WEBVTT 1 00:00:00.200 --> 00:00:03.200 Right now, the building you're sitting in is trying to 2 00:00:03.240 --> 00:00:04.160 tear itself apart. 3 00:00:04.320 --> 00:00:06.599 Oh. Absolutely, it's constant, it really is. 4 00:00:06.679 --> 00:00:12.839 It is waging this massive, invisible, entirely relentless war against gravity, 5 00:00:12.960 --> 00:00:17.559 the wind, the soil, and well its own crushing weight. 6 00:00:17.879 --> 00:00:20.199 Yeah, we look at a house and see a static, 7 00:00:20.719 --> 00:00:24.079 quiet box of brick and wood, but that perception is 8 00:00:24.199 --> 00:00:25.079 just a total. 9 00:00:24.800 --> 00:00:28.800 Illusion, completely and exposing that invisible battlefield is exactly what 10 00:00:28.839 --> 00:00:31.519 we are getting into today. Welcome to this deep dive. 11 00:00:31.960 --> 00:00:32.640 Glad to be here. 12 00:00:33.000 --> 00:00:35.439 Whether you are an engineering student trying to wrap your 13 00:00:35.479 --> 00:00:39.159 head around coursework, a young professional, or just someone who 14 00:00:39.200 --> 00:00:42.560 is insanely curious about how the physical world is put together, 15 00:00:42.880 --> 00:00:44.200 we are thrilled to have you with us. 16 00:00:44.320 --> 00:00:47.759 Definitely. Our mission today is to explore the scientific principles 17 00:00:47.799 --> 00:00:50.640 and the practical applications of building construction. 18 00:00:50.399 --> 00:00:53.240 Right and we are pulling heavily from excerpts of a 19 00:00:53.280 --> 00:00:57.359 really fascinating textbook, Andrew J. Charlotte's Fundamental Building Technology. 20 00:00:57.640 --> 00:01:00.399 It's a great source. The goal here is to mentally 21 00:01:00.439 --> 00:01:04.480 construct a house from the ground up, translating complex civil 22 00:01:04.480 --> 00:01:08.640 engineering concepts into clear, actionable knowledge. 23 00:01:08.400 --> 00:01:11.959 Which means looking at everything the physical mechanics of the 24 00:01:12.000 --> 00:01:15.760 dirt beneath our feet, the actual chemistry of concrete, and 25 00:01:16.079 --> 00:01:18.560 you know the physics of how load bearing walls even 26 00:01:18.599 --> 00:01:19.480 manage to stand. 27 00:01:19.319 --> 00:01:23.040 Upright exactly, Because before a single brick is laid, we 28 00:01:23.159 --> 00:01:26.079 have to understand the forces the building is going to exert. 29 00:01:26.439 --> 00:01:28.400 You can't design a solution if you don't even know 30 00:01:28.400 --> 00:01:32.599 the parameters of the problem, right, So the book talks 31 00:01:32.640 --> 00:01:35.879 a lot about the science of loads. What exactly is 32 00:01:35.879 --> 00:01:37.319 a house pushing down with? 33 00:01:37.719 --> 00:01:43.120 Well? Engineers categorize these forces into three specific types of loading. First, 34 00:01:43.159 --> 00:01:43.799 you have the dead. 35 00:01:43.680 --> 00:01:46.200 Load, which is just the materials themselves. 36 00:01:45.879 --> 00:01:49.000 Right right exactly. It's simply the weight of the building materials, 37 00:01:49.280 --> 00:01:52.599 the bricks, the timber, the concrete foundation, the roof tiles, 38 00:01:52.680 --> 00:01:56.519 stuff that doesn't move exactly. It is a constant, unchanging force. 39 00:01:56.879 --> 00:01:59.319 Then second, you have the live load, okay, and that's 40 00:01:59.599 --> 00:02:02.680 the very weight of everything inside the building, so the occupants, 41 00:02:02.719 --> 00:02:05.840 your furniture, the washing machine, the bookshelves. 42 00:02:05.280 --> 00:02:07.519 Because it changes based on how the space is used 43 00:02:07.519 --> 00:02:08.639 over time decisely. 44 00:02:08.840 --> 00:02:10.960 And the third one is the superimposed load. 45 00:02:11.319 --> 00:02:14.840 Superimposed that sounds like weather stuff, You got it. 46 00:02:15.680 --> 00:02:19.000 This accounts for environmental forces interacting with the outside of 47 00:02:19.000 --> 00:02:22.360 the structure. Late wind wind. Yeah, think of the lateral 48 00:02:22.400 --> 00:02:25.199 pressure of a gale force wind pushing against the side 49 00:02:25.199 --> 00:02:28.599 of the house, or you know, the sheer physical weight 50 00:02:28.639 --> 00:02:30.919 of a heavy snowfall accumulating on the roof. 51 00:02:31.000 --> 00:02:33.560 I am trying to visualize the sheer scale of this. 52 00:02:34.159 --> 00:02:37.680 If we take a typical brick built house, what kind 53 00:02:37.719 --> 00:02:40.159 of numbers are we actually talking about pressing into the 54 00:02:40.159 --> 00:02:40.840 dirt well? 55 00:02:40.879 --> 00:02:44.520 A standard brick built house exerts a uniform downward load 56 00:02:44.639 --> 00:02:48.800 of roughly thirty kilin Newton's per meter run on its foundations. 