Today is not a normal episode of the Civil Engineering Academy Podcast. Whether you’re getting ready for your Geotech Depth Exam on the PE or the California Seismic Exam, this audio version of our last live workshop will make you feel guilty if you haven’t participated in it. Seriously.
Isaac and Mark walk us through some seismic, Geotech, and deep foundation materials, explaining some key concepts that will most likely be tested on the PE and California Seismic exams, both as theoretical and problem-solving types of questions. In addition, they lay out the resources we can use to prepare for these exams and how to approach each type of question, as well as solving real questions, step by step, which will be valuable for all of us getting ready for the next PE Exam regardless of our depth exam choice.
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Transcript of Show
You can download our show notes summary or get our transcript of the show below!
Isaac Oakeson: Hey! What's going on, everybody? Isaac here with Mark. What's going on, Mark?
Mark Oakeson: Hey! Just enjoying the opportunity to join you again, Isaac.
Isaac Oakeson: Nice! So today I'm running the show. So you guys are in for a treat. We're going live today. Today, we're going to hit some topics detailing seismic material, as well as some deep foundations. It's kind of all over the place. And really this is a depth topic that you're going to find for geo-tech. Not necessarily general stuff, or broad stuff, for everybody for all topics. So if you are in need of something this specific, or you need other specific material covered, feel free to email us. You can email me at [email protected] But we also have a depth course. You can go check that out. And it should cover all these topics for you if you are needing a quick dive into your own particular area. So you can go check that out at civilengineeringacademy.com if you'd like. Anyway, with that, I say we get going. What do you say?
Mark Oakeson: Let's roll, baby? Yep.
Isaac Oakeson: All right. So I'm going to add this to the stream. We're going to go straight to a big board, and I'll maximize this. All right. So the first thing we want to do is review some seismic waves, very common questions that come up just as part of our seismic review on this. And I'm going to get rid of this banner. If you need a course, go to civilengineeringacademy.com. Alright. So, first thing we want to do is review seismic waves. And one of the things we get asked quite a bit is kind of the difference between these. So, you know, this is -- A lot of seismic stuff that comes up specifically on the geo-tech depth can be theory. So that's kind of where we hit it a lot. All right. Quick reveal: When an earthquake hits, you have two types of waves hitting. So for those in the audience, joining us, if you're studying seismic material, do you know what kind of waves are hitting? What are the two types of waves that are hitting when we have an earthquake? Anybody? We'll wait for just a second.
Isaac Oakeson: Let's look at the comments. We're doing great work. Well, thank you. I appreciate that.
Mark Oakeson: Yeah. Always appreciate that.
Isaac Oakeson: All right. I don't know if anybody knows, but if you do know, leave a comment in the chat room and we will definitely check it out. So, when an earthquake hits, you have two different types of waves hitting. And so what are they? They are called Body Waves and Surface Waves. And within that, I kind of gave a hint here, but within the waves, there's two different types of waves as well -- Oh, Morgan got it. You got P Waves and you got S Waves. So if you do any research online whatsoever, you're probably going to come across some videos. This is a goofy one I found. No offense to any professor that might be watching this.
Mark Oakeson: What if somebody online is actually video?
Isaac Oakeson: I don't know. But you know what I mean? You do any sort of research online and you can get a good feel of what a P Wave and what an S Wave is. You can also get a good feel of what a Surface Wave is. So, you know, I just threw this in here because I thought it was a fun video just showing some humans. Let's see if I can -- Hopefully you guys can see this. So, here's your example of your P Wave. P Wave stands for Pressure Wave. So the guy jumps. Hits a guy. These guys are acting like molecules. But basically when you have a P Wave, these are called Primary Waves or Compression Waves. So if I can write this down. Let's see. P Waves are Primary Waves. Or They're sometimes called Compression Waves. And they, just like this example showed, a guy hits them, and they just kind of move back and forth. Kind of like a slinky, if you were just to give it a kind of a push. It just move back and forth.
Isaac Oakeson: An S Wave. Slightly different. I don't know if there's a good example this guy does. I think it's right here. I won't show the whole thing. But basically he has them interlock their arms and he shakes one of them, and they kind of shake like a snake. So an S Wave, if you think of a slinky, it's kind of doing that "S". it's kind of making that "S" snake move. So you've got your S Waves. They officially are called Secondary Waves. Or they're called Shear Waves. Okay?
Isaac Oakeson: So those are the two different waves that you have within a Body Wave. So again, you've got an earthquake hits, you're going to be hitting two different kinds of waves. One set are called Body Waves, the other are called Surface Waves. Hopefully that makes sense to you. Within the surface waves, you have two different kinds. One's mainly called the Rayleigh Wave and the other one is called the Love Wave. So these are named after the professors that discovered these. There's not some hidden meaning here on the Love Wave there. So for surface waves, we deal with Rayleigh and Love waves. Rayleigh causes some rolling and heaving as the wave moves, and love waves deal with sideways movement.
