All right. We're going to go ahead and get started. There's a few more people trickling in, but I think we've got the substantial amount in. So, good morning to those of you on the West Coast, and good afternoon to all those out East. Welcome again to the Post-Tensioning Institute's February webinar. My name is Kyle Boyd. I'll be moderating today's session again, and I'm the chair of PTI's Education Committee, which, as we discussed, is the one that really hosts these webinars. A reminder, PTI is hosting one of these webinars the second Wednesday of every month. We're planning on doing it for the foreseeable future. Put on the calendars. We're trying to look at least three months out. We're starting off with a bunch of building topics, but we're eventually going to get more into rock and soil, bridges, those as the year progresses on here. Today's topic is a very popular topic. It's one that has a lot of questions within the building industry, and that's post-tension podiums. We at PTI get a lot of inquiries about it. There's a lot of nuances to it, and so we felt it appropriate to do it as one of our more technical first sessions that we presented to you guys after last month's introduction to the new manual. Before we introduce today's speaker, though, we do have to go through a few classic housekeeping items on there. The first one is the exciting one. If we go to the slide that shows the RCEP continuing education credits, everybody on today's webinar is going to get a continuing education credit for being here. It's through RCEP, and RCEP is the one that issues it. At the end of today's webinar, what we'll do is we will send you, we'll send the information of everybody who's been on for the hour to RCEP, and then they'll issue your credit for your PDH there. To get that credit, you do have to be on for the entire hour. If you're not on for the entire hour, it will not generate that credit for you. If you do have to leave for whatever reason, you can log in online, and online you can watch the recorded webinar, do a quick quiz, and that's true for this one in February. You can also go back and watch the one from January. You watch it, you take the quiz, and then you'll be issued that continuing education credit there. We do have a copyright slide that just essentially says, thou shall not copyright. We're keeping our legal counsel happy by putting that one in there, and then we have a slide that just kind of goes over some of the fundamentals for the logistics for today's presentation, and that is that all you attendees, you're all in listen-only mode, so we can't hear you. If you got a question, answer it, ask it in the Q&A function, not the chat function. What we'll do at the end of today's presentation is we will take that Q&A, and we'll go through and we'll answer as many questions as we can. The ones we can't get to, we'll show you how to get a hold of us or the presenter on there. Now, we get to introduce our speaker, Mr. Brian Allred. For those of you who know him, he's got strong opinions, and most of them are very, very valid. He's the vice president of a very well-established engineering firm out of California that does specialize in post-tensioning. In fact, their group was really one of the groups that did create some of the founding principles of PT, as we know today. Brian's written books on the topic. He teaches at universities, webinars like he's doing today, travels around the country and teaches it. So, when we're trying to figure out who's best to do podium design, really, Brian was the most qualified person out there. I was talking to Brian earlier this week, and we were discussing he could really talk for about three weeks on podium design and still just be at the surface level. He's trying to condense it down to about 45 minutes. I was going through the slides. We probably even have a couple hours worth of content here. So, that being said, we really need to get to Ginn. So, Brian, take it away. Hey, thank you, Kyle. Like Kyle said, this is a very fascinating topic. There's a lot of podium slabs out there. A lot of podium slabs are going to be constructed, hopefully with post-tensioning. The aspect of a podium design is not unique to post-tensioning, so I'm not going to focus on the global characteristics of how do you do podiums in general. I'm going to focus on the fact that when you use a podium design and you use PT, what are the things to look for outside of the usual things with podiums? Post-load, smear loading, hold down bolts, hold down anchors, uplift, stuff like that. Those are all common to whatever material you're going to be using, whether it's precast, rebar only, or post-tensioning. What I'm going to focus on are hopefully PT-specific items. Now, having said that, to add on to what Kyle said, this is a huge topic. I am going to do my best. I'm not that good of a teacher in 45 minutes to cover everything. I'm going to leave my email address at the end of the slide. Feel free to email me with any questions, specific or general, and I'll get back to you as soon as I can. But first of all, I'm going to cover the benefits of post-tensioning, general philosophy, the recommendations that I think are important for podium slabs to help you keep you out of the oak-paneled rooms or having an expert tell you how wrong you did it. A big thing I'm going to talk about is caps or stud rails. I see a lot of new engineers potentially getting themselves in trouble because they don't understand that caps and rails are not interchangeable, especially in podium structures. And then I'm going to talk about conduit, which is if you do anything with parking and you're dealing with EVs and solar, PV, Wi-Fi, all this kind of stuff, the conduit has gone up exponentially these years, and it's now literally our biggest issue to deal with to the point where the second-place problem doesn't even exist in my mind. So conduit was always it. In general, podium slabs are typically a concrete structure that supports a wood frame or metal steel structure up above. They're typically two-way flat plates. You can use long-span beam systems like a long-span garage structure that you see at stadiums or casinos or hotels, stuff like that, or the really, really 2,000, you know, 3,000-car garages, stuff like that. The only issue really with the long-span beams is they create path-to-travel issues for the MEPs, the conduit, and also long-span anything, even if it is post-tensioning, unfortunately, has some deflection and vibration issues. And when that translates up into the wood, that's where you get upset people. Obviously, the podium slab is designed for the vertical load, but also if you're in high seismic like I am in California, you have to take all the horizontal and lateral loads of the superstructure. That's just part and parcel of the system. This is a very simplistic viewpoint of one. Obviously, slab on grade down here, if you can see my cursor, elevated parking deck, and then elevated podium slab with about four stories of conventional wood framing. Now, when I say podium structure, I typically am referring to something that has a bearing wall system. If you're spanning 30 feet with steel columns, it's really not a podium structure. They're not set up for stuff like that. It's more of the 2x8s, 2x10s, the TGI equivalent that goes to bearing wall. So you have more of a uniform surface loading than columns at 30 feet on center. Here's another photograph of a podium slab. You can see here the multiple steps at the courtyards between the walkways and the units, which I'll cover in great detail later. Relatively small columns. Again, it's a two-way slab, so the moment transfer is not that large. And then our favorite column caps and set of stud rails, and I'll talk about that in a little bit. But basically, you're just designing a two-way podium slab. It's just for really, really, really heavy loads. So the philosophy is the same, punching shear, flexural stresses, all that kind of stuff's the same. It's just more than a hotel, apartment, or a garage. And that's effectively how you would do it. Now, in terms of lateral