Load path

A house is a stack of systems: a roof, two floors of walls, a foundation and the soil under it. A load that lands anywhere on it has to be handed from each system to the next until it reaches the ground. Every handoff is a junction, a place where two systems meet and a connector carries the force between them. The figure draws the reference house used across this collection, opened up so the frame shows, and follows four kinds of load down through it one junction at a time. Press N and the camera moves to each junction in turn, lights the members that meet there, and gives the force arriving and leaving.

Dead load

The weight of the building itself. For this house: about 12 psf (0.57 kPa) of roof and ceiling on plan, 10 psf for each floor, 11 psf of 2×6 wall with its sheathing, siding and gypsum. Concrete weighs 150 lb per cubic foot (24 kN/m³).

Live load

People, furniture and stored things, set by code rather than weighed: 40 psf (1.9 kPa) on the first floor, 30 psf in the bedrooms upstairs.

Snow

Taken from the snow on the ground at the site, reduced for a roof that sheds some and warmed for one over a cold attic. A 30 psf ground snow becomes about 23 psf on this roof.

Wind

A pressure that grows with the square of the wind speed. It pushes on the walls facing it, pulls on the walls and roof slopes away from it, and lifts the roof.

Earthquake

Not a pressure at all: the ground moves, and the building’s own weight resists being moved. The heavier the house, the larger the force.

Every member carries the load from the area around it, half way to its neighbours on each side. A truss set every 24 in (610) carries a 2 ft strip of roof across its whole span, 52 sq ft with the eaves; under 35 psf of snow and roofing that is 8.1 kN (1,825 lb), half to each wall. A stud at 16 in (405) carries 16 in of whatever is above it. Each basement column carries the girder halfway to the next support, and the girder carries the joists from half the house. Following the strips from the roof down is the whole of the arithmetic in the figure.

Every connector in that column says the same thing. Under gravity each piece simply sits on the one below it, and the nails only keep things in line. Wood is weakest pressed across its grain, which is how a heel bears on a plate and a rim on a sill, but even here the heel presses about 110 psi where spruce is allowed 425. Choose Break a link under gravity and nothing moves: the weight finds its way down with or without the ties. That is why a house can be framed, roofed and lived in for years with a weak connection that nobody notices.

The trusses span from wall to wall, so the roof never touches the floors. The floors bear on the outside walls and on a line down the middle: a bearing wall upstairs, a glulam girder and two steel columns in the basement (see Beams and Columns). The footings widen the wall so the soil is pressed no harder than it can take, as Spread footings shows in detail. The column pads here are 3 ft (915) square; a 2 ft pad would press the soil at nearly twice what it is allowed.

Wind blowing along the ridge of a gable roof turns the whole roof into a wing. The suction is strongest near the end the wind comes from, and air leaking into the house pushes up from inside at the same time. At 115 mph (51 m/s), a common design speed inland, the roof near that end is pulled up at about 15 psf; only 60 per cent of the roof’s 12 psf may be counted against it, so each end truss pulls up on its heels with about 0.9 kN (206 lb). The path is the gravity path run in reverse, and every junction must now hold in tension:

• the sheathing nails are pulled straight out of the top chords, in withdrawal;
• a hurricane tie at each heel, bent steel nailed to truss and plate, carries the pull in strap tension (Truss connections takes one apart);
• the top plate is fixed to the studs only by nails driven into their ends, and a nail pulled from end grain holds almost nothing, so the sheathing, nailed to plate and studs alike, carries the pull down the wall in nail shear;
• straps nailed to a stud above and a stud below cross the second floor;
• and anchor bolts with plate washers hold the sill to the concrete, whose weight is the last thing keeping the house on the ground.

Each storey’s weight takes a little off the pull on its way down, so at 115 mph the straps carry only about 0.24 kN (54 lb) and the bolts nothing at all. Raise the wind and watch the junctions run out of weight one by one: at 160 mph each strap carries 3.5 kN (790 lb) and each bolt 2.9 kN (650 lb). Take the ties away at any speed and the end trusses lift off their plates, held only by toenails good for about 100 lb.

