All right, let's get started. So good morning to the West Coast and good afternoon to those out east again. Welcome back to the Post-Tensioning Institute's monthly webinar for August. My name is Kyle Boyd, and I'm again the moderator of today's session. I'm also the chair of the Education Committee, EDC 130. EDC 130 is a committee within the Post-Tensioning Institute that sponsors this monthly webinar. For those of you that this is your first time joining us, welcome. We do host this webinar every single month at the same exact date and time, so the second Wednesday at whatever time zone you're in, this same exact time right here. As you can see on the screen, today's topic is around dual-banded post-tensioning layout. So this is a topic for me personally because it's very, very near and dear to my heart. It's one that I know I've spent a lot of time researching and analyzing and looking at how to apply it to projects, thus I'm really excited to host the presenters today to go through this one. This one for me in particular is a really exciting topic. A little different today. Normally we just have one presenter. Today we're going to have four different presenters trying to go through this topic start to finish because it is very broad, complex, and everything we've done with it over the last several years. We have Asif Baxi, who's representing the engineering consultant side of things. He has a company that focuses on post-tensioning design. We have Tim Crissle. He's the executive vice president of the Post-Tensioning Institute, so we're getting that PTI Industry Association representation right there. We have Jonathan Hirsch. He's the lead software developer of an industry-leading FEA program that designs post-tension slabs out there. So we're also getting that computer analysis side input to this. And then we have Dr. Karen Roberts-Wolham, who's representing the research from Virginia Tech, who did partial-scale testing of these actual slabs. So I'm going to introduce them in greater detail, but before we do that, we always have to go through those housekeeping sides, the first one being the continuing education. As long as you sit at your desk and watch this thing for the full hour, you will get one continuing education credit from RCEP. All you got to do is just sit there. If you have to step away for whatever reason, you can go online and we'll talk about how you can get that credit through the recording here in a second. Copyright material, this is one that we just always have to put in. Once again, a housekeeping slide, plagiarism's a giant no-no, don't do it. If you need anything, just reach out to us, we'd be happy to try and help you. And the webinar protocol is all y'all that are listening, you're all muted, you're all cameras off, we can't see or hear you. So if you have questions, ask them through the question feature and we'll try and answer as many as we can at the end. We're anticipating a lot of questions, so for those we don't get answered, we're going to show you guys how to get ahold of us at PTI and the presenters individually to ask them on there. The webinar is being recorded, so if you miss it or you have a colleague that couldn't make it, you can go online to PTI, you can go to the webinar section and you can watch these pre-recorded webinars, you can take the quiz at the end and you can get the credit just by going online and it's all free, everybody there. So with that, I'm going to dive in and introduce the speakers into a little greater depth. Asif Baxi, PhD, licensed engineer, he focuses only on design, not only, but mostly on design of post-tension slabs throughout the world. He is a very specialized design consultant and he spent a lot of time working on how we get dual-banded post-tension into the world through the consultant side of things. He also is involved with every committee there is out there, it seems, I'm not quite sure how he has time to do his day job, but he finds it. He's a fellow of PTI. He is on ACI 318T, which has to do with the post-tensioning side of 318, and he is also on DC20, which is the main design committee, and those are the ones I really associated with the topic today. The next one's Tim Crissel, Tim's the executive vice president of the Post-Tensioning Institute. He's really responsible for the direction that the PTI organization has gone over the last several years. It's gaining a lot of momentum, we're doing a lot of really great things, and he's the one at the wheel leading us right now. He's a licensed engineer, he's got well over 30 years of experience within the Post-Tensioning Institute, and it's been great having him help with PTI over the last several years. Jonathan Hirsch is the next presenter. Jonathan is a little bit of a double-edged sword. Not only did he decide he wanted to become a licensed engineer, that wasn't enough studying for him, so he decided he wanted to become a software developer. And with that, he is the lead developer of RAMConcept, which is, for anybody who's in the design side, you know what RAMConcept is. It's an awesome software that gets used heavily, and it also leads a lot of other efforts within the concrete side of things over at Bentley, so it's great to see his input on how this relates to FBA design on there. Last presenter we have is Dr. Karen Roberts-Wolin. She is very involved in the concrete industry. She's a fellow of ACI. She sits on ACI 318, so that's a big deal for anybody who knows anything about ACI. And she is the one who led a team that did the physical research at Virginia Tech on this dual-banded stuff, so she's going to talk about what that research looked like in the testing we did there. So with that, I'm going to hand it over to Tim. We have a lot of slides to cover. These presenters are going to be talking fast. Once again, we're trying to get four hours of stuff into, you know, 60 minutes here, so it's going to go quick. Tim, go for it. All right. Thanks a lot, Kyle. Appreciate the introductions. Yeah, like Kyle said, we have quite a bit to cover, but we want to give it all appropriate time to get each speaker to address their section. So just as an outline of what we're talking about today, we're going to have a little bit on the history of two-way PT slabs and how tendons have been laid out throughout history going back even to the 1950s, the reasons to pursue a code change, which is ultimately where this dual-banded layout is headed and what it was kind of based upon in the first place as far as impetus to get this into the code. We'll talk about analytical modeling and the computer-related studies of dual-banded layouts and how two-way slabs perform, so that's a section that Jonathan will spend time on. And then, of course, the really important research project. There was various entities involved in that research, but at Virginia Tech, led by Karen, we'll talk about