system, because you're typically using shear walls as the lateral system, the concrete box or masonry box, if you're using masonry, is unbelievably stiff compared to the wood. So you can do a two-stage analysis, which is almost imperative because if you had to translate the seismic forces up to the roof level of the wood, of the concrete weight, there's no way it could take it. So you almost have to justify a two-stage analysis. For people like me who specialize in concrete, a lot of times the podium structure engineer is not the same superstructure engineer. We do a lot of design-build projects where we come in as a concrete quote-unquote expert. We bring in our equivalent wood expert to get the best bang for the buck. Below the podium slabs, you're typically looking at parking or retail. We've done some structures where they kind of build almost like a little city. They'll have a dry cleaner, maybe a 7-Eleven where you park your car and you can kind of live almost in your group for a small amount of time, of course. But for the most part, parking and living go hand in hand, especially in California where everybody has to have their cars. The fire separation is provided by the podium slab and that creates one issue I'm going to talk about in a little bit. As I mentioned before, numerically, the design is very similar to a slab. It's just more of it. The beauty or the devil or the god in the details, whatever you want to say, it's the coordination of all the trades going into your slab. This is the proverbial five pounds of stuff in a five-pound bag and there's no extra leftover and there's no, you know, the bag is full and there's nothing left on the job site. Plumbing, penetrations, conduit, kitchens, toilets, air conditioning, anything you have for four stories of wood has to go through that deck to a certain extent. That could go through columns, that could go through band anchorages, stuff like that. Trying to do the dance so everyone is equally unhappy with the design and nothing really is bad, that's where you make or spend your time or make your money either way. For post-tensioning, the interaction or the step between the courtyards and the units has a big impact on the design and performance. I'll talk about that in a little bit. A lot of times if you have large landscaping elements, those can govern the design or need a thicker slab. So if you're talking about four stories of wood with some live wood, you're probably in the 300, 250 pounds a square foot range. You have landscaping at four feet of dirt or three feet of dirt with a whole bunch of queen and palms, you're going to kill that and then some. So be very careful with landscaping, especially mounds of dirt, which obviously weigh more than or almost as much as concrete, and the trees or mature weights can get 20 to 25,000 pounds. One of the things I'm going to hit on at the very end is I recommend placing all unit conduit or the vast majority of it under our slab, not in it. The mountain of conduit they try to put in there and try to justify is scary if you've never seen it. It's hard to justify having a 12 inch deck when you have 100 pieces of conduit three inches apart that are 1.5 inch OD. I mean you don't have 12 inches, you have six up and six bottom. That's not the same thing as 12 inches with lack of composite action. So the conduit again is part of that tradesman and we have to get all the stuff in the five pound bag. Now this is a typical architectural cross section of the podiums that we deal with. Obviously slab one grade, parking one, parking two, and then this says the podium slab and you can see here that obviously the bearing walls for the wood structure are not aligning with the columns. So therefore the podium structure itself is taking the load. So if we zoom in on this part right here, you can see it's called that as a three hour podium. That's the fire separation. So when you go from parking or something else to units and you change occupancy, typically you're going to have a different fire rating for podiums. You're going to go up to three hours. Now the reason that matters to you is that there is a section of the code 721.1. It's a table, the unrestrained bays. And again, just write this stuff down. We can talk about it later, have a different cover requirement than everything else. So you can have a 20 span two way slab spans. One in 20 are going to have slightly different cover requirements. If there are three hour or two hour rating, then spans two through 18. So you want to know a what's the fire rating and B where your end spans are, because that's going to affect the actual mechanics of your design. The other thing is to help the architects out. As you can see here, there's a pool vault, which a lot of times happens for podium slabs. Everyone loves a nice pool. The one thing about pools is obviously you need a lot of depth for that. So if you help your architect out, I typically would budget about seven feet for a pool vault. So if you want to drive underneath there or have a legal space, make sure you account for that. So let's say you have four feet of water, you have six inches of coping top and bottom. So coping above the waterline and then six inches of gunite. So that's going to be five feet, 18 inch thick slab. That'll be 6.5 feet. And then six inches for lines underneath it. So at least seven feet for that cavity, if the architect wants to have anything usable beyond that space. And we found that not that they can't do the math, but there's just a lot of things in a pool that keeps adding and adding and adding to the depth. So at least seven feet minimum for those types of structures. So if you're new to post-tensioning, why am I here besides it being free? Why do we use post-tensioning and podiums? Well, it comes down to one big actual thing, a thinner slabs, reduction rebar. We can do longer spans, speed of construction. We don't reshore anything in post-tensioning. So once you take your slab aside from poor strips or construction joints at that edge, once you pour the slab on a Thursday or Friday, it hits the brakes. You stress it successfully. You pull the shores. It's good to go. So there's no 28 day reshore requirement. What does that all come down to? Money. It's the only thing that matters. It's the only thing we're actually doing that's beneficial for the most part in terms of the owner's eyes. And we're saving mountains and mountains of money. And that's why we go through these quote unquote headaches. Post-tensioning is the most challenging numerical floor system out there. We have to design for allowable stresses and ultimate strength. We have restraint to shortening issues and transfer stresses and all these different things. But because we are so effective, we can save mountains and mountains of money. I have never once in 30 years lost a job because the rebar equivalent deck costs less than mine. Typically, in my experience on a podium slab versus a well-designed rebar deck, I can take three or four inches out of the slab, which usually results in plus or minus four pounds a square foot of rebar. That's where the money is saved. Yes, less concrete is good, but massive reduction in rebar is where all the money takes place. The other performance aspect of post-tensioning is that with rebar concrete and to the similar effect, post-tension concrete, it's not the strength issue. I think our two-way flat plate slabs, whether the PT and rebar are way stronger than we give them numerical credit for. Where I've seen issues, and this comes in a legal sense for HOAs or apartments where they get the engineer in trouble for the podium, it's not so much a lack of strength or a punching shear issue or something like that. It's the slab starts to creep, deflect, and those long-term creeps can put a factor of three. So even if you camber out a slab and put a rebar deck only and camber it up one inch and then build the wood on top, if that thing goes down two or three more inches, it doesn't matter where it started and where it ends because they're going to drag that wood with it. So the long-term multiplier on deflection