All right, guys, we're just letting the last few people kind of trickle in here. We're a minute past the hour, but good morning to those on the West Coast and good afternoon to those out East again. Welcome to the Post-Tension Institute's monthly webinar for March. My name is Kyle Boyd. I'll be moderating today's session. I'm also the chair of the Education Committee, EDC 130, which is really the committee within the Post-Tension Institute that sponsors today's monthly webinar. As you can see on the screen today, the topic has to do with restraint cracks. This is what we believe at PTI to be a very important topic that one historically probably wasn't discussed enough and there wasn't a ton of educational content out there. So the last few years, PTI, the design committee, has been focusing on trying to get more information out to the public on restraint, restraint mitigation cracks, and to the point that there's actually a document that's been produced and that's what's going to be discussed today as well in there. Before we go too far down today's topic, one thing I did want to address was last month's webinar. For those of you who are on here, it was the webinar for podiums. Zoom did have a technical issue and had a fatal crash about three quarters of the way through. It kicked out everybody, including us as presenters. This was obviously very frustrating for us as we were trying to get rebooted and it wouldn't start back up for us. We believe we discovered away the root cause of it and we think we have a measure in place to keep it from happening again. But once again, it was a technical issue. However, what we did was we recorded the rest of the webinar. It is posted online and you can go watch it. If you watch this quiz, then you can get your credit on it through that happening right now. It's a great topic. It's definitely worth the watch on there. And like I said, recommend going and seeing if you were on last month's webinar for podiums. All right, now we can keep going for today. So if we go to the next slide, continuing education. On PTI, what we've done is we've worked with RCEP for continuing education. And upon completion of today's webinar, meaning you are logged in and you've watched the webinar, you'll get a credit email to you. If for whatever reason though, you have to drop and leave the webinar, it will notify and you will not get that credit. We are still working on a program for conference rooms if there's a conference room full of individuals to figure that out. However, it's something that we're going back and forth with RCEP still and we don't have a final solution for it yet. Hopefully in the coming months, we'll have that figured out on there. The next slide is just our boilerplate. Hey, here's our copyright stuff. Don't plagiarize, we all know that. And then the following slide that we're gonna go into has to do with our just general housekeeping items for today's webinar itself. And so as you can see, all attendees, you guys are in listen-only mode. We cannot see you or hear you on there. So if you have questions, ask it through the Q&A function on there and we'll go through them at the end of it. Write the question, be very specific in how you write the question. Try not to be vague in it itself. The vague ones we typically skip over just because there's not a great way to answer them on there. The webinar is being recorded like I mentioned earlier. Every webinar, every month is recorded. It's online, it's available to watch for free. You can take the quiz afterwards and you can get the credits. It's a great way to keep up to speed with your compliance for your state registrations on there. And yeah, so from there, I get to introduce our presenter today, Dr. C. Baxey. So C is the person who I personally go to when I have a technical question or need to brainstorm something off of. He manages a company that focuses in consulting of just post-tensioning. And he does it, he's based out of the Houston area, but he doesn't have any boundaries to where he actually goes and practices. He's very, very active in the Post-Tensioning Institute and ACI. You can see the long list of different committees that he's involved in, to the point that he's actually a fellow in both ACI and PTI itself. And in PTI, one of the committees is the Design Committee, DC20. And that's one that worked towards putting together this document on restraint cracks. And within that, C was really the guy who spearheaded it and led the document creation itself. And so he is our industry expert right now on restraint cracks, and obviously the most qualified person to talk about today. So that, C, go for it. Carl, you are very, very kind and thank you for the generous introduction. So the presentation outline for today, background and update of the PTI restraint document, overview of what we worked on in the DC20.22 document. So we'll start with what is RTS, restraint to shortening? What are the types, different types of RTS cracks? Crack mitigation philosophy and strategies to mitigate cracking, crack mitigation detailing. There's a section on structural evaluation and repair. How to compute shortening, which is always a challenge. I'm gonna present some basic theoretical background and an example. And then we've got to a point where we can, there's a little bit of finite element modeling that people are dabbling in right now. And some of the considerations to be made when computing shortening. I'll end with a summary and what I would like at least the viewers to take away from this presentation. So learning objectives. So at the end of this presentation, you will be able to understand hopefully the sources or the reasons for restraint or shortening, the different types of RTS cracking, basic crack mitigation strategies, common crack