All right, we'll go ahead and get started. So good morning to the West Coast and good afternoon to those of you out East. Welcome again to the Post-Tensioning Institute's monthly webinar for April. My name is Kyle Boyd. I'll be the moderator of today's session. I'm also the chair of the Education Committee, EDC 130, and that's the committee that sponsors this monthly webinar. As we've discussed in the past, this monthly webinar happens at the same time every month. So it's the second Wednesday of every month at the same time, depending on where you are within the U.S. itself. As you can see, today's topic is post-tension design best practices. And this one, you know, there's a lot of material that's out there for very clean textbook type examples, right? A square orthogonal slab, one that has very unique column layouts. You know, that information is out there. But folks, when they first start getting into PT design, then they go to design the first slab and they see this is much more complicated. There's these weird slab steps. There's this really weird column layout. There's how do we align our tendons with this really weird bay that's going on here. We have a column that comes down and that's, you know, they call them walking columns where they step over a little bit for whatever architectural scheme is going on in there. So today's presentation really dives deep into those items you don't necessarily see within the textbook itself. And so there's a lot of really good information that we're going to go over today. But before we dive too much further, we just have a couple of general items that we need to go over. So the first one is the continuing education. Everybody attending today's session, as long as you personally registered for today's session, you attend the entire session, you will get one hour of continuing education credit through RCEP on there. And that is you need to be logged in and attend the session itself. The next slide, we just have our typical copyright material. This just says thou shall not plagiarize, it makes our lawyers happy, and on there, if you do want to use any of this material, feel free to reach out to us at PTI and we can talk to you about it. So for the webinar protocol today, all attendees, you guys are all in listen-only mode. We can't hear you. We can't see you. If you have questions, ask it in the Q&A box, not the chat function, there's a Q&A thing in there. At the very end, we'll go through the questions. Try and be very precise and accurate with your questions. The more of a detailed question you give, the better chance it's going to be answered. For those questions that don't get answered, we'll give you some information on how to get ahold of us so that we can't get them answered at the end there. We talked about you need to be present for the entire webinar just to get that credit there. It's being recorded. If you do have to step away because a client comes walking in the office or something else happens or you missed today's session, you can go watch this online. This whole thing is being recorded. There'll be several questions afterwards. You take those questions, and then you get that continuing education credit. That's true for any of the previous webinars that we've done as well on there. With that, I get to introduce our speaker today. Our speaker is Anantha Sathour. He's with BASE Engineering. Many of you, I'm sure, have heard of BASE Engineering. He leads their Chicago office. BASE is one of the leaders in PT design. They've completed millions and millions of square feet of PT design, a lot of it's very complex, unique projects. In fact, many times during our annual post-tensioning institute convention, they are presenting something technical about one of their very unique projects that they had and how they handled that situation. With that, we really thought that they are truly one of the most well-equipped to present on these one-off situations and how to handle them and the most qualified to do that. Anantha leads their Chicago office. Within the company itself, Anantha is one of the best to present on there. He's also very active in the post-tensioning institute design world and within the design committee at PTI. With that, Anantha, I'll let you take it away, go for it, and I'll see everybody towards the end for Q&A. Thanks, Kyle. Good morning, everyone, if you're joining from the West Coast or from Hawaii, and good afternoon if you're in the East Coast and Midwest. As Kyle mentioned, today's webinar isn't going to dive into typical stuff, isn't going to dive into the details of how to do a two-way post-tension slab design or a one-way slab design or a beam design, but it's going to focus a lot on nuances of post-tension design, best practices as we would like to call it for design and detailing. It's going to dive into a lot on constructability and coordination issues. We'll talk about a handful of advanced topics that Kyle alluded to earlier, such as when you have sloping columns or walking columns and things of that nature. We'll end the presentation with the opportunity for you to ask some questions. So with that, this is a slide that outlines a typical lifecycle of a post-tensioned building. You start with design, go through aspects of design, such as sizing the slab, column layout, wall layout, how to lay out the tendons to snake through and hit the columns that the architect has provided, talk about long-term deflections, talk about coordination items. There's MEP, items that need to be coordinated, glazing embeds, slab slopes, slab steps that are detailing aspects for tendon sweeps, slab steps, crack control, rebar and tendon layering, and stud rails. And we'll talk a little bit about considerations for detailing balconies, and then we'll spend a little bit of time talking about construction items, you know, shoring, reshoring, deflections. Again, deflections are extremely critical when it comes to construction coordination. We'll talk about slip form and jump form considerations. And we'll spend a little bit of time talking about special items such as, you know, sloping columns or diaphragm design, again, get into the nitty-gritty of pore strip design or how to deal with amenity decks. And we'll end the presentation with some oops items, which are mainly things you'll see when things don't go as planned in the job site. So no post-tension presentation, in my view, should start with load balancing. Load