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FX153 SERIES
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FX153 SERIES
TRANSCRIPT
Obstructed Construction: Bar Joists with Fireproofing
INTRO If you’ve been following along this series, you’ll never guess what we’re covering today – that’s right – another example of Obstructed Construction. Here we’re talking about Bar Joist Construction with Fireproofing, which is our seventh example of Obstructed Construction that we’re given out of the Annex of NFPA 13. We’ve covered Beam and Girder, Concrete Tees, Composite Wood Joists, Panel Construction, Semi-Mill Construction, Wood Joist Construction, and now we’re covering Bar Joist Construction with Fireproofing. DEFINITION Now the definition we’re given here is that our ceiling or roof construction is considered to be Bar Joist Construction with Fireproofing if greater than 30% of the area of the joist as obstructed. Pretty straightforward, right? Have at it - QUESTIONS Well, except – first question – how do we know when the steel joists are even going to be fireproof? And second question – how do we know if the joist is going to obstruct more than 30% of its profile? Well, the answer is, just wait till the job’s done, walk inside the building, take a tape measure, come-on man! You got it! Okay but seriously, there’s a little more nuance to this answer. WHEN? So, our first question – when will steel joists get fireproofing? This answer is based on the building’s construction type, and whether that joist is supporting anything which is fire-resistance-rated. When we’re talking about open-web steel joists, or bar joists, same thing. These are usually (not always) but usually supporting a floor or a roof directly. So, when the floor or the roof is fire-resistance-rated, that joist also has to be rated. When it’s rated, it gets fireproofing. So, if the joist gets fireproofing when the floor or the roof is fire-resistance-rated, our question then becomes, well, when is the floor or the roof rated? FIRE-RESISTANCE RATINGS A floor or roof is rated according to the building’s construction type. Now under the International Building Code, Table 601 identifies fire-resistance rating requirements for different building elements. If we're under a different building code, then we wanna refer to that other code. For instance, if we have NFPA 5000, look at NFPA 5000. In the United States, IBC is most common. That's the example we're gonna use here. Now the floors and roofs, and their associated secondary structural members (that’s in this case, our bar joist), need to be rated according to this table. When we say secondary structural members, we mean the structure that's supporting whatever our element is. So, the floors and the roofs and their associated secondary structural members need to be fire-resistance-rated according to this table. A sidenote here, I repeatedly say the word “rated” but in more technical terms, I mean fire-resistance-rated. Fire-resistance-rated is the term that's used in the IBC and that's the proper term I really should be using throughout here. In Table 601 of the IBC, floors and roofs need to carry a rating whenever we have Type I-A, I-B, II-A, III-A, and any Type IV, and type V-A. Now building types III, IV and V allow combustible interior structure, so like wood on the inside. So, we usually don’t see open-web steel joists being used in these types of buildings. When we're allowed to have wood construction on the inside, well, we might use composite wood joists or solid wood joists or wood trusses or something to that effect because we're allowed to. That can be cheaper. That can be the overall goal for the construction type of the building. So, if we're not using steel joist in Type III, IV and V, and you can, just not often, it's just that for those types of construction, we don't often have choice. So, if we usually would use wood in Types III, IV, and V, let's scratch those out here because that's not where we're gonna commonly see open-web steel joist. That really leaves Type I-A, I-B, and II-A as the construction types where we find fireproofing for open-web steel joists. NFPA CODES & STANDARDS Now that’s the case for the International Building Code, but let’s step back and look at NFPA Codes and Standards that could dictate this as well. Let’s look at if NFPA 5000 is our Building Code, or our adopted building code uses NFPA nomenclature here. Well, NFPA has its own naming convention for different construction types. In the NFPA series of construction types, we would have rated floors and roofs under types I (442), I (332), II (222), II (111), III (211), IV (2HH), and Type V (111). So, Type I, II, III, IV, and V are similar to the IBC nomenclature. However, NFPA gives us that three-digit designation as well as our main construction type. So, the first digit in that three-digit designation refers to the fire-resistance rating of an exterior bearing wall. The second digit is for columns, beams, girders, trusses and arches, supporting bearing walls, columns or loads for more than one floor. And our third digit is floor construction. So, if our three-digit NFPA construction type ends in a zero, that’s when we know our floors are not going to be rated. And when our floors are not rated, it does not need fireproofing. So, Type III, IV and V again allow combustible interiors, so we’re not likely going to see steel bar joists used in that configuration. In which case, if we're going to see steel bar joists and we're going to see fireproofing on those bar joists, it's going to be under Types I (442 and 332) and Type II (222 and 111). SUPPORTING RATED CONSTRUCTION The only other time a steel joist would need fireproofing is if it's directly supporting fire-resistance-rated construction. Let's say we have a 1-hour shaft, or a trash shoot or a one-hour fire barrier that's supported by the floor. Well, any supporting structure for that floor also would then need to be fire-resistance-rated. Now – is that support going to be an open-web steel joist? Well, if it's a really heavy load or a concentrated load, probably not. Joists are not best equipped for that. It could be a solid beam or something like that. But if we had a scenario where the floor needs to be rated because it's supporting something above, which is fire-resistance-rated, then our joist is going to need to be fire-resistance-rated as well. Most projects, even non-combustible projects, are not these types of construction that need fireproofing, the Type I (442 and 332) or Type II (222 and 111). We see many projects where we use Type II-B construction as the IBC convention, or in NFPA terms Type II (000), which has no rating for floors or roofs. So, for most projects, the majority of buildings that are built, which have open-web steel joists, we’re not concerned about fireproofing because it's not required. But let's say we have a project that does use one of these construction types. Let's run out that example and explore this case as if we have one of these so we get an idea of how it works together and what we need to be looking at. PERCENTAGE OF OPENNESS So back to our first question, we now