57 00:02:48.919 --> 00:02:51.599 Wait pause right there, What exactly is a meter run? 58 00:02:51.759 --> 00:02:53.400 I mean, I can guess, but no. 59 00:02:53.280 --> 00:02:55.639 It's a good question. Think of it like taking a 60 00:02:55.680 --> 00:02:58.719 slice out of the house. If you imagine isolating just 61 00:02:58.800 --> 00:03:02.280 one linear meter of exterior wall running from the roof 62 00:03:02.319 --> 00:03:03.479 all the way down to the foundation. 63 00:03:03.639 --> 00:03:04.960 Okay, I have that mental picture. 64 00:03:05.280 --> 00:03:07.840 The total weight of that one meter slice pressing into 65 00:03:07.879 --> 00:03:10.319 the ground is about thirty killin Newtons. 66 00:03:09.960 --> 00:03:11.680 Which means what in plain English. 67 00:03:11.719 --> 00:03:14.400 To put that into everyday terms, it is roughly the 68 00:03:14.439 --> 00:03:18.039 equivalent of stacking three small cars on top of every 69 00:03:18.039 --> 00:03:19.919 single meter of your foundation. Trench. 70 00:03:20.120 --> 00:03:23.400 Wow, that is an immense amount of weight. And the 71 00:03:23.400 --> 00:03:26.080 book mentions that engineers don't even use that number when 72 00:03:26.120 --> 00:03:28.599 they design the foundation, right, They use something called a 73 00:03:28.639 --> 00:03:29.639 factor of safety. 74 00:03:30.000 --> 00:03:32.560 They do, yeah, because designing a building right to the 75 00:03:32.599 --> 00:03:35.639 edge of its breaking point is just a recipe for disaster, 76 00:03:35.919 --> 00:03:39.159 right obviously, So the factor of safety for foundations is 77 00:03:39.240 --> 00:03:43.000 usually two. This means we design the foundation to safely 78 00:03:43.080 --> 00:03:45.520 handle double the calculated load. 79 00:03:45.719 --> 00:03:48.199 So instead of thirty killing newtons, we engineer it to 80 00:03:48.240 --> 00:03:49.520 support sixty. 81 00:03:49.400 --> 00:03:53.680 Exactly sixty killing newtons per meter run. This ensures that 82 00:03:53.759 --> 00:03:57.120 even if there is an unprecedented snowfall or a massive 83 00:03:57.159 --> 00:03:59.919 structural shift, the foundation won't fail. 84 00:04:00.199 --> 00:04:03.439 Okay, So we have these immense stresses pushing down into 85 00:04:03.439 --> 00:04:06.520 the earth, and I like to visualize stress in materials 86 00:04:06.560 --> 00:04:09.280 in a very physical way, you know, like compression is 87 00:04:09.319 --> 00:04:12.800 like crushing something, tension is pulling or stretching it, and 88 00:04:12.879 --> 00:04:14.560 sheer is like tearing it apart. 89 00:04:14.840 --> 00:04:16.240 That's a perfect way to look at it. 90 00:04:16.480 --> 00:04:20.120 So we are pushing all this compressive stress into the dirt. 91 00:04:20.800 --> 00:04:23.800 But obviously not all dirt is created equal. 92 00:04:23.680 --> 00:04:26.600 Oh far from it. In soil mechanics, the ground is 93 00:04:26.639 --> 00:04:30.879 generally classified into three types. First, you have solid rock. 94 00:04:30.959 --> 00:04:32.120 Which is the dream scenario. 95 00:04:32.160 --> 00:04:35.959 I'm assuming pretty much. It obviously has an immense bearing capacity, 96 00:04:36.240 --> 00:04:39.639 provided it isn't severely weathered or fractured. Then you have 97 00:04:39.959 --> 00:04:42.720 non cohesive soils. Think of dense gravel and sand. 98 00:04:43.000 --> 00:04:44.839 Now, how does sand hold up a house? If I 99 00:04:44.879 --> 00:04:47.480 step on dry sand at the beach, my foot just sinks? Right? 100 00:04:47.519 --> 00:04:50.839 In true, but it comes down to confinement and frictional resistance. 