the experimental testing side and the resulting findings. Then wrap things up with a seat talking about the code change proposal itself. Ultimately, what's happening here is this is going to get into, first and foremost, the ACI PTI 320 code, which is under development, the first version of which will be the 2025 version. And then, like Kyle said, we'll have time at the end for some questions from the audience. So the learning objectives in concert with RCEP requirements, the learning objectives here again is for you to have a better understanding of the history leading up to this point, how we got to where we are today and where we're headed, talking about, again, the analytical modeling and how that has aligned with the research, the lab tests, and the comparison of banded-distributed versus banded-banded specimens and things of that nature. And then, of course, talk even further about the advantages for design and construction utilizing a dual-banded tenon layout for two-way slabs. So there's quite a few things that are obviously hugely beneficial by having that flexibility. So starting off with the history lesson here. Some of you may know this, some may not, but way back in the day, the very pioneering days of post-tensioning essentially for building slabs. We used to have what was kind of referred to more like a basket-weave tenon layout for two-way slabs. You can kind of see that logical mindset that was dealing with flexure and bending and deflections and post-tensioning pre-stress forces in a two-way directional mindset. So kind of a symmetrical equivalency. So back in the day, it used to be a basket-weave, very cumbersome to install. You can kind of see here on my little laser pointer, see here where there was kind of a concentration of tenons on the column lines and then a little wider distribution out in what they used to call the middle strip. So that was how it was done for quite a while there leading into the 60s. Obviously again, very difficult and laborious to install and detail. Everything kind of was above and below in various different ways of each other. So that was how it was leading up to a point where back in the late 60s, there was a pioneering project actually at the Watergate Apartments in Washington, D.C., where the design team, based on the geometry of the buildings, based on various other challenges and whatnot, they came up with a very innovative concept of doing more of a banded distributed layout. And that worked really well in the configuration of those structures and led to the advent of banded distributed and ultimately into the 1970s where that was also taken into the research mode, and we'll talk about that here in a second, at UT Austin, looking at ways to codify banded distributed so that that was a major improvement beyond basket-weave to try and make installation easier, allow for greater flexibility. Again, there's a lot of cost reduction and a lot of other things that really made that a major change in the industry for the better back in the 70s. So touching on some of that history, this is again primarily the icon here of Ned Burns at UT Austin and what transpired with the testing. Here's some imagery of what happened back in the day in the 70s. You'll see here soon some similarities with the ideology of that specimen and the ideas for testing back even then compared to what happened at Virginia Tech. And where there's ultimately what came out of that was a integration into 318 code of a banded distributed layout with certain restriction. So this example here in the 83 code, for example, section 18.12.4, which I have snippeted up here, is where we started to see the eight times the slab thickness or five feet maximum for the distributed tendon direction. And then some other elements there, we were writing 125 PSI minimum pre-compression. Ultimately leading up to the current code of 318.19, section 87.23, which is how that currently reads today, which still has the max spacing for distributed tendons at 8H or five foot. And then talks again in the commentary regarding the origins of that work from the 70s that led to that code language. So we've lived with that for a long time. It's served us well. It's still very economical structures, very good PT designs, but ultimately the advent of trying to take it the next step has been evolving over the last 25 years or so. And so if you look here, I point out PTI DC 20.804, that publication shown here in the appendix A, there's another reiteration of tendon layout history there. So for those who want to access even greater detail about that history, you can find it in the back of that book. And then you can see here some things spotlighted that have happened over the last many years evaluating the potential for dual banded kind of layouts and what that might entail. So here's an example of a paper, technical paper authored by Scanlon Shocker. This was from 2012. This was looking at, again, the performance of two-way slabs with a dual band integrated in there. So that paper that was ultimately in the PTI journal is worth your look to see how that described the evaluation. And then ultimately it's two of the folks that are on this webinar today, Aseet and Jonathan plus another colleague had done a presentation in 2012 at the PTI convention, ultimately talking about this same topic. So this has been in the works for quite some time. And then that leads to where kind of starting around 2016, 17, there was a greater effort in the DC20 committee to put together a task group and start to work on elements of what this would entail to be taken to the research level, both the analytical modeling and the research side combined. So I point out, and this will all come up later when Karen talks, but point out the end result of the research is a technical paper that was published in July of 22 in the ACI Structural Journal. This is, again, with Karen as one of the authors and her grad student here who worked through this. So this is a great paper to review. Also noteworthy is just recently won the ASE TY Lin Award for this paper. So that's an outstanding achievement for Karen and Ty for this article and this paper. That's an award that ASE gives out every year. Now it has to do with kind of a combination of PTI, PCI, ACI and inputs for the most meritorious paper, if you will, for the pre-stress concrete industry. And so that's a great acknowledgement of how great this paper is. And then lastly, the dual-banded post-tensioning layout was covered in great detail in the PTI technical note issued in 2023, which is shown here on the right, which we'll mention again later on in our presentation. So the reason to pursue the code change, which has evolved over the years, we want to have greater flexibility for design engineers to be able to create layouts without the tight restriction on the eight times the slab thickness or the five