Speaker 1: Right now, the building you're sitting in is trying to
Speaker 1: tear itself apart.
Speaker 2: Oh. Absolutely, it's constant, it really is.
Speaker 1: It is waging this massive, invisible, entirely relentless war against gravity,
Speaker 1: the wind, the soil, and well its own crushing weight.
Speaker 2: Yeah, we look at a house and see a static,
Speaker 2: quiet box of brick and wood, but that perception is
Speaker 2: just a total.
Speaker 1: Illusion, completely and exposing that invisible battlefield is exactly what
Speaker 1: we are getting into today. Welcome to this deep dive.
Speaker 2: Glad to be here.
Speaker 1: Whether you are an engineering student trying to wrap your
Speaker 1: head around coursework, a young professional, or just someone who
Speaker 1: is insanely curious about how the physical world is put together,
Speaker 1: we are thrilled to have you with us.
Speaker 2: Definitely. Our mission today is to explore the scientific principles
Speaker 2: and the practical applications of building construction.
Speaker 1: Right and we are pulling heavily from excerpts of a
Speaker 1: really fascinating textbook, Andrew J. Charlotte's Fundamental Building Technology.
Speaker 2: It's a great source. The goal here is to mentally
Speaker 2: construct a house from the ground up, translating complex civil
Speaker 2: engineering concepts into clear, actionable knowledge.
Speaker 1: Which means looking at everything the physical mechanics of the
Speaker 1: dirt beneath our feet, the actual chemistry of concrete, and
Speaker 1: you know the physics of how load bearing walls even
Speaker 1: manage to stand.
Speaker 2: Upright exactly, Because before a single brick is laid, we
Speaker 2: have to understand the forces the building is going to exert.
Speaker 1: You can't design a solution if you don't even know
Speaker 1: the parameters of the problem, right, So the book talks
Speaker 1: a lot about the science of loads. What exactly is
Speaker 1: a house pushing down with?
Speaker 2: Well? Engineers categorize these forces into three specific types of loading. First,
Speaker 2: you have the dead.
Speaker 1: Load, which is just the materials themselves.
Speaker 2: Right right exactly. It's simply the weight of the building materials,
Speaker 2: the bricks, the timber, the concrete foundation, the roof tiles,
Speaker 2: stuff that doesn't move exactly. It is a constant, unchanging force.
Speaker 2: Then second, you have the live load, okay, and that's
Speaker 2: the very weight of everything inside the building, so the occupants,
Speaker 2: your furniture, the washing machine, the bookshelves.
Speaker 1: Because it changes based on how the space is used
Speaker 1: over time decisely.
Speaker 2: And the third one is the superimposed load.
Speaker 1: Superimposed that sounds like weather stuff, You got it.
Speaker 2: This accounts for environmental forces interacting with the outside of
Speaker 2: the structure. Late wind wind. Yeah, think of the lateral
Speaker 2: pressure of a gale force wind pushing against the side
Speaker 2: of the house, or you know, the sheer physical weight
Speaker 2: of a heavy snowfall accumulating on the roof.
Speaker 1: I am trying to visualize the sheer scale of this.
Speaker 1: If we take a typical brick built house, what kind
Speaker 1: of numbers are we actually talking about pressing into the
Speaker 1: dirt well?
Speaker 2: A standard brick built house exerts a uniform downward load
Speaker 2: of roughly thirty kilin Newton's per meter run on its foundations.
Speaker 1: Wait pause right there, What exactly is a meter run?
Speaker 1: I mean, I can guess, but no.
Speaker 2: It's a good question. Think of it like taking a
Speaker 2: slice out of the house. If you imagine isolating just
Speaker 2: one linear meter of exterior wall running from the roof
Speaker 2: all the way down to the foundation.
Speaker 1: Okay, I have that mental picture.
Speaker 2: The total weight of that one meter slice pressing into
Speaker 2: the ground is about thirty killin Newtons.
Speaker 1: Which means what in plain English.
Speaker 2: To put that into everyday terms, it is roughly the
Speaker 2: equivalent of stacking three small cars on top of every
Speaker 2: single meter of your foundation. Trench.
Speaker 1: Wow, that is an immense amount of weight. And the
Speaker 1: book mentions that engineers don't even use that number when
Speaker 1: they design the foundation, right, They use something called a
Speaker 1: factor of safety.
Speaker 2: They do, yeah, because designing a building right to the
Speaker 2: edge of its breaking point is just a recipe for disaster,
Speaker 2: right obviously, So the factor of safety for foundations is
Speaker 2: usually two. This means we design the foundation to safely
Speaker 2: handle double the calculated load.
Speaker 1: So instead of thirty killing newtons, we engineer it to
Speaker 1: support sixty.
Speaker 2: Exactly sixty killing newtons per meter run. This ensures that
Speaker 2: even if there is an unprecedented snowfall or a massive
Speaker 2: structural shift, the foundation won't fail.