101 00:04:51.399 --> 00:04:54.240 When sand and gravel are densely packed together under ground, 102 00:04:54.639 --> 00:04:57.879 the individual particles grind against each other. Oh, I see, 103 00:04:57.879 --> 00:05:00.519 because they have nowhere to spread out sideways. That friction 104 00:05:00.639 --> 00:05:03.519 locks them into a solid mass that's capable of supporting 105 00:05:03.519 --> 00:05:04.199 tremendous weight. 106 00:05:04.319 --> 00:05:07.160 That makes sense. But the third type is the tricky one, right. 107 00:05:07.160 --> 00:05:11.759 Very tricky. Cohesive soils like clay. Yes, clay cohesive soils 108 00:05:11.800 --> 00:05:15.639 bind together tightly, but their physical properties change drastically depending 109 00:05:15.680 --> 00:05:16.680 on their moisture content. 110 00:05:16.959 --> 00:05:18.199 So water messes them up. 111 00:05:18.439 --> 00:05:23.000 Exactly. If clay dries out completely during a drought, it 112 00:05:23.120 --> 00:05:26.759 physically shrinks, and that can cause the foundation sitting on 113 00:05:26.800 --> 00:05:27.720 it to drop. 114 00:05:27.800 --> 00:05:29.079 And if it gets wet, If it. 115 00:05:29.079 --> 00:05:32.600 Gets saturated with rain, it swells and basically turns to. 116 00:05:32.600 --> 00:05:35.519 Mush s hikes. And there's also the freezing problem. 117 00:05:35.600 --> 00:05:35.759 Right. 118 00:05:35.879 --> 00:05:39.319 The book mentions frost heave, which just sounds incredibly destructive. 119 00:05:39.480 --> 00:05:44.199 Frost eve is a fascinating, albeit destructive mechanism. Water expands 120 00:05:44.240 --> 00:05:46.879 by about nine percent when it freezes, right, So when 121 00:05:46.879 --> 00:05:49.439 you have wet clay, the moisture inside it doesn't just 122 00:05:49.480 --> 00:05:52.879 freeze in place. It forms what are called ice lenses. 123 00:05:52.920 --> 00:05:56.519 Ice lenses, Yeah, and these lenses draw in more surrounding 124 00:05:56.600 --> 00:06:00.480 water as they freeze, growing larger and larger. Wow, hydraulic 125 00:06:00.519 --> 00:06:05.240 pressure generated by this expanding ice is so powerful that 126 00:06:05.279 --> 00:06:09.120 it can physically lift a concrete foundation upward, totally cracking 127 00:06:09.160 --> 00:06:10.560 the walls of the house in the process. 128 00:06:10.839 --> 00:06:13.240 Man, So clearly we can't just look at a patch 129 00:06:13.279 --> 00:06:15.000 of grass and guess if the dirt underneath is going 130 00:06:15.040 --> 00:06:18.040 to shrink, swell or you know, hold up three small 131 00:06:18.079 --> 00:06:20.040 cars per meter. Yeah, we have to test it. 132 00:06:20.120 --> 00:06:22.319 Absolutely. Site investigation is non negotiable. 133 00:06:22.439 --> 00:06:24.120 Right, And I was reading the section on that and 134 00:06:24.160 --> 00:06:27.240 it mentions this thing called the standard penetration test. I 135 00:06:27.319 --> 00:06:29.519 have to admit it sounded surprisingly low. 136 00:06:29.439 --> 00:06:31.959 Tech to me. It does sound a bit basic. 137 00:06:31.759 --> 00:06:34.879 From what I understood. They bore a hole, stick a 138 00:06:34.920 --> 00:06:38.319 metal tubein, and basically just drop a heavyweight onto it repeatedly. 139 00:06:38.720 --> 00:06:42.120 If we are dropping a sixty three point five kilogram 140 00:06:42.199 --> 00:06:46.040 weight down a borehole, how does counting those impacts actually 141 00:06:46.120 --> 00:06:48.480 translate to ensuring a house won't sink. 142 00:06:48.600 --> 00:06:52.680 Well, it sounds rudimentary, but it provides incredibly precise data 143 00:06:52.759 --> 00:06:56.800 regarding the soil's relative density. So the standard test uses 144 00:06:56.839 --> 00:07:00.000 a thirty five millimeters hollow tube called the split spar 145 00:07:00.040 --> 00:07:03.040 boone sampler. Okay, they drop a standard sixty three point 146 00:07:03.079 --> 00:07:06.680 five kilogram weight from a very specific height exactly seven 147 00:07:06.720 --> 00:07:08.680 hundred and fifty millimeters onto the top of this tube, 148 00:07:08.720 --> 00:07:10.959 and the counting by counting exactly how many blows it 149 00:07:11.000 --> 00:07:13.680 takes to drive the sampler specific distance into the earth, 150 00:07:14.040 --> 00:07:15.959 engineers calculate the bearing capacity. 