Isaac Oakeson: So a common question that they might ask you on an exam is something to deal with this: What causes the most destruction to buildings? Is it going to be P Waves? Is it going to be S Waves? Are they going to be Rayleigh waves or Love Waves? Does anybody know? Michal Coleman seems to be on it. He's got P and S waves listed. P waves. If you could take a stab at problem one, what causes the most destruction to buildings? Good question. And really guys, to really dive into these topics, you either need a good geo-tech book that you can dive into that it's going to detail some of this. Or, like, Goswami. You need a book where it details some other stuff, like Goswami. He dives into a little more detail on the depth sections versus the CERM. CERM doesn't hit really seismic material, which is a big fault that they need to update on. So we've got some answers coming in. Let's highlight some of these.
Mark Oakeson: Yeah. We got some answers, Isaac,
Isaac Oakeson: Kevin Morgan: Raleigh, because it includes both P and S. One guy on Facebook said P Waves. And I think Kevin, you are correct. So Rayleigh waves are produced with P and S waves. They involve both vertical and horizontal particle motion, and it can cause quite some damage. Love waves do as well, obviously. But from everything that I have read and experienced, the Rayleigh waves are surface waves and they produce ground motion in both vertical and horizontal directions. And you basically can have buildings rolling so they're being lifted up and they're dropping to the ground.
Mark Oakeson: Hey, Isaac. Just a way to visualize a Rayleigh wave is maybe, like, the surface of a lake or an ocean where there's not really things going on underneath. If you were to draw a picture of Rayleigh wave, maybe draw like the surface of the ocean with some undulating waves. Those waves are acting at the surface.
Isaac Oakeson: You got some waves up top.
Mark Oakeson: Yeah. But down below, there's really not much going on. So those Rayleigh waves -- There you go. That's perfect for Rayleigh Wave illustrations.
Isaac Oakeson: So yeah. These are surface waves. You know, this is the ocean. This is what you're dealing with on the surface. The P and S waves are bouncing around underneath the surface and they can bounce off the surfaces, they can refract, they can change direction, they can reach to the top. They can do all kinds of weird stuff. They can just go straight through. So these guys are the Rayleigh. And the Love. And these guys are like the P and the S [waves]. Okay?
Isaac Oakeson: So, you know, this is a good theory type question. But it also tests -- I mean, this is basically starting comprehension of seismic waves and seismic material. Is there any questions surrounding this from anybody?
Mark Oakeson: I'm not seeing any questions pop up, Isaac. It looks like you're nailing it.
Isaac Oakeson: Anyone getting on the love train?
Isaac Oakeson: All right. Cheesy dad jokes. All right, let's go to problem two. All right. Problem two we're going to talk about real quickly is liquefaction. So quick review of that is that liquefaction occurs when saturated sands break down rapidly due to an applied load, like an earthquake. As the shaking continuous the loosely packed soil attempts to move closer. There's not enough time for the water to escape. So the water ends up being confined and the soil doesn't get closer. So to make this happen, you basically need three things to occur: You need a saturated soil, so this means you got water. Okay? You need rapid loading, you need an earthquake. And three, you need soils that are susceptible to this, basically a sand. So something round clean uniform has a low effective stress, low density. We're talking about basically sands here for this. So again, this is a topic that can range in a lot of different areas, but really I just wanted to hit some conceptual stuff here with you on this, and we'll dive into other topics here shortly.
Isaac Oakeson: So problem two I wrote here, which combination will most likely lead to liquefaction during a high magnitude earthquake? So giving that quick little review, we got to use some engineering judgment here. And if you're in the audience, you can participate if you would like. So let's look at it: A) Ground made up of multiple layers with a large amount of groundwater; B) sand with no groundwater; C) soil strata made up of multiple soil layers with little groundwater; or D) ground made up of rock with little groundwater?
Isaac Oakeson: So let's just go through these a little bit. A) if you have a ground made up of multiple layers, you have a good chance one those layers is sand. And if you have a large amount of groundwater, then that's definitely a choice we want to select. B) sand with no groundwater. If you have no groundwater, you're not going to have liquefaction. You need to have groundwater, right? Some sort of water involved here. Saturated soils. Kevin's got it. And let's see. Jess Smith got it. Excellent.
Mark Oakeson: Yeah. It looks like most people are honing in on the right answer, Isaac.
Isaac Oakeson: You got it. So C) soil strata made up of multiple layers with little groundwater. You have no groundwater again. And then D) ground made up of rock with little groundwater. Again, no groundwater. And you're dealing with rock. So rock's a good thing. You're not going to have liquefaction with the rock. It's not low density. So good job, guys. The answer here is A) for sure. That's your best choice. You have a best option of hitting sand and you're dealing with water. Let's just look at this one. Problem 2a: It's been determined that a proposed construction site is on a liquefaction susceptible location. Which statement is true about this location? Do you want to avoid construction here? Do you want to improve the soil strength, drainage, and density? Which one of these is true? True: Improve the soil, strength, drainage, and destiny. C) Do we want to design the foundation to resist the effects of any liquefaction? Or do we want to do D), all the above? Any thoughts on that one? If we go through each one of these, we could straight up -- We're getting some answers here.
