where we have beams that rest on top of heavy timber girders. These large beams can affect both heat movement and sprinkler protection substantially. That’s our definition of obstructed construction. If we have a heavy timber building, look to see if there are beams that rest on top of the girders or if each beam effectively creates its own pocket. Also, look to see if column spacing is more like 8-10 feet, like Standard Mill Construction, or if the columns are spaced further apart, like say 14-16 feet, which would be Semi-Mill Construction. These drivers can steer us one way or another to determine if we have Obstructed Construction. That covers our burning questions on Semi-Mill Construction. Next up in the series on Obstructed Construction is Wood Joist Construction. I’m Joe Meyer, this is MeyerFire University.
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