mitigation details, and then some basics of computing shortening in buildings. Of course, we urge all the design professionals and whoever is attending to get a copy of the document. So there was a restraint document, a very well-written document been used in the industry since 1988. It was authored by Bijan Alami and Florian Barth. Was a wonderful document, been used for almost 20 years. It was that good, did not need an update, but obviously industry caught up with a lot of the changes and it was high time that this document was updated. And we started working on this document roughly in 2018, 19 timeframe, and it took three or four years to finish this document. And that's the new updated document you see on the slide here, right here. So what is new? What's the reasons for acquiring this document? So document required a much needed update, although the original document was detailed, well-written and provided excellent details. Obviously needed an update so that you would become current with design and construction practice, current design and construction practice. And I would even venture to say that this document is very comprehensive and represents the state of the art on this topic at this time. We've added, so besides the general, pretty much rewrite of the entire document and all the figures, we've added a section under crack mitigation techniques. New sections have been added on concrete mix design, use of shrinkage compensating concrete, use of lockable dowels. Some of you guys have heard about that. I heard about a manufacturer providing lockable dowels and use of that technology. Additional details for crack mitigation have been provided. The original document had quite a few details, but we have the benefit of having excellent post-tension design engineers on our committees, in our committee. And we have collected details from many engineers who have used these successfully for all these years. So that is available now for public consumption. There was some information on computing shortening in the previous document, but now we have, it's very organized now. For a young designer or any designer for that matter, to compute slab shortening, there are several methods presented. This is the first time this has been done, especially for post-tension members, because it was never done in a, it was, there were documents in ACI, but it was never in a format where designers could use it. And then, like I said, we have a chapter on finite element modeling too. So what is RTS? RTS is an acronym for restraint or shortening. You have, obviously, when you have elevated post-tension slabs, the elevated post-tension slabs are supported on columns. You have walls or those supporting elements that you can have in a typical flat plate member. The supporting, the type of connection can be either rigid or it could be partially rigid or partially flexible, whatever word you want to use, or the slab can be free to translate or rotate with respect to the supporting elements. The one thing to consider when you talk about movement or slab shortening, what are the sources of slab shortening in a member? Obviously, it's the shrinkage of the concrete, creep of the concrete, which is the strain in the concrete due to constant stress, state of stress. And in the case of pre-stressed concrete, it'll be the pre-compression. And of course, you have elastic shortening due to the imparted pre-compression in the concrete. And then temperature changes, which is you can have a temperature differential. So you can have either expansion or contraction in the concrete. Now, where does the cracking come from? It is basically the restraint created in the member. If the restraint created is of, creates a magnitude where the tensile stresses exceed the capacity of the tensile or the inherent tensile strength of the concrete at a given location, you're going to have some cracks occur. Now, where can the cracks occur? It's the weakest link. You can have cracks in the slabs or in the columns or in the walls or even in both. This is an example of a typical floor plate. You see the shear walls here and the column layout. I would say the, a lot of the stuff on restraint or shortening is about identifying the movement of the building relative to what I would like to call the line of zero movement. So roughly in this case, the line of zero movement is about right there, about in this direction. And then for movement in this direction, it'll be about, roughly about this line here. And again, like I say, what are the sources of shortening? You have elastic shortening, shrinkage, creep, temperature change. Majority of the, most of the movement obviously will be away from the line of movement. So what kind of cracking can you get due to a restraint or shortening? You can have overall cracking in the member, you can have localized cracks. So overall cracks will be caused because of poor layout of the shear walls, columns, or other lateral load resisting elements. It all depends on the connection details between the slabs, the foundation walls, and again, the lateral load resisting elements. You can get cracking potentially from slab irregularities. This is a case where you have, you know, walls and sort of put away from the line of zero movement and you have general cracking in the slab. Now, keep in mind that post-tension slabs obviously use post-tensioning as the main form of reinforcement. So you obviously have less reinforcement in the deck. So you have to account for that. And it's, we're talking about unbonded post-tensioning not bonded post-tensioning. Slab irregularities can obviously cause cracking at the hot spots. And so that is something you have to consider. Another thing to consider is the effect and the interaction between the slab and the walls. When