Mark Oakeson: Yeah. We've got an answer B.
Isaac Oakeson: B is a good answer. Improve the soil, strength, drainage, and density. And D is also a good answer. So now the secret here is this is the kind of crap they might throw at you on the PE, where you're going to have good answers. So you have to really think about it. In this case, you could straight up avoid construction here. That is an answer that is acceptable. If you found liquefaction in an area, you could avoid it. You know, it's probably not ideal if you have to build there. But if you could avoid it, you could. That's a true statement. B) Improve the soil, strength, drainage, and density. Yes, we could do that. There's lots of methods that you could use to improve the soil, the drainage, and the destiny to try to remove liquefaction potential. C) Design the foundation to resist the effects of liquefaction is also a possibility. You could go deep. You could build to a rock layer. Get a deep foundation going. So the answer here really is D) all the above, okay? And so these are the kind of questions that you could potentially see on the PE exam, where you have lots of good options, and you just need to think through every option here and make the best selection. So you guys definitely were catching on there. Good job, everybody! Any questions just loosely regarding liquefaction before we dive into something here a little deeper with liquefaction? Probably not yet.
Mark Oakeson: Yeah. I'm not seeing any questions here. I think you nailed this one again, Isaac. Good job!
Isaac Oakeson: I like that. I keep nailing these things. If I screw up, I'm sure the audience will let me know quickly. All right. Let's dive into something a little more juicy here. Problem number three. So we want to do another quick review of what CSR and CRR are -- Wow! That's a lot of R's. To evaluate liquefaction potential, you can use two methods to evaluate. One of them is called the CSR and the other one is called the CRR, which stands for Cyclic Stress Ratio and Cyclic Resistance Ratio. So what CRR means is it represents the capacity of the soil to resist liquefaction. That's simply what it is. And CSR represents the seismic demand on the soil and is a ratio of the horizontal sheer stress to the vertical effective stress. Pretty straight forward. This could be likened, I believe we said, to Poisson's Ration.
Mark Oakeson: Yes. The CSR could be likened to the Poisson's Ratio.
Isaac Oakeson: So I can't even spell Poisson. The Poisson Ratio. Very similar to that. So, if we look an equation -- I'm going to show you the equations as we solve these problems. But if you are diving into these topics, my hope is that you have a reference or some sort of material that you can reference to determine these equations, okay? But we'll go through this anyway. So problem number three: Groundwater for a site was 12 feet at the time of the testing. The average horizontal sheer stress was 500 pounds per square foot. Based on corrected blow counts, the CRR was determined to be 0.4, which also includes the correction for a seven and a half magnitude earthquake. Which is a severe earthquake, right? That's a doozy.
Mark Oakeson: Yeah. And I think that's the benchmark -- Well, it's not the benchmark, but it is classified as a severe one for sure.
Isaac Oakeson: Yeah. So the question here is: What is the factor of safety against liquefaction for the sand below the water table, given the density is 115 pounds per cubic foot and the saturated density is 118 pounds per cubic foot? So we got a little diagram here. You've got sand going down. You've got water table down here at 12 feet. You've got a clay layer down here at the bottom. So what we could do is--- The first thing that we need to do is determine the vertical effective stress at the mid point of the saturated sand layer, which is going to be right here. So first thing we're going to do is solve for that. So, what's effective stress? We have to take that first layer, which was determined as 115 pounds per cubic foot. And I'm not going to write down all the units here. We're going to multiply that by the thickness here, 12 feet. That's the layer that we're dealing with. Now we have to add. The next layer here was 118 pounds per cubic foot. And we're going to subtract out the poor water pressure, which is 62.4 pounds per cubic foot, and multiply that by four, which gets you down to halfway point of that sand layer. Is everybody clear on how to solve for effective stress? Hopefully so.
Isaac Oakeson: If you're in the Geotech world and you're studying for this exam, you should know how to solve for the effective stress at a certain level down there. All right. So if I punch that out, I get an answer of 1602.4 pounds per square foot. So that gives you your vertical effective stress at the mid point of the sand layer. The next thing we have to do is calculate the cyclic stress ratio, which is CSR. And the equation for that is this. We said it was a ratio, right? Up here. It's a ratio of the horizontal sheer stress to the vertical. So there you go. So we've got a ratio of the horizontal sheer stress to the vertical shear stress, or effective stress. Sorry. And that's your equation for CSR. We were given what the horizontal sheer stress was. And in all honesty, I'd probably write down what each of these values were right over on the side. Just so it's a quicker reference for me, you know? Horizontal sheer stress was given to me as this, CRR was given to me at 0.4. But it said this also includes a correction for seven and a half magnitudes. So that's for a CRR of seven and a half magnitude. We're solving for the factor of safety. And you were given these, you could write in right in the layers if you wanted to. So this is pretty straight forward. This was given. It's 500 and you're going to divide that by what we just saw for it: 1602 0.4. And I get an answer here of 0.312 is my ratio. Okay? Now that we got that, we can solve for our factor of safety. There is an equation in that lists to solve for the factor of safety. It's equal to your CRR of 7.5 and you divide that by the CSR, and that gives you your factor of safety against liquefaction. So, if you've simply just solve that out, your CRR of 7.55 was given, a 0.4, and divide that by the CSR we solve for, 0.312. If my math is right, I get an answer of 1.28.