feet, greater opportunities to allow for tendon-free zones, if you will. There are types of structures, hospitals, research labs, office buildings, and other things where owner-developer mindset of having greater flexibility for tenon rotation and or major penetrations through the slab have really desired to have a more dual-banded leaning tendon layout. So that's coming from that mindset. So we want to try and eliminate that max spacing for distributed tendons, overall reduce construction cycle and placement costs. So just as basket weave moving to banded distributed greatly improved the economics of everything, this would also take another step in the right direction for construction cost, economy, and efficiencies, and ultimately, as well, try to reduce congestion or reinforcement in the mid-bay of the slab. And then my last piece here, talking about other reasons for code change, again, owner flexibility, being able to create new openings, as you can well imagine, there's buildings that have a certain layout for stair elevators and mechanical penetrations and things. And then over time, through a long 50 to 75-year service life, there's changes that might need to take place. And it's a lot easier to do that. It can still be done today with banded distributed, but there's a lot easier way to do that flexibly if you have a dual-banded layout. But again, ultimately, that's another component of the reasons for trying to pursue code change. So with that, I'm going to turn it over to Jonathan and let him talk next about the analytical side of things. Thanks, Tim. Well, Before we set off to do a bunch of expensive experimental work, we wanted to do some analytical investigations just to vet the idea and make sure there weren't going to be any surprises when we got to the experimental phase. For many years, it was thought that the banded-banded system should have similar behavior to the banded-distributed system. Banded-distributed has a long, successful track record, and banded-banded essentially extends the same theory in the one direction to the orthogonal direction, so you'd have banded tendons in two orthogonal directions. The purpose of the analytical study was to compare banded-distributed behavior to banded-banded behavior in otherwise identical slabs. We studied many different variations, including different slab thicknesses, span lengths, concrete strengths, loads, which affects the span-to-depth ratio of the slab, and also various amounts of pre-compression. While we studied many different test configurations in the analytical study, the observations in this presentation are based on this prototypical slab model. We had 8 1⁄4-inch slab thickness, 30-foot spans, fairly typical loads, and the slabs were designed to ACI 318, and so while ACI 318 doesn't currently permit the banded-banded tendon layout, ACI requires designs to be done on full-width strips, so essentially there is no special, anything special required in the design to accomplish that for the banded-banded case. The ACI code does not account for the distribution of tendons within the strip, and because we're using full-width strips, the quantity of tendons and the drape on the tendons is identical for both cases, so we were able to do a design for both banded-distributed and banded-banded, which were essentially identical. The reinforcement in the slabs is identical, again, for both the banded-banded and banded-distributed. The top reinforcement is fairly typical, we have some top rebar concentrated over the column regions, and in the bottom rebar case, we've got some bottom rebar in the end spans, which was driven by the service flexural stress limits, and the center panel does not have any reinforcement at all, so no rebar and no tendons in that panel. The tendon layouts compared are shown on this slide with the banded-distributed on the left and the banded-banded on the right. Just want to point out again that the total number of tendons in both slabs is the same, so essentially the distributed tendons in this slab are simply concentrated along the column lines in the banded-banded case, and the total number of tendons in the north-south directions is identical in both slabs. There is one practical issue which was not considered in the analytical models, and that is the high points of the distributed tendons over the bandlines can be achieved in the banded-distributed layout, but in the banded-banded layout, because all the tendons conflict at the column regions, from a practical standpoint, one set of these banded tendons is going to need to dip under the other and lose a little bit of drape, but again, this was not considered because this was meant to be an analytical study considering as much apples-to-apples as possible, and so in these analytical models, these tendons are occupying the same space over the columns. We'll get into looking at some of the plots that we did. These are pre-compression plots in the east-west direction, so left-right on the screen, and we can see that both slabs in the banded direction have fairly similar shapes. We get these triangular wedges at the edge of the slab where the concentrated banded tendon force is spreading out into the slab. In the banded-banded case, we get these bubbles where the perpendicular bands come in, and these are actually bursting forces where these forces are also spreading out in the slab, and it creates a small tension in this direction, but it's important to note that everything in this region is still in compression. In the north-south direction, things look a little different. In the banded-distributed case, things are more uniform in the north-south direction or up and down on the page. This slab also has the bursting forces from the perpendicular bands, so both slabs have that in common, and the banded-banded case has these similar wedges where the banded tendon forces are spreading out in the slab. So, the banded-banded case is fairly similar to the banded-distributed in one direction, just rotated and applied in both directions. We start looking at stresses at the transfer of prestress. These plots we're looking at here, just to point out first, are max stress plots in any direction, so they represent the peak stress at any location and in any direction. We're only going to focus on effects that are significant to transfer of prestress here, so you'll see some other behaviors that might be more significant for other load cases, so I'm only going to discuss the ones that are really significant for transfer of prestress here. In the banded-banded slab, the only potentially significant behavior here are these bubbles over the bandlines. We get these regions where the concentration of bands is causing a little bit of top stress in this region, and we don't see that behavior in the banded-distributed case. The reason for that is simply that, in one direction, the