Speaker 1: Okay, So we have these immense stresses pushing down into
Speaker 1: the earth, and I like to visualize stress in materials
Speaker 1: in a very physical way, you know, like compression is
Speaker 1: like crushing something, tension is pulling or stretching it, and
Speaker 1: sheer is like tearing it apart.
Speaker 2: That's a perfect way to look at it.
Speaker 1: So we are pushing all this compressive stress into the dirt.
Speaker 1: But obviously not all dirt is created equal.
Speaker 2: Oh far from it. In soil mechanics, the ground is
Speaker 2: generally classified into three types. First, you have solid rock.
Speaker 1: Which is the dream scenario.
Speaker 2: I'm assuming pretty much. It obviously has an immense bearing capacity,
Speaker 2: provided it isn't severely weathered or fractured. Then you have
Speaker 2: non cohesive soils. Think of dense gravel and sand.
Speaker 1: Now, how does sand hold up a house? If I
Speaker 1: step on dry sand at the beach, my foot just sinks? Right?
Speaker 2: In true, but it comes down to confinement and frictional resistance.
Speaker 2: When sand and gravel are densely packed together under ground,
Speaker 2: the individual particles grind against each other. Oh, I see,
Speaker 2: because they have nowhere to spread out sideways. That friction
Speaker 2: locks them into a solid mass that's capable of supporting
Speaker 2: tremendous weight.
Speaker 1: That makes sense. But the third type is the tricky one, right.
Speaker 2: Very tricky. Cohesive soils like clay. Yes, clay cohesive soils
Speaker 2: bind together tightly, but their physical properties change drastically depending
Speaker 2: on their moisture content.
Speaker 1: So water messes them up.
Speaker 2: Exactly. If clay dries out completely during a drought, it
Speaker 2: physically shrinks, and that can cause the foundation sitting on
Speaker 2: it to drop.
Speaker 1: And if it gets wet, If it.
Speaker 2: Gets saturated with rain, it swells and basically turns to.
Speaker 1: Mush s hikes. And there's also the freezing problem.
Speaker 2: Right.
Speaker 1: The book mentions frost heave, which just sounds incredibly destructive.
Speaker 2: Frost eve is a fascinating, albeit destructive mechanism. Water expands
Speaker 2: by about nine percent when it freezes, right, So when
Speaker 2: you have wet clay, the moisture inside it doesn't just
Speaker 2: freeze in place. It forms what are called ice lenses.
Speaker 2: Ice lenses, Yeah, and these lenses draw in more surrounding
Speaker 2: water as they freeze, growing larger and larger. Wow, hydraulic
Speaker 2: pressure generated by this expanding ice is so powerful that
Speaker 2: it can physically lift a concrete foundation upward, totally cracking
Speaker 2: the walls of the house in the process.
Speaker 1: Man, So clearly we can't just look at a patch
Speaker 1: of grass and guess if the dirt underneath is going
Speaker 1: to shrink, swell or you know, hold up three small
Speaker 1: cars per meter. Yeah, we have to test it.
Speaker 2: Absolutely. Site investigation is non negotiable.
Speaker 1: Right, And I was reading the section on that and
Speaker 1: it mentions this thing called the standard penetration test. I
Speaker 1: have to admit it sounded surprisingly low.
Speaker 2: Tech to me. It does sound a bit basic.
Speaker 1: From what I understood. They bore a hole, stick a
Speaker 1: metal tubein, and basically just drop a heavyweight onto it repeatedly.
Speaker 1: If we are dropping a sixty three point five kilogram
Speaker 1: weight down a borehole, how does counting those impacts actually
Speaker 1: translate to ensuring a house won't sink.
Speaker 2: Well, it sounds rudimentary, but it provides incredibly precise data
Speaker 2: regarding the soil's relative density. So the standard test uses
Speaker 2: a thirty five millimeters hollow tube called the split spar
Speaker 2: boone sampler. Okay, they drop a standard sixty three point
Speaker 2: five kilogram weight from a very specific height exactly seven
Speaker 2: hundred and fifty millimeters onto the top of this tube,
Speaker 2: and the counting by counting exactly how many blows it
Speaker 2: takes to drive the sampler specific distance into the earth,
Speaker 2: engineers calculate the bearing capacity.
Speaker 1: So if it goes in super fast.
Speaker 2: Right, if the weight drops and the tube slides right
Speaker 2: in with only two or three blows, you know you
Speaker 2: were dealing with loose, weak soil, got it. But if
Speaker 2: it takes fifty blows you have highly dense load bearing soil.
Speaker 1: Yeah, okay, So we take that density number, we run
Speaker 1: it through the formulas and we figure out how big
Speaker 1: our foundation needs to be, which leads to a crucial transition.
Speaker 2: Digging the actual trench exactly.
Speaker 1: We've mathematically quantified the soil. We know the ground can
Speaker 1: handle the house if we spread the weight out properly.