you pre-stress the slab, when you have, if the slab is free to move, then all of the post-tensioning remains within the slab. If the slab and the walls or the walls in this case are connected, some of the post-tensioning will be diverted into the wall. And it is important to keep in mind you want to keep the most of the pre-compression within the slab and to have as little as possible diverted to the wall. Now, localized cracks. Once you pour concrete in a member, localized cracks can occur in a few hours after the concrete is placed. And even before the application of any pre-stressing, this is a case where you have a corner condition in a slab and you have walls here. So obviously you're going to get some shrinkage and temperature cracking at the early stages. You have an opening here with, whether it's an opening with walls or without walls, you can have some distress or some restraint, shrinkage cracks at the corners. It is very important that the post-tension slabs be stressed as quickly as possible. The guideline is to stress the slab within 72 hours. Of course, the concrete has to have attained a strength. The typical industry standard is around 3000 PSI for 5000 PSI concrete, or even for 4000 PSI concrete. So you can get cracks in the slabs. You can get cracks in columns. The case where it's more severe are cases where you have really short columns. They make the connection much stiffer. A lot of moment is attracted to the column and that can cause cracking in the short columns. There is a way to release the amount of movement by either using pin conditions and putting confinement reinforcement in the column or to fix it and then to let it behave like a cantilever. Again, these are all conditions that are very project specific, but these are also things that you have to consider during the design of the columns and the slabs. You can have a case where you have infill walls and you may not realize it, but by restraining or connecting the wall and the columns, if you have a partial height wall, you can create a short column condition which can severely crack the columns. This shows a case where there is no connection or there is the wall and the column are sort of monolithic or connected. This is where you have a partial height release or a full height release. So these are things that you have to consider during the design or detailing of the members. Long slabs. You can have slabs that are really long, regardless of how long they are. You're going to have movement. You typically have the first elevated decks of a high-rise building where you have the most amount of movement and the distress, whether you pin the slab or you have a partially fixed condition or a rigid condition, these are distress locations where you're going to have a restraint to RTS cracking. As the slab wants to shorten here, you're going to introduce moments and shears in the columns. Cases where the slab and the walls are connected, whether it's a CMU wall or a concrete wall, you have to consider the connection between the slabs and the wall and how the forces are being dispersed. If they are rigidly connected, there is a possibility that you are going to get some restraint to RTS cracking. You can have a case, this is a localized case of cracking in a CMU wall where the slab wants to move in this direction and it's also deflecting downwards. And if the CMU wall and the slabs are connected, then you can potentially get some cracking at this location as well. So what are the techniques for crack mitigation? There are several techniques shown here and I'll go over those and you can use them individually or you may have to use one or both of them. So layout of the restraining members, how you lay out the columns. Columns can be 24 by 24s, you can even have 48 by 12 columns or columns that can actually cause restraint issues if they are longer in the one direction over the other direction. Layout of the walls. You can mitigate RTS by creating a permanent structural separation. So depending on the length of the building and the geometry of the building, you might consider a permanent structural separation. A good use of a way to mitigate cracking is to use four strips. And I'll give you some examples of that as well. Sequencing of construction can reduce... The building can be built in a manner where you can not necessarily solve every issue, but you can definitely reduce the amount of shortening. Use of slip details. Details can be permanent slip details or temporary slip details. Changing the concrete mix design. Use of specialty add mixtures. And use of strategically placed non-pre-stress reinforcement that is adding reinforcement at the hot spots. And even the way you detail the tendons can mitigate cracking. So this slide shows, like I said earlier, this is an example of a slab where you have a line of zero movement in the horizontal direction is here, in the vertical direction is here. And by placing the walls close to the line of zero movement in both directions, I would consider that to be a more favorable arrangement of the restraining walls. A less favorable arrangement is where all the shear walls and the... Are far away from the line of zero movement. Now, having said that, I know that you, we structural engineers don't have control on where all the shafts go in a building. But in initial discussions with the architect, it is important that we bring these topics up and see if there is a way to somehow lay out the walls so that you have the least amount of... Or you're closer to the line of zero movement in both directions. And in cases where you can't, then you use the other crack mitigation techniques that we'll go over. Structural, permanent structural separation is required in cases where you have really long slabs. PTI, even the