Wind on the long face of the house pushes on the front walls and pulls on the back ones, together about 13 psf at 115 mph. The studs span from floor to floor like small beams and hand half their load up and half down, to the roof and the second floor. Those two are the house’s diaphragms: flat, deep beams lying on their sides, 36 ft (11 m) long and 24 ft deep, made of sheathing nailed to trusses or joists. Each hands half its load to the walls at its two ends, and the top plates along its long edges work as its flanges, squeezed on one side and stretched on the other, exactly like the chords of a truss.

The end walls are shear walls. A stud wall alone is a row of four-sided frames, pinned at every corner by a few nails, and folds like the unbraced bay in Lateral stability. Nailed to its edges every 6 in (150), a sheet of 7⁄16 in (11) OSB stops the frames changing shape; it is good for about 365 lb per foot of wall under wind. Here the lower end walls carry 138 lb/ft, 38 per cent of that. Take the sheathing away and the house racks over.

A shear wall pushed at its top tries to tip over about its far bottom corner, lifting the corner nearest the wind. The tipping moment is the push times its height; the wall’s own weight, and whatever bears on it, resists with its weight times half its length. Whatever is left over has to be held down at the windward end by a hold-down: a steel bracket bolted through the end post, with a rod into the concrete. In this light house at 115 mph the two nearly balance, and the hold-down carries about 0.2 kN (44 lb). At 170 mph it carries over 9 kN (2,000 lb); remove it and the end post simply pulls its nails out of the sill, and the house tips. Straps across the second floor do the same job for the upper walls.

Everything the end walls collect, plus the wind on the lowest few feet of wall, has to cross from the sill to the concrete: 20.9 kN (4,703 lb) along each end wall at 115 mph. Five ½ in (13) bolts share it, each loaded in bolt shear between wood and concrete, about 940 lb each against roughly 1,040 lb allowed. A wood sill on a foam sealer can’t be counted on to stay put by friction, so with the bolts gone the house slides off its foundation. From there the walls and footings pass the push into the ground, by friction under the footings and by the earth bearing against the basement walls.

An earthquake follows the same path as the sideways wind, from diaphragms to shear walls, straps, hold-downs and bolts, but the force comes from the house’s weight rather than its surface. With design shaking of SDS 1.0 g, common in coastal California, wood shear walls are designed for a base shear of SDS ÷ 6.5 of the weight (the 6.5 allows for how much a nailed wood wall can bend and yield without collapsing). That is about 22 kN (4,940 lb) at working level, shared so the roof, which swings furthest, takes the larger part. Shaking up and down lightens the house at the same time, which is why the hold-downs work harder here than the base shear alone would suggest. For this light wood house the earthquake pushes about half as hard as a 115 mph wind; a house of brick or concrete, several times heavier, would turn that the other way round.

The figure works the loads the way a designer checks a house on one sheet, and simplifies as such a sheet does. It uses allowable stress design: wind and earthquake are scaled to working level, and only 60 per cent of the dead load is counted when it is holding something down.

• Snow: ASCE 7 flat-roof snow, 0.7 × 1.0 × 1.1 × the ground snow, unreduced for the 6:12 slope of a shingle roof; never less than the 20 psf roof live load.
• Wind: ASCE 7 low-rise envelope method, Exposure B (suburban), flat ground, velocity pressure 0.00256 × 0.70 × 0.85 × V², internal pressure ±0.18. Uplift is taken with the wind along the ridge, sideways force with the wind on the long face.
• Earthquake: equivalent lateral force, R = 6.5, weights from the dead loads above with 80 ft of partitions per floor; the vertical effect taken as 0.2 SDS of the dead load.
• Capacities are typical rather than exact: about 500 lb for a single hurricane tie, 365 lb/ft for 7⁄16 in OSB nailed at 6 in, 1,040 lb for a ½ in bolt along the grain, 2,000 psf for a sandy soil.
• When a link is taken away, the figure asks whether anything else on the path could carry its force: toenails count for a little, nails pulled from end grain and friction under a sill count for nothing. Failures are drawn many times larger than life.

It is an illustration, not a design. Real houses are checked against the code that applies where they are built, by tables in the residential code or by an engineer.