Speaker 1: But we can't just build a brick wall on top
Speaker 1: of raw grass. It would settle unevenly.
Speaker 2: No, we need a rigid anchor deep in the ground,
Speaker 2: which means we have to dig a trench.
Speaker 1: In reading the book, I was really surprised that simply
Speaker 1: digging a hole is treated as a major engineering hazard.
Speaker 2: Oh. Trench excavation is an active battle against sheer failure.
Speaker 1: Sheer failure tearing apart.
Speaker 2: Right When soil is resting naturally, it is held in
Speaker 2: place by lateral earth pressure. The dirt on the left
Speaker 2: is pushing against the dirt on the right, keeping everything stable.
Speaker 1: So they balance each other out exactly.
Speaker 2: But the moment an excavator scoops out a trench, you
Speaker 2: remove that lateral support.
Speaker 1: So the walls just want to cave in.
Speaker 2: Yes, the soil walls on either side suddenly have nothing
Speaker 2: pushing back against them. Gravity pulls down the soil, shears
Speaker 2: or tears away from the main mass and collapses inward
Speaker 2: into the trench.
Speaker 1: And that collapse could bury a worker or totally ruin
Speaker 1: the foundation line. So how do we stop the sheer failure?
Speaker 2: We use earthwork supports, generally referred to as shoring. We
Speaker 2: place vertical planks of word called polling boards against the
Speaker 2: raw walls of the trench. Okay, Then we lay horizontal
Speaker 2: beans called wallings across those boards, and finally we wedge
Speaker 2: heavy struts across the width of the trench, pushing the
Speaker 2: wallings outward.
Speaker 1: Oh so you're basically artificially replacing the missing lateral earth pressure.
Speaker 2: That is exactly what we're doing.
Speaker 1: And I imagine the worse the soil, the more wood
Speaker 1: you need to use.
Speaker 2: Right, that is the basic principle. Yeah. Yeah. In very loose,
Speaker 2: crumbly soil, we use full or closed sheeting. Whether polling
Speaker 2: boards are tightly packed side by side to hold back
Speaker 2: every single inch of dirt makes sense. In slightly more
Speaker 2: stable soil, we might use half sheeting, leaving small gaps,
Speaker 2: and in very stiff cohesive clay that can mostly hold
Speaker 2: its own shape, we might only use open sheeting, just
Speaker 2: putting a board every meter or so as it fails.
Speaker 2: Safe precautions exactly just in case. Okay, so the trench
Speaker 2: is dug, the walls are secured. Now we have to
Speaker 2: actually pour the foundation. The book distinguishes between shallow foundations
Speaker 2: like strips or pads and deep foundations like piles.
Speaker 1: Right, depending on the load and soil.
Speaker 2: But regardless of the type, the goal is to manage
Speaker 2: something called bulbs of pressure. What is the fil mechanism
Speaker 2: happening there?
Speaker 1: So a bowl of pressure is the way the load
Speaker 1: from the foundation actually travels through the dirt. When a
Speaker 1: concrete foundation presses down, that force doesn't just shoot straight
Speaker 1: down in a narrow vertical column.
Speaker 2: It spreads out.
Speaker 1: Yeah, it radiates outward and downward into the soil in
Speaker 1: semi circular layered zones. If you slice the ground open,
Speaker 1: it would look like the layers of an onion radiating
Speaker 1: out from the bottom of the concrete.
Speaker 2: I can picture that.
Speaker 1: And as this pressure bulb expands deeper and wider into
Speaker 1: the earth, the force dissipates. So the trick is making
Speaker 1: sure the top of that onion, the most concentrated point
Speaker 1: of pressure, right where the concrete touches the BRT doesn't
Speaker 1: overpower the dirt strength.
Speaker 2: Exactly the case, and for a standard strip foundation. The
Speaker 2: math is elegantly simple, love simple math. You take the
Speaker 2: downward load in killing Newton's perimeter run and you divide
Speaker 2: it by the baring capacity of.
Speaker 1: The soil, and that gives you the width.
Speaker 2: Right. That direct ratio tells you exactly how wide your
Speaker 2: concrete strip needs to be to safely spread the pressure bulb.
Speaker 1: Which brings us to the concrete itself. And this is
Speaker 1: a point where I really need to completely correct my
Speaker 1: own assumptions. I have always thought that concrete just you know,
Speaker 1: dried out in the sun. A lot of people think that,
Speaker 1: but the text makes it very clear that this is
Speaker 1: a total myth.
Speaker 2: Yeah, if concrete merely dried by evaporation like a pubble
Speaker 2: in the sun, it would crumble back into loose dust. Really,
Speaker 2: concrete is actually formed through a one way chemical reaction
Speaker 2: called hydration hydration.
Speaker 1: So what is actually happening at the molecular.
