Truss connections

Transfer force through truss joints and into the supporting building.

Expose the plates, fasteners, bearing surfaces and movement provisions at each handoff.

Context. An educational model. Geometry, material assumptions and the reference building are described in the illustrated study.

A truss is only as good as its joints. The members are simple sticks that push or pull along their length; it is at the joints that forces change direction, pass from one member to the next, and leave the truss for the rest of the building. Most of the engineering in a roof sits in these places, and so do most of the failures. Each joint in the figure is also a place where systems meet: roof structure on wall structure at the heel, roof on roof at a girder, roof over an interior wall at a partition. Each is drawn with enough of its neighbours to see what bears on what, what fastens to what, and in what order it all goes together.

Every fastener in the figure does one of three things. It carries shear, a force across its shank, like a nail holding a strap to a stud while the strap is pulled along the stud. It resists withdrawal, a force along its shank that tries to pull it out of the wood. Or it transfers load by bearing, pressing on the wood or steel around its hole.

Nails are good in shear and poor in withdrawal. A 16d common nail in spruce-pine-fir holds two to three times as much sideways as it does along its length, and a slanted toenail does worse still. Good connectors are designed around this: a hurricane tie turns the upward pull on a truss into shear on ten nails. Bolts work in shear and bearing. Where a bolt passes through three layers, as in the timber and steel joints here, it crosses two shear planes and carries roughly twice as much as it would through two layers. That is double shear.

Light wood trusses are joined by connector plates: galvanized steel, usually 20 gauge (0.9 mm), punched so that rows of teeth stand up from one face. At the factory the members are laid in a jig and a press drives a plate into each face of every joint. Each tooth is a tiny nail; together, in typical spruce-pine-fir, they grip somewhere near 120 lb per square inch of plate on each face (0.8 MPa). The truss designer sizes each plate for the force it must carry from one member to the next. Take the heel apart in the figure and its plate comes off with its teeth showing.

A plate must not be bent, pried or nailed through on site. A truss with a loose plate is a truss with a missing joint, and it is sent back for repair rather than patched. Plates on both faces keep the joint from twisting.

The heel is where the roof structure sits on the wall structure, inside the enclosure. Under snow, the top chord thrusts down and outward into the heel; the bottom chord, the truss’s tie, pulls back inward; and what is left over, the truss’s share of the roof’s weight, goes straight down into the doubled top plate. With 20 psf (1 kPa) of snow on this 24 ft (7.3 m) roof, the top chord pushes about 1,730 lb (7.7 kN) into the heel and the bottom chord pulls about 1,540 lb (6.9 kN) out of it, both through the toothed plate. The wall below only feels the remaining 770 lb (3.4 kN) pressing down.

Wind reverses the problem. Air flowing over a roof sucks it upward, and wind getting under the overhang pushes it up from below. With the method the codes use, a 115 mph (185 km/h) wind on this roof pulls each heel up with about 200 lb (0.9 kN) more than the roof’s own weight holds down. Three toenails can resist only about 130 lb (0.6 kN), because they are being pulled out along their length. So every heel gets a hurricane tie, a bent strap nailed to the truss and to the top plate, rated here at about 500 lb (2.2 kN). Choose Left off in the figure and raise the wind: past about 105 mph the toenails are overloaded and the heel lifts.

The tie is one link in a chain. The upward pull has to reach the ground through every connection below: roof sheathing nails into the truss, truss into tie, tie into top plate, plates into studs, studs into wall sheathing, down through the floor to the anchor bolts and the foundation. A continuous path like this is the subject of the load path figure. A roof is lost at its weakest link, which is usually the one nobody thought of.

The heel also decides how warm the house is. A standard heel brings the top chord down onto the wall, which leaves only about 4½ in (115) for insulation over the inside face of the wall once the vent channel is kept clear, about R-16 instead of the R-49 in the middle of the attic. That cold band along every exterior wall is where ceilings get damp and where, in snow country, ice dams start. A raised heel (or energy heel) stands the top chord on a short post at the wall line, 16 in (405) up here, so the insulation keeps its full depth right out to the wall. The baffle keeps the insulation off the sheathing, so air from the soffit can still flow up under the roof.

On site the order runs like this. The trusses are set on the plates and toenailed to hold them in place. The ties go on as each truss is set. Blocking goes between the heels, then the roof sheathing goes on top. Inside come the baffles, the gypsum ceiling and, last, insulation blown in from above.

The flat triangle at the end of a gable roof is the part most often lost in hurricanes. Wind sucks it outward, and unless it is tied back it can only resist by hinging on the toenails along its foot. On this house the triangle is 72 sq ft (6.7 m²), and at 115 mph suction pulls on it with about 1,100 lb (4.9 kN).