banded tendon, or sorry, the tendons are distributed in one direction, so there's no concentration of force in this direction to cause this bubble, and in the other direction, the distribution of banded tendons, sorry, of distributed tendons over the bands is kind of counteracting that effect, so this is an effect that's unique to the banded-banded case, and again, nothing to be concerned about, just something that will need to be considered in the design of these slabs. And when we look at the bottom stresses at transfer of prestress, we see that the only – we see some different behaviors that we'll cover more when we look at service cases because they're more significant to the service cases. The aspect that's interesting to point out for the banded-banded case is that the – we get some bottom stresses near the columns, and this is really just due to the concentration of banded tendons in both directions. ACI has provisions in place to handle this. I expect they'll work just fine, but again, something that is maybe a little more significant in the banded-banded case and would need to be considered in the design of the slab. When we get into the service stresses, you can see that these plots look quite different. The banded-distributed case has high peaks over the column regions, so we get really high stresses, and oftentimes, you'll see some cracking in the slabs at these locations in the banded-distributed layouts, and when we look at the banded-banded layout, if we took the integrated stress over the entire width of the structure or even one bay, the total integrated stress should be the same as the banded-distributed, but you can see that the top stresses are much better distributed, and again, this is due to the fact that the concentration of tendons along the column lines in both directions is reducing the peak and just spreading out, kind of smoothing out the top stresses over the entire structure. Similar to the bottom stresses, if we look at the first, the banded-distributed layout, we see along the column lines, that's where our peak stresses lie for bottom stresses because that's where the slab is the stiffest along the column lines. In the banded-banded case, it's actually the opposite because of the concentration of tendons along the column lines, it counteracts the stresses there, so that's actually where the lowest stresses are on the bottom, and it pushes the peak stresses out into the middles of the panels, but one thing to note, again, is that if we took integrated stresses across the entire structure, they would be the same for both structures, but the distribution of stress is different between the two, and the peak stresses are actually higher in the banded-distributed case along the edge of the slab here. For deflections, if we look at linear elastic deflections, the behavior is actually quite similar and that shouldn't be too surprising, again, the total number of tendons and the drape on the tendons is the same, the distribution is a little different, which causes some differences in these plots. You can see that the banded-banded distribution is a little more symmetrical because the layout of that slab, or the layout of the tendons in that slab are more symmetrical, but all in all, the peak deflections are essentially the same, and the behavior is quite similar. When we get to the crack deflections, the behavior becomes quite different, so we can see that in general, the crack deflections in the banded-distributed slab are quite a bit, they're slightly larger, but there are many more locations that have higher deflections, and the explanation for that is simply that the concentration of bands along the column lines there is reducing that peak top stress, which is where a lot of the cracking is occurring, which is contributing to the deflections in this case, so by reducing the top stress at the column locations here, and thereby reducing the cracking at these locations, we reduce the overall deflections caused by cracking in the slab, and that's in the banded-banded case. So getting into some of the strength considerations we looked at. We wanted a way to calculate the flexural strength considerations, and because the ACI code uses full panel strips, there really wasn't any way to use that. We wanted to look at these regions out in the middle of these panels that may not have any tendons and have just minimal reinforcement, or in some cases, no reinforcement, and so we needed a way to calculate the strength in those regions, and the way that we arrived at was by using a column-middle strip approach, so we did use the ACI column and middle strips for this study, and then essentially what we used is we applied the secondary force approach to this, whereby we were calculating a strength in these middle strips based upon the secondary axial and secondary moments in these strips. So because the tendons are applied at the column strips, these forces still bleed over into these middle strips. The slab doesn't know that there's some boundary between the column and middle strips, so we do get forces over here, we get beneficial balance forces from the prestressing, and we get beneficial pre-compression from the prestressing. The details of how this calculation is done is outside the scope of this presentation, but I provided a reference here on how the mechanics of that worked, if anyone's interested in reading that, but here we're looking at, in this case, the secondary axial forces for both slabs. You'll see that they're very similar in the banded tendon direction for both slabs, but they're different in the north-south direction for the banded-banded case and the banded-distributed case. And likewise, this is a plot of the secondary moments for both slabs, you'll see that they're very similar in the east-west direction where both sets of tendons are banded, and then very different in the north-south direction where we have a difference between the banded and distributed tendon direction. Again, refer you to the paper for more details on how that calculation was done. But this did enable us to calculate a strength out in these panels where there was otherwise very small or no reinforcement. As far as shear strength goes, the shear strength effects in the studies we did there were much more qualitative than quantitative because shear behavior is a little more difficult to understand in the context of the tendon distribution. Just from a qualitative standpoint, we know that the stresses in the slab, the shear stresses in the slab increase as you get closer to the column. The load path for all the loads eventually has to get back to the column, so that should be no surprise. We get this huge peak near the column region, whereas if we look at the mid-span region, it's much more uniform, although the peak stress is still along the column line there. The thought process there is