Isaac Oakeson: So these are the equations that you're going to need to know if you're solving for CRR and CSR, which will help you out. And so like I said earlier, you need to have a good reference, I could book, a Geo-tech depth book. Use Goswami's reference material to really hit some of the seismic material, or good geotech book so that you can solve this type of equations and problems that come up. Any comments, Mark?
Mark Oakeson: Well, Isaac. It looks like you're nailing it again. I'm not seeing any questions there. But this is a good one. I like how we've related your CSR to Poisson's Ratio. And it's kind of cool to see that the factor of safety is really just a gauge. It's a measurement on when the horizontal stress --- Or excuse me, when that vertical stress exceeds that horizontal stress and everything starts moving, and you get that liquefaction.
Isaac Oakeson: So if your factor safety is 1.28, you're saying basically you're 28% able to resist your liquefaction by this amount.
Mark Oakeson: Yeah. It's like any factor safety, right? You're over strengthed, or your factors of safety, you're almost 30% greater than the capacity that you need to resist your stresses.
Isaac Oakeson: Which is good. So maybe you did something to the soil, you improved it in some way, you know? Maybe you are shooting for a factor of safety of that. Or maybe you need more. Maybe you need a higher factor of safety and you need to improve the soils. So that helps you to determine that.
Mark Oakeson: It's a good one. You nailed it, Isaac. Good job.
Isaac Oakeson: All right. Any questions? it doesn't look like we have any. If you do leave them in the comments. We can always come back to some of these problems. I'm going to march forward beause we got some doozies in here. All right. Problem number four. And this is a good one. So now we're going to start diving into the code. So as part of this review of seismic material, I wanted to cover seismic base shear. And according to the ASCE 7-10, which is what's used on the exam right now. We haven't upgraded it. I believe they have a 7-16 out now, and probably a new one shortly after.
Mark Oakeson: Yes. Seven ACI --- Or excuse me. ASCE 7-22 is going to get released here fairly soon.
Isaac Oakeson: Yeah. So it seems like -- You know, everybody lags behind on when they're actually released versus when they're adopted. And it sounds like the NCEES just hasn't gotten around to updating that.
Mark Oakeson: Yeah. It's all good.
Isaac Oakeson: So hold your questions on the standards. We're sticking with 7-10, uh. According to 7-10 ASCE 7-10, the seismic base shear using the equivalent lateral force procedure in a given direction is determined by this equation V=CsWs. And so what we want to do is quick really review the ASCE 7-10, which kind of gives you the whole breakdown of how to solve for this this type of equation. And then we'll do a problem. Quick problem. So seismic base share, just like I said, is given by this equation, this 12.8-1, where vehicle CS times W. CS is the seismic response coefficient. So, if you likened what we're trying to solve here to F=ma, we're finding a force that's hitting the building, that's happening on that very ground level. And you're taking some sort of earthquake information that we're gonna plug into the Cs value, and we're going to multiply it by W, which is basically the weight of that building, the effect of weight. And sometimes, you know, when you're solving these things like California, we're in Utah, we have high seismic loads here in Utah. Sometimes these values for Cs -- I mean that you're multiplying by the weight of the building can get quite high. Almost to two times the weight of the building. I mean, if you took like a 10-story building and you can imagine how much that weighs, and then you're multiplying that by two, that's a lot.
Mark Oakeson: Yeah, Isaac. You can attribute that, or you can equate that equation to Newtons. You know, good old F=ma equation, right? So you can have a given mass, which kind of relates to your W, but the higher the acceleration, right? The bigger the force, or the bigger that V, the bigger the shear is going to be. You can think of it that way as well.
Isaac Oakeson: Yeah. Good deal. So as part of that, like I said, we have to find out what the Cs value is. And it kind of just runs you through a lot of equations that you will need to find some variables for. So if you jump to 12. 8-2, you have to find what that Cs value is, which is the seismic response coefficient. And that you can see here that it's equal to SDS divided by R over I. SDS is the design spectral response acceleration parameter in the short period range, as determined by these equations, 11.4.4 or 11.4.7. And so that's why we've included some of those sections kind of over here as you start diving into what you really need to solve this. R is the response modification factor pulled from a table. Ie is the importance factor determined in accordance with the section 11.5.1. And they can't exceed the values of CS computed. You know, you have to follow these rules as part of this code. So your period, your short period T, has to be less than TL for this equation, 12.8-3. T is greater than TL. Then you've got to use 12.8.4. And it can't be less than this guy, 12.9-5, where Cs is equal to this funny equation.