151 00:07:16.040 --> 00:07:17.839 So if it goes in super fast. 152 00:07:17.639 --> 00:07:19.519 Right, if the weight drops and the tube slides right 153 00:07:19.519 --> 00:07:21.920 in with only two or three blows, you know you 154 00:07:21.959 --> 00:07:24.279 were dealing with loose, weak soil, got it. But if 155 00:07:24.319 --> 00:07:28.199 it takes fifty blows you have highly dense load bearing soil. 156 00:07:28.279 --> 00:07:30.839 Yeah, okay, So we take that density number, we run 157 00:07:30.920 --> 00:07:33.439 it through the formulas and we figure out how big 158 00:07:33.480 --> 00:07:37.879 our foundation needs to be, which leads to a crucial transition. 159 00:07:38.079 --> 00:07:40.079 Digging the actual trench exactly. 160 00:07:40.759 --> 00:07:45.120 We've mathematically quantified the soil. We know the ground can 161 00:07:45.160 --> 00:07:47.319 handle the house if we spread the weight out properly. 162 00:07:48.040 --> 00:07:50.480 But we can't just build a brick wall on top 163 00:07:50.519 --> 00:07:53.360 of raw grass. It would settle unevenly. 164 00:07:53.639 --> 00:07:56.199 No, we need a rigid anchor deep in the ground, 165 00:07:56.399 --> 00:07:57.839 which means we have to dig a trench. 166 00:07:58.240 --> 00:08:00.639 In reading the book, I was really surprised that simply 167 00:08:00.720 --> 00:08:04.120 digging a hole is treated as a major engineering hazard. 168 00:08:04.279 --> 00:08:07.439 Oh. Trench excavation is an active battle against sheer failure. 169 00:08:07.560 --> 00:08:09.000 Sheer failure tearing apart. 170 00:08:09.240 --> 00:08:12.439 Right When soil is resting naturally, it is held in 171 00:08:12.480 --> 00:08:15.920 place by lateral earth pressure. The dirt on the left 172 00:08:15.959 --> 00:08:19.160 is pushing against the dirt on the right, keeping everything stable. 173 00:08:18.800 --> 00:08:20.920 So they balance each other out exactly. 174 00:08:21.480 --> 00:08:23.959 But the moment an excavator scoops out a trench, you 175 00:08:24.040 --> 00:08:26.000 remove that lateral support. 176 00:08:25.680 --> 00:08:27.399 So the walls just want to cave in. 177 00:08:27.800 --> 00:08:31.000 Yes, the soil walls on either side suddenly have nothing 178 00:08:31.040 --> 00:08:34.960 pushing back against them. Gravity pulls down the soil, shears 179 00:08:35.039 --> 00:08:37.879 or tears away from the main mass and collapses inward 180 00:08:37.960 --> 00:08:38.919 into the trench. 181 00:08:38.919 --> 00:08:42.000 And that collapse could bury a worker or totally ruin 182 00:08:42.039 --> 00:08:45.399 the foundation line. So how do we stop the sheer failure? 183 00:08:45.759 --> 00:08:49.159 We use earthwork supports, generally referred to as shoring. We 184 00:08:49.279 --> 00:08:52.360 place vertical planks of word called polling boards against the 185 00:08:52.440 --> 00:08:55.840 raw walls of the trench. Okay, Then we lay horizontal 186 00:08:55.879 --> 00:08:59.720 beans called wallings across those boards, and finally we wedge 187 00:08:59.720 --> 00:09:02.399 heavy struts across the width of the trench, pushing the 188 00:09:02.440 --> 00:09:03.200 wallings outward. 189 00:09:03.240 --> 00:09:07.159 Oh so you're basically artificially replacing the missing lateral earth pressure. 190 00:09:07.399 --> 00:09:08.639 That is exactly what we're doing. 191 00:09:08.759 --> 00:09:10.559 And I imagine the worse the soil, the more wood 192 00:09:10.600 --> 00:09:11.120 you need to use. 193 00:09:11.200 --> 00:09:14.279 Right, that is the basic principle. Yeah. Yeah. In very loose, 194 00:09:14.519 --> 00:09:18.759 crumbly soil, we use full or closed sheeting. Whether polling 195 00:09:18.799 --> 00:09:21.840 boards are tightly packed side by side to hold back 196 00:09:21.919 --> 00:09:24.919 every single inch of dirt makes sense. In slightly more 197 00:09:24.919 --> 00:09:28.320 stable soil, we might use half sheeting, leaving small gaps, 198 00:09:29.279 --> 00:09:32.600 and in very stiff cohesive clay that can mostly hold 199 00:09:32.639 --> 00:09:35.600 its own shape, we might only use open sheeting, just 200 00:09:35.639 --> 00:09:38.080 putting a board every meter or so as it fails. 