and then the impact on the wood, the facades, the doors, the windows, that's where I've seen a lot of people get into issues. Now with podium slabs and PT, we don't have that particular issue because if you use PT correctly, you do high balance loads based on the concrete self-weight, your initial deflection is almost close to zero, and I'll show you that in the next slide. You can take a multiplier of five or six or 10, and if you're basically starting out at zero deflection, multiply it by 10, it's still going to be zero. I rarely see well-designed, even poorly designed, but well-constructed PT slabs have deflection issues. The continuity effect, the pre-compression, the balance load effect really helps the deflection issues. Not saying it goes away, but it's a lot less prevalent in PT than it is in other structures in comparison to rebar only. Now, if you look at this structure, obviously it's done for dramatics effect. If we do this in post-tension, and again, this is a real slab I designed, 16 feet, 28, 30 feet, 29, so those are decent spans, 28 and a half foot wide trips, so again, roughly a 30 foot on center grid, 11 and a half inch slab, which you can see right there. The resulting deflections of dead plus balance load, which is the PT upward force, let's call it, some of you may call it equivalent loads, so all the dead load, concrete, wood, partitions, whatever, we're talking about, sorry, 0.011, L over 32,000. If you're used to wood and steel, you're talking about L over 360. I'm two zeros above you. This is effectively zero. The live load, L over 5,000, L over 18,000, the 30 foot bay, 0.01. So take your multiplier, hit it by a factor of 10. That's still only 0.1 of an inch. There is minimal deflections when you do it right. So again, not to say you don't check it, not to say it's not a thing to obviously look out for, but typically when you do this correctly, the deflections are laughably minimal, and that has a beautiful impact by no one sees anything in the facades. So this is what it looks like out in the real world. In typical two-way slabs, you have banded tendons, which are grouped over one direction of the columns. You can see here they're about five feet wide, typically groups of four to five strands and nine inch bundles, and they go right across, very similar to a steel girder line, if you're more used to that type of framing. The uniform strands, you can see here there's a bundles of three and four. They're about three or four feet on center, depending on the specifics. Those are more like a joist or a one-way slab system where you just have this uniform, you know, force distribution. The reason, without getting too far in the details, is why one direction's banded or one direction's uniform has nothing to do with engineering or numerics. It's purely for speed of construction and simplicity. Back in the day, they did a basket weave system, the exact same number of strands, everything was the same, but trying to basket weave things when things are draping high and draping low and they're perpendicular was very difficult. So years and years and years ago, they developed the banded system, and this is what you're seeing here. So bands here, you have top rebar over the columns, almost no top rebar at mid span, which is typical. The PT tendons are transitioning through, excuse me, you have anchor bolts here, drain lines in the courtyard. As you can see, you have a whole bunch of anchor bolts over in these parts. So it's getting all this stuff in while at the same time maintaining structural integrity. That's the real, you know, engineering challenge that you're dealing with in these types of structures. This is just an opposite viewpoint. You can see here the banded tendons right here, going along the column lines, top rebars, we have a column cap here, anchor bolts along this dropped edge, and you can see the unit line goes all the way along this side. So this is the edge of the units. There's a small courtyard, and then you can see other units there. But again, you can see the anchor bolts over here, a lot of plumbing penetrations in the background for back-to-back kitchens or bathrooms. But again, if you look at this and compare it to, let's say, a rebar-only slab, there is substantially less reinforcement. If you're more used to a two-way rebar podium slab, you're going to have roughly no chance to actually touch the forms because there's top and bottom rebar each direction from grid one to grid, you know, 32. In this case, you actually have plenty of room to touch the forms, guaranteed to get your pants dirty, but that just shows you the effectiveness of the post-tensioning with the drapes, the force of the strands, and how it works. We need very, very little rebar in terms of our, you know, competitors, let's say, for lack of a better word. And that's the beauty of the efficiency, and that's where we save the money with this layout. So again, very similar to any other two-way slab, but like anything, it's just more of the same. So typical two-way slabs, what should you expect if you're new to doing it? I would say typically between 10 to 14 inches with column caps. Again, I'm going to stress that, no pun intended, for a little bit. Typically, much like steel, we're at a 30-foot on center range. The 30-foot on center is not a magic number, but typically, if you have parking below, which you typically will, 30-foot will get you three normal stalls, and even with these massive EV cars these days. So typically, the 30-foot, 28 range will get you three stalls, and then 28 to 30 will go over the drive aisle. Again, I'm going to hit this, caps and rails are not interchangeable, which I'll cover in a little bit. When you design post-tensioning, there's something called equivalent or a balanced load. Now, some engineers, and again, this is where we disagree. I think Kyle mentioned I have strong opinions, and he said I'm mostly right. I disagree with that. I'm always right. But aside from that, some engineers will compare equivalent loads to total dead load. If it's a garage, it's an extra five pounds a square foot. Nobody cares. If it's an apartment building, maybe it's 25 to 30. When it's a podium structure, it could be up to 300 pounds a square foot and your deck's only 150. To minimize the chance of blowouts, you can compare it to whatever your industry or your office standard is. However, you must compare it to the concrete self-weight because when you pour that concrete on a Friday and you stress the 33,000 pounds each one strand, you only have naked gray concrete to resist that force. So if you deck at 12 inches thick, weighs 150 pounds a cubic foot, and you push up with 450 because you're trying to balance out the future apartment deadload, which won't be there for nine months, it can A, lift up severely, B, crack, or you get some pretty serious blowouts because you're literally pushing up on something with three times its weight. So, and again, I'm not trying to scare anybody or say don't use PT, but you have to be careful because PT is active reinforcement. It's not passive. If you put a 14 by 22 in, but they build an 18 by 35 steel beam, it works by default. Nobody's gonna care. If I show 14 strands, but they put in 28 strands, that's active force, I'm pushing on it 24 seven. It has a huge impact. So it is active, not passive. Being conservative, grossly conservative, can be somewhat of an issue. We'll cover that in a little bit. As I said before, the wood structure will not be there for months. So when you check balance loads, it's self weight of concrete. Be careful about using substantial overbalancing to make the stress checks work. As I mentioned, we designed for stress and allowable stresses and ultimate strength. It's not A or B, it's A and B. One way to use a very, very thin slab and make anything work effectively is substantial overbalancing. But again, when you stress those strands, when there's only concrete, but you're pushing up with four times the weight of it, good things are not gonna happen. Now it's a really cool, slick, numerical, three-dimensional time history, upside