Mark Oakeson: Isaac, it's worth noting that seldom does that equation 12.8-5 apply. It's very rare that that happens.
Isaac Oakeson: Okay. Good to know. So you know, diving into this, you've got to find what that SDS is. So if you're going to go find what SDS is, we're going to section 11.4, and you'll see the SDS is equal to these guys. So it's STS is equal to two thirds SMS. And then you gotta go find out what SMS is, which leads you to this section over here. And I've kind of just copied and pasted a lot of this for you guys to look real quickly, but you would have to thumb through this, and hopefully you've got this tabbed up pretty well for your own exam, so you're not thumbing through things like crazy. Or you've practiced enough problems that you just know what you need is really the ideal situation.
Isaac Oakeson: So, okay. So eventually you drill down to what SMS is, which was titled here, site coefficients and risk targeted maximum considered earthquake spectral response acceleration parameters. The MCE spectral response acceleration parameter for short periods, which is what SMS is. And one second periods, so SM1, adjusted for specific site class effects shall be determined by these equations. So this stuff is pulled from maps and data. You've got seismologists that are out there pulling. you know, they've got measured locations on maps, and you can pull this information from those locations. SS is listed here. So if you look at what SMS equals, it's equal to Fa times SS, and SM1 is equal to FE times S1. So NSS is the mapped MCE spectral response acceleration parameters at short periods, as determined in accordance with this section 11.4.1. And S1 is the mapped MCE, spectral response acceleration parameter at a period for one second, as determined in accordance with 11.4.1, Where site coefficients FA and FV are defined in tables.
Isaac Oakeson: Mark, you brought some good comments about this in some of your studies before. What did you, what did you say on that?
Mark Oakeson: Just that those mapped accelerations are always expressed in terms of percent of gravity, percent of the acceleration of gravity. And like you were talking about, there's two separate maps, one for the long period and one for the short period. One for the long period is the one second period. And the short period is the Ss value. You need to make sure you're looking at the right maps as you're kind of thumbing through your ASC 7. But they're just spectral response values that are derived or determined from seismologists that have actually gone out there and hooked up a size monitor and measured percentage of gravity accelerations for given events, given seismic events. And so they think they got a pretty good beat on how much acceleration is going to happen in a given area. based on those maps.
Isaac Oakeson: Okay, good. So I just pulled up here what Fa is. Like we said, it comes from tables. So in table 11.4-1, we have the site coefficient Fa. So you just look at what site class you've been determined where you're building. So if you've been given a certain site class, which is dependent on soil conditions and such, you can go look up what your value should be. And you've been given these different values for what Ss is, which is based on what comes from the map, right? So you go find an S. Let's say you're in class D and you've got an Ss of 1.0. So if you were a class D, If I'm reading that straight across, it should be 1.1. And then lastly, I don't know if I hit this, but you've got this design spectral acceleration parameters of SDS, and SD1 for the short term and the one-second duration. And those are the equations for those.
Isaac Oakeson: So my point in showing you all of this is that you take a simple equation V=Cs times W, and you realize that there are a lot of different pieces that you may need to solve for this equation. And the truth of the matter is that on the PE Exam, any one of these pieces could be a question, a piece of the puzzle that you may need to encounter. So when you're practicing, and you need to solve for a seismic base shear type question, it's good to pay attention to all the steps that you need to see in order to solve that, because any one of those other steps could be a potential question that you might get hit with. So I guess before we carry on, any questions on the code that I'm diving into for seismic base shear? If there is leave them in the comments and we will get to them.
Isaac Oakeson: So let's get to an actual question that we might have to solve for this. So here we go. Problem four: A 12-story building listed on a site class D, this is all good information. So, I mean, you could be jotting this down, right? 12- story, class D. Has a response modification factor R=5.6, and a Seismic Weight W=3000 Kips. Sorry for the handwriting. The mapped acceleration parameters Ss, so they got this from a map, 1.25, and S1 is equal to 0.45. The seismic importance factor, so that's, IEE, is equal to one. Can you guys read this? I don't know if you can. That's a four and a five. Let's redo that.
Mark Oakeson: Yeah. I can read it.
Isaac Oakeson: All right. The fundamental period T, so T is our fundamental period, and that was given us 2 seconds. And the long period TL is equal to 3 seconds. What is most nearly the seismic base shear for the structure? So as soon as you say, seismic base share, what's the equation we want to use? V=CsW. So the first thing I need to look up, we know what W is. We don't know what Cs is and now we've got to go through all those steps. So, first thing we do is we're going to write out what Cs is. Cs is equal to SD1 divided by T times R over IEE. That comes right from the code that we just reviewed, and I could show you again up here, okay? 12.8-3. And that is specifically for T less than or equal to TL. So T here is less than or equal to TL. That's why we're using this, right?