PTI manual, this document, there's another PTI design guide. It has guidelines on what lengths of slab you should put permanent structural separations. Usually 350, it can vary from 350 to 400 feet where you may want to consider a permanent structural separation. Obviously those numbers will change depending on the location of the building. If you're in Houston or Hawaii, that would be different than if your building is in Denver, not Denver, in Detroit, where in one day you can have a temperature differential of 80 or 90 degrees. So that does affect that length at which you may want to consider a expansion joint. When you have structural irregularities, of course you want to, like in this case, you want to separate this piece of, this portion of the building versus this one here. Or if you have a reinforced concrete deck here, like a smaller appendix to this slab, you want to create a permanent structural separation. Use of four strips, like I said, and or a combination of four strips and expansion joints. This is a building actually that was, is one of my jobs built in 2016, 2016-ish. Very long building, 440 foot in one direction, 430 foot in the other direction. It also had retaining walls all around. And as the engineer of record, we were very, very concerned about restraint on this job. So we actually put expansion joints, like is shown here. It was a combination of expansion joints and four strips. Now to that end on this job, I was concerned enough that I actually asked the contractor to measure the amount of movement we got at the expansion joint. And so we actually kept a record of the movement at 30 days, at 90 days, at a year, and I think even at two years. And I even computed the shortening, expected shortening, and I was pleasantly surprised that we were able to predict the movement quite well. And it's been 10 years now, well, about seven, eight years now in this, we have really not had any issues related to any cracking on this building. Techniques for crack mitigation besides the layout of the columns and the walls is use of release details. So this A shows where the slab is free to move. Again, it depends on the specifics of a project on which detail is more applicable at a certain push in a certain area of the structure. Permanent release detail is where the slab can move permanently relative to the walls. So you essentially transfer the force through dowel action. This is a case where these are fully connected and there could be cases, high seismic zones or such where you do have to have the full connection between the slab and the wall. And in that case, there are things you can do to provide the connection. You can have a temporary slip detail and then lock it in and then add additional reinforcement. So there's ways you can do that, but this is a case where you have full connection between the slab and the wall. And I would also alert the engineers that when you have a condition like this, you can divert a lot of the post-tensioning force into the wall and you might have to consider the reduced amount of pre-stressing in the slab and design for it accordingly. This is a very popular detail, the D, which is a temporary slip detail where you grout the, you leave a sleeve at the dowel, you stress the slab and then grout it later. That timeframe could be three days to seven days, 14 days. I mean, the longer you wait, the better. It depends on the construction schedule and what you can work out with the contractor. This is a case where you have a non-load-bearing wall. So you have movement vertically and, I mean, vertically and horizontally. This is the slip detail I was talking about with the two sleeves when you have two layers of steel. I do want to alert one thing here that it is very important that the top of the wall, which is poured to the underside of the slab, have a smooth travel finish. I've seen many times where we, even though we specify a smooth travel finish, you go to the field and it's roughened. Sometimes they don't have the felt paper or poly. I also recommend strongly, you should always have two layers of poly or felt paper, whatever you want to use, not just one. You need that, both of them, to reduce the friction there. Another concept to use is to use embed plates in the slab and in the wall, and then you let the tube move, you lock it in at a certain given point in time. This slide shows permanent release details. The release detail is you can put a foam, insulated foam or a pipe insulation in the plane of the slab. So whatever movement you get is in this area, the slab can flex within this area, allows a little bit of movement, and still ties the slab and the wall together. This detail here shows the foam, but it's inside the wall. And again, there's a little give in this area, which allows the slab to move relative to the wall. Now keep in mind when using this detail, it depends on the bar size. If you have number eight bars, you're not going to get any movement in here. So you might have to consider another detail. Another very good way to control cracks, and I've used it successfully, and a lot of others in the industry have used it successfully for many, many years, is to provide crack control reinforcement at the stiff elements. So you provide reinforcement that is parallel to the walls. This is a detail for corners, detail at interior walls, and this is for shafts. I must say that in the main, in the document, there's a lot of other details that have not been included here. So you will have the option of looking at those and seeing what applies best for your project. Layout of tendons, you want to always have a crack inhibiting layout versus a crack promoting layout. Obviously, in this case, as you can see, there's tension created behind the dead ends and that has the potential