down, fourth dimensional analysis to show it works. But you could have some very serious consequences. So stay within the parameters I'm gonna give you. Don't try to be ultra hero and be careful how much force you're putting in. This is a blowout, obviously on a podium type slab. You can see here there's anchor bolts for the future wood structure. And you can see this very large pimple that came off a podium slab. This was due to overbalancing. They draped the strands too much. Who draped them is debatable, but obviously the slab was 12 inches thick. It went down way too far. And when those tendons started to straighten out, it lifts up. I mean, just like a shoestring between your fingers. And if you way overkill it, or it's built incorrectly, and instead of it being at 10 inches from the form, it's one inch and you have this way extra huge drape and this force from that, this is what can happen. And this is a large chunk of concrete. It lifted up about a foot and you can see here this whole big circular area just got pulled up. So if you look at the backside of it, again, that just blew out six or seven, eight inches of concrete and just lifted this whole thing up. And unfortunately, or fortunately depending on how you look at it, this is the power of PT. It is wonderful. You do it correctly. You can have thin sections, saves money, no deflections. But if you don't know what you're doing or not paying attention, or, you know, in California, the marijuana is legal these days and you take a little too much, this, you know, joking of course, this kind of stuff can happen. So one thing I tell new engineers or the students that I teach, you be accurate with PT, you be conservative with rebar. Rebar has never caused a blowout, never caused cracking or camber up a slab. Post-tensioning is precise. Rebar, throw it everywhere. It's a party. Everyone loves rebar. So again, not to dissuade you, but just keep an eye on how things actually can be in these situations. So two-way podium slabs, they're same as two-way flat plates. The six square root number is still required. The code requires 125 PSI minimum. And that includes everything structural. So if you use a slab band, a beam, a drop cap, a panel, anything they're using structurally has to do with the 125. The 125 value has nothing to do with restraint. Now, restraint and shortening, that's a whole different topic. Probably equally as important to podium slab because a lot of times podiums are subterranean. If you lock up a PT building, typically it doesn't have good effects on the cracking or the performance of the structure. However, the 125 minimum has nothing to do. It's just a minimum like 0018 or 0033 for rebar concrete. It's just a place to start. Now, having said that, I will tell you honestly that if your design works at 125 PSI, most likely your system's too thick. I would typically see you more in the 175, 200 range. That's a quote unquote good design. But again, the 125 is a prescriptive minimum. For some reason, if you lose a strand or someone drills through it, now all of a sudden you're at 119, no one's gonna die. It's not gonna be the end of the world. It's just one of those minimums and another way to make sure you're having a reasonable design without being grossly over or under. There is effectively no code maximum pre-compression and this applies to anything. So basically what some engineers do is that if you have a slab that's too thin and you caught it too late, you literally just start putting in more and more and more pre-compression. The stresses like anything else are P divided by A plus or minus M over S. If you can't change the M and you can't change the S, obviously you can't make your slab thicker. You add way more P over A, more compression to knock down the tensile stresses so it works. Unfortunately, there is no real maximum pre-compression value. So if you're in the four or 500 PSI range to make your system work, A, you're doing something wrong or B, your slab is way too thin. Remember, if anything in PT, if you take one thing from this webinar besides caps and rails are not interchangeable, in PT, less is more. More PT does not mean more better. Punching shear controls and like anything, if you do it correctly in quotes, the deflection should be relatively small to the point where if you're used to steel and wood, they'll almost look like they are zero. So the keys to efficient design, the step detail between the courtyard and the unit, I will cover in a little bit. Obviously you wanna determine the full deadload and reduce live load. Podium structures have historically and through code used reduced live load, not a hundred pounds a square foot times four. It's usually reduced live load like it is a foundation. I would strongly recommend doing that. Obviously nothing wrong with higher live loads. You'll just need more deck. As I mentioned before, columns are roughly in a 30 foot grid for parking. If you go more than that, obviously you can do it. The slab are gonna get thicker of course and the longer the spans are, the more deflection potential or vibrations you're gonna have. And again, that's not the best scenario for having people living on a vibrating system. The slab thickness, as I mentioned before, I can typically take three inches out of a well-designed rebar deck. Don't be a hero. We already win the slab thickness competition. There's no one even close to us on that. No one's gonna beat PT going 30 feet, holding up whatever it is. Don't make it 10 inches when it's supposed to be 12. We've already won the game. Don't, like I said, don't be a hero. I would also make the argument that a slightly thicker deck that uses less PT and rebar is probably more economical than taking one or two inches more than you should and then packing it in with PT and rebar. So again, the slab thicknesses are in the PTI manual. Adjust them as needed, but don't try to be a hero by going from 12 to 10 and a half or something. It's not gonna work out well. Column caps or stud rails make that choice. Also check the slab thicknesses with rails and caps are not the same. Pre-compression levels typically less than 250. In my opinion, that's not like a PTI number. That's a Bryan number. If you're over 250 consistently, not just in localized areas, I would start looking at your design. The reason I say that is because, A, pre-compression leads to movement, and that goes into restraint. I would say in my experience, 95 plus percent of all cracks I see in post-tension structures are restraint to shortening, which is related to the pre-compression force to a certain extent. Basically, it's the more you push on a concrete system, PT in this case, the more it's gonna move, the more it moves, higher potential for RTS, higher potential RTS, more cracking. So if you're way over 250, I think your slab's too thin, I would look at that and go from there. Typically, the top slab rebar required by the code minimums control. So if you have 20 spans and 19 of them are controlled by the code minimum at a 175 PSI compression, to me, that's a good one. If 19 columns are there, and all 19 are controlled by strength, not minimums, I would say that's an indication your deck is probably a little too on the thin side. And again, goes through steps one, two, and three. Right slab thickness, you should have minimal slab deflections, which is key. Things that you don't wanna do is do not use over-balancing, make a thin slab work. If you're over-balancing substantially more than the concrete self-weight, I would say roughly about 125%, I would start reevaluating things. It's a numerical way to do it. A lot of times the new software may not tell you what your balance load is, but if you are balancing 300 pounds of square foot, when you stress a 150 deck, the issues are gonna happen. If your software is telling you that you need bottom rebar at the slab column joint, at the slab column joint, you need bottom rebar, meaning you have either way too much pre-compression or actually tension, that should send off some flax. Gravity load should always be tensile stresses on top at the supports and not at the bottom. So use those things to check, to make sure you're not going too far down the rabbit hole. Again, don't use large pre-compressions, make a thin slab work. Obviously, if your slab works with low balance loads, 20, 30%, and you're barely at 125, you may be too thick. So adjust accordingly, but use those percent balance loads and pre-compression as a good indicator of where you are. Add rebar where you're nervous, add rebar where you're not nervous, it's not gonna do anything bad. Huge notes about conduit and landscaping, and then obviously like anything else, you have to overlay the penetration plans and make sure you have good path of travel of your tendons while maintaining structural integrity. So real quickly, I'm gonna talk about the step detail between the courtyard and the unit. So this is a typical podium example we're gonna go through. 