Isaac Oakeson: All right. So next thing we needed to find here. We don't know what SD1. So we got to solve for what SD1 is. And if we go back up to our code, SD1 is two thirds of S1. Two thirds S1, which equals two thirds times 0.45, which equals 0.3. This is your one-second spectral response. So now we can say that CS is equal to 0.3 divided by two times -- Two was given as T right? Plugged into. And Rwas given as 5.6. Divide that by one. And that equals 0.027. So there's your seismic response coefficient. So there's some other parts to this. We can't just assume that's all we need here. We have to calculate now the minimum Cs. Make sure it's all valid here. So next thing to do is calculate the minimum. That equation was given up here as Cs is equal to 0.044 times SDS times IE. and that's to be greater or equal to 0.01.
Isaac Oakeson: So now we have to find what SDS is. To find SDS, you have to solve for SMS, which is -- So SDS is the short period. To solve for that, we have to find the max period, which is what SMS is. And we use equation 11.4 dash one and the table that we showed. So if you go up there and look at it, SMS is equal to FaSs. And to solve for Fa, RSS was given here as 1.25. And if you go up and look at the table for 1.25 for the site class D, you can see that it's 1.0, okay/ Hopefully you can see this up here: 1.0. So if you're going to solve for SMS, your Fa is one and your SS is 1.25. Mark, am I missing anything? Are you checking me?
Mark Oakeson: No, you're doing great.
Isaac Oakeson: All right. Let's keep going. So the max considered spectral response is From equation 11.4-3, and that says that SDS is equal to two thirds SMS, which equals two thirds times 1.25, which equals 0.833. So what's the minimum coefficient? So the min seismic coefficient. -- So now we can plug it in here. So Cs is equal to 0.044 times 0.833 times one, and that has to be greater or equal to 0.01. If I work that out, I get an answer here equal to 0.037, which is okay. So therefore you're going to use 0.037, okay? So now we go back up and we say: Okay, our seismic base shear equals CS times W. Now we plug in our 0.037, and we're going to multiply that by our 3000 kips, which was given to us. And that gives me 111 kips. Yeah, that's a lot of kippige
Isaac Oakeson: So that my friends is a quick rundown of using the ASCE 10 code solving for seismic base shear and seeing how that works. Now, you're getting a seismic base shear at the bottom. A deeper dive, would be that you could solve for this as it applies to other levels of the building. Right?
Mark Oakeson: Well, yeah. So we can say, Isaac, that that base shear is kind of a reaction to all the shears that are being imposed on every story going up through that building, right? So you could divide that base shear just using statics. As thinking of the concepts that we're looking at here, that base shear is kind of the main reaction to all the shears that are getting distributed throughout the stories on that building because of the seismic event that's occurring. So if you were to draw a big arrow at the bottom of that building that you've got there and label that V, then you could draw little arrows on every story going up the building and they'd be in the other direction, right?Because they'd be reaction to the -- There you go. So all those little v's that are being imposed at every story -- Although that up to the big V, which is the base here. That's the big V.
Isaac Oakeson: So yeah. I mean, hopefully this whole problem just gives you a new respect, if you aren't studying this yet, of knowing what seismic base shear is. How it works, using the code to solve that. You need to be familiar with that as part of this, the ASCE7-10. Probably spent a lot of time on that.
Mark Oakeson: And it's worth mentioning too, Isaac, this problem is a little kind to us in the fact that it, it just gives us the importance factor, it just gives us the fundamental period, you know? It just gives us the response modification factor. All those items are -- You know, that's a whole other subject for calculation that you'd have to get into.
Isaac Oakeson: Do you have any questions on this one?
Mark Oakeson: It looks like we've got one here, Isaac. It says: And what do you do if your Cs mean is not greater than or equal to 0.01? Do you use the other Cs solve for? Does that mean we did it wrong? It happens so infrequently that I would worry about having done something wrong. It's the short answer on that.
Isaac Oakeson: Okay. That's good to know.
Mark Oakeson: So, in a test situation, if you're finding out that that is the minimum, then I would go back and look at my calcs. Because you're probably doing something wrong.
Isaac Oakeson: Good question. All right. Any others? We will move to the next one here. I know we try to keep these to an hour. This topic, and the other ones are just long topics. All right. Quick review of this. Let's go through it real quick. Piles. We're going to do deep foundations now. So piles, they're slender foundations driven or drilled into the ground. They can be still, timber. They can be steel casing, which is just a big steel kind of thick ring with reinforced concrete. You have friction piles. They get their load bearing capacity from the friction from the soil along the pile shaft. You have in-bearing piles, they get their bearing capacity from the soil layer below the pile and transfers the load to the rock or a hard layer at the very bottom.