The gable-end truss stands on the end wall and bears on it all along, so it needs studs rather than webs, at 24 in to carry the sheathing. To make a rake overhang, it is dropped: built with its top chords lowered by the depth of a 2×4, so that outlookers can lie across it. Nailed to the first common truss behind and cantilevered out over the wall, they carry a fly rafter and the edge of the roof. Diagonal bracing runs from the gable studs down to a 2×4 laid across the bottom chords of the next trusses, handing the gable’s top reactions to the ceiling plane. Steel straps under the sheathing tie each gable stud down to a wall stud below.

Where a roof changes direction, steps down at a hip, or must leave an opening, some trusses cannot reach a wall. They hang instead from a girder truss, a heavier truss made of two or three identical plies nailed together. Here a flat-topped girder at a hip end carries jack trusses that run from the end wall to its face. Each jack’s end sits in a face-mount hanger: a steel stirrup whose flanges are nailed to the girder and whose seat carries the jack.

The hangers can only be nailed to the near ply, yet both plies must share the load. So the plies are nailed together to a pattern the truss designer specifies, and the hanger’s face nails are long enough to reach through into the far ply. At 30 psf of snow each jack brings about 315 lb (1.4 kN), half of which has to cross to the far ply. Hide the near ply in the figure to see where the nails go. On site the girder goes up first, braced and fully nailed, before any jack is hung from it.

Trusses span from outside wall to outside wall, so the walls inside carry nothing from the roof. They still need their tops held in line. Nailing the bottom chords down to them seems the obvious way, and it is wrong, because in winter many trusses rise. The top chords, out in the cold, damp attic, take on moisture and lengthen. The bottom chord, buried in warm, dry insulation, dries and shortens. The truss arches upward, often ¼ to ¾ in (6 to 19) at mid-span and sometimes more, then settles back in spring.

A truss clip lets it. The clip is nailed to the partition’s top plate, and a nail through its vertical slot into the chord holds the wall in line while letting the truss slide up and down. The ceiling board is screwed to the chords except within about 16 in (405) of the partition. There its edge rests on drywall clips on the top plate, so it can bend rather than tear. Try the three ways of fastening in the figure. Screw the ceiling right up to the wall and a crack opens at the corner every winter. Nail the chords down and the rising truss pulls the wall up off its floor plate.

Large timber trusses are joined with steel. The neatest way is a knife plate: a single steel plate let into slots sawn down the middle of the members and bolted through. The timber hides the steel, which looks clean and keeps the steel out of a fire. In the figure’s heel, a 40 ft (12 m) glulam truss at 12 ft centres carries about 10,800 lb (48 kN) onto its column under 30 psf of snow.

The two chords hand their forces to the plate in different ways. The bottom chord’s pull, about 21,600 lb (96 kN), goes through eight ¾ in bolts. Each crosses two shear planes, wood to steel to wood, and carries about 2,700 lb (12 kN). The top chord’s push goes by bearing: its square-cut end presses on a plate welded across the knife plate, at about 300 psi (2.1 MPa) on the end grain. The bolts in the top chord only hold it in place. Bolts have to stay set back from the ends and edges of the wood, typically seven diameters from a loaded end and four between bolts along the grain, or the wood splits before the steel yields. Hide the near half of the timber in the figure to see the plate in its slots.

In a steel truss each member is often a pair of angles back to back, and at every joint a gusset plate is sandwiched between them. A bolt through angle, gusset and angle is in double shear. The members’ centre lines are made to meet at a single work point, so the joint carries no bending. With 48 kips (214 kN) in the diagonal, each of its three ¾ in bolts carries 16 kips (71 kN). The limit here is bearing on the ⅜ in (10) gusset, about 19.6 kips (87 kN) a bolt, not the bolts’ own shear strength.

Choose Welded and the bolts give way to fillet welds along the angles’ edges, about 13 in (330) of ¼ in weld for the same force. Welding suits the shop, where the work can be turned to the welder; bolting suits the site, where it needs no welder and is easy to inspect. Trusses are commonly welded into shipping pieces in the shop and bolted together where they meet on site.

Wind follows the code method in its simplest form: velocity pressure for 115 mph in open suburban terrain (Exposure B), net pressure coefficients for the roof edge and the overhang, and 60 percent of the wind against 60 percent of the roof’s weight, as allowable-stress design does for uplift. Nail, bolt and plate values are typical allowables for spruce-pine-fir, Douglas fir glulam and A325 bolts, increased for the short duration of wind or snow. The values for the tie and hanger are typical of products of their size. These are round numbers for learning how a joint works, not values to design from.