Floors

One floor per room, on top of the walls

The Floor tool, on the LSF card, places floor joists in a closed room with one click. The joists are created on top of the walls, span from centerline to centerline of the supporting walls, and each end bears on a stud. When there is no stud below, the tool shifts the wall grid until there is one.

In the single-story example house, the ground floor is the raft slab, outside the steel model. What this tool places on top of the 2.80 m walls is the intermediate floor: the base of an upper story. What it is for explains why the example does this in a single-story house, and what to do with the floor before the roof.

In the example house: three floors, one per room. PS1 in the left room, with a 4.50 m span; PS2 and PS3 in the two rooms on the right, with a 3.50 m span.

Joists from wall to wall, and the track becomes a joist

The floor carries the weight of the slab, the finish and the people using the room down to the walls below. Each joist spans between two supporting walls and bears on a stud; the wall takes the load to the ground. The plan shows the three floors of the example house:

Why an intermediate floor in a single-story house

The tool has no elevation of its own: the floor always sits on top of the walls of the story where it was placed. The ground floor sits on the foundation, outside the steel model. In a single-story project, then, what the tool places is the plane at the top of the ground floor walls.

The example's choice

The example house is single-story and will get a truss roof (see Roof trusses and Roof). It does not need a floor on top of the walls. Even so, this page places the three floors in it, to show the tool on a plan already familiar: the same walls, the same openings, the same column. It is exactly what is done on the ground floor of a 9 x 7 m two-story house.

If your house will stay single-story, undo the three floors before placing the trusses (see Roof trusses): each placement is one Ctrl+Z step, which brings back the floor, the joists and the studs that moved. The Delete button also removes floor and joists, but the studs stay where the floor put them.

Joist over stud, and L/350 deflection

What the standard does not say

These come from the product, not from ABNT: the 400 and 600 mm spacings (600 the default), the sections of each role and the default loads of the Brazil market, taken from mCalcLSF and from the CalcSteel product study; the double boxed rim joist as the stick-built default, and the 300 mm lattice joist with the rim switched off as the roll-formed default; and the floor board restraining the top flange every 0.30 m, a choice that follows AISI S100, I6.3, an American standard.

Shortest span, joist over stud

• 0.60 m: between joists: the spacing of the governing wall
• 12.9 mm: L/350 on the 4.50 m span; 10.0 mm on the 3.50 m one
• 20 mm: of tolerated eccentricity between joist and stud
• 3 Hz: minimum natural frequency of the floor, by hand

1. Joists on the shortest span. It is what the tool picks without you asking. In the left room it is 4.50 m along X against 7.00 m along Z: the Z direction is also accepted, and the choice is yours.
2. Every end over a stud. Check the rings in the preview: orange bears on a header or truss, and the header then carries the joist; amber has no stud below. Both call for your decision.
3. The joist to rim connection is yours. With the double rim, the joist reaches the box through an angle, designed separately. With the single rim, the engine checks the web crippling of the joist on the flange.
4. Board screwed every 0.30 m. The engine counts on the board holding the top flange. Without the board screwed at that spacing, the computed lateral-torsional buckling is unconservative.

1. Arm the tool in plan view

In the plan of the house, click Floor on the LSF card. The box opens asking for the room and shows the sections that apply to the next floor, the ones from the project settings. The bottom line sums up the keys: click a room, X or Z, G switches the governing wall, Enter places, Esc exits.

2. Move the cursor over the left room

The room must be closed by walls. Over it, the tool already draws the floor that would be created, without saving anything.

3. Click and read the draft

The click opens the new floor in the box and the preview on the plan. The tool chose by itself: joists along X, "the shortest span", a 4.50 m span; 11 joists at the 0.60 m spacing, "the project one". The governing wall is Wall 3, the left side wall, for being "the exterior one". The legend counts 2 ends that "bear on a header or truss": they fall on the header of the window in the left side wall, and appear on the plan as orange rings.