balancing is the most important concept that everybody should understand and appreciate. This is in very simple terms, two components. You have a force, which is the P, and you have the drape, which is the eccentricity. So the combination of the force and the drape is what makes the post-tension slab do what you're trying to make it do. I mean, in essence, you're like a puppet master pulling the strings to make the slab behave the way you want it to. It is an active system. So what you put in, there will be a reaction from the slab as a result of your inputs of load or drape. So this is in a very simple term. You have your P times E, which is your moment, and W is the equivalent balance loading, and L is the span. So the equivalent balance loading comes out to be eight times your moment, which is P times E over span squared. This is an extension of the same concept in multi-span situations where you have a tendon with a reverse curvature and reverse parabola, which is what is shown here. So you have lift provided at mid-span, and then as it reverses, you're dumping that load on top of the columnar support. This is, again, what the equivalent load translates into in terms of the load pushing the slab up and what it translates into pushing it down. I have a little example here that will help you appreciate load balancing a little bit more. We have a cantilever, an end span, a short interior span that's bounded by relatively long spans on either side, and a long end span. The system is designed to have balancing of anywhere between 60 to 80 percent, and what you end up seeing here is the tendon profile as a result of that load balancing. And a handful of things to keep in mind here is short spans, we need to make sure that you're not overbalancing. And what I mean by overbalancing is, as you can see here, you don't have the tendon draping a lot or profile a lot because the long span adjacent to the short span is going to put the slab in a uplift situation. It's going to get benefited by the longer spans, so you don't need to put as much drape to lift the slab up. And then similarly, the cantilever, the cantilever again benefits the end span because you have some deflections happening on the cantilever that help the end span. But again, you have to be very careful with the ratio of the cantilever link to the back span. If you have a short cantilever, then the end span could cause the cantilever to lift upwards. And on the contrary, if you have a long cantilever, it can help lift the end span up. So that's something to pay close attention to. So this is, again, trying to get into the weeds of load balancing. The first image up here is what the slab will do when you're subject to self-weight. You can see you have a nice moment diagram. The short span, as you can see here, is mostly all in negative moment. And the tendon profiled will provide a balance loading. And the final design is a combination of the dead and balance loading. The most important thing to remember here is that the balance load will tremendously help the column moments. Because this is, again, an example looking at a roof where there is no column above. The balance load from post-tensioning almost helps cut down the column moment by half. So that is something to keep in mind when you're going through building design. Oftentimes you're running through RAM structural system or ETABs, and you'll find that roof columns end up having more reinforcing than the column on the level below. So in those scenarios, it is advisable to take benefit of the post-tensioning balance load that you're getting to offset some of those moments. We'll discuss a little bit on slab thickness and P over A consideration. And P over A is just short for free compression. It's force over area. Typical post-tension slabs are designed with span-to-depth ratios of L over 45 to L over 54. And one of the things to realize and appreciate is that the span is really what you're looking at is a clear span. So if you end up having walls or columns that are fairly long, you want to be able to capture that in your analysis in your modeling. Because as you can see here, two are exact same systems, center-to-center spans are the same. But if you let the slab deflect within the footprint of the supporting element, you end up having higher deflections than a scenario where the slab does not deflect over the support. So if your modeling software allows you to incorporate this, this is something that you can use to benefit your deflection studies. Compression again, ideal range is to stay somewhere between 175 to 250. More than likely, a wall proportion slab will probably be closer to 175 to 200 PSI. You want to be careful once you start hitting north of 275 PSI, 275 to 300, you can do it. But once you get past 300 PSI, you want to be cautious in terms of other implications. Strength again, typically 5,000 to 6,000 PSI. Although markets such as Chicago, you will get slabs with 8,750 PSI concrete, which I'll go over a little bit more on the 8,750 and its significance. The other thing to keep in mind is to try and stress the slabs at 3,000 PSI. There's a lot of archaic notes and drawings that you will find slabs being stressed at 75% of the design F'C. So if you end up having a scenario where you have a 6,000 PSI slab, asking the contractor to wait to get to 75% F'C may cause a delay in the construction cycle because the slab will take some time to get up to that strength. We'll talk a little bit about tendon layout. This is where Kyle, as he mentioned earlier, typical buildings don't have columns on an orthogonal grid at an even spacing. Markets will like to provide columns where it makes sense for unit layouts. Things of that nature that will force you to go and provide the most optimal direction for your banded and distributed tendons. And for those who are not familiar, banded tendons are groups of tendons, oftentimes 15, 20, 25 tendons grouped together in groups of three or four running, connecting the columns in a line. Distributed tendons are perpendicular to banded tendons and often spaced at anywhere between two and a half to five feet on center. The code requirement is no greater than eight times the slab thickness or five feet. Again, one of the most important things to remember is drape is far more beneficial than free compression. And what I mean by that is if you have a slab with a tendon profile for an eight inch slab going from seven inch at the high