of creating cracks at this location. So by using this detail, you are mitigating or reducing the amount of tension that can be created here. So this is just one example. In general, when you lay out the tendons, always think about what you can do so that you have a crack inhibiting layout, not a crack promoting layout. There's sections in the chapter on, like I said, on concrete mix design to reduce the shrinkage. Also a section on lockable dowels and on shrinkage compensating concrete. There's also a chapter on structural evaluation. So either it is your own job or it can be a job that you are making an evaluation on if you get a call and say, okay, we have cracks in the deck, what do you do? First thing you're going to do is evaluate the crack, the cracking for serviceability and strength and make sure you can, if you find any deficiencies in what needs to be addressed. But based on millions of square feet of post-tension slabs over the last 50 years, restraint cracks generally do not affect the slab strength. They do not also, they also do not lead to excessive deflections. They usually do not affect the slab structurally at critical locations. Now, you can get cracking that may increase exposure to corrosion. And there are instances where if you have a situation where something either didn't get detailed properly or didn't get built properly, you can have restraint cracks that are full depth of the section. The general solution in these cases is to seal the cracks, especially where corrosion and durability is important. There's a lot more information about this in the document. Computational shortening. So for the longest time, you know, the designers always say, okay, we want to know what the amount, how much is the slab going to move? I always think that we can, as engineers, we cannot give hard numbers. We can say, these are the, this is the estimated shortening. So the reason for that is because determining the true shortening in a post-tension slab is extremely complex and virtually impossible. There are too many variables in the design and with materials, and also the way the slab is built. Poor sequencing, poor dates. I mean, there's just too many variables to know exactly what the shortening of a slab is, the true shortening of a slab is. However, we can estimate a reasonable value based on good engineering judgment and assumptions that will give a good estimate for the amount of movement you can get at different stages of construction. And these values then can be used to calculate moments and shears for your restraining members, the width of expansion joints, and then just to calculate overall volume change that can be used to facilitate the detailing of the facade elements. So this document has three methods presented for calculating shortening. The ACI 209 method, which basically uses the ACI 209 provisions for creep and shrinkage of concrete. The PCI method, this is not really a method. This is a term that I used. It's really a method that uses the PCI design aids. And so that's what I call it a PCI method. The simplified method just uses equations. It simplifies the calculation. You don't have to go to too many places. It'll give you a quick number for the estimated shortening. Now, if you want to get into details, the ACI 209 method will obviously be the most detail oriented, but it'll also be lengthy and will take time. So it depends on what your need is for a given project. So like I said, when you compute shortening, you're computing the strains due to shrinkage, due to elastic shortening, due to creep and temperature change. Another pie chart here. Most of the shortening occurs due to shrinkage. Short, elastic shortening is around nine to 10%. Temperature, depending on the location of the project is around 18%. It can go up and down depending on the location, like I said. And then creep is around 13%. In the ACI 209 method, I'm not gonna go through all the details. There's a lot of mathematical Greek symbols here, but I'll give a highlight of what I wanna convey. And there's a lot more details and formulas given in the document for cases where you need to do a more precise analysis. So in the 209 method, the concept is based on providing correction, calculating the shrinkage strain based on correction factors applied to a base shrinkage strain. So the base shrinkage strain is 780 micro strain, which is based on a relative humidity of 40% and a volume to surface ratio of one and a half. Now you can apply all these different correction factors here for curing and slump and fine aggregate, cement content, et cetera. But for most designers, we always use a number of ones. So the critical values are the relative humidity and the volume to surface ratio. The same thing for creep strain. It is based on a creep. The 209 method is based on a creep coefficient value. And that is multiplied to the amount of the permanent strain, which is based on the permanent stress. So that will determine the total creep strain. Same concept. You apply all these correction factors for the different, if you want to get into very specifics, depending on the type of concrete you have. Otherwise you can use a value of one and then apply correction factors for your relative humidity and your surface to volume ratio. PCI method is basically using the design aids in the PCI manual. I know I say seventh edition here, but the same charts are available in the most current version of the PCI manual. So you should be able to use those. The concept using this method is again, based on correction factors applied to a shrinkage strain. So the design aid 41113 has a base shrinkage strain based on a relative humidity of 70%, a volume to the surface ratio of 1.5, average P over A of 600, and a concrete release strength of 3,500 PSI. And then