Isaac Oakeson: When are they used? They're used when shear strength of upper soil is insufficient or too weak to support the load. So if you have crappy soil on the top and you've got to get deep. They're used when you have expansive soils or collapsible soils, or really compressible soils that are present. They're used when foundation soils have scouring effect. So if you're thinking of building across the river or something where you've got moving water, then you're probably going to want a deep foundation. They're used to resist horizontal forces and support vertical loads. They also resist lateral and uplift forces in structures, like transmission towers or offshore platforms. I have used these a lot in my own industry for some transmission structures and they can get very large when you start modeling these things.
Isaac Oakeson: So like I said here, get a good geotech textbook for these kinds of problems that come up. You know, you could also be dealing with group piles, skin friction, installation methods, that those are usually theory questions. Pile dynamics, testing, integrity tests, these are all topics that come up. Today, we're just going to hit capacity using skin friction. You want to check out the CERM chapter 18 for this. There are several different ways of calculating skin resistance of pile and clays. One of the methods is called the Alpha Method, and it's a very common method to solve. There's also methods called the beta and lambda methods. I'm not covering those, but the Alpha Method calculates skin friction as a function of the cohesion of the soil. So let's go through this.
Mark Oakeson: Hey, Isaac. You said CERM chapter 18, but I think you mean meant CERM chapter 38.
Isaac Oakeson: Yeah. I think I wrote it right and said it wrong. 38! Sorry about that. All right. So let's look at this question: A two-foot diameter pile, so we've got two-foot diameter, is driven 50 feet into the ground. That's going to be our length, this diameter, into a normally consolidated clay. That's good to know. The soil strength varies linearly from zero at the surface. So if we have this thing drawn out. At zero at the surface down to 1200 psf at the pile tip. The estimated adhesion factor is 0.8. So that's what this symbol is, is the adhesion factor alpha. What is the total capacity of the pile?
Isaac Oakeson: So if you go to chapter 38 and you read up on piles, you'll find that our total capacity Q is equal to the Qs plus Qp, or the skin plus the tip pile capacity. And you will also see that Qs is equal to the summation, if you had multiple layers of alpha, times, Su, which is our undrained shear strength, times your Delta L,or length, times the perimeter. So here we go. We've got some variables. We can start looking at what we need to do here. To solve for Su -- What do we get here? The soil strength, which is our undrained shear strength. We could write that here. Su is equals 1200 psf. In the case of the equation, though, we want to find the average soil strength. So to solve for Su we take 1200 plus zero at the top divided by two, and you get 600 psf. That is the average soil strength. on this pile.
Mark Oakeson: We understand that because in the problem, it says that it varies linearly from zero down to 1200. Okay.
Isaac Oakeson: It varies literally, which is why I tried to draw this out like that. We know the length is given to us. We need to solve for perimeter. P is equal to pi times D. So that's equal to 3.14 basically times two. So something like 6.28. And now you can solve for what Qs is. Qs is equal to 0.8 multiplied by 600, times 50, times 6.28, nd that equals 150,720. Or, I'm going to just round it, about 151 kips, okay? Are we cool there, everybody? Did I put you to sleep yet? Wake up, Kevin!
Mark Oakeson: I'm not seeing any questions, Isaac. So you must be nailing it.
Isaac Oakeson: All right. Well, now that we've solved for Qs -- I'm going to scroll this down a little bit more. All right. Now we need to solve for the tip. So Qp is equal to, and these are equations in the CERM, but Qp is equal to NsAP times C. Now NC is equal to always nine for these kinds of problems. And that's because it's coming from the Terzaghi cohesion factors. You can look that up. Table 36.2 in the CERM. If you remember, you know, solving for bearing capacity, there's these factors. Well, if you're dealing with driven piles, this is typically always nine for driven piles.
Isaac Oakeson: And then we've got area Ap, which is the area of the pile tip. And we can just solve that right here if we want it to. pi force D squared. So PI force times, diameter was two feet. So that cancels out, got 3.14 feet squared. That was nice. How about that? All right. So now we can solve for Qp. And that equals Nc, which was nine, times Ap, which was 1200, and C, which was 3.14. So I get an answer here of 33.212, or approximately 34 kips. Now, I'm putting approximate numbers here, but you probably would keep it in that on the real exam. So then your Q total is equal to -- I just add them up, right? So we had about 33.9 kips plus one 150.7 kips. And that equals 184.6 kips. Check my math. All right. Looks good.
Mark Oakeson: Yep. It looks good. That's a nice pile too. If you get 184 kips out of your pile, you're doing pretty good.
Isaac Oakeson: That's a good one, huh?
Mark Oakeson: Yeah. When we drive pile, we're usually in the 30 foot kind of depths, just because of soil conditions. And we're usually just north of a hundred kips per pile. So 184 is a nice pile.