that while it's difficult to quantify anything, these additional tendons, the banded tendons, completely concentrated along the column line should only serve to increase the shear strength there, given that's where the highest shear demand is. That's for the banded-banded case. We would expect the one-way shear strength to be similar to or superior to the distributed tendon case. A similar story with punching shear. Punching shear is complicated and difficult to quantify analytically, so we have to look at it qualitatively. The idea there is, again, the presence of more reinforcement and pre-compression in the column regions should only serve to increase the punching shear capacity. This is just a table of the summary of the entire analytical study, showing all the peak stresses. The only location where we had maybe a slight disadvantage of the banded-banded case was at the transfer. Again, it's just something that would need to be considered during the design. We do get a positive peak tension stress at transfer, whereas there was no tension at transfer in the banded-distributed case. However, every other aspect, the banded-banded really shows superior behavior, so it has lower tension stresses at the top and bottom at service case, lower compression stresses. The linear elastic deflection was very comparable. The cracked service deflection was actually superior, and we also expect the flexural strength and the shear strength to be essentially equivalent or superior to the banded-distributed case. The banded-banded distribution, we expect to be equivalent or superior in essentially every way. We also did some analytical predictions that before the experimental tests that were done, and they showed good correlation with the test results, even well into the inelastic range. I'm going to hand it over to Karen to talk more about that experimental work. Okay, I think I'm in control now. Okay, so I'm very excited to be talking about the dual banded tendon layout experimental study. I wanted to note that I'm going to be presenting on two of five specimens that were done in this project. Both of them had typical mild reinforcement as required by ACI, one with the banded distributed layout and one with the banded banded layout. The other three specimens in the project use fiber reinforced concrete rather than mild reinforcement. And I'm hoping to get some of those results out in papers in the next six or eight months or something like that. So that'll be out there too. But today we're just going to talk about conventional reinforcement. So again, I led the study with my PhD student Taiyojo. There were also several master's students that worked on the project, but they worked on some of the different slabs. We worked in conjunction with a task group that was established by PTI to help us out. It was industry professionals and other academics to make sure that the specimen was designed per ACI requirements. We did an independent check on that design, just using conventional equivalent frame method. So on the left, you can see the prototype of our slab, our specimen, with 30 foot by 30 foot bays, three in each direction. On one side, we had an overhang of seven and a half feet. Now this is very, very similar to the prototype that was used by Burns and Hemicom in their research back in the 1960s, with the only difference really being that they also had a second overhang on one of the other sides up here, it would be at the top of this prototype. So our one third scale model, basically dimensions all scaled down exactly by one third. So now we've got a three inch thick slab, 10 foot by 10 foot bays, again, the overhang on the one side. And also note for this specimen, we sort of oversized the columns a little bit so we wouldn't have to worry about punching shear, and we didn't have to add any punching shear reinforcement. So the prototypes were all 33 by 33 columns, except the two corner columns, which were 36 by 36. And we just scaled that down by a third in our specimens. This shows some of the other information comparing the prototype to the scaled specimen, we've already talked about the dimensions, the tendon diameter, or actually would have been cross sectional area wasn't scaled down exactly to a third. But what we tried to target was getting exactly the same pre compression in the slab, which we wanted it to be just a little bit above the ACI minimum of 125. And so both the prototype design and our scaled specimen design, we were shooting at 128 psi of compression. And again, same with the bar diameters, they didn't scale down exactly one third, but we made sure that the cross sectional area total cross sectional area did. You can see the the self weight, obviously, you lose a lot of self weight when you scale from nine inches down to three. And so we had to return that self weight back into the specimen. And we did that with dead load compensation, which includes some blocks that we placed on the top, similar to what you saw in the photograph of Burns work. And also we had a loading system that we refer to as a whiffle tree hanging beneath the slab. And that also contributed to this dead load compensation. Our mild steel was actually deformed bars that can be used in welded wire fabrics or welded wire grids. So it had a little bit higher yield strength. Our pre stressing was a little bit lower, but we didn't worry about that because we didn't get anywhere near yield because it's our ultimate strength. And so again, we tried to go for equivalence in every way that we could between our prototype and our scaled specimen. This shows the tendon layouts of the two on the left, the control specimen, which we refer to as T1 with again, banded in one direction distributed in the other. And I'm also going to note the north arrow here because I'll talk about the north south direction and the east west direction. Specimen T3 and again, if you're wondering why it's three and not two, it's because the second specimen was identical to the first, except it had FRC instead of mild reinforcement, but we're talking about T1 and T3. So T3 is our first banded banded again, it's got exactly the same number of tendons in the east west direction, but now we have them concentrated along the interior column lines with five tendons at each interior column line and three tendons at each exterior column. The top reinforcement was exactly the same in both specimens, no differences at all. And we also placed the little red numbers that you see are our strain gauges. So we put strain gauges on selected reinforcing bars over a typical interior column, over a typical column near the overhang and on one of our corner columns. And those were again, the same in both specimens. Bottom reinforcement, there was a little bit of a difference. So in the control specimen, the required bonded reinforcement in the positive moment regions in the