201 00:09:38.080 --> 00:09:41.440 Safe precautions exactly just in case. Okay, so the trench 202 00:09:41.480 --> 00:09:44.120 is dug, the walls are secured. Now we have to 203 00:09:44.120 --> 00:09:48.919 actually pour the foundation. The book distinguishes between shallow foundations 204 00:09:49.159 --> 00:09:53.159 like strips or pads and deep foundations like piles. 205 00:09:53.159 --> 00:09:54.600 Right, depending on the load and soil. 206 00:09:54.799 --> 00:09:57.360 But regardless of the type, the goal is to manage 207 00:09:57.399 --> 00:10:00.799 something called bulbs of pressure. What is the fil mechanism 208 00:10:00.799 --> 00:10:01.440 happening there? 209 00:10:01.799 --> 00:10:04.200 So a bowl of pressure is the way the load 210 00:10:04.240 --> 00:10:07.480 from the foundation actually travels through the dirt. When a 211 00:10:07.519 --> 00:10:11.279 concrete foundation presses down, that force doesn't just shoot straight 212 00:10:11.320 --> 00:10:13.080 down in a narrow vertical column. 213 00:10:13.080 --> 00:10:13.840 It spreads out. 214 00:10:14.000 --> 00:10:16.679 Yeah, it radiates outward and downward into the soil in 215 00:10:16.799 --> 00:10:20.240 semi circular layered zones. If you slice the ground open, 216 00:10:20.320 --> 00:10:22.559 it would look like the layers of an onion radiating 217 00:10:22.559 --> 00:10:23.960 out from the bottom of the concrete. 218 00:10:24.000 --> 00:10:24.720 I can picture that. 219 00:10:24.919 --> 00:10:28.200 And as this pressure bulb expands deeper and wider into 220 00:10:28.240 --> 00:10:33.480 the earth, the force dissipates. So the trick is making 221 00:10:33.519 --> 00:10:37.000 sure the top of that onion, the most concentrated point 222 00:10:37.000 --> 00:10:39.799 of pressure, right where the concrete touches the BRT doesn't 223 00:10:39.840 --> 00:10:41.720 overpower the dirt strength. 224 00:10:41.440 --> 00:10:45.120 Exactly the case, and for a standard strip foundation. The 225 00:10:45.159 --> 00:10:48.320 math is elegantly simple, love simple math. You take the 226 00:10:48.360 --> 00:10:51.799 downward load in killing Newton's perimeter run and you divide 227 00:10:51.840 --> 00:10:53.200 it by the baring capacity of. 228 00:10:53.200 --> 00:10:54.919 The soil, and that gives you the width. 229 00:10:55.039 --> 00:10:58.799 Right. That direct ratio tells you exactly how wide your 230 00:10:58.840 --> 00:11:02.559 concrete strip needs to be to safely spread the pressure bulb. 231 00:11:02.519 --> 00:11:05.039 Which brings us to the concrete itself. And this is 232 00:11:05.039 --> 00:11:07.480 a point where I really need to completely correct my 233 00:11:07.480 --> 00:11:11.440 own assumptions. I have always thought that concrete just you know, 234 00:11:11.559 --> 00:11:13.440 dried out in the sun. A lot of people think that, 235 00:11:13.600 --> 00:11:15.720 but the text makes it very clear that this is 236 00:11:15.720 --> 00:11:16.480 a total myth. 237 00:11:16.639 --> 00:11:19.960 Yeah, if concrete merely dried by evaporation like a pubble 238 00:11:20.000 --> 00:11:23.840 in the sun, it would crumble back into loose dust. Really, 239 00:11:24.039 --> 00:11:27.080 concrete is actually formed through a one way chemical reaction 240 00:11:27.279 --> 00:11:28.879 called hydration hydration. 241 00:11:29.240 --> 00:11:31.799 So what is actually happening at the molecular. 242 00:11:31.440 --> 00:11:34.720 Level, Well, when you mix water with Portland cement, which 243 00:11:34.759 --> 00:11:36.879 by the way, is essentially a fine powder made from 244 00:11:37.200 --> 00:11:41.480 crushed limestone in clay that has been fired at extreme temperatures, Okay, 245 00:11:41.639 --> 00:11:46.440 a molecular transformation occurs, the water molecules chemically bond with 246 00:11:46.519 --> 00:11:47.480 the cement particles. 247 00:11:47.559 --> 00:11:49.240 Oh, they bond, Yes. 248 00:11:49.200 --> 00:11:53.519 They grow into a dense, interlocking crystalline structure that essentially 249 00:11:53.600 --> 00:11:57.559 glues the sand and gravel aggregates together into artificial rock. 250 00:11:57.720 --> 00:11:59.559 So the water isn't just floating around. 251 00:12:00.080 --> 00:12:03.840 The water is entirely consumed by this reaction. It becomes 252 00:12:03.840 --> 00:12:05.759 a physical part of the solid structure. 