13-inch slab at 163 pounds a square foot. I have a topping slab everywhere at roughly 69. So the question's gonna be, why am I using a topping slab everywhere? And that's what I'm gonna cover. So the wood structure right there is at 205 pounds a square foot. Again, conventional framing, nothing funky, we're not trying to win architectural digest, but that's typically where I'm in the 200 PSI range for about four stories of wood with the roof on it. The live load on the structure is reduced. So five times 24 pounds a square foot for each level, plus the rough live load. Now, obviously if you're in Colorado or Wyoming or someplace where they actually have weather, that snow could be a lot obviously larger. So I'm just using it for my neck of the woods, but obviously adjust for snow. We always reduce the live load on our podium slabs because their tributary area is very similar to foundations. And in fact, it basically is a foundation. I've never once had a problem justifying that, but obviously going from 100 down to 40 or 40 down to 24 has an impact on the slab thickness. This is what a typical column cap at a podium slab would look like. Relatively small columns, as I mentioned before, one, two, three parking stalls will get you that 29, 28 foot range. But the cap right there is used for punching shear, obviously, but also it has a huge impact on flexural strength, flexural stresses, and the slab thickness. So the one thing about column caps and drop panels, if you're more used to rebar concrete, is the fact that there is no code requirement. You have to use a column cap at a certain size. Basically with PT, the drop panel requirements do not apply. It's the wild west. You can pretty much do whatever you want. I typically wouldn't go less than six foot square. The L over six requirements do not apply to us. Now, if you want to use them, that's great, but they don't apply to us directly. As I mentioned before, when you add a cap, the cross-sectional area will increase. So make sure you check your 125 PSI. And again, it's on the structural cross-section that you're using. If for some reason the architect does X, Y, or Z, and they got to flare out some concrete or something like that, I wouldn't get overly concerned. But if you're adding in structure, then make sure the 125 is there for you. The column cap and dimensions, as I mentioned before, the cap will increase the section modulus. When you increase the S, the M over S stress immediately goes down. So you can technically use less PT. Lower flexural stresses can often generate thinner slabs. And so in addition to the post-tensioning effect, the fact that we can use a cap usually means we can have an impact on the slab thickness. And then when you flip out rails for caps, because someone doesn't want to see the beauty of a cap, do not immediately assume those slab thicknesses are the same. Caps, in my opinion, are way more useful for punching shear. And I would say they're probably more useful for flexure and slab thickness than just straight punching shear. So quick comparison of how that works. We have a 12 inch thick PT slab, seven foot six square cap by 12 inches thick. So it's pretty stout. So if we look at the slab without a cap, so just 30 feet times 12 is 4320. The section modulus is 8640 cubic inches. Now we compare it with the cap itself. The A goes to 5,400. You just trust me that the math is correct. I won't go through all the numbers. Center of gravity is 15.6. The moment of inertia using our favorite moment of inertia equation for college is 189,000, which means the top S is now 22,526. So if we start comparing those two segments, the increase in area goes from 5,400 from 4,300, which is 25%. Not a big deal, but obviously it's an increase. But this is where you get all the benefit, 22,500 to 8,600. The S goes up by 2.61, where the moment is typically the highest. So your M over S stress, if everything was exactly the same, just went down 250%. So if you're basically just making it at six square root and you drop it down by 250%, you can take out post-tensioning and still satisfy stresses or make the slab thinner. This is where you get the benefit of slab thickness. So to compare this to a solid slab, besides using slabs and caps, because caps are strategic at their location. If we take that and convert it using the standard equations, the 12 inch with a seven foot six square cap is equivalent to a basically a 19 inch thick deck. Now, obviously it's not exactly apples and apples because you would get more drape with your PT and stuff like that. But my point is, if you were to take a 12 inch slab with a cap, design it beautifully, and then take that cap out, you're gonna lose 250% S. So the only way you can account for that is a massive amount of PT to knock down those stresses or massive overbalancing to knock down your M, which are not always the good solutions. So if you use caps for punching and slab thickness and you change them out for stud rails, do not immediately think that A and B are equal. Because in my experience, I've seen a lot of engineers get in trouble by doing that where they schematically did a 12 inch slab with caps. Somebody comes in and says, we hate caps. We don't want them. Get rid of the cap, start designing it. And they realize they're at 400 PSI or they're 200% overbalanced. So just make sure you're taking care of that. Why a topping slab? So the main issue with post-tensioning is you wanna make sure you have path of travel of your strands and everything in post-tensioning is gradual, up, down, vertically, left to right, laterally. Everything should be gradual, nothing funky, nothing extreme because that's where typically bad things would happen. This is a typical courtyard detail. You can see here, this is a courtyard slab below the podium slab. You're typically about five, five and a half inches. We'll have a courtyard topping slab, some water surface, a weep screed, and then you climb into your units. So you typically have, let's say a five and a half to six inch step all over the place. What we typically do is this is our typical slab step detail for a podium. You can see here, this is a courtyard slab coming through this way. You have your six inch step there. That's the unit framing. And then it flares out. The key thing about this detail is we slope or ramp the bottom form. That dotted line you can see in there, that is the actual 12 inch or 10 inch structural slab that you're designing. This concrete on top is just superfluous. The beauty of this detail is that if this was perfectly flat, you're just out in the middle of the courtyard or in the unit, the chair that the PT supplier will be right here. That chair would be the same if it was perfectly flat or if it's at a slope. So the post-tension supplier really doesn't need to know where that step occurs. Because as you slope the form like a mini ramp or for like a parking garage, everything's the same. And this is what it looks like out in the real world. Hopefully you can see here, that's a two by six. That's gonna be the step indicator or the step form. You have the courtyard, excuse me, right there. This is the courtyard. It comes up, steps up. Hopefully you can see the transitioning of the forms right through this