Roof trusses

A truss from wall to wall

The truss is the roof piece that spans between two walls. On the LSF card it has no button of its own: it lives in the Roof box, in Single roof truss mode. One click on the wall, the distance to its end, the span to the opposite wall, and the sheet opens with the type, the pitch and the panels. Generate truss writes the bars on top of the walls.

The single truss is a one-piece truss: to study a span, compare types, cover an annex. The roof of the whole house, with eave, battens and tiles, comes from By outline mode, shown on the Roof page. What the calculation does with the single truss is in What the engine does with it.

In the example house: T1 on the stud 0.60 m from the left front corner, with a 7.00 m span to the back, trapezoidal at 30%; T2, copied 0.60 m further along, on the next stud.

Chord, vertical and diagonal

The truss is a flat, upright lattice. The top chord takes the roof and slopes down in two pitches to the ends; the bottom chord is straight, from support to support, and the ceiling hangs from it. Between the two, verticals and diagonals close triangles, and the load travels through the bars as axial force to the two supports, on top of the walls. T1 of this house, as the sheet generates it:

The sections appear on the sheet with the catalog name (C_90x40x12x0.95 is the Ue 90x40x12x0.95 of the other boxes). They come from the project roles, and the sheet only shows them.

Five types, and one truss at a time

The sheet generates five types, and the Half truss box turns any of them, except the Portal, into a single pitch, from end to end of the span. All of them are created in the vertical plane that runs from one wall to the other, with the bottom chord on top of the walls:

Single truss or By outline

Both come from the same Roof box, and both draw trusses. The difference lies in who decides where each one goes and in what the engine knows about them:

Truss on a stud, and L/250 deflection

What the standard does not say

These come from the product, not from ABNT: the chord, vertical and diagonal sections, the 600 mm spacing and the roof loads of the Brazil market (tiles 15, insulation 5, ceiling 15 and utilities 0 kgf/m², live load 25 kgf/m²), all from the CalcSteel market conditions table, which reproduces mCalcLSF; the 10% pitch the sheet opens with; and the panels per half proposed from the span, one every 1.20 m, between 3 and 12.

On the stud, across the shorter span

• 0.60 m: between trusses: the wall stud spacing
• 1.05 m: of height at 30% on the 7.00 m span: half the span times the pitch
• 28 mm: L/250 of the whole 7.00 m truss, a hand reference: the engine checks the chord segment by segment
• 20 mm: between the truss web and the stud web, at most

Put the truss on a stud, on both walls. The distance the tool asks for is measured from the nearest end of the wall, centerline to centerline. Read the studs first: on the front of this house they are 0.60 m and 1.20 m from the left corner, and the back follows the same 0.60 m grid. At 1.80 m the front has door P1, and a truss there would land on the track, with no stud below.

Span the shorter direction. In the house, the 7.00 m side is the shorter one that runs from outside to outside; the interior wall of the right room cuts the span to 3.50 m. A shorter span is a lower, lighter truss.

Choose the pitch by the tile. The standard gives no number; the tile does. The Roof box shows the minimum pitch of each market tile (ceramic asks for 30%; see Roof). The sheet opens at 10%; the house uses 30%.

Two trusses over the house

The house is the one from Bracing: a 9 x 7 m perimeter, two interior walls, door P1, the windows, the headers, column PL1, strap F1 and the blocking. If you followed Floors, first undo the three floors with Ctrl+Z: the house is single-story, and the truss sits on top of the walls.

1. Open the Roof box in plan view

In Plan, click Roof on the LSF card. In 3D the button is greyed out, with the hint "Abra a planta: o telhado se lança sobre as paredes" (open the plan view: the roof is placed over the walls). In the box, under Mode, choose Single roof truss.

2. Hover the front track

The tool picks the wall within 0.30 m of the cursor. Over it, a cyan strip shows the truss that would be created: from the cursor point, into the house, up to the first wall that line crosses. In the left room it crosses the house to the back. The hint next to the cursor is the usual one: the piece underneath (V6 · top track), the section and the length of the piece.

3. Notice where it stops

In the right room, the same gesture gives a 3.50 m truss: the line hits the interior wall before the back. The proposed span is always up to the first wall, and step 5 lets you change it.

4. Click, and give the distance from the end

The click fixes the wall and its end nearest the cursor, here the left corner. Next to the cursor the offset m from the end box opens, with the click distance (0.72 m in this example). Type 0.6 and Enter: the truss moves onto stud M1001, 0.60 m from the corner. Without typing, the distance follows the cursor along the wall, and a second click sets it. It always stays between zero and the wall length.