Speaker 2: Level, Well, when you mix water with Portland cement, which
Speaker 2: by the way, is essentially a fine powder made from
Speaker 2: crushed limestone in clay that has been fired at extreme temperatures, Okay,
Speaker 2: a molecular transformation occurs, the water molecules chemically bond with
Speaker 2: the cement particles.
Speaker 1: Oh, they bond, Yes.
Speaker 2: They grow into a dense, interlocking crystalline structure that essentially
Speaker 2: glues the sand and gravel aggregates together into artificial rock.
Speaker 1: So the water isn't just floating around.
Speaker 2: The water is entirely consumed by this reaction. It becomes
Speaker 2: a physical part of the solid structure.
Speaker 1: Wait, if the water is consumed in a chemical reaction
Speaker 1: rather than just evaporating, doesn't that mean getting the exact
Speaker 1: water to cement ratio wrong could structurally ruin the foundation.
Speaker 2: It creates a massive vulnerability.
Speaker 1: Absolutely, because the book says we often add extra water
Speaker 1: beyond what is chemically required just for workability. So the
Speaker 1: concrete is soupy enough to pour into the trench, right.
Speaker 2: And that excess water takes up physical space inside the
Speaker 2: concrete mix while it.
Speaker 1: Is curing, and it doesn't chemically bond exactly.
Speaker 2: Eventually, over months or years, that unbonded water will evaporate out,
Speaker 2: and when it does, it leaves behind microscopic empty voids
Speaker 2: inside the crystalline structure. Well, that sounds bad. It is
Speaker 2: those empty pockets drastically reduce the compressive strength of the foundation.
Speaker 2: You are essentially turning your solid anchor into a rigid
Speaker 2: sponge just to make it easier to shovel on day one.
Speaker 1: That is an incredible penalty for taking a shortcut. And
Speaker 1: speaking of structural vulnerabilities in concrete, I want to talk
Speaker 1: about fire.
Speaker 2: Oh yeah, this is an important one because.
Speaker 1: We usually think of a concrete foundation or a concrete
Speaker 1: wall as entirely fireproof. I mean, it's rock, it doesn't burn.
Speaker 1: But there's a hidden danger here involving the aggregates.
Speaker 2: It is a terrifying phenomenon known as centering.
Speaker 1: Centering. How does that work?
Speaker 2: Concrete often uses natural aggregates like river gravel or certain
Speaker 2: types of flint mixed in with the cement. These stones
Speaker 2: naturally contain microscopic pockets of moisture trapped deep inside them.
Speaker 1: Okay, so there's water inside the little rocks.
Speaker 2: Right, And if a concrete wall is exposed to the
Speaker 2: intense sustained heat of a severe building fire, that trapped
Speaker 2: moisture rapidly boils and turns.
Speaker 1: Into steam, and steam expands.
Speaker 2: It expands violently because the steam is locked inside solid rock.
Speaker 2: The pressure builds until the aggregate physically explodes.
Speaker 1: You're kidding, It explodes.
Speaker 2: Yeah. The concrete literally blows itself up apart from the
Speaker 2: inside out, throwing off chunks of material, and that exposes
Speaker 2: the steel reinforcement inside to the fire, which leads to
Speaker 2: total structural failure.
Speaker 1: Wow. Okay, So we've got this perfectly hydrated, carefully measured
Speaker 1: rock solid anchor sitting safely in the ground distributing its
Speaker 1: pressure bulbs. Now we have to actually build the.
Speaker 2: House upwards right time to go vertical.
Speaker 1: But as we rise out of the earth, the building
Speaker 1: faces a relentless new enemy, the weather.
Speaker 2: This brings us to masonry. The exterior walls are the
Speaker 2: primary defense, most often constructed from fired clay bricks.
Speaker 1: Let's focus on those clay bricks for a moment. They're
Speaker 1: fired in kilns, which gives them that beautiful finish and
Speaker 1: phenomenal fire resistance. But the firing process also creates a
Speaker 1: really unique problem called efflorescence.
Speaker 2: Ah, yes, effluorescence.
Speaker 1: What exactly is that white powdery stain you sometimes see
Speaker 1: ruining the look of a new brick wall.
Speaker 2: It actually goes back to the chemistry of the clay.
Speaker 2: Because clay is a natural material dug from the earth,
Speaker 2: it contains various soluble salts. Okay, when a brick wall
Speaker 2: gets saturated by heavy rain, the water penetrates the porous
Speaker 2: clay and dissolves those trapped salts.
Speaker 1: And then when it dries out.
Speaker 2: Later, when the sun comes down the wall begins to dry,
Speaker 2: the evaporating water draw those dissolved salts outward to the surface.
Speaker 2: The water vanishes into the air, leaving the white salt
Speaker 2: crystals behind.
Speaker 1: So it's basically just salt stains. Does it damage the wall?
Speaker 2: It usually doesn't harm the structural integrity, but it is
Speaker 2: visually frustrating for homeowners.