point down to one inch at the low point and then back up to seven inches, you have a drape of six inches. Now imagine if you have the same slab with tendon at four inches at the CG of the slab going down to one inch and then back up to four inch at the CG, you only have a drape of three inches, which effectively means you only have half the benefit. So in order to get an equivalent system, you have to put twice the amount of pre-compression. And once you start putting a lot of pre-compression, then you run the issue of blowouts, you know, things of that nature. So pay close attention to the profile of tendons and making sure that you're looking at overbalancing. And by overbalancing, I mean balancing more load than the self-weighted slab. Important consideration again, tendon and reinforcement layering. And this is a diagram that we put together because we had some experience with jobs where we found tendons and rebar ended up in four layers. You know, you had the banded tendon, distributed tendon, top rebar in one layer and top rebar in the other layer. But the intent is to have the top reinforcing parallel to the banded tendons being the same layer. And the uniform tendon typically over the column is below the banded tendon. And the reinforcing parallel to the uniform tendon direction is in the same layer as well. So in effect, you end up with only two layers of reinforcing and tendons. So that was a lot of text, a lot of history. And this is some examples of how you choose tendon directions. Key considerations here, this is again a high-rise building, central core, and a couple of stair cores. As you can see here, the banded tendons are oriented left to right. And the goal with that is you're trying to hit the lines of columns with minimal horizontal curvature or what we call as sweeps. So if you chose a direction which is indicated in blue here, you would end up having a significant amount of horizontal curvature in this lab. And just like load balancing, horizontal curvature will impart a horizontal thrust on this lab. So in this scenario, banded tendons are laid out left to right, primarily to avoid how many horizontal sweeps we have to make. The other thing also to keep in mind is you want to look for the direction where you have balconies. You want to try and take your uniform tendons where possible into the balconies to benefit longer cantilevers if they're provided. This again is another layout where we laid this project out in two different ways. We laid out banded tendon in the left-right direction, and then we laid out banded tendons in the north-south direction. And you need to watch out for MEP shafts, that anything that comes in the way of a tendon trying to go from column to column to column. So you end up having a MEP shaft here, stair shaft here, another MEP shaft adjacent to the column. So the direction of banded tendons made sense here to run them in the north-south direction because they had the least amount of interruptions. And the uniform tendons could start and stop at the openings. The other thing, the highlighted areas, again, this is you want to maximize your tendon profile. As you can see here, we're not anchoring the tendon at the face of the support, but trying to provide a high point at the core. Again, you're trying to maximize your grade. This again is another example where banded tendons are running left to right. Again, you're trying to pick the straight line for the banded tendons, uniform tendons running through the balconies, all principles that we talked about. But you do end up getting into these unique scenarios, which happen a fair amount. So what you have here is a true end span scenario where the tendon is at mid-depth at the core and at mid-depth at the slab edge, and potentially with maximum drape, which on an 8-inch slab, you only end up getting 3-inch of eccentricity. So one of the strategies that we've used is to provide minimum P over A and not really force putting a lot of tendons to balance the loads, and by providing groups of banded tendons in the distributed tendon direction with a high point at the core. And it's got two benefits. You know, a typical end span with the core will have the negative moment more biased towards the shear wall because the columns are not as stiff on the perimeter. So you will end up having a fair amount of negative moment at this interface. So having tendons at high point to counteract that is beneficial. You'll end up in scenarios such as this one, where we end up using banded tendons in two directions, especially just concentrating those tendons to provide lift in very specific areas where you have a very long span, because you don't want columns within the corner unit. So there are those strategies as well, look for opportunities to maximize tendon drape, look for opportunities to provide groups of concentrated tendons within the distributed tendon direction. So this again is a slide that shows the horizontal curvature of tendons. As you can see, as it sweeps from this column, hits this column and then continues on to the next column that may be further away, you end up having to turn the tendons horizontally in the slab. And just like the load balancing principle, you have horizontal thrust as a result of the tendons wanting to straighten out. And this is something that is resisted by hairpins that are provided in the slab to restrain the tendons. One of the important things to keep in mind is if you're sweeping your tendons closer to high points or low points, there's not enough concrete there to resist the horizontal thrust. And PTI recommendations are that these tendons within the group be separated by two inches so that there is enough concrete for the tendon to thrust against. And in a scenario, if you have tendons right next to each other and they try to straighten out, the tendons will push against each other and because they're circular and sheathed, there is not a whole lot of resistance and the tendons will tend to roll over the adjacent tendon causing what is a blowout where the tendon displaces laterally and there's not enough concrete cover so it just blows the concrete cover. So pay close attention to large tendon sweeps, make sure sharp drawings indicate tendon hairpins in them. And it's something to be watchful for, tendon sweeps happen on 95% of the projects, but it's something just to be cautious of and watchful for. We'll talk a little bit about slip form construction