north south direction, our banded direction, we concentrated along the column lines, which is again, a typical practice for the banded direction. But when we did the testing, we found that we would get, when a crack would form here in the east west direction, we would get narrow crack widths near the column lines and much wider crack widths out in the middle of the bays. And so we decided to take a look at how we might improve that performance and try to get more uniformly small crack widths. And so in the banded banded specimen, we distributed that bottom reinforcement uniformly in both the north south and the east west direction. And again, the cryptic little numbers that you see here and there and everywhere are just electrical resistance gauges that we placed on that mild reinforcement. We had a lot of other instrumentation on this lab, which I'll talk about in this slide. We put load cells on the live end and the dead end of one tendon in each direction. And so, you know, obviously we monitored everything during stressing, but we put these on to be able to keep track of any pre-stress losses that might have occurred between the time of stressing and the time of testing, which was typically two to three weeks. We had a lot of work to do to get prepared, wrecking the form, setting up the whiffle trees, et cetera, et cetera. So we wanted to keep track of those losses. We also had column reactions measured on eight of our 16 columns, and we used symmetry to make sure that we still understood the loads to every type of column in the specimen. Again, as I mentioned, we had electrical resistance gauges on a lot of our reinforcing steel. We put on surface strain gauges during testing. We measured the mid-span panel deflection. We had some longitudinal concrete strains using vibrating wire gauges at the mid-depth. Again, that was kind of in conjunction with trying to keep track of the pre-stress losses in particular. And we kept careful track of crack formation, crack widths, all the way through the testing regime. This is just a couple of pictures showing what these specimens looked like. So on the left, again, the banded uniform, you can see the red tendons banded along the column line one way and distributed the other. You can see that we used, we kind of custom created these little bar chairs to make sure that we were getting our tendons always to the proper elevation to match the as-designed tendon profile. You can also see in this one where we had the mild reinforcement in the positive moment regions concentrated along the band in one direction, but uniformly distributed in the other. On the right is T3, where the big differences are. You see now the banded tendons in both directions. And you see the uniformly distributed bottom steel also in both directions. The other thing I wanted to point out is you see these little white PVC pipes sticking up out of the slab. So those were there so we could attach our loading system. So they just created a pass through that we could pass an eye bolt through, put a washer on top and be able to pull down. But the other advantage of having these little dudes was when the contractor was leveling the slab for us, finishing the slab, it gave him a guide to make sure that he was getting it exactly three inches everywhere. So this is a photograph of the slab almost ready to test. So again, you can see the dead load compensation blocks on top. And you can see our whiffle trees hung below. So to look a little bit closer at the whiffle tree, basically it's just a system of levels of spreader bars. And so down here at the bottom level, this is where the load is applied, pushing down. And that spreader then distributes the load to a spreader at each end. So now we got two spreaders, up to four spreaders, and then finally eight spreaders up at the top level. Then again, an eye bolt that passes through those blockouts up to the top of the slab with a little steel plate and washer and nut to attach it to the slab. So then we didn't have any tension actuators, which were actually used by Burns and Hemicom. We had compression rams. And so to accommodate that, we would bolt these beams to the strong floor this direction. And then in each bay, we had a little portal frame right at the middle. We would bolt the ram to the top beam of the portal frame and it would dangle there. And then that ram would push downwards on the bottom level of the spreaders or the whiffle tree. All nine bays were connected to a single manifold and a single pump. So they all had exactly the same pressure, exactly the same load. The overhang was attached to a different manifold and pump because it was a little bit smaller load just because of the size of those overhangs. The other thing I wanted to note on here is we just had these little stub columns below. We had pedestals so we could get the slab up high enough off the floor to accommodate the whiffle tree. And again, at the base of each one of these columns is either a load cell or a swivel head to allow rotation. Now we start talking about results, we'll first look at two examples of load deflection plots. So this is Bay 10. And if we just quickly go back and let you know where Bay 10 is, you can see it here. So it's on the south side of the slab, the middle bay. So we're just going to compare those head to head for the purposes of this load deflection discussion. The other thing to point out is the different colored lines here. Each is associated with a certain kind of milestone in the loading of the specimen. The first is the reduced service load, which was reduced live load plus superimposed dead load for 44 PSF. And again, that's just what we apply. It always is carrying its own essentially 112 PSF of self weight. 64 PSF is the unreduced service load, 84.4 the factored load. And then at 100, so if we look at those three levels, you can see both of these slabs have a linear relationship between load and deflection, and it's almost identical. At about 100 PSF, then they begin to start showing some nonlinear behavior. So the load deflection plot starts to soften a little bit, and then eventually we get to a point where we get considerable additional deflection with increasing load until we reach the failure load. And so in specimen T1, that failure load was 212 PSF, again, applied. Also had that 112 self weight. So it's over 300 pounds per square foot before this specimen failed. And same with the specimen T3 at 200 PSF plus the 112 of self weight. Looking at cracking patterns, we saw extremely similar behavior in the two. These are both of specimen T1. The top slab cracking is left, the bottom slab cracking is right. And so by the end, as we're approaching failure, we had, you know, continuous cracks along the column lines on the top slab in both directions, where these cracks initiated at around 100 PSF and then propagated and finally kind of