you make the correction factors for relative humidity and volume to surface ratio. So this is the chart I was talking about. These are the assumptions. And you make corrections to the shrinkage strain. So for a moist cured case at P equal to final, you would have a base shrinkage strain of 560 PSI. Now, the good thing about this is without getting into too much formulas, you can just look at the chart and calculate the shrinkage strain at P equal to one day or 10 days or 30 days or whatever the timeframe and compute the shortening for all the different cases. The correction factors are based on the relative humidity. This is a chart which is there also in the PCI manual, design aid 41115, and apply the appropriate correction factors. The correction factors for volume to surface ratio are also provided for creep and shrinkage. The creep strain is also... Now, there's a little difference between the 209 method and the PCI method. So in the 209 method, there was a creep coefficient which was applied to the elastic shortening strain. In the PCI method, you actually have a base shrinkage strain based on certain variables and then you apply the correction factors. So the base shrinkage strain, for example, for normal weight concrete at T equal to final, you have a base shrinkage strain of 315 micro strain, which is based on a release strength of 3,500 PSI, P over A of 600, relative humidity of 70% and a volume to surface ratio of 1.5. You apply, you can make corrections to these values if you have a different P over A, a different release strength, and it's perfectly okay to interpolate between these values. I've done that for all my calculations and it works out just fine. You apply your correction factors for relative humidity. You apply your correction factors for your volume to surface ratio. Temperature strain. There is a chart in the PCI design manual again, design 8.4.11.11 that provides a chart for the U.S. for the temperature differential in the country, in the country. And then depending on whether your building is heated or unheated, you apply the, you can get the strains from here. The elastic shortening is computed very easily. You have just compute the P over A divided by the modulus of elasticity at stressing. So it's not the one at 28 days. The simplified method is essentially very similar to the two methods, but it's based on a paper by Zia and others back in the, I think in the late 70s, early 80s, and it's sort of based on the PCI and the 209 method, but it simplifies it. And it's just a couple of equations that can be used to calculate the shrinkage strain again, on a base shrinkage strain with correction factors. When you don't have charts, you are limited in the amount of stuff you can, all the other variables that affect the strain, but it can be a quick way to calculate the amount of creep and shrinkage strain. The equation for calculating temperature strain is delta T, which is the temperature differential times the coefficient of expansion, thermal expansion. There is a correction factor of 0.75 applied for unheated buildings for what is called a reduction due to thermal lag. And it's 0.5, it's a heated building. The elastic shortening, like I said, is also computed just stress over strain. I'm going to give you a quick example of a slab, which is a hundred foot in length. It's eight inches thick, 5,000 PSI concrete, relative humidity of 70%, P over A of 150, stressing at three days, temperature differential of 25 degrees. Using the ACI 209 method, you can go over all these equations like I presented earlier. You have a shrinkage strain of 368, creep coefficient, again, it's not based on a shrinkage, on a base creep strain, but coefficient is 1.494. You have the temperature strain, which is 112, shortening strain, 51. Add it up, you get about a shortening of 0.73 inches on a hundred foot deck. PCI method, you do the same thing, apply all the correction factors, you get a shortening of three quarters of an inch. Now, one of the differences in this method is the correction factors are applied to a shrinkage strain, not to a coefficient, but again, these formulas are sort of interrelated based on how they were developed over the last 50 years. But I personally like the use of the PCI design aids. It's very intuitive and there are things you can do. You can look at the charts and figure out what the shortening is pretty, with a lot of comfort. Simplified method also, like I said, it's a simplified, based on simplified equations and you can run through the numbers, you'll get a three quarter inch movement. Now it's amazing, the movement is about the same, but the reason for that is the results are quite close because some part of portions of the equations, they sort of came from the same source. For the calculations presented, this was an example of an unrestrained member. It is not a restrained member. If you have an actual member in a building frame where you have the resistance from columns and shear walls, the actual total movement is a lot less. So, and again, I wanna say that for the shortening of three quarters of an inch, essentially for a hundred foot slab, which is unrestrained, you're getting three eighth inch on either sides. Now, like I said, the example presented was for unrestrained shortening. The actual shortening in a slab is significantly less depending on the many factors that affect restraint. Building frames will have an overall shortening value reduced by a factor between two and four. The PCI design handbook has design aids for building frames, which can be used. However, I would urge or rather caution that engineering judgment should be used to interpret these values. In the PTI document, we have three additional examples that one includes the poor strip where you can calculate the