and for those not familiar with slip form or jump form, this is a method of construction where the shear walls, the central shear walls or the core walls are constructed ahead of the slab. As you can see here, the shear wall probably has been constructed three or four floors above the level that is being poured and as a result of doing this, you end up having some sort of pullout bars or form savers and tendons are anchored outside of the slab edge. And this is again, indicating what happens when the tendon is anchored inside the slab edge versus tendon anchored outside the slab edge. The important thing is if the tendon is anchored inside the wall thickness, the tendon is fully effective and all the force of the tendon is fully effective by the time you get to the face of the wall where the moment is. So this is again, a factored moment diagram for traditional case versus slip form. You don't see a whole lot of difference between the forces, but where it gets interesting is in the traditional case where the tendon is anchored inside the shear wall, all the force is effective. And as a result, the post-tensioning is sufficient to resist the moment and the reinforcing that is provided is often just to satisfy the code of minimum reinforcing. But in a scenario where the tendon is anchored outside the shear wall, there is nothing at the face of the shear wall other than the rebar that you have to design to provide. So when it comes to slip form and jump form construction, it is important to make sure that the tendons are modeled where they will be placed to make sure that you don't miss out on providing negative moment steel where it's required. We get into a little bit on diaphragm design. I mean, post-tensioning, you know, everybody's familiar with thinner slabs, longer spans and things of that nature, but it also benefits in resisting diaphragm forces. And this is codified. And all the references that you'll see here from ACI are from ACI 3.18.19. And code is pretty clear in saying, it shall be permitted to use pre-compression from pre-stress reinforcement to resist diaphragm forces. And the commentary goes on to say that, you know, when you have two transient loads, such as a live load and a wind load or a live load and a seismic load, the assumption is that they're not both acting with the same load factor as they would when they're independently acting on the building. So you end up having residual or additional capacity from the pre-stressing that can be used to resist the diaphragm forces. So as an example, this is again, a layout of a traditional high-rise. Yeah, you have your central core and you have columns and then you have your diaphragm. So in this scenario, we're assuming you had 40 PSF of wind load applied to the facade of the building that the diaphragm needs to transfer to the core, 10 foot floor to floor height, 400 PLF of horizontal load. The slab is cantilevering 92 feet from the face of the core, which results in about 1,700 kip feet of moment. The diaphragm is 84 feet deep and you end up with a diaphragm section modulus of 1.35 million cubic inches. Do your M over S, you end up with a 15 PSI reduction in pre-stress on the tensile side of the diaphragm. So if you imagine you have slabs upwards of 150, 170 PSI, you're only losing 15 PSI of pre-compression. And this is again, what the code is talking about in terms of the load combinations that are occurring simultaneously. So traditional load combination, just under gravity loads is 1.2 dead plus 1.6 live, you end up with 208 PSF. But when you have gravity load in combination with lateral loading, such as wind or seismic, you have a smaller factor, which reduces it down to 164 PSF. So you have actually provided 208 PSF worth of post-tensioning in your slab, but under a transient load event, you end up having excess capacity, which is what is referred to as a residual or leftover pre-compression that can be used in the diaphragm design. Same thing with the allowable stress. Key thing here to remember is that when you're doing allowable stress design, if you're calculating your wind under factor loads, you are allowed to reduce your wind down by 0.6 factor. And when you're applying live load and wind load simultaneously, you're allowed to take a further 25% reduction. So by the time you're said and done, even if you only had 150 PSI, your diaphragm stresses from lateral load is only 6.75 PSI. So typically you will find that for wind and moderate seismic, the diaphragm will have enough residual pre-compression to handle the moments. So now we've talked about residual pre-compression, but there are scenarios where there isn't residual pre-compression or pre-compression starts to bleed away, which I am calling it decompression. It's something that can happen on projects. Again, it's more localized. It's not a global behavior. This is again in the center, a slide that shows a 40 story building that had columns sloping up to the 13th floor. And then you had a straight shot of column going up all the way to the 40th floor. So as you start building on the gravity load on the column, the raking column is going to thrust outward. And the level 13 diaphragm, which is highlighted in blue here, as the gravity load starts building up, it's going to stretch out. And when it's gonna stretch out, it's going to take out the pre-compression that you put into the slab. So you may have a scenario where depending on the loads, you could be losing up to 120 to 125 PSI of pre-compression. So if you ended up putting a system 250 PSI, you may be only left with 150 PSI to resist your loads. Again, it may not be a strength issue, but it may factor in deflections. So it's something to keep in mind. A lot of times my preference is to provide additional tendons, put the lateral thrust load into your analysis. So you have the pre-compression bleeding studied and you will provide additional reinforcing to cater to that. In this particular project, we had these corner columns sloping in both directions, which is why you'll see additional tendons added in the north-south direction and additional tendons added in the east-west direction. This phenomenon can happen again on columns where you're walking columns. You have a residential level and you may have a amenity or a podium level and the columns may need to move out of the way to create space for a drive aisle. And what you end up having is a localized effect where this column load from this