know under the IBC or NFPA series of codes when fireproofing would be applied to joists. Well, what about the second question? The level of openness? How can we know ahead of time when we're in the design stages if a steel joist is going to be fireproofed and it'll obstruct more than 30% of its profile? This can be a very difficult question to answer ahead of time. There’s not really a good way to get this information from a structural engineer. Often times during design, a structural engineer will not know the exact makeup of the profile of the joist as they defer that design responsibility to a truss fabricator. So, asking a structural engineer here might not result in an answer that gives you the full picture. What we can do though is make a few conservative assumptions and get a rough idea of where we stand in terms of this percentage of openness. If we know we're gonna be close to 30% and that information becomes critical, then we can dive in, ask a lot of questions and run this analysis all the way out so to speak. Let's draw up a structural member and get an idea of where our break point is. EXAMPLE DRAWING So first here, let's draw a 14-inch deep 24-foot clear span, and we're gonna start with doing a case series, open-web bar joist and see what level of openness we get just with using rods for web members. Now, this is essentially the slimmest or skinniest profile that we get the most open that we would get. And then we'll do a few examples, see where our break point is and where we fall. Now, our web members are panel points where the points where the web members touch the top cord. These are usually about two feet on center. The other thing to note when we're drawing up the profile of a bar joist, is that the vertical members usually show up wherever we have a V-shape. The other thing to note is that other than the V’s showing up about two feet on center is that these can be shifted so that we have a V shape on either end of the bar joist. We can't end up with a A-shape that doesn't work structurally. We need to end up with a V-shape, and this is specific to an under slug parallel cord configuration. We don't need to know all of that, but in structural terms, that's what we're using is the parallel cord configuration. That's what we see most often with these joists. So, I'm gonna go ahead and continue to rearrange here so that we have V members on either side. And our top cords, we're assuming an inch and a quarter thickness on the top cord web members. We're assuming a three-quarter inch rod. So, this gets us pretty close to what we're gonna see for a case series bar joist using a few assumptions on there, but you know, this is gonna be pretty close to what we see. So, I'm curious to what our level of openness is for this joist. So, taking into account what portion of this is solid. Our solid area is 7.42 square feet. Now let's get the total area. Our total area is 26.9 square feet. Now, when we compare these two, the open is gonna be 26.9. Solid is gonna be 7.42. So, when we take 7.42 divided by the 26.9, that's gonna give us a solid area of 27.6% is the solid area. Now this is about as lean as we're gonna get from a case series open-web steel joist. So, a few things that are interesting here to consider is if we have cords that are thicker and that's determined by the steel truss manufacturer, or we have welded angles used for web members, we're probably gonna be over our 30% solid threshold before we even apply the fireproofing. Now, from a fireproofing perspective, if we're trying to achieve a one-hour fire resistance rating for that joist, then we're gonna need at least an inch and a half thickness all around. That's every web member, every cord everywhere in order to achieve a one-hour fire resistance rating. That's gonna put us well over a 30% solid obstruction. If we go up to an hour and a half fire resistance rating, our UL standard is kicking us to two and a quarter inch thickness. And then if we get to a two-hour fire resistance rating, we're getting even thicker fireproofing. So really, this is around our break point. One thing to note here is we went with a 14-inch-deep truss. I would contend that if we have a deeper truss, let's say a 16 or a 18 or a 20-inch, we're probably gonna be slightly more open. Whereas when we have shallower truss, let's say we've got a K-10 or a K-12 or something like that. The more squat that our truss is, the less open it's gonna be, because those web members aren't changing, they're staying the same, but those cords then start to impact the overall openness. So as an exercise, you know, when we have fireproofing, even when we have rods more than likely, we're going to be more than 30% obstructed and qualified for this. So, the big takeaway here is when we're doing this exercise, let's say we have welded angles and we're somewhere around a 14-inch-deep joist. Well, once we add that fireproofing, we know we're gonna be more than 30% obstructed. Let's say we have a very deep joist and it's 24 inches and we have rods for the web members and we only have a one-hour fire resistance rating for that joist. Is there a chance that we could be less than 30% obstructed? Maybe. Maybe we run that out, draw this up real quick and get an idea. Or if we know we're gonna be close, we reach out to the structural engineer and ask for all of these dimensions. How wide will those web members be? How tall will the cords be on the top and the bottom and how thick does that fireproofing need to be so that we can run that calculation ourselves. RAMIFICATIONS So, what are the ramifications here? Why does this whole exercise matter? Well, what if we assumed that our bar joists are open, they have thin members, and we space our sprinklers under unobstructed construction? What if our pipe was run through those open-web portions of the joists, and we only later find out that fireproofing made this obstructed construction? Well for one, our sprinkler discharge would be affected. We would likely need to lower the deflector to an elevation that throws beneath the fireproofed joists. And to do that, we’d likely need to lower the branch pipe serving those sprinklers as well. We’d essentially be lowering the entire system to a new lower elevation that works to throw underneath those joists. That could be a ton of rework! Like most things in design, it’s better to spend slightly more time getting it right upfront before anything is laid out or arises on site than requiring massive changes on the backend in order to get it right. SUMMARY This was our seventh example of Obstructed Construction that comes from the Annex of NFPA 13. We introduced this concept of bar joists being more than 30% obstructed as qualifying for Obstructed Construction. We ran down a wormhole of calculating on what that openness is actually in reality just to get an idea on where things typically fall. We then also talked about why this all even matters. In our next segment, we’re going to touch on our eighth example of Obstructed Construction, which is Steel Purlin Construction. I am Joe Meyer, this is MeyerFire University.
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