four or five feet. And if you can look right at this post-tension, you can see it's coming up. It is vertically going up because the forms are up, but it's a smooth transition. There is nothing sharp, funky, eccentric, anything about it. It's just a nice smooth transition that looks like it's gonna work beautifully. Having said that, you can see there's a lot of work involved to hang these forms. And the more funky or more unique or more architectural those forms become, the more complicated the step detail is to construct. And that's where we get into the, how do we do this better for post-tensioning? So again, similar photograph here, you can see the two by six is hung for the step between the courtyard and the unit. But again, this chair that you can see right here and right, sorry, where was I? Oh, right there. That's holding up this PT. That would be the exact same chair if this was perfectly flat. So as they're ramping up, because the ramp and the slab, is doing the exact same thing. Everything's the same. The drape's the same. It matches your drawings, matches the shops. It all works out, hopefully very well. But again, forming a little more challenging than if everything was rectilinear. Now, when using the unit to courtyard structural step detail the one potential issue that comes up is when you get a very funky courtyard step layout like you can see here, when you try to break the forms list this way, that way, this way, that way, it can get very complicated very quickly. And then you end up with some thicker slabs. No one really knows where the step's going to be. The forming becomes challenging and it's generally a very difficult procedure. You can still do it, of course, but then you want to make sure you eyeball the post-tensioning to make sure you have a smooth transition and make sure you don't get any extreme steps from the post-tensioning which can create blowouts or cracking. So this can be very challenging to form and also for the rebar reinforcement placement. So expect complaints. Sloping in both directions is always not a fun thing to do. So when you have something like this the typical step detail may not be the best solution. So there's other options that we'll talk about in a few slides. So before we get into the option B this is really a construction issue. I mean, the numerics of your analysis is not going to change. It's the same procedure where they use equivalent frame finite elements or you do it by hand. This is basically how to get this built well so it matches your design. Also sloping in two directions are going to add concrete which does add some weight depending on how big your step is and also making the strands or excuse me making the strand placement more difficult. Now, one thing that form work and the field will try to do is square off the step. You can see here the dotted line is where I would want the form. And there are a lot of people who do that. And technically there's nothing wrong with this. obviously, it's probably a little simpler than trying to form it. The problem that comes up is the PT supplier is going to provide a chair. And unless they somehow are clairvoyant, oh, exactly how they're going to square it off, they're going to put the chair on the bottom of the form, assuming that is the bottom of the structural slab. When you square the step off the bottom of the structural form, and where this quote unquote slab is, could be off between one inch and five and a half inches, which can be a very large condition. So when you have a structural slab step that is flattened, which I get I would not recommend, sometimes you have to eyeball the tendon to make sure you don't get a really odd placement of the strand or a kink in that strand. So if you're, you know, everything being equal, the slabs are squared off, and the people in the field are just taking the chairs that they get from the PT supplier who really has no way to know where the forms are going to be, or excuse me, where the thickened sections are going to be placed how long those forms are going to be, they're going to ship out shares basically off a 12 inch thick slab. And when that person puts the slab bolster or the slab chair where it's supposed to be on the plan view, but now the section is 18 inches thick, and not 12 inches, you have a reversal or a dip in the strands, just because of the incorrect or the correct chair placement, however you want to look at it. And anything that's funky inside of a post tensioning system that has stress, typically, whether it's horizontal or lateral, is going to want to straighten out lift up potentially crack the system. So anything that you do like that, and this, you know, this can happen with the wrong chair, even on flat slabs, but more pronounced when the slab gets thicker for this condition, you develop a very localized balance load, that's going to lift up create large uplift forces during stressing, and potentially blow out the slab. Now, obviously, this isn't a huge blowout similar to the ones we saw previously. But anytime you can get whoop de dos, either laterally or vertically in those strands, you run the risk of creating some fairly substantial large forces that can really, you know, chew up some concrete. If you look over here, you know, these are very large pieces of concrete that may have only been caused by maybe an inch or two inches difference in chair heights. When you take that two inches of drop over one or two feet, the actual upward force from those strands is actually extremely large. So when using the structural slab, it's usually beneficial or most efficient when it's straight up and down, nothing funky, nothing curving stuff like that. When you're doing it in both directions, this can be extremely challenging. The benefits are it minimizes a slab thickness, obviously, there's a cost impact to that downside, for lack of a better word, the bathroom and kitchen traps will be in the structural slabs. But that's kind of the nature of the beast. As I mentioned previously, forming will be very challenging, expect complaints, I would recommend against squaring off the forms. But if that's your preferred method, just make sure you put a lot of information on the detail to show where the tendon is supposed to be, whereas the quote unquote, structural slab, give yourself a lot of, you know, a lot of information to the field to say just don't put the chairs from the PT supplier, exactly where they are on plant account for the thickened sections. A lot of times you the engineer or someone from the PT supplier may need to eyeball the strands. Myself and my business partner have done that a few times when we allowed a squared off form, because they really have no way to know where that quote unquote structural slab is when it hasn't been poured yet. They don't want to guess and so we end up going out there with a tape measure and, you know, making the strand as smooth as possible. So be aware that that may have to occur. The biggest issue with this and, and having it done or not done, depending on if you want to do it, is when the estimators do this, and they're obviously on the front side of the job, they don't see the cost of the form work, the potential delays, all the extra rebar, all the extra concrete, when the transition is not simple. So if you have a very, very funky, interesting architectural curvature to your courtyard step, make sure the estimator understands the potential downside of doing that. Now, this is option B, which is the, you know, kind of a fail safe for all conditions. It's a topping slab option. The major difference here is you have your structural slab. And instead of breaking the slab up, and creating this to be the structural topping surface, you effectively keep the slab flat, you put a topping surface non structural for the units, and a slightly smaller topping surface or slab in the courtyard, which creates that break between the two systems. Now, the benefit of this is that basically your slabs are perfectly flat, there's no major jumps, you're not hanging forms, and the chairs are going to work whether they're under the