253 00:12:06.159 --> 00:12:08.879 Wait, if the water is consumed in a chemical reaction 254 00:12:09.120 --> 00:12:12.879 rather than just evaporating, doesn't that mean getting the exact 255 00:12:13.080 --> 00:12:17.240 water to cement ratio wrong could structurally ruin the foundation. 256 00:12:17.639 --> 00:12:19.320 It creates a massive vulnerability. 257 00:12:19.320 --> 00:12:22.480 Absolutely, because the book says we often add extra water 258 00:12:22.639 --> 00:12:26.039 beyond what is chemically required just for workability. So the 259 00:12:26.159 --> 00:12:28.919 concrete is soupy enough to pour into the trench, right. 260 00:12:28.759 --> 00:12:31.960 And that excess water takes up physical space inside the 261 00:12:32.000 --> 00:12:33.600 concrete mix while it. 262 00:12:33.600 --> 00:12:36.120 Is curing, and it doesn't chemically bond exactly. 263 00:12:36.320 --> 00:12:41.039 Eventually, over months or years, that unbonded water will evaporate out, 264 00:12:41.399 --> 00:12:45.720 and when it does, it leaves behind microscopic empty voids 265 00:12:45.799 --> 00:12:49.080 inside the crystalline structure. Well, that sounds bad. It is 266 00:12:49.440 --> 00:12:53.320 those empty pockets drastically reduce the compressive strength of the foundation. 267 00:12:54.080 --> 00:12:56.799 You are essentially turning your solid anchor into a rigid 268 00:12:56.840 --> 00:12:59.720 sponge just to make it easier to shovel on day one. 269 00:13:00.120 --> 00:13:03.639 That is an incredible penalty for taking a shortcut. And 270 00:13:03.679 --> 00:13:06.519 speaking of structural vulnerabilities in concrete, I want to talk 271 00:13:06.519 --> 00:13:07.240 about fire. 272 00:13:07.399 --> 00:13:09.679 Oh yeah, this is an important one because. 273 00:13:09.440 --> 00:13:11.799 We usually think of a concrete foundation or a concrete 274 00:13:11.840 --> 00:13:15.679 wall as entirely fireproof. I mean, it's rock, it doesn't burn. 275 00:13:16.600 --> 00:13:19.679 But there's a hidden danger here involving the aggregates. 276 00:13:19.919 --> 00:13:22.200 It is a terrifying phenomenon known as centering. 277 00:13:22.320 --> 00:13:23.519 Centering. How does that work? 278 00:13:23.759 --> 00:13:27.679 Concrete often uses natural aggregates like river gravel or certain 279 00:13:27.720 --> 00:13:31.120 types of flint mixed in with the cement. These stones 280 00:13:31.200 --> 00:13:35.159 naturally contain microscopic pockets of moisture trapped deep inside them. 281 00:13:35.240 --> 00:13:37.759 Okay, so there's water inside the little rocks. 282 00:13:37.720 --> 00:13:40.519 Right, And if a concrete wall is exposed to the 283 00:13:40.559 --> 00:13:45.080 intense sustained heat of a severe building fire, that trapped 284 00:13:45.120 --> 00:13:47.080 moisture rapidly boils and turns. 285 00:13:46.919 --> 00:13:49.320 Into steam, and steam expands. 286 00:13:48.799 --> 00:13:52.919 It expands violently because the steam is locked inside solid rock. 287 00:13:53.320 --> 00:13:56.399 The pressure builds until the aggregate physically explodes. 288 00:13:56.679 --> 00:13:58.159 You're kidding, It explodes. 289 00:13:58.200 --> 00:14:00.399 Yeah. The concrete literally blows itself up apart from the 290 00:14:00.480 --> 00:14:03.840 inside out, throwing off chunks of material, and that exposes 291 00:14:03.879 --> 00:14:06.960 the steel reinforcement inside to the fire, which leads to 292 00:14:06.960 --> 00:14:08.240 total structural failure. 293 00:14:08.399 --> 00:14:12.799 Wow. Okay, So we've got this perfectly hydrated, carefully measured 294 00:14:12.879 --> 00:14:16.039 rock solid anchor sitting safely in the ground distributing its 295 00:14:16.080 --> 00:14:18.320 pressure bulbs. Now we have to actually build the. 296 00:14:18.279 --> 00:14:20.200 House upwards right time to go vertical. 297 00:14:20.480 --> 00:14:22.440 But as we rise out of the earth, the building 298 00:14:22.480 --> 00:14:25.519 faces a relentless new enemy, the weather. 299 00:14:25.759 --> 00:14:28.679 This brings us to masonry. The exterior walls are the 300 00:14:28.720 --> 00:14:32.440 primary defense, most often constructed from fired clay bricks. 