location has to get transferred to this location and causes a rotation in the system, which is resisted by the diaphragm in a tension-compression couple. The reality of this, this is a simplified model, obviously. The reality is more slabs will participate in resisting this moment, but most of the decompression will occur right at the level where the transfer is occurring. So again, it's something to pay close attention to in terms of design and detailing of tendons. We'll talk a little bit on construction considerations because we've talked a handful about design considerations, but it's important to understand the percentage of cost for formwork materials and labor. So as you can see here, the concrete material and reinforcing material only contributes to about 30% of the cost of construction of a floor slab or any concrete building. Remaining 70% is in formwork material and formwork labor or rebar labor. So we'll look at a handful of scenarios of detailing, things to watch out for. So this is a fairly common detail that you will find in drawings where you have slab edge bars with a 180-degree hook. It's difficult to install, and it may seem like a good idea to provide a 180-degree hook, but oftentimes it is difficult to install because you cannot really drop that 180-degree hook bar. And so oftentimes we'll show 90-degree hooks at the slab edge because it can be placed after the tendons are installed or your dead-end bursting steel is installed. It's easier to just drop them in place. So consider providing 90-degree hooks at slab edges. There's also a discussion about using a stirrup hook instead of a standard hook, which has an even shorter tail length. The only thing is that the stirrup hook needs to be anchored around a horizontal bar, which oftentimes you will have in a peaky slab. Another consideration is sizing the concrete strength for the floor and column system. ACI is very clear in terms of their recommendations that the floor system shall have at least 70% of the column strength because you want to make sure that the load is traveling through the slab into the column below. So there's a handful of different strategies. You want to make sure that when you size your columns, try and size them in a manner where you don't end up having to have the contractor provide localized high-strength concrete. I mentioned earlier that Chicago uses 8750 PSI concrete fairly often, and that 8750 PSI number just didn't come out of thin air. That came up because engineers are using 12,000 PSI concrete for the columns and considering the 0.7 factor on the floor in order to avoid puddling you up the concrete strength of the floor slab. But again, oftentimes it's not necessary. You may be able to just use 6,000 PSI on your slab, as long as you consider 6,000 over 0.7, 8,500 PSI on the column. You know, that may be one consideration. Size your columns for 8,500 PSI. If you're an interior column, the code also lets you use 35% of your slab strength plus 75% of your column strength. So if you end up doing that, 9,600 PSI versus 10,000 PSI is not a huge difference. So that's something you want to consider using. Again, important thing to remember is that it only applies for interior columns where you have confinement from the slab on all four sides. It does not apply to edge or corner columns, at which point you're better off either making them larger or providing additional dowels as needed to make up for the bearing area transfer. Slab steps. I mean, this is something that you will often find yourself with on high rises where there is a parking and then there is a podium on top of the parking and then you have a tower that continues up. It doesn't have to be limited to high rises. You even have it on mid-rises and low-rise buildings. You know, best case scenario is you're trying to make the step happen at column lines. That's the cleanest way to try and do it. Oftentimes, we'll find that slab steps range from anywhere between six inches to 12 inches. And the reason for that is most of these will be waterproof. The thickness of the step needs to be sufficient for them to terminate the waterproofing to be warrantied. So oftentimes, you'll end up with a fairly substantial step, a step that is thick enough to be able to stress your tendons. So any of those scenarios, anytime I find myself in a scenario where I can actually stress my tendons at the step, I'll go ahead and do it, as opposed to trying to take the tendon in from the high slab down into the low slab and risking the profiling and localized curvature of the tendons and causing blowouts. This is something that has worked pretty well over the years, and contractors haven't complained much about it either. So try and locate your steps where possible at column lines. There will be scenarios where that doesn't happen. You know, this was, again, a project where we had an amenity deck and a clubhouse. And the clubhouse ended up being a transfer slab because they wanted setbacks around the perimeter for this clubhouse to be able to walk around the edges. So on this particular slab, it was simpler to just design the slab as a flat slab with additional loading applied with the topping. Again, it was done for formwork ease. You know, it may have a little bit more material, but, you know, the total cost ends up being less because once you consider the 70% cost of installation versus 30% cost of material, the economics work out to having a slab with additional, a topping slab, oftentimes done with a rigid installation, a geofoam, and, you know, a three or four-inch topping slab. Now we get into shoring, reshoring. This is, again, something that is extremely important, extremely critical. Key things here to remember is when you're pouring your live deck, the reshoring is designed considering what is the reserve capacity of the slabs. And with that, what I mean by that is the design live load and superimposed dead load combined, those loads are not in existence during construction and then end up being used as a reserve capacity to support the load of the live deck. Important things to consider are that the slab is stressed at 3,000 PSI. So these slabs are at 3,000 PSI and gaining strength. And the reshoring loads that you're applying are going to be oftentimes the highest load that the slab will ever see. The contractors are really doing us a favor by load testing the slab for free. So that's something you want to keep in mind. But again, don't