units at the transition or in the courtyards. Now, obviously, the downside is a little extra material, but the amount of work is substantially less form the formers. So basically, what's going to happen is they're going to pour the structural slab effectively flat with a little drainage in the courtyard, they're going to, you know, shot pan a few things in to create the formwork of a very interesting slab edge, that if you did break the slab, this would be a lot more challenging, come back in two or three days, make sure the anchor bolts are in the right location, nothing is, you know, bent or broken or at an angle, come back in, pour the system up, and you know, you're done. So the forming is substantially easier. You do have a chance to check the anchor bolts, it's a non structural topping surface. So it can be, you know, 2500 psi concrete, you can embed conduit in there. I've never met a contractor who didn't like this method. I've met a lot of contractors who didn't like the structural step method. And again, there's nothing wrong with that. But depending on your geometry, it may be a little more challenging than it needs to be. So when you have a highly variable step, like I've shown you, the topping slab option may be a very nice option B for the system, it typically adds about a half inch of the structural slab thickness. So again, it's not without its negative. But typically, if you have a 12 or a 12 and a half inch thick slab, I don't think that's going to make or break the entire job. Obviously, the forming is simplified, there's no additional rebar, because of the breaking of the forms and just trying to crack control. Basically, the slab is effectively flat, you can put conduit in the topping slab. This is I said before, preferred by every field person I've ever spoken to and the engineers who takes that whole construction detailing, where's the slab where the tendons out of the equation. The secondary benefit is all of your traps will be in the structure or sorry, you're all of your traps will be in the topping slab, not in the structural slab. So you have this five and a half plus or minus inches of fluff that people can put stuff in, but it doesn't impact punching shear or flexural strength of your actual structural system. Similar to the structural step, the estimators only see the added material cost of the topping slab, obviously, and the extra material cost in the structural slab, the slab thickness. But again, I think this is a much simpler way to do it. And if you don't have a completely orthogonal slab step, this may be something to look at. If you really get into it with the inspector, not the inspector, excuse me, the estimator, have the forming and rebar contractors provide an opinion, they are usually very boisterous and very opinionated, and they will explain their point of view greatly. Like anything with a podium slab penetrations are an issue. Now one of the main benefits of post tensioning, and you can see it from this slide, is hopefully the these are the banded tendons, you can see here, the tendons, we have groups of five here, here and here, they're going to be spaced out at plus or minus nine inches, 10 inches, 12 inches on center. But as you can see here, there's a larger gap here, a smaller gap on this side, and so forth. The actual placement of the strands really doesn't matter. So you can curve these things over to miss a penetration, all you really need is some alleyways, it does not have to be exactly 12 inches on center or whatever it is. Luckily, concrete is not smart enough to know the difference. So outside of being it directly over the columns for the integrity strands, the exact placement is almost irrelevant. So you have a lot of flexibility as you can see with this strand here, the uniform strand curves around this trap comes up over the column curves back goes underneath, and then makes it right to the column core. So again, this spacing across here, let's say is three feet right here, and maybe five feet plus or minus. So you have a lot of flexibility, you can use two strands, you can use one. And then all you have to do is just thread, you know, kind of thread the needle because again, it's unbonded reinforcement. The fact that you have one inch of cover right here, or maybe even no cover, let's say worst case scenario, it's not going to change the strength of the strand, or the action of the concrete with the strand. So typically, what I do is I get on the deck before they start putting in the reinforcement, and just see if we can move this or that just to get me some alleyways to curve this stuff through. But again, the actual position of the tendons is almost completely arbitrary, there is a spacing requirement of eight times a slab thickness or five feet. As long as you say within that you have lots of room to curve adjust gradually to avoid these penetrations. The last thing I want to talk about is conduit. And like anything, conduit is the biggest problem. This is one of the first photographs I got from a concrete or excuse me, a conduit supplier, installer. And the reason I keep it is because he was so proud of how well how straight that the conduit was installed, and he couldn't understand why I couldn't appreciate the beauty of the system. I'm like, yes, it's beautiful. The conduit is perfectly straight, it belongs in the loof. The problem is you just destroyed my structural slab. They seem not to get that somehow, you know, their mind if it's straight, and rectilinear, everything is fine. But when you have concrete like this, and you just chew it up side by side by side, you don't have a 12 inch slab anymore, you have five and a half inches on top five and a half inches on bottom. Without that composite action. That's very different to five and a half inch slabs compared to let's say a 12 inch slab. If you do the section properties, a more recent picture would be something like this, where they're running all the unit conduit from, you know, let's say 200 units all the way through your structural slab to the electrical room. And as these get closer and closer, you can barely see the form work, you have a hard time seeing the post tensioning to the point now where you can't even pour concrete and see the forms you're walking on the conduit. And I don't care if this is post tension, if this is rebar, if you make it two or three inches thicker, when you start putting in literal slip planes in the slab, the slab doesn't work as it was supposed to mean your whole analysis is based on a 12 inch or a 13 inch thick system that is solid, not having an entire slip plane over let's say a 30 foot wide segment. This just doesn't work. This conduit was taken for a parking deck, not a podium, but it was below a podium slab. And this is what people are trying to put in for EV stations, EV, solar, Wi Fi, security, carbon, lighting, video, whatever you have, everything is just gone exponentially up. And when you start putting in this type of conduit through any deck, again, PT or rebar, but especially for podium slabs, and especially with PT, because we're typically three inches thinner, it has a much larger impact. So the conduit strongly recommend you put notes on the beginning of your drawings, even if it's a 25 DD set, no unit conduits in the slab, you just can't have a structural concrete slab with this much penetrations. This is what we basically make them do this is the best type of conduit. You have it hung off unistrut. Here is our structural slab way up here. They hang it down here, they run it underneath the system, they can go all the way across however they want to, they have access to the lines. But with the amount of conduit you're talking about, especially for four or five stories of wood, and you only have maybe one or two electrical rooms, there's physically no way just to get the rest of that conduit and concrete slab together and still be structurally sound. This is the review, the sorry, the reviews, reverse view of that picture. Again, look at all this conduit side by side by side by side by side going all the way across this entire deck. If you look in