301 00:14:32.919 --> 00:14:35.159 Let's focus on those clay bricks for a moment. They're 302 00:14:35.240 --> 00:14:37.720 fired in kilns, which gives them that beautiful finish and 303 00:14:37.759 --> 00:14:41.679 phenomenal fire resistance. But the firing process also creates a 304 00:14:41.720 --> 00:14:43.919 really unique problem called efflorescence. 305 00:14:44.320 --> 00:14:46.799 Ah, yes, effluorescence. 306 00:14:47.159 --> 00:14:51.000 What exactly is that white powdery stain you sometimes see 307 00:14:51.159 --> 00:14:53.000 ruining the look of a new brick wall. 308 00:14:53.480 --> 00:14:55.960 It actually goes back to the chemistry of the clay. 309 00:14:56.559 --> 00:14:59.120 Because clay is a natural material dug from the earth, 310 00:14:59.440 --> 00:15:03.240 it contains various soluble salts. Okay, when a brick wall 311 00:15:03.240 --> 00:15:07.159 gets saturated by heavy rain, the water penetrates the porous 312 00:15:07.159 --> 00:15:09.919 clay and dissolves those trapped salts. 313 00:15:10.120 --> 00:15:11.559 And then when it dries out. 314 00:15:11.600 --> 00:15:14.039 Later, when the sun comes down the wall begins to dry, 315 00:15:14.519 --> 00:15:19.000 the evaporating water draw those dissolved salts outward to the surface. 316 00:15:19.679 --> 00:15:22.480 The water vanishes into the air, leaving the white salt 317 00:15:22.519 --> 00:15:23.440 crystals behind. 318 00:15:23.879 --> 00:15:27.000 So it's basically just salt stains. Does it damage the wall? 319 00:15:27.120 --> 00:15:30.399 It usually doesn't harm the structural integrity, but it is 320 00:15:30.519 --> 00:15:32.720 visually frustrating for homeowners. 321 00:15:32.279 --> 00:15:34.440 I can imagine. But you know, the real magic of 322 00:15:34.480 --> 00:15:36.879 a brick wall isn't just the brick itself. It's how they. 323 00:15:36.720 --> 00:15:38.799 Are stacked, the bonding right. 324 00:15:39.200 --> 00:15:41.799 I've always wondered why we can't just stack bricks straight 325 00:15:41.879 --> 00:15:44.039 up and neat vertical columns. Why go through all the 326 00:15:44.080 --> 00:15:47.519 trouble of creating all these complex overlapping patterns. 327 00:15:47.320 --> 00:15:50.559 Because a wall has to transfer all that superimposed weight 328 00:15:50.960 --> 00:15:53.960 the roof, the snow, the wind down into the foundation. 329 00:15:54.360 --> 00:15:56.320 So if they were just stacked straight. 330 00:15:56.039 --> 00:15:59.240 Up, if you stack bricks with the vertical joints lining up. 331 00:15:59.559 --> 00:16:02.159 Any the point load pushing down on the top brick 332 00:16:02.440 --> 00:16:05.159 will just travel straight down that single column. It would 333 00:16:05.200 --> 00:16:06.600 likely just crack the whole column. 334 00:16:06.679 --> 00:16:09.200 But by overlapping them, you spread it out right. 335 00:16:09.240 --> 00:16:12.639 By overlapping the bricks in specific bonds, you force the 336 00:16:12.720 --> 00:16:15.360 load to spread out horizontally as it goes down, like 337 00:16:15.399 --> 00:16:18.840 a pyramid. Precisely, if a heavy load hits one brick, 338 00:16:19.200 --> 00:16:21.559 that brick rests on two bricks below it, which rests 339 00:16:21.600 --> 00:16:24.519 on three below them. Right, Patterns like the English bond, 340 00:16:24.519 --> 00:16:28.120 which alternates horizontal rows of bricks laid lengthwise and with wise, 341 00:16:28.799 --> 00:16:31.519 or the Flemish bond, which alternates them in the same row, 342 00:16:32.240 --> 00:16:35.039 ensure this diagonal load transfer works flawlessly. 343 00:16:35.360 --> 00:16:37.879 So the bricks are doing a great job managing gravity, 344 00:16:38.320 --> 00:16:40.840 but there is a silent invader trying to ruin this 345 00:16:40.960 --> 00:16:43.480 wall from the bottom up. We already talked about rain 346 00:16:43.559 --> 00:16:46.600 hitting the outside, but what about ground moisture. 347 00:16:46.320 --> 00:16:48.120 Oh, ground moistures relentless. 348 00:16:48.399 --> 00:16:51.360 How does water from the dirt get up into the wall? 349 00:16:51.399 --> 00:16:52.759 I mean gravity is pulling it down. 350 00:16:52.840 --> 00:16:56.600 It uses capillary action. Brick and mortar are heavily porous. 