Speaker 1: I can imagine. But you know, the real magic of
Speaker 1: a brick wall isn't just the brick itself. It's how they.
Speaker 2: Are stacked, the bonding right.
Speaker 1: I've always wondered why we can't just stack bricks straight
Speaker 1: up and neat vertical columns. Why go through all the
Speaker 1: trouble of creating all these complex overlapping patterns.
Speaker 2: Because a wall has to transfer all that superimposed weight
Speaker 2: the roof, the snow, the wind down into the foundation.
Speaker 1: So if they were just stacked straight.
Speaker 2: Up, if you stack bricks with the vertical joints lining up.
Speaker 2: Any the point load pushing down on the top brick
Speaker 2: will just travel straight down that single column. It would
Speaker 2: likely just crack the whole column.
Speaker 1: But by overlapping them, you spread it out right.
Speaker 2: By overlapping the bricks in specific bonds, you force the
Speaker 2: load to spread out horizontally as it goes down, like
Speaker 2: a pyramid. Precisely, if a heavy load hits one brick,
Speaker 2: that brick rests on two bricks below it, which rests
Speaker 2: on three below them. Right, Patterns like the English bond,
Speaker 2: which alternates horizontal rows of bricks laid lengthwise and with wise,
Speaker 2: or the Flemish bond, which alternates them in the same row,
Speaker 2: ensure this diagonal load transfer works flawlessly.
Speaker 1: So the bricks are doing a great job managing gravity,
Speaker 1: but there is a silent invader trying to ruin this
Speaker 1: wall from the bottom up. We already talked about rain
Speaker 1: hitting the outside, but what about ground moisture.
Speaker 2: Oh, ground moistures relentless.
Speaker 1: How does water from the dirt get up into the wall?
Speaker 1: I mean gravity is pulling it down.
Speaker 2: It uses capillary action. Brick and mortar are heavily porous.
Speaker 2: If you look at them under a microscope. They are
Speaker 2: full of tiny interconnected.
Speaker 1: Two right, like a sponge.
Speaker 2: Exactly when the bottom of a brick wall sits on
Speaker 2: damp earth, the surface tension of the water allows it
Speaker 2: to grab onto the sides of those microscopic tubes and
Speaker 2: physically pull itself upward, defying gravity.
Speaker 1: That's wild.
Speaker 2: The dampness will just keep climbing, ruining the interior plaster
Speaker 2: and inviting mold.
Speaker 1: And the engineering solution to this is so simple it's
Speaker 1: almost beautiful. The damp proof course or DPC, you basically
Speaker 1: just slice a permanent roadblock into the wall.
Speaker 2: You are severing the capillary tubes entirely. Just above ground level.
Speaker 2: Engineers lay a continuous, impermeable barrier directly into the mortar joint.
Speaker 1: Like a piece of plastic.
Speaker 2: It could be a tough sheet of polyethylene plastic yeah,
Speaker 2: or a layer of mastic asphalt. The ground moisture climbs
Speaker 2: up through the lower bricks, hits this waterproof layer, and
Speaker 2: it's physically stopped dead.
Speaker 1: So the wall above the DPC stays bone dry for
Speaker 1: the life of the building. Okay, we have this brilliant
Speaker 1: solid wall holding everything up and keeping the water out,
Speaker 1: but then we deliver compromise the whole thing.
Speaker 2: By adding windows and doors.
Speaker 1: Right, we punch massive holes in it. The section on
Speaker 1: bridging these openings fascinated me because it's a massive structural liability.
Speaker 1: The bricks directly above a wide window have absolutely nothing
Speaker 1: underneath them to push against.
Speaker 2: Gravity wants to pull them straight down into the living room.
Speaker 2: To span those gaps safely, we use lentils lentils right.
Speaker 2: A lentil is a horizontal beam laid across the opening
Speaker 2: to catch the weight of the bricks above and transfer
Speaker 2: it to the solid walls on either side. What are
Speaker 2: they usually made of today? Pressed steel lentils are very
Speaker 2: common because steel has incredible tensile strength. It really resists bending.
Speaker 1: But the book points out a huge flaw with steel lentils.
Speaker 1: Steel is a fantastic thermal conductor.
Speaker 2: Yes, it acts as a cold bridge coal bridge. In
Speaker 2: the winter, the warhair inside your house heats up the
Speaker 2: interior side of the steel lintel. The steel conducts that
Speaker 2: heat rapidly through the wall and radiates it straight out
Speaker 2: into the freezing exterior air.
Speaker 1: So you are essentially bleeding your heating budget directly outside.
Speaker 2: You really are to prevent this, the hollow cores of
Speaker 2: these steel linkles must be packed with specialized insulation to
Speaker 2: break that thermal bridge.
Speaker 1: Or we look at the older alternative solution, which is
Speaker 1: my absolute favorite piece of engineering.
Speaker 2: The arch ah the arch Classic.