this plane, you can't even see the structural slab, it's just isn't visible. And that's the amount of conduit they wanted to put in this deck, that a has to span 30 feet. And then on top of that has to hold up four to five stories of wood and all the people loading. So again, the conduit just in my opinion, can't go in the PT slabs. If it's a lighting, if it's minimal stuff, okay, that's one thing. But when you're running this much conduit in a structural slab, it is unbelievably challenging to try to justify how it is still a structural slab with literally an entire slip plane in that system and still trying to achieve composite action from the top and bottom of the slab. So with that, I will turn it over to Tim, thank you very much for your time. And if you have any questions, please let me know my address is right there. All right, Brian, thank you very much. That was excellent. I'm stepping in here for Kyle to kind of wrap things up here for this webinar. So yeah, the next section we've got here is an opportunity to kind of go through some of the questions that have come in during the webinar, we've had a handful submitted as we've gone along. So I'm going to pull out a few of these and run those past Brian, as Brian mentioned, whatever we don't get to here, there's an opportunity to email him, you can also email post tension Institute through the technical inquiries email address. So regardless, let's get a couple of these in here. So one of the questions we received, it goes as such, it says, as we understand PT slabs are good options for gravity loads. However, when a podium slab is supporting superstructure, it does see reversible moments due to lateral loads. Does that make PT slab less effective? In my opinion, no, actually, if you look at the podium slab is effectively a foundation, let's say, and going back to our example, the wood structure, I believe was about 200 pounds a square foot. Let's see have an SDS of one and an R of five, you're talking about a seismic coefficient of 20%. That's only 40 pounds a square foot of quote unquote seismic load. The 12 inch deck by itself weighs 150 pounds a square foot. So it is substantially almost a factor of four more than the quote unquote lateral overturning weight of the wood structure. In addition, that 12 inch slab is anchored down with columns at you know, 30 feet on center and most likely a whole series of returners concrete shear walls that are going to pin it to the ground. The biggest issue I think, laterally is just the hold down forces. I mean, most wood buildings are have a TC couple, the anchorage obviously needs to be done correctly. So you develop the strength of the concrete. But in my experience, the concrete, you know, 5000 psi, PT reinforced rebar uplift plates, the stick framing is not going to break the concrete system. To answer your question more thoroughly, if you were using, let's say post tensioning in a moment frame beam as the primary reinforcement, and obviously that system is meant to go, let's say, east, west and north, south, then yes, post tensioning probably wouldn't be the best choice. But as effectively a foundation that is substantially heavier, and pin down to the actual real foundation. I don't have a problem with that. And I've never once seen, let's say a wood structure actually damage the much stronger concrete reinforced structure. Okay, that makes sense. All right. Another question here is what percentage of the self weight, would you recommend for balancing with PT at the podium decks since the superimposed load is heavy? The the value I have always used, regardless of it's a podium or not, but typically I don't go above 100. If it's not a podium is 125% of the self weight of concrete. When I was a young much thinner had more hair person, pretty much the God of post tension concrete Ken Bondi, who has forgotten more than I'll ever know, told me 125 is as high as you go. I don't have any type of proof for that. I just have, you know, gajillions of square feet. It has seemed to work for me. I don't have a fourth order differential theoretical backup for it. But I typically don't go anywhere past 125. And I try not to even sniff 120 less, I really, really have to. But 100 for most conditions is the right answer for podiums, I will go a little bit higher, but I don't go past 125. Again, of self weight of concrete has nothing to do with the total dead weight on the system is purely the self weight percentage. Okay, good. And then one more, we'll sneak in here. It's a question about, do we need any minimum bottom mat reinforcement to avoid cracking due to shrinkage? That's a very highly debated topic. It's kind of like a West Coast, East Coast thing, like we're rappers. A lot of people on the West Coast use a bottom out of rebar for is at 24, four is at 30. For crack control, diaphragm continuity, just feel good steel. However, there is no code requirement for it. People in the East Coast, if they have a mid span or bottom tensile stresses at mid span that is less than two squared f prime c you technically per code need no rebar on the bottom anywhere. And there's been you know, millions and millions of square feet that have been built like that, that have performed just fine. A post tensioning is an unbelievably, unbelievably great strong material, I would have no objection to it. Personally, I always put a bottom mat of rebar in whether it's a parking garage, an office building or a podium, the thicker the slabs go I typically tighten the spacing. So an eight inch deck, I may have fours at 36, or fours at 30, a 12 inch and higher, maybe fours at 24, or something like that, depending on the loading. But I'm a big fan of just putting a small amount of bottom rebar again, for crack control, diaphragm continuity, and then just low distribution during the life of the structure. But that's much more of an engineering preference than a code requirement. Makes sense. All right, let's move on to the next slide here, Brian, and we'll wrap things up. So, again, appreciate Brian's time. And this is a very popular topic, we had a lot of call for so certainly glad to have a chance to finally present podiums down the road. I'm sure we'll have other webinars that cover just the subject matter of two way slab design and a variety of other things in that realm. So I know this is just kind of getting started in that direction. So as a preview to what's coming up at the PTI monthly webinars, our next three months look ahead here I want you to be aware of in March on March the 13th. Our next webinar is going to be a overview of our DC 20.2 publication, which really talks about restraint cracks, cause effect and talks about ways to mitigate. So that's a great overview that's coming up next month. In April on the 10th, we're going to have a PT design best practices webinar, several case study examples. So this kind of, again, as it says, best practices covers a variety of topics in the building realm that get into recommendations and things of that nature from a practicing consultant with various cases. And then in May, the international residential code code change for the 2020, sorry, the 2024 version. There's some things that affect post tension slab on ground designing construction. So we're going to highlight that for the slab on ground industry, that's kind of our first chance to kind of step from buildings in the slab on ground. And down the road, we've got other things coming this year related to bridges related to rock and soil anchors, and a variety of other things repair as well. So keep your eyes open as we go through. And so lastly, we'll just kind of bring up the final slide and just remind everyone again, if you want to get ahold of us, you saw the question and answer slide. This is just again to thank Brian for his time today and his expertise. And with that, we'll wrap up this February webinar. Thank you. Thank you, everyone.

post-tensioning

podium structures

structural integrity

design considerations

reduced rebar usage

live load reduction

structural performance

crack control

engineering considerations

design principles