abuse it because you want to make sure that the slab has enough reserve capacity to handle the reshoring loads. You want to take a look at what the slab does in terms of initial strength gain during the early age. Type 1L cement has had issues as far as strength gain in the early age, so something to be cautious about. Worry about built-in deflections because if you have long spans and the slab is deflecting from the reshoring loads, you end up having that built-in deflection on the upper floor. And then again, you want to consider long-term deflections when it comes to reshoring loads because you're loading the slab when it is at its weakest and it is potentially likely to crack, and that crack is going to stay in the system. Poor strip strategies, again, you know, this was a project that we recently completed. It was 425 feet long. Had restraining elements at the end and in the middle. You know, for those of you that attended last month's webinar on restraint to shrinkage, it's an important consideration for a slab that are long. The strategy there could have been to add two-pore strips and break the slab into three pores, but from discussions with the contractor, what was preferred was to do two pores, and we ended up just isolating that central core from the edges, and that allowed the contractor to just have shoring in place localized and have all this other area available for interior fit-outs. Things to consider, you know, oftentimes 28-day pore strips is, again, a relic. It's a good number. I'm not saying 28-day pore strips are bad, but it's, on projects, a good idea to have the contractor do a shrinkage study on the particular mix that's going to be used to look at how the shrinkage occurs over time. So on this project, we found out that, you know, bulk of the initial shrinkage happened between day seven and day 14, and then another small amount between day 14 and day 21. So when we got to the upper levels of the project, we let the contractor close the pore strip at 14 days, and we didn't have any issues as a result of it. This is, again, you know, I'll show you a couple of different slides here on traditional pore strip strategies and alternate pore strip strategies. And what this shows is, you know, you have a pore strip that is placed in the, close to the inflection point of, say, a 30-foot span, you know, so it's somewhere between 0.25L and 0.33L. That's your pore strip. The key thing here is this long span on the left side does not have any capacity to support itself. So you start out at day zero, shore this slab, pour this slab. Seven days later, you come back and pour this slab. So this particular shoring is now carrying the weight of two levels. And then you move up to the next shore level. So you start accumulating loads on the formwork, and then you start to close the pore strip at seven days after, 28 days after your first pour, kind of work your way up. And then you get to 49 days when the last level is closed and the time it takes to gain design strength. That's when you can start stripping the formwork at the topmost level and then work your way down. So there is a significant amount of time that the slabs have to remain shored in a traditional scenario. And this may not be a big deal on a four or five-story building, but it could be a huge problem on 15, 20-story buildings. So this is an alternate strategy where you design the pore strips to have a balanced cantilever where each cantilever is self-supporting. And once the slab is stressed, you can remove the shoring, slab supports itself, and you go back and put the reshores to carry the weight of the wet weight of concrete from the levels above. So the nice thing with this is each slab is assumed to be taking equal amount of load. The formwork actually sheds load as you go down because the load is being transferred to the cured slab. Things to keep in mind as you're designing, there is a locked-in force that you have to deal with. When you have a slab that is cantilevering, you design it for the pore strip plus reshoring loads and once the reshoring loads are removed, the slab will rebound. And then once the slab is cured, you have the SDL and live loads that get transferred as if it would in a continuous scan. And these two values, the value in a closed position with the locked-in force needs to be less than what the slab capacity is when it was designed as a cantilever. Roof slopes, again, important thing here to keep in mind is slabs will need to pitch to drains at the roof. And one of the things to keep in mind is it's often economical to build the slope within the parent slab to accommodate the slope. The benefits are twofold. One, you see that you end up using less concrete thickness to achieve that same slope and you benefit from having a larger tendon drape. You can do the math on here to figure out, you end up with 1.5 inches of thickness at the low point and 3.75 inches of buildup. And then you end up adding to a 12 and a half inch thick total concrete. The important thing to keep in mind is that the additional concrete isn't an average thickness, but it's two thirds the thickness of the buildup added to the base slab. Transfer girders, again, you have to be pretty careful designing and detailing them. We often like to push the ends of our transfer girders beyond the columns to account for the number of anchors that need to be provided in all the bursting steel and trying not to conflict that with the column reinforcing that's going to be hooking in there. So some detailing considerations, just make sure that your end of the transfer girder extends beyond the column. Next few slides, we're going to talk about the importance of tolerances and studying deflections. This is something that I read when I started practicing as an engineer and it stuck to me always. Strength is essential, but otherwise unimportant. And how many times have you gotten into trouble because the slab has deflected too much or a slab is cracked, right? I mean, you typically had issues related to serviceability of the slab rather than strength of the slab. You want to make sure that you study the long-term deflections in consideration of the different loading that happens on a slab throughout its life cycle, such as from shores drop, you apply a reshoring loads. And then once the reshoring loads are removed to interior build out and things of that nature. You want to study these different stages along with the loading