Speaker 1: I saw the diagrams in the text, but the physics
Speaker 1: still boggle my mind. How do stacking bricks in a
Speaker 1: curve over an empty space not just immediately collapse?
Speaker 2: It is an ingenious manipulation of forces. An arch is
Speaker 2: typically built using wedge shaped bricks or standard bricks with
Speaker 2: wedge shaped mortar joints. You build a curve up from
Speaker 2: both sides over a temporary wooden support.
Speaker 1: Right, and then the final piece.
Speaker 2: The final piece is the keystone, placed right at the
Speaker 2: top center.
Speaker 1: It's like the final piece of a three D puzzle.
Speaker 1: The moment you wedge that keystone in, it magically converts
Speaker 1: the downward pull of gravity into a horizontal compressive force,
Speaker 1: squeezing the bricks together.
Speaker 2: That's exactly it pometry takes over. Gravity is pulling straight
Speaker 2: down on the keystone, but because the keystone is wedge shaped,
Speaker 2: it cannot fall straight down without pushing sideways against the
Speaker 2: wedge shaped bricks next to it.
Speaker 1: Wow.
Speaker 2: Those bricks, in turn, are forced to push sideways against
Speaker 2: their neighbors. The entire downward pull of gravity is instantly
Speaker 2: converted into horizontal compressive force.
Speaker 1: It takes a point of ultimate weakness and turns it
Speaker 1: into a self reinforcing bridge of pure compression. That is incredible.
Speaker 2: It really is elegant.
Speaker 1: Okay, So masonry handles compression beautifully, but houses more than
Speaker 1: walls to span across wide ceilings and cap the building
Speaker 1: with a roof. We need a material that has high ductility, right,
Speaker 1: something that bends. We need something that can handle tension
Speaker 1: without instantly snapping. We need timber.
Speaker 2: Wood is a remarkable engineering material. It has a high
Speaker 2: strength to weight ratio due to its cellular biology. How So,
Speaker 2: wood is essentially made of millions of tiny, hollow cellulars
Speaker 2: tubes bundled together, almost like a bundle of drinking straws.
Speaker 2: This makes it strong in both compression and tension.
Speaker 1: But all wood isn't the same.
Speaker 2: No, to use it safely, you have to choose the
Speaker 2: right part of the tree. The outer rings the sapwood
Speaker 2: are living in active transporting.
Speaker 1: Water, so you don't use that part right.
Speaker 2: You want the inner core, the heartwood. It's dead, but
Speaker 2: it is heavily saturated with natural resins and extractives that
Speaker 2: make it incredibly dense and naturally resistant to decay. That
Speaker 2: is your structural gold.
Speaker 1: But wait here is where I get completely stuck. It
Speaker 1: feels completely counterintuitive that a material we literally use as
Speaker 1: firewood is trusted in structural safety.
Speaker 2: Yeah, people ask this all the time.
Speaker 1: I mean, how thick does a beam need to be
Speaker 1: for this charring trick to actually work? Shouldn't we just
Speaker 1: be using steel for everything to stop fires?
Speaker 2: That is a very common assumption, but the reality is counterintuitive.
Speaker 2: What exposed to extreme fire, a steel beam will rapidly absorb.
Speaker 1: The heat, and steel bends when it's hot.
Speaker 2: Yes, it will lose its structural strength, warp buckle, and
Speaker 2: fail catastrophically, often with very little warning. Timber, on the
Speaker 2: other hand, is combustible, but structural timbers are intentionally oversized.
Speaker 1: Meaning they are thicker than they actually need to be
Speaker 1: just to hold the weight.
Speaker 2: Exactly, when a massive timber beam is engulfed in flames,
Speaker 2: the outer layer, what we call the sacrificial timber, ignites
Speaker 2: and burns.
Speaker 1: But it doesn't burn all the way through.
Speaker 2: No, because As it burns, it forms a thick layer
Speaker 2: of charcoal. Charcoal is a tremendous thermal insulator. It blankets
Speaker 2: the inner core of the beam, shielding it from the
Speaker 2: extreme heat and choking off the oxygen supply.
Speaker 1: That's crazy.
Speaker 2: The combustion rate slows down to a very predictable creeping crawl.
Speaker 2: An oversized timber beam will charg slowly while maintaining its
Speaker 2: structural integrity, far longer than equivalent steel beam, buying the
Speaker 2: occupants the critical time they need to evacuate.
Speaker 1: That is brilliant engineering hiding in plain sight. Let's talk
Speaker 1: about where we use this timber. The floors are a
Speaker 1: major and ground floors face an entirely different threat than
Speaker 1: upper floors.
Speaker 2: Yes, with a suspended timber ground floor, the enemy is
Speaker 2: stagnant moisture coming from the earth beneath.
Speaker 1: It, just sitting there in the dark.
Speaker 2: Exactly. If you build a timber floor too close to
Speaker 2: the damp ground without proper ventilation, you invite a fungal
Speaker 2: attack from cerpulolacramens, commonly known as dry rot.