Isaac Oakeson: It's solid. So this was a quick example of a deep foundation. Like I mentioned earlier, there's lot of different things they could test you on. And, you know, it's a whole other live that we probably could do problems on dedicated to that. But regarding this particular problem, any questions you guys have watching this? And if you have any, feel free to email us too, if you're watching a replay of anything. We'll also be putting this on the course eventually. Kevin asked a question: If the question did not say varies linearly, then C in Q's equations would have been 1200, assuming the soil profile was all the same, correct? I would assume so, Kevin. Yes, if they did not say that, if they did not say varies linearly in the problem. Yeah. Yeah. This is one of those things where you got to pay attention to some of that wording in the problem. Good catch. Never seen an example that says varies linearly. So all good to see. Yeah. That's to throw you off, Kevin. Make you think a little bit more when you're solving these PE problems. Okay? If you have any more questions, let us know in the comments, I'll come back to it. Let's get to these last few that I have. And then we'll let you guys get back to your weekend.
Isaac Oakeson: All right. Problem number six. This is a theory question. Deep foundations theory. Most problems I specifically remember were more theory geared on the exam dealing with these. All right. Many deep foundation theory problems can pop up on the exam. Here's problem number six that we've got for you: Which of the following is true for the use of crosshole sonic logging used for quality control for deep foundations? How many of you out there have heard of cross hole sonic logging? I don't know if many people have, but this basically falls under dynamic pile testing. It's actually a non-destructive test for quality control. It's what this is. Sorry.
Mark Oakeson: Yeah. And it's typically used on big caissons. So when we do big bridge work, and we've got big piers that we're constructing this is how they test whether the concrete has got a nice uniform consistency across the --
Isaac Oakeson: Michael's heard of them. And I, to be honest with you, I use these all the time. I do project management now, but in the transmission world, if we hit groundwater and we could not get -- If it was considered a wet hole, then we would use crosshole Sonic logging to test the installation of their work and see if there was anything wrong with that foundation at specific levels of that foundation. Because you want your foundation to be solid. And there's a specific point on that foundation where it sees the maximum loading on it. And if you have problems at that spot and you've probably got problems,
Mark Oakeson: Yeah. You just don't want big honeycombs, big voids in your concrete matrix,
Isaac Oakeson: Blow outs with soil coming into it.
Mark Oakeson: Exactly.
Isaac Oakeson: So, okay. Let's look at this, which of the following is true for the use of Crosshole Sonic logging use for quality control for deep foundations? And you're thinking in your head, when you're looking at this, "Do I even know acrosshole sonic logging is?". Like, I've played Sonic the hedgehog, but I have no idea what this is talking about. And maybe that's you. But you need to read up on it. So A) CSL is not sensitive to the surrounding soils or pile length; B) It can be used to find defects and determines where the defects are located; C) it cannot find diameter changes or bulges; and D) all the above.
Isaac Oakeson: So, these things can do quite a bit. Definitely CSL is not sensitive to the surrounding soils or Pile length. It can be used to find -- I know it can be used to find like soft bottoms if the tubes extended the bottom of the shaft if you go that deep. It can find multiple defects and determines -- It can determine like a quadrant where those defects are located. It cannot find diameter changes or bulges. It's like a pulse of energy, which is probably the one that's going to throw you off. And it cannot see the cage and does not detect centering or covering a problem. I'm reading some other things about crosshole Sonic logging that maybe you don't know. So yeah. The answer I'm going to go with here on this, guys, is D? Michael, do you agree? You work for a drilling company. Throwing Michael out there.
Isaac Oakeson: So yeah. I mean, these are the kinds of questions you're going to see on the exam typically, if you're going to get a theory type of question. But that's the answer for crosshole sonic logging. D).
Mark Oakeson: Yeah, pretty straight forward. You just got to understand your CSL.
Isaac Oakeson: So like I said, to try to wrap this up and keep you guys here for an hour, get a good textbook. Get a good geotech textbook that you can dive into some of this material, if you haven't already. I mean, we're sitting in April already, so the exams coming up. I mean, hopefully you've got something going already. But for future test takers, make sure you do have a good textbook. Like I mentioned before, Goswami's review manual does dive into some seismic material and this material where the CERM doesn't necessarily hit it as hard. But I think between a good textbook and that, you should probably be okay on the exam. But if anybody else has any other resources, we are game to hear them. Another good resources, just generally studying some seismis material in general. And I know many people in California do have to take the seismic exam, and there's some good resources built for that exam that could be applicable to this particular depth exam. Mark, any last thoughts before we wrap this up for the day?
Mark Oakeson: No, man. I think you nailed it. I think you did a great job here. I haven't seen a lot of questions. So that means everybody understood exactly what you did.
Isaac Oakeson: Good. I hope so. Like I said, guys, if you guys need help with anything with the PE exam, don't hesitate to email us. We are around. You can reach Mark at [email protected] I'm [email protected] If you do need a course, whether that's FE, PE, or specifically to your depth, our course for the depth section includes all materials. So you're going to get it all, even though you might not want it all, but that's pretty sweet. And you can go check it all out at civilengineeringacademy.com. And that's going to probably wrap it up for us.
Mark Oakeson: Yeah. That's going to be it. Thanks, Isaac. It was fun.
Isaac Oakeson: No problem. Everybody have a good weekend and we'll see on the next live workshop.
Mark Oakeson: See you.
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