that is applied in those stages to arrive at a true long-term deflection value. And oftentimes it's good to compare that with the ACI values of three times the immediate deflection. Long-term deflection, again, kind of an important topic when you start designing thin peaky slabs spanning 30 feet and 35 feet. You want to pay attention to how much your slab is deflecting at the slab edge to allow for enough tolerances for glazing. You know, a lot of that tolerance is accounted for at the head joint of the glazing. And some of it is often in the range of three-eighths of an inch to half inch, sometimes three-quarter inch pushing it. You know, as a firm, we design our slabs to have no more than five-eighths of an inch deflection long-term. And that's something that has worked historically pretty well for us. We try and stay, you know, well upwards of L over 700 or 750 when it comes to slab deflection at flooring installation. So you can study all these different stages of deflection in your analysis software and arrive at what the slab deflection will be over a period of time. And it's good to have this information and be upfront with the contractor and be transparent with it. Because next time you get a phone call and saying, oh, all this is because your slab is deflecting, you have the data to at least have a discussion. Is it slab deflection or is it construction tolerance where the contractor didn't pour it to the right tolerance? So it's good to have the information and be willing to share it early on in the project. You know, just play it transparent, play it open book. Another common myth is, you know, post-tension slabs get blamed for everything. Oh, the slab is moving too much because of post-tension. It's not the case. You know, elastic shortening from post-tensioning is often in the five to 10% range. The remaining 90% comes from temperature creep and shrinkage. So post-tensioning only contributes a small amount to the overall shortening of the slab. So something to keep in mind. Balconies, again, typically you'll have 3 1⁄4 inch step or 1⁄2 inch step and then sloping away, but pay close attention to what the backspan is doing. When you have a short cantilever slab with a long backspan, the tendency of the backspan is to deflect and lift the cantilever up. As you can see here, you know, if you had a scenario in this particular case, the cantilevered balcony was pitching upwards 1⁄4 inch under dead and balanced loads. It's important to look at this deflection just under dead and balanced loads and no live loads because that's typically gonna be the controlling case. You know, when it's pouring down rain, nobody's gonna be standing in the balcony. So there's gonna be zero life load in that balcony. So you wanna pay close attention to how much your balcony is pitching upwards. Pay close attention to punching shear at balconies because you have rebar that gets dropped to go into the balcony, which is depressed 3⁄4 inches. So you end up with less effective depth for the rebar. Again, you wanna pay close attention to whether you have pre-compression. You know, you may have a balcony that's projecting out and you have tendons running in the north-south direction. A lot of that pre-compression isn't going to bleed into the balcony, so you wanna make sure you provide temperature and shrinkage reinforcing in there. Same thing when you have slip form construction or space within the inside of the elevator lobby. You wanna make sure you design these slabs as RC slabs in one direction because you don't have the benefit of any pre-compression in there. This again is a slide that shows some of the problems and we got called in to look at a slab that was cracking. And sure enough, the majority of it was due to the sawtooth balconies and pre-compression not getting into the sawtooth. And there was a slip in this wall that almost created a column-like scenario. And you ended up having a lot of cracks in this yellow shaded area. Stud rail layout, again, you wanna be careful in terms of how stud rails are laid out. This is an orthogonal layout, but if you have a scenario with columns that are following an angled orientation, these studs will get in the way of you running tendons over the column. So a modified layout could be to orient the tendons at an angle and follow it and create some pathways for the tendons to go through. Similarly, you wanna try and lay out stud rails around a round column in an orthogonal fashion rather than a radial fashion because the tendons here in red will have a hard time going through from one side to the other interrupted by the stud rails. Some of these things, again, I have a title here, Who Sandboxed? Because there's plenty of players on projects. You have your tendons, the idealized tendon in red here. And then what ends up happening is tendons get placed as shown in green here because you have MEP penetrations, you have conduits. All those things are something that you have to pay close attention to. Again, be proactive during design and during coordination. Make sure that you explain what the no-fly zones are and don't paint yourself in a corner, especially when it comes to punching shear. Don't paint yourself in a corner. If a column is working at 80% DCR, demand capacity ratio with an opening right next to it, don't hesitate to provide stud rails in there because more often than not, you will find that there's a couple of more sleeves next to the column that you were not told about during design. So give yourself the cushion, provide stud rails. They're cheap insurance. Watch out for glazing embeds. You know, the code requires you to have two tendons running over the columns, but oftentimes you'll find that there is a glazing embed right behind the column or at corners where it is impossible to take tendons and stress them at the slab edge. Thankfully, ACI appreciates that scenario and they have requirement to add integrity steel as a alternate to running tendons over the column. So you can provide reinforcing that is calculated as a function of the slab strength and the width of the column in the direction under consideration. And you can provide reinforcing that is anchored beyond the face of the column to allow for the tendons to be moved out from over the column. Again, handrail attachments at balconies. As again, it's a common coordination issue. It's always good to discuss that with the contractor, try to get them to coordinate that as an