Speaker 1: Now, dry rot sounds harmless, but it's not actually dry
Speaker 1: at all.
Speaker 2: Is it not the slightest the spores require dark, damp,
Speaker 2: unventilated conditions to germinate. Once they do, they grow root
Speaker 2: like threads called hyphaei that spread rapidly.
Speaker 1: What do they do to the wood?
Speaker 2: These threads literally secrete enzymes that digest the cellulose, branding
Speaker 2: the wood together, turning your massive structural joists into brittle,
Speaker 2: crumbling dust.
Speaker 1: So how do you stop a microscopic fungus from eating
Speaker 1: the floorboards with airflow?
Speaker 2: You elevate the timber joists on sleeper walls. But these
Speaker 2: aren't solid walls. They are built in a honeycomb pattern,
Speaker 2: leaving regular gaps between the bricks.
Speaker 1: Oh, so the air can flow through.
Speaker 2: Right. This allows a constant cross breeze of fresh air
Speaker 2: to sweep under the entire floor plan, whisking away the
Speaker 2: dampness and keeping the moisture content of the wood well
Speaker 2: below the threshold where dry rod can survive.
Speaker 1: Okay, so the ground floor is ventilated, but upper floors
Speaker 1: don't have the dry rock problem.
Speaker 2: No, they have a mechanical spanning problem.
Speaker 1: Right. If you have tall, thin wooden joist spanning a
Speaker 1: wide open living room ceiling, any movement from above, like
Speaker 1: people walking, creates a bouncing effect that can cause those
Speaker 1: tall joists to twist or buckle sideways.
Speaker 2: To prevent that lateral twisting, engineers use herring bone streutting.
Speaker 1: Herring bone strutting, what does that look like?
Speaker 2: Small thin pieces of timber are nailed in X pattern,
Speaker 2: criss crossing the gaps between the joists. This ties every
Speaker 2: single joist together.
Speaker 1: So they can't move independently exactly.
Speaker 2: If a heavy load pushes down on one joist, the
Speaker 2: struts transfer that force to the joists on either side.
Speaker 2: It physically vents them from twisting out of alignment and
Speaker 2: turns a dozen individual bending beams into a single, rigid,
Speaker 2: unified floor system.
Speaker 1: And finally we reach the apex. The roof. The book
Speaker 1: calls it the fifth wall because it has to enclose
Speaker 1: the space and fend off the weather, just like the brickwork.
Speaker 1: Let's talk about pitched roofs first.
Speaker 2: Okay, pitched roofs.
Speaker 1: I was looking at the diagrams and the engineering really
Speaker 1: seems to scale aggressively based on how far the roof
Speaker 1: has to.
Speaker 2: Stretch it does. The core problem with a pitched roof
Speaker 2: is lateral thrust. If you take two plane cards and
Speaker 2: lean them against each other on a smooth table to
Speaker 2: form a triangle.
Speaker 1: What happens The weight pushes the bottom edges out and
Speaker 1: they slide apart.
Speaker 2: Triangle flattens. Right, Yeah, the same thing happens with the roof.
Speaker 2: The weight of the sloped rafters wants to push the
Speaker 2: exterior walls of the house outward until they collapse.
Speaker 1: So for a tiny span like a narrow shed, you
Speaker 1: can use a simple couple roof, just rafters leaning against
Speaker 1: each other because the walls are strong enough to resist
Speaker 1: that small outward put.
Speaker 2: Yes, but as the house gets wider, that lateral thrust multiplies.
Speaker 1: And you have to neutralize it.
Speaker 2: So you step up to a close couple roof. You
Speaker 2: lay a horizontal ceiling joist straight across the bottom, tying
Speaker 2: the lower ends of the opposing rafts.
Speaker 1: Together, oh, holding them like a string across the bottom
Speaker 1: of the triangle.
Speaker 2: Right now, instead of pushing the walls outward, the rafters
Speaker 2: are just pulling harmlessly on that horizontal tie. If you
Speaker 2: need more headroom in the attic, you move that horizontal
Speaker 2: tie slightly higher up the rafters, creating a collar roof.
Speaker 1: But what if you were spanning a really wide modern house,
Speaker 1: The rafters would be so long they would start to
Speaker 1: sag in the middle under the weight of the tiles.
Speaker 2: That is where the double roof comes in. We introduce
Speaker 2: heavy horizontal beams called pearlins. These run lengthwise across the
Speaker 2: roof space, supporting the rafters at their midpoint to completely
Speaker 2: eliminate the.
Speaker 1: Sagging that covers pitched roofs. But what if a building
Speaker 1: has a flat roof. That seems like a completely different
Speaker 1: physics problem, very different. With a flat roof, the book
Speaker 1: sizes that you aren't just fighting rain falling on the outside,
Speaker 1: you are fighting invisible vapor attacking from the inside. This
Speaker 1: concept of interstitial condensation, it is.