shortening. At, I think it's T equal to 30 days, one year, and then the final movement. So what are these shortening values used for? We can use the shortening in the slabs to determine the moments and shears. Remember, I showed you a picture of the slab and the column interaction and how the delta, the movement in the slab affects the moments and shears in the columns. You can determine that. You can determine the expansion joint widths, overall volume change. When you wanna use sleeves, if you have a really large deck and you're trying to figure out, should I use a two inch sleeve or a three inch sleeves for your temporary sleeves, if you know what your movement is gonna be, you're then considering the size of the bar and then the amount of movement you're gonna get. It's very important, obviously, that you have to put the, you have to, in the field, the installation should be such where the bar is in the center of the sleeve. If it's to the one side, especially if it's touching it and it's moving in the wrong direction, then there's no point in having a release detail. And of course you can use it to calculate the facade movement and tolerances. There's a chapter on finite element modeling, which, as you have more and more tools to design post-tensioning buildings, and where now you're going from a designing one level to designing multiple levels. And there are softwares that can do that. You can incorporate that even in a calculation of shortening and doing FE modeling for those. Now, I must put this in right here. The idea is not to suggest that engineers must perform finite element modeling for routine post-tension designs. It should be considered as an available tool if you want to do a very precise analysis or you have a very unusual design or construction situation. I still think that the methods we have provided in the document can get you really close to providing good values for the shortening to be expected for a project. Summary and takeaways. For those who are just starting your career in post-tensioning or those engineers who have not done a lot of post-tensioning design, always think about restraint or shortening on your project. It is a real thing. There is a reason why you have those issues. Project size and geometry. If you have a long deck or if you have an irregular geometry, you may need to provide expansion joints or pore strips for your project. Very important. Just make these discussions with the architect and the owner and the contractor at the very outset of a project. I've had situations where we, as a specialty post-tensioning engineer, come in to a project towards the midway or the end of the project, and then we say, oh, we want expansion joints and pore strips, and it creates a lot of issues. So when you start a project, you want to make the architect, owner, and the contractor aware that this is where we need our expansion joints, and then you design all your details based on that. A myth buster, RTS occurs only in post-tension buildings, and the answer is no. You saw that the pre-stress contribution from elastic shortening is only less than 10%. You have the same amount of issues, not issues, you have the same amount of restraint in a reinforced concrete building. The only difference is you have a lot more rebar in a reinforced concrete building, so it helps you. But you are obviously doing a post-tension building because it is more economical, and it is truly economical. The issue is that you have to consider the effects of RTS. And just by following some of the basics in this document, good layout, providing slip details, providing some additional reinforcement, for not too much money, you can make your overall project economical and not have any issues. Make judicious use of slip details. Ensure that the pre-stressing force does not go into the walls. And if it does, keep it to a minimum. Think about that during your design. Add crack control reinforcement at hot spots. It is cheap insurance to prevent problems. And like I said, the amount of shortening can be computed using techniques that are available in this document. With that, I can take questions. Now, I do want to say a couple of things. I was checking the cost of this document. It's like 60 bucks or something, which is about 10 cups of Starbucks coffee. So for the amount of information and the wealth of information, for the countless number of hours we have spent developing this document, it is truly worth it. I encourage everybody to use it in their offices. And then I also want to say one thing that I've been in the industry for about 30 plus years now. And I was very fortunate enough to work under Dr. Burns at UT. And the one thing he told me was, he always encouraged me to join technical institutes. He said, I see it's an investment in your career. And I want to urge you, I'm sort of towards the end of my career. I encourage, especially the younger design professionals, join technical institutes, be a part of our institutes and contribute at any level. You get to interact with other engineers. You get to learn new ideas. And most importantly, you participate in committee activities. You get to work on some of these very challenging and technical documents and are promoting our industry and let it thrive in the long-term. Thank you. I'm ready to take any questions. Awesome. Thanks, Asif. As you guys saw, that was a presentation with a ton of information. We're trying to shove four or five hours worth of information into an hour there. And just remind everyone, this will be posted online. You can go back, you can rewatch it. You can see the information there. And for those who couldn't make it today, they could go on there and take a quiz at the end. So I see a couple of questions for you there coming in. I'm going to paraphrase