alternate if there are dead ends at the balconies, trying to push them in a foot from the slab edge so that there is no tendon in the way of when they go to install some of these post-installed anchorages. And, you know, lastly, this is a slide of, oops, you know, top four items that I will typically see on projects. You know, one of them could be, oh, we just got notified that the electrician drilled into two tendons. I mean, how many of you have got that email? It's pretty common. It's always the electrician. I don't know for what reason, but it's always seems to be the electrician that's doing this. The key things there, you know, some of the strategies are to see if they are integrity tendons. And if they're not integrity tendons, then just make sure that you are meeting the minimum P over A requirement. Oftentimes we design our slabs to be somewhere between 140 to 150 PSI precisely to avoid the scenario where if you lose two tendons, you're less than the 125 PSI minimum rule. You ask the contractor to provide you asphalt forces. You know, look at the slab strength. You may have more strength in the slab than what you considered live load reduction. And this is something that as a firm, we do not use live load reduction during the initial stages of design because we want to leave ourselves opportunities to reduce live load in future for things like an electrician drilling into the slab and you're losing tendons. You'll find sleeves getting missed and the contractor wanting to core through the slab. Again, we want to make sure that they scan the slab before they core so they don't core through a tendon. Oftentimes it's acceptable to core through PT slabs as long as they're not hitting tendons or reinforcing. This again, we've seen this happen more recently as a trend with the new type of cement where the slab doesn't get up to specified strength at 28 days. I mean, again, there you want to consider the impact of punching shear. You know, if you assumed a 6,000 PSI concrete in the slab, it only gets up to 5,000. You want to make sure that there are no punching shear issues. You want to look at long-term deflections. Again, you want to look at live load reduction as it's applicable and see if you can justify a slab at a lower strength. This is again, another common one where tendons are outside of the tolerance of plus or minus 7%. So my strategy always is, you know, if the tendon is stressed way more than the 7%, even if it's at the 10 or 11%, you want to make sure that you leave those tendons alone because if you go to try and fix it, you run the risk of breaking that tendon because you've already stretched that tendon far. If it's in the plus 7% range, you know, there's a lot of energy in that strand because deflection is just a proxy for force. So the more you've elongated it, the more force you've built into it. And in order to release the wedges, you have to go in and yank on it even more and you run the risk of breaking a tendon. So you're better off leaving some of those tendons alone. Anything less than 7%, you look at it. You know, you get an asphalt force if it's one or two tendons in a band of plenty of tendons, chances are that you can leave them alone. I have found out that anytime you go and try and fix a problem that is not a problem, you create more problems for yourself. So just something to keep in mind. PTI had a couple of good articles in July of 2007 and February, 2012 that addressed issues related to short elongations and how to address some of those. And we're at the end of the presentation. I did not want to end the slide or end the presentation on a oops note. So I wanted to end on a more positive note in terms of what post-tension can do and possibly only what post-tension can do efficiently, economically and reliably. You know, long cantilevers, transfer girders holding up multiple floors or even cantilevering very far to not have columns. And this is something where we had to create a loading dock in the corner of a building. And we ended up using the upturned crash rails and put some post-tensioning in there to cantilever 20 odd feet to make it work. So with that, I like to thank you for your attention and want to thank PTI for the opportunity to present this to everyone. All right, awesome. As we mentioned, you know, we're trying to fit several hours, if not days worth of information into a single about 50 minutes worth of presentation time. So if you have any more questions on this, try and get more in the weeds, feel free to reach out to us or reach out to the presenter here and all of us will be happy to answer them as you bring them to us. We have time for one quick question and then we'll tell you what the next three webinars are gonna be. So a real quick question for you. Is there guidance for the radius or offset for the tendon sweeps? I've seen allowable offset, but this does not seem to address the radius which results in the force. How aggressive of a radius or sweep is common in your experience? So talking about sweeping tendons, how aggressive? Sure, yeah. So typically, you know, one in 12, fairly common, one in six slope, you wanna be very careful. You wanna make sure that anything exceeding one in six, you're paying close attention to providing hairpins in this lab. PTI has some good recommendations. Some of the post-tensioning suppliers will have a very good set of typical details on how they address that. So, you know, I'll be happy to provide more information to the person that asked the question via email. Yep, awesome. All righty, so our next three webinars, like I said at the beginning, it's always the second Wednesday of the month. It's always at the same time depending on where you are in the country. So for the eighth IRC, so the residential code, we're gonna get out the IBC 2024 code changes for PT side on grade. We're gonna go over that. June 12th, we're gonna switch topics. We're gonna get more into the bridge side of things. So resilience of PT segment on bridges. Then July 10th is evaluation of existing PT concrete structures. So that's the next three that are on deck. Like I keep saying, same time, we'll be sending out emails for it so you can register for it. And we look forward to seeing you guys all here in a month. Enjoy the rest of the day. See ya.

post-tension design

best practices

slab design

balcony detailing

formwork strategies

construction considerations

load balancing

tendon layout

structural design