Speaker 2: A profound design challenge. The warm air generated inside a
Speaker 2: living space holds a significant amount of water vapor. Because
Speaker 2: warm air rises, it migrates up through the ceiling. In
Speaker 2: a traditional cold deck flat roof design, the thermal insulation
Speaker 2: is placed directly above the plaster ceiling but below the
Speaker 2: wooden roof deck.
Speaker 1: Which means the wooden deck itself, sitting out under the
Speaker 1: night sky, is freezing cold.
Speaker 2: Precisely when that rising warm invisible vapor hits the freezing
Speaker 2: cold timber deck inside the roof structure, it rapidly cools
Speaker 2: and condenses back into liquid water.
Speaker 1: So the roof structure literally sweats from the inside out, and.
Speaker 2: That leads to severe rot and structural decay right above
Speaker 2: your head.
Speaker 1: So how do engineers fix a problem that's happening inside
Speaker 1: the ceiling. You have to move the battle lines.
Speaker 2: You change the entire thermal dynamic by switching to a
Speaker 2: warm deck design. You move the inside lation layer to
Speaker 2: the very top, placing it above the structural roof deck,
Speaker 2: right beneath the waterproof membrane, so.
Speaker 1: The wood stays warm.
Speaker 2: Now the structural timber stays warm, matching the interior temperature
Speaker 2: of the house, so condensation can never form on it.
Speaker 1: Or are They use an inverted roof right where the
Speaker 1: waterproof membrane is laid directly on the deck and a
Speaker 1: special waterproof insulation is placed on top of everything, held
Speaker 1: down by heavy gravel Exactly.
Speaker 2: That protects the membrane from both the freezing cold and
Speaker 2: the cracking yuvy rays of the sun. It is all
Speaker 2: about managing where the temperature exchange happens.
Speaker 1: Before we wrap up, let's actually test this out. We've
Speaker 1: covered everything from testing the dirt to capping the roof
Speaker 1: for the learners listening, Let's run a quick thought experiment
Speaker 1: to apply this knowledge, tying the roof way back to
Speaker 1: the dirt.
Speaker 2: Let's do it.
Speaker 1: Imagine you are building a house. You have a massive
Speaker 1: load bearing wall carrying a total of forty five kilo
Speaker 1: unones per meter run, and your standard penetration tests on
Speaker 1: the site reveals you are building on a cohesive clay
Speaker 1: soil with a bearing capacity of seventy five kilin newtons
Speaker 1: per square meter.
Speaker 2: Okay, so we need to determine the absolute minimum required
Speaker 2: width of the strip foundation to ensure the pressure bulb
Speaker 2: doesn't cause a sheer failure in that clay.
Speaker 1: Right. So, taking the formula from earlier, we simply take
Speaker 1: the downward load of forty five and we divide it
Speaker 1: by the clay's bearing capacity of seventy five. Forty five
Speaker 1: divided by seventy five gives us point six.
Speaker 2: So the minimum safe width for that concrete anchor is
Speaker 2: point six meters or six hundred.
Speaker 1: Milimeters waid in that trench pour the hydrated cement, and
Speaker 1: the house stands firm.
Speaker 2: It is deeply satisfying when the physics translate perfectly into
Speaker 2: the practical application.
Speaker 1: It really is we've seen today that a building is
Speaker 1: a carefully calibrated machine managing compression, chemical reactions, and environmental
Speaker 1: defense every single day. But I want to leave you
Speaker 1: with a final provocative thought from the text to ponder.
Speaker 2: On your own life cycle costing.
Speaker 1: Yes, we've talked extensively about strength, extreme durability, and resisting forces,
Speaker 1: but life cycle costing introduces a new variable.
Speaker 2: It forces us to look at the dimension of time.
Speaker 2: If an the engineer selects a highly expensive, incredibly durable
Speaker 2: material that will last for three hundred years, but the
Speaker 2: functional usefulness of the building itself, the floor plan, the
Speaker 2: technological integration, will be completely obsolete in fifty years, is
Speaker 2: that actually a good engineering choice.
Speaker 1: It's a fascinating question. Are we engineering homes to be permanent,
Speaker 1: unyielding monuments that will stand unchanged for centuries or should
Speaker 1: we be engineering them to be adaptable machines designed to
Speaker 1: evolve or be easily dismantled as the loads, the technologies,
Speaker 1: and the needs of society inevitably change over the next
Speaker 1: hundred years. The next time you walk up to a house,
Speaker 1: don't just see a static box. Look at the masonry
Speaker 1: and see the thirty kila newtons pushing down. Look at
Speaker 1: the mortar and see the DPC fighting the climbing water.
Speaker 1: Look at the roof and see the ties fighting the
Speaker 1: lateral thrust. The invisible forces are always there, and now
Speaker 1: you know how to see them,