reached cracks all the way across in both directions by the time you got to about 190 PSF. On the bottom slab, the specimen showed first cracking at about 100 PSF. And almost initially, when we hit 100, that crack extended in these outside bays, kind of from end to end, and then eventually the cracks in the orthogonal direction would occur. You'll see extremely similar behavior with the banded-banded specimen, T3. Again, these horizontal cracks initiating at around even a little bit higher loads of 120 PSF, but eventually propagating all the way across the slab in both directions. Bottom slab cracks occurred at a slightly lower load, again, eventually propagating all the way across the slab in the outer bays from end to end. But very, very similar cracking behavior, very, very similar crack widths of all the two specimens. In terms of their failure mode, they were a little bit different. T1 failed in punching shear at column B4. So again, if we see where B4 is, this is the row four in the column B. So it was this column right here. We got a punching shear failure. Specimen T3 failed in flexure in bay nine, which is this corner bay right here. So again, looking at those two, this was kind of surprising. All of the researchers were actually on the far side of the specimen with all the instrumentation and all of a sudden things went boom and like, what happened? And we had to walk around the slab before we found that this is the failure that occurred on that slab. T3 just had a very gradual flexural failure in bay nine. Eventually it wouldn't take any more load. And when we looked at the top surface of the bay, or actually in several bays above the cracking, you could see crushing of the concrete initiating. So overall, they exhibited very similar behavior. Deflections were linear at service loads and we could fully recover that when unloaded prior to when we initiated cracking, but even that was very recoverable. Our reinforcement did not yield at service loads and tendon stress is hardly changed. Our actual failure loads exceeded the ACI load by quite a bit, 151 for the T1 and 137 for T3 percent. And the failure load would exceed even what we would predict using a yield line analysis. Again, I mentioned that the bottom crack widths of T1 were erratic. They were very small cracks along the column lines where we had that concentrated reinforcement much wider out in the mid bays. And so in T3, when we distributed that reinforcement more uniformly, we saw more uniform crack widths. The tendon stress increases that we did measure as we approached ultimate were between 17 and 35 KSI, which was extremely similar to what you would calculate using the ACI equation of 30 KSI. And so based on the very comparable performance of these two specimens in almost every measure of performance, we saw hardly any differences. And so that would lead us to believe that this limit on the maximum tendon spacing of either 8H or 5 feet for one direction can be removed. And now I'm going to hand it off to Asit to discuss code changes. Okay, thanks Karen, Jonathan, and Tim. And a shout out to the others in our PTI dual-banded task group. Thank you very much for this great work. Personally, for me, it has been a privilege and frankly a lot of fun to be a part of this significant research and the opportunity it will give structural engineers basically another tool in their design kit to design post-tension buildings. While there were a few instances where this has been done in the past, I think in other parts of the world, at PTI, like Tim had mentioned earlier, we started tinkering with this idea of dual-banded about 15 years ago. And after many discussions, extensive analyses, as Jonathan presented, and Kyle was also a member of the group, and we developed the research program, we had funding challenges, but we finally were able to make it, and finally of course the testing. And we're glad that this has come to fruition and we can introduce some design provisions that can be used by design professionals. So to recap, what is this going to do for us? So the dual-banded research will allow us to use banded-banded layouts, banded-uniform layouts, and hybrid layouts. Basically, there will be no restriction of or the ultimate goal is not to have a restriction on how you place the tendons in a post-tension slab. Again, to recap, it will allow flexibility to make future slab penetrations without any fear of breaking tendons or the cost of repairing tendons, and as Tim also mentioned, reduce slab congestion and faster construction cycles. Okay, so the dual-banded layout, you have several options depending on a project. In the case of an orthogonal layout, it's pretty straightforward. You have tendons in both directions. If you have a project with columns that are in different directions, then it adds a little complexity due to that. And then for projects, say, like you have in Florida, where you have tendons that curve, that might add some additional complexities. But, you know, those can be handled through detailing. And of course, this is a new system which is going to evolve as more and more designers start using it. Designers will also have the option to use the conventional banded uniform layout where tendons can be spaced at eight times slab thickness or less than five feet. And, you know, you can extend the tendon spacing beyond the eight times slab thickness or five feet, if a designer chooses to. The other case is going to be a hybrid tendon layout, where for some reason, if there is an opening in a slab, and I'm not talking about a future opening, I'm talking about an existing opening, and if a designer chooses to place some tendons along the edge of the opening, like so, then they will be, you will be able to do it and, or even in the case of concentrated loads. So, what are the code change proposals? As Tim indicated, it's not going to be in 3.18 per se, but it's in the 3.20 code, which is the first joint ACI PTI post-tension code. And this is the very first version of the code. And essentially, we're at the last leg of the the cycle where, you know, everything should hopefully has passed the ballot in the committee, but it's at the last leg of the cycle. So, so what is the code change? The code change essentially allows you to exceed the eight times slab thickness or five feet, provided you, you, you reinforce the area between the banded tendons with an opening that is less than five feet. So, you reinforce the area between the banded tendons with an amount of steel, which is a minimum amount of steel, and it's 0.1% of the gross area of the concrete. So, for example, if you have an eight inch slab, then that, that amounts to 0.096 square inches. There is also a requirement for the spacing of the reinforcement, which is going to be three times the slab, which cannot exceed three times the slab thickness or 24 inches. What