351 00:16:56.600 --> 00:16:58.440 If you look at them under a microscope. They are 352 00:16:58.440 --> 00:17:00.399 full of tiny interconnected. 353 00:16:59.799 --> 00:17:01.279 Two right, like a sponge. 354 00:17:01.320 --> 00:17:04.400 Exactly when the bottom of a brick wall sits on 355 00:17:04.480 --> 00:17:07.400 damp earth, the surface tension of the water allows it 356 00:17:07.440 --> 00:17:10.519 to grab onto the sides of those microscopic tubes and 357 00:17:10.599 --> 00:17:13.599 physically pull itself upward, defying gravity. 358 00:17:13.720 --> 00:17:14.519 That's wild. 359 00:17:14.559 --> 00:17:17.759 The dampness will just keep climbing, ruining the interior plaster 360 00:17:17.839 --> 00:17:19.000 and inviting mold. 361 00:17:19.160 --> 00:17:22.319 And the engineering solution to this is so simple it's 362 00:17:22.400 --> 00:17:26.920 almost beautiful. The damp proof course or DPC, you basically 363 00:17:26.960 --> 00:17:29.400 just slice a permanent roadblock into the wall. 364 00:17:29.599 --> 00:17:33.720 You are severing the capillary tubes entirely. Just above ground level. 365 00:17:33.759 --> 00:17:38.279 Engineers lay a continuous, impermeable barrier directly into the mortar joint. 366 00:17:38.160 --> 00:17:39.079 Like a piece of plastic. 367 00:17:39.319 --> 00:17:41.640 It could be a tough sheet of polyethylene plastic yeah, 368 00:17:41.759 --> 00:17:45.119 or a layer of mastic asphalt. The ground moisture climbs 369 00:17:45.200 --> 00:17:48.799 up through the lower bricks, hits this waterproof layer, and 370 00:17:48.839 --> 00:17:50.119 it's physically stopped dead. 371 00:17:50.319 --> 00:17:53.559 So the wall above the DPC stays bone dry for 372 00:17:53.640 --> 00:17:56.160 the life of the building. Okay, we have this brilliant 373 00:17:56.240 --> 00:17:59.039 solid wall holding everything up and keeping the water out, 374 00:17:59.200 --> 00:18:01.480 but then we deliver compromise the whole thing. 375 00:18:01.400 --> 00:18:02.759 By adding windows and doors. 376 00:18:02.839 --> 00:18:06.079 Right, we punch massive holes in it. The section on 377 00:18:06.160 --> 00:18:10.720 bridging these openings fascinated me because it's a massive structural liability. 378 00:18:11.119 --> 00:18:14.880 The bricks directly above a wide window have absolutely nothing 379 00:18:15.000 --> 00:18:16.559 underneath them to push against. 380 00:18:16.839 --> 00:18:19.519 Gravity wants to pull them straight down into the living room. 381 00:18:20.000 --> 00:18:23.640 To span those gaps safely, we use lentils lentils right. 382 00:18:23.720 --> 00:18:26.519 A lentil is a horizontal beam laid across the opening 383 00:18:26.559 --> 00:18:29.039 to catch the weight of the bricks above and transfer 384 00:18:29.079 --> 00:18:31.160 it to the solid walls on either side. What are 385 00:18:31.160 --> 00:18:34.599 they usually made of today? Pressed steel lentils are very 386 00:18:34.640 --> 00:18:39.200 common because steel has incredible tensile strength. It really resists bending. 387 00:18:39.759 --> 00:18:42.359 But the book points out a huge flaw with steel lentils. 388 00:18:42.839 --> 00:18:44.920 Steel is a fantastic thermal conductor. 389 00:18:45.079 --> 00:18:48.279 Yes, it acts as a cold bridge coal bridge. In 390 00:18:48.319 --> 00:18:52.079 the winter, the warhair inside your house heats up the 391 00:18:52.119 --> 00:18:55.640 interior side of the steel lintel. The steel conducts that 392 00:18:55.720 --> 00:18:59.119 heat rapidly through the wall and radiates it straight out 393 00:18:59.160 --> 00:19:00.960 into the freezing exterior air. 394 00:19:01.200 --> 00:19:04.480 So you are essentially bleeding your heating budget directly outside. 395 00:19:04.559 --> 00:19:07.920 You really are to prevent this, the hollow cores of 396 00:19:07.960 --> 00:19:11.519 these steel linkles must be packed with specialized insulation to 397 00:19:11.680 --> 00:19:13.039 break that thermal bridge. 398 00:19:13.240 --> 00:19:16.240 Or we look at the older alternative solution, which is 399 00:19:16.240 --> 00:19:18.960 my absolute favorite piece of engineering. 400 00:19:19.240 --> 00:19:21.680 The arch ah the arch Classic. 401 00:19:21.839 --> 00:19:23.880