5. Check the span and confirm

The box changes to span m, with 7.00: the distance to the back, wall centerline to wall centerline. Moving the cursor, the truss stretches or shortens, and within 0.30 m of the far wall it snaps back to it. Enter or a click confirm; Esc cancels everything and disarms the tool. Once the span is confirmed, the truss sheet opens.

6. Choose the type and the pitch on the sheet

The sheet opens with the 7.00 m span in the header and a Trapezoidal truss: 6 panels per half (one every 1.20 m of span), heel height 0 and a 10% pitch, which on the 7.00 m span gives 0.35 m of height. The sections come from the project roles, read-only, and the preview below counts the bars. For the house, change only the pitch to 30: the height goes to 1.05 m.

7. Generate

Generate truss writes Truss 1 on top of the walls, already selected, and the footer becomes its own: Copy, the trash can, and the readings of bars, Span 7.00 m and Base 2.80 m, the height of the bottom chord, which is the top of the walls. One Ctrl+Z undoes the whole truss.

8. Copy 0.60 m further along

Copy, in the footer, arms the Copy tool. Click Truss 1: the click point becomes the anchor, and a ghost of the truss starts following the cursor. Take it 0.60 m toward the door and click: Truss 2 lands on M1002 at the front and M1013 at the back, 1.20 m from the corner. Each copy is one Ctrl+Z.

9. See both in plan and in 3D

In plan, each truss is a line over the studs, from front to back. In 3D, it is an upright lattice, with the bottom chord on top of the walls and the ridge 1.05 m above. Up close, the truss heel is the track node, on top of the stud: the load goes straight down.

10. Read a bar with Tab

In 3D, the cursor over the truss says OBJECT Truss 1: the truss is a single object. Tab goes to the piece, and the hint says WHOLE PIECE DG3 C_90x40x12x0.8 0.68 m: the diagonal, the section and the length. Another Tab goes back to the object. Clicked, the bar lights up; its section and length are the ones in the hint.

What each field does

Each field works within the range in the table.

The defaults of each type, and the sheet previews on this 7.00 m span

Trapezoidal from the button: 5 panels, heel height 0.6, plateau 0, 10%, 43 bars. From the freshly opened sheet: 6 panels, heel height 0.

Where people go wrong with the truss

1. Nothing lights up under the cursor. The cursor is more than 0.30 m from any wall. Bring it closer to the track.
2. I closed the sheet and the truss vanished. Esc or a click outside the sheet close it without generating. Redo the clicks on the wall.
3. Truss over the door. At 1.80 m from the corner, the front has P1: the truss lands in the middle of the track, with no stud below, and the track starts to bend. Skip the door opening, or check the track and the header (Headers).
4. Parallel chord upside down. From the button, at 10%, the height (0.35 m) is smaller than the heel height (0.6 m), and the bottom chord drops in a V. The height has to exceed the heel height: on this span, a pitch above 18%, or a smaller heel height.
5. Double truss. The single truss and the By outline roof are independent pieces. Both on the same pitch are two trusses in the model: delete the single trusses before generating the roof (Roof).

A lattice in the model, with its own roof strip

The engine calculates the whole house as a single model, and the truss enters it together with the walls. It recognizes the truss by the type of its bars (BZ, MT, DG), not by the label.

How the truss enters the model

Where each load enters

What the engine checks

Buckling length: in the plane of the truss, that of each bar, node to node. Out of plane, the top chord goes up to the largest gap between the battens, purlins or rafters that cross it, including those drawn by hand; with none, it is the whole chord, and the screen warns. The bottom chord goes up to the largest gap between braced nodes: the walls the truss sits on, the bottom-chord bracing line or a diaphragm bar. With a ceiling screwed underneath, it drops to the fastening spacing of the market ceiling board, when that is smaller; the torsional length stays that of the braced nodes. Diagonals and verticals are pinned, K = 1.

Negligible compression: a bar with compression up to 1% of its resistance does not trigger the compression slenderness limit (the case of many diagonals under self-weight only).

Deflection: L/250 of each chord segment, node to node (NBR 16970-2, 8.2), on the single truss and in By outline.

What stays with the designer

The path, in one line

Plan → LSF card → Roof → Mode Single roof truss → cursor on the track → click → distance from the end and Enter → span and Enter → sheet: type and pitch → Generate truss → Copy in the footer for the next ones → 3D.

The example so far

• 2: single trusses, T1 and T2
• 6: panels per half, in each one
• 7.00 m: of span, from front to back
• 1.05 m: of height, at 30%