about the length of the reinforcement? So, for interior bays, the length of the reinforcement will be three quarters of the clear span length. And in exterior bays, the, the rebar will have to be extended to, into the support at least six inches. Now, when you have corner bays, then you, you obviously have the rebar extended in both directions, going into the support by six inches. So, these are, again, minimums a designer can choose to adjust it according to their project. Now, the current code cycle is, is going to allow for use of banded, banded tendon layouts only. We, within our committee, we have still not been able to come to an agreement or on hybrid layouts because there are so many different conditions to consider. So, that item has been, will be addressed in a future code cycle. And as I think Jonathan had indicated, there is, there are no changes to the basic structural integrity provisions. You know, the requirement in section 87562, where you have to have the two uniform tendons minimum go under the band, really does not apply to the banded, banded tendons. And for the banded, banded tendons, you, you just place the band in one direction, above the band in the other direction, and you will still be not violating any structural integrity provisions. Now, one of the changes, and it's added as a commentary, it's added to the commentary, is what Karen talked about, the, the distribution of the reinforcement and how it affects the crack control. Many designers over the years, myself included, would, in a banded uniform layout, concentrate the rebar required for service, service or for strength, sort of in the, in the column strip, right under the band. But this research has shown that you have better distribution of the cracking and, and smaller crack widths if you take that rebar and distribute it uniformly across the bay width. So, regardless of tendon layout, meaning whether it's a banded, banded or a banded uniform, it is advisable to distribute the, the reinforce, the reinforcement across the, the bay width in both directions. And then to add, to, to end, like Tim had indicated earlier, the DC20 committee has a great document which was published in October of last year. It, it talks about, it's sort of the state-of-the-art or whatever we, whatever the committee has come up with, deal banded post-tensioning. It's a, it's a great resource for engineers who want to use this in, in their designs. And it, it covers, it has additional items on detailing. So, how to handle, what to do when you have tendon curves or splitting stresses between tendon bundles and initial stressing and even state stressing. So, that's a great resource for, for designers to use. With that, I end the presentation and I'm going to hand it back to Kyle. Thank you very much. Yep. All right. So, we got one, and they have to do with detailing of reinforcing. In particular, when you're looking over the column head, how does the interaction between the, having two banded systems over the column head with all your top bonded steel and potentially stud rails, how does that have an effect? Has there been any considerations to lay out priorities there? And the second is more the classic question of what about the shear lag effect? And now you kind of have that in both directions from the pre-compression and any thoughts to anything to be unique compared to this first traditional banded distributed layout. Kyle, what was the first question? Was it on the... Yeah, it had to do with over the column head. We have a lot of congestion now where, where there's banded directions in both and you have competing for which one has priority with your CDS elevation while also having your mild steel in there and having potentially stud rails. Yeah. So, this is going to be, this is something that's going to evolve over time. And I think at some point in time within PTI, we have discussed having some further detailing guidelines on, you know, for design professionals. But to answer this specific question, designers are going to have to consider the, especially at curved or inclined banded layouts as to how to fit the banded tendons in both directions, along with stud rails, along with sleeves and, you know, other embedded items and stuff like that. So, it's a great tool. You need to think the congestion part of it at the columns through. You're going to have a great time in the middle of this lab because you don't have to deal with any of the tendons. But, you know, you will have to consider that. Absolutely. All right. I want to take this. So far, shear lag effect, if you want. Yeah. So, it's an interesting question. The shear lag effect are those triangular wedges that I was showing during the analytical studies. And those have existed in the banded distributed slabs forever. There's a lot of debate about whether or how they should be reinforced. Just to be frank, there's many, many millions of square feet of post-tension slabs been built out there without any special reinforcement in those zones, without any known issues. There are some engineers that choose to put some reinforcement in those triangular wedges. I think there's a PTI technical note that says something to that effect. So, there is quite a bit of debate about what the right thing to do there is. I just want to point out that with the banded banded case, there really isn't anything new that we're introducing. We just have the same effect in two orthogonal directions instead of just one direction. Yeah. I agree. So, if you guys have any specific questions to the individual presenters, the information is shown with their email here. There's also a way to get ahold of us with the Post-Tensioning Institute, and we show that on our closing slide. I do recommend everybody go online and just download that tech note from PTI. That has a lot of questions that are being asked. A lot of information is discussed there. And when we look forward to the next three webinars, because we're always looking three ahead, once again, same date, same time, we're looking at September 11th. It's going to be high-strength PT bar revisited reminders and recommendations. October 9th, we're going to do post-tension bridges bonded PT durability and improvement. So, we're looking at some material presentation and then one on bridges, and then go back to buildings on November 13th for a two-part series on slab on ground design. We've had a ton of requests, you know, a little more technical on it. So, the first one in November is going to be on geotechnical. Then we're going to have part two in December. So, with that, thanks for joining, guys, and we will see you next month, second Wednesday. Have a good one. Bye.

dual-banded post-tensioning

Kyle Boyd

Post-Tensioning Institute

Tim Crissel

Jonathan Hirsch

Karen Roberts-Wollam

Asif Baxi

historical overview

analytical modeling

experimental studies