some of these questions a little bit, but one is on the temporary slip details that you show where you have to go back and do something later on to permanently fix it there. Can you talk more about your experiences with those, any challenges you've had with coordination and or making sure the contractor actually goes back and does whatever element needs to be done to secure it? Well, I mean, they better do it because they want to make the next elevated deck, right? Now, the one thing I have, the common question I get is, hey, can I, when I pull my shear walls, can I let that concrete be part of the grout? You know, fill that in the sleeve. And I mean, I think if the engineer feels that you can get adequate consolidation, that could be an option. But a good thing is pre-construction meetings, have a discussion with the contractor and kind of go over these slip details. I know there is a lot of push and resistance to these things but when they realize that they have a deck that does not have any cracks, they appreciate that. You know, I always, when I'm working in a new region of the country or with a contractor who is not familiar with post-tensioning and the more I explain to them and when they see the benefits of it, then on future jobs, then they feel very comfortable with doing some of these things. Yeah. And to further elaborate on that, you know, in my experience, it is very common to see these temporary slip details throughout the country. It's not a one-off thing. Will, would you agree with that? Absolutely. And you're from, you worked in Denver. And so I know there is a very popular detail in Denver of using embed plates, but these details are used across the country, yes. And have been used successfully for, I mean, I've done, I don't know, more than 400 deck, I mean, buildings, and I've used these all the time, never had it. The only pushback I can get is, I see this is seven to 14 days. Can I grout it in three days? And then as the engineer, you make a call, you know, as to if you can reduce the amount of, the timeframe in which you grout those back. Right. Next question. So we show this plan view of some sort of concrete wall called a shear wall. And then next to that wall for a distance of 10 feet out, we show bar being spaced there to, because there's no pre-compression, essentially in that slab to help with that right there. Is there any logic behind that 10 foot distance of distributing that bar? Are you talking about the length of the bar that is parallel to the wall? Correct. I have to go back and see, was that 10 foot in there? Or maybe- Yeah, it's about 10 foot distribution of the width that you distribute parallel to the wall. I typically use six feet. I think we probably got that detail from one of the other engineering firms or something. So I think six to 10 feet is a good number. I mean, I use six feet all the time, never had any issues. Yeah. Another question for you is- And I did want to mention one thing. The percentage of reinforcement you put is usually, and ACI 423 has a guidance on it. It's a 0.15% of the slab thickness. And you can put half of it top and half of it bottom. Now, if it's a podium slab, then obviously you want to not put number fours at 24. You want to put it a little closer. So it's whatever the engineer is comfortable with. Yeah. So where we have slip details, where the slab's intended to slip on top of the wall, there's a, we'll call it a, not a frictionless material, but a material to minimize the friction between the two. Can you give some examples of different materials that can be placed on top of that wall there? I have used felt, two layers of felt paper, or you can use 10 mil poly, two layers, but always make sure it's two layers. It's not just one layer. And I even said that during the presentation, you got to make sure that the top of the wall is smooth trowel finished. And they oftentimes, they don't do that in the field. All right, guys. So there's a ton of great questions coming in here. We don't have time to answer any more because we do have to go through one or two more things before we leave. So you do see Asit's information on there. You can email him. And then at the very last page, we'll show you another way to get ahold of us too. The next webinars, as we've mentioned, this is a monthly webinar. It always happens at the same time on the second Wednesday of every month. The next webinar that we're going into for April is going to be PT Design Best Practices. So it's going to do a lot with unbonded post-tensioning design for slabs. Think things like slab steps, weird situations that aren't what you seem to call textbook and some different tips and tricks on that. Then we kind of transfer down to slab on grade. And we're going to talk about the IRC changes for PT slab on ground there. Then June 12th, we kind of leave the building industry and we go more to the grouted industry, although there's some grouting in the building industry. We change the topic a little bit and we go into resilience of PT segmental bridges on there. So those are the next three upcoming webinars. Like I said, same time every month on there. And then if we go to the very last slide here, this is the information. You saw Seat's information, get ahold of him earlier. And then this is another way of getting a hold of people at the Post-Tensioning Institute, that info at post-tensioning.org right there. And you can submit questions through there as well. So with that guys, we thank you for your time and we look forward to seeing you here in a month. Have a good one. All right. See y'all. Thank you.

restraint cracks

post-tensioned structures

mitigate restraint cracks

computation of shortening

slip details

crack control reinforcement

layout of tendons

ACI 209 method

PCI method