4. Switch the governing wall, if you want

The G key, or the Switch button, hands the grid to the other supporting wall, here Wall 1, the interior wall between the rooms, and the box says "chosen by you". The joists move to fall on its grid, and the one that follows becomes Wall 3. In this example the rule's choice works: press G again to go back.

5. See the other direction, and go back

The Z key turns the joists: a 7.00 m span, 7 joists, and Wall 6, the front one, governs because it "has a column or truss" (PL1; see Columns). On the back wall the studs would shift for up to 5 joists, in cyan. The tool accepts that span, and the choice is yours: go back to X.

6. Place

With the joists along X, Enter or Place. The box answers "Piso PS1: 11 vigas." (Floor PS1: 11 joists.) and the floor joins the Project floors list. Floor, joists and the studs that moved go in a single Ctrl+Z step.

7. The two rooms on the right

Click the front room on the right: joists along Z, 3.50 m span, 7 joists, Wall 6 governs, "the longest". Place it (PS2). Then the back room on the right: Wall 4, the interior wall between the two, governs because it is already "governed by another floor": the grid PS2 set on it applies to PS3, and the joists of the two rooms fall in line with each other. Place it (PS3).

8. Reopen a floor from a joist

With the tool armed, click a joist of PS1, or its name in the list. The box opens Floor PS1, with the 4.50 m span, the direction, the governing wall and the sections. Changed something? Apply (Enter). Delete removes the floor and the joists.

9. Check the wall view

Open the front wall. At its top, the top track gave way to the rim joist, as deep as the floor joist, and the studs got shorter to fit under it. Label and type stay those of the track.

The floor box

The refusals, and what to do

1. "Nenhuma parede apoia as vigas nesta direção. Troque para X ou Z." (no wall supports the joists in this direction; switch to X or Z). In the requested direction there are not two supporting walls. Fix: the other key.
2. "O cômodo é estreito demais para uma viga no passo da obra." (the room is too narrow for a joist at the project spacing). The room is narrower than the spacing: no joist fits between the edges.
3. "Fora de cômodo fechado: o piso pede paredes fechando o contorno." (outside a closed room: the floor needs walls closing the outline). The floor is created from a closed room: outside one, the box keeps asking you to move the cursor over a closed room and click. Fix: close the outline by joining the walls (see Wall joints).
4. Deleted the floor and waited for the studs to come back. Delete leaves the studs where the floor put them. To bring everything back, use the Ctrl+Z of the placement.
5. End in orange or amber. Orange: the joist bears on the header or the truss, which then carries it; check the header (see Headers). Amber: the end falls between 20 and 80 mm from a fixed piece, or on a wall that already belongs to another floor, and no stud fits below; it calls for a header or column, or another governing wall.

The full-span joist, pinned

In the model. Each floor joist is a member, with the floor joist role given by the VP label. It is pinned at both ends: fixed, the end rotation twisted the track and tipped the upper wall out of plane. The node of an upper stud bearing on it splits the member into pieces, but the engine measures the joist over the full span.

Checks. Bending by NBR 14762, 9.8.2 (with lateral-torsional and distortional buckling), shear by 9.8.3 and combined forces by 9.8.4. Deflection is L/350, by NBR 16970-2, 8.2, Table 3, measured against the straight line between the two supports of the full span. The limit is fixed: a limit typed on the member only applies if it is stricter.

• 4.50 m: PS1: L/350 = 12.9 mm
• 3.50 m: PS2 and PS3: L/350 = 10.0 mm
• 7.00 m: PS1 along Z, if placed: L/350 = 20.0 mm

Rim joist. Checked as a beam: its web works in bending, and the slenderness limit that applies is that of a beam web; the one for a compressed track web becomes a warning. Deflection is also L/350, measured over the span between the studs that support it.

Load by strip, and web bearing

Loads. The dead and live loads of the intermediate floor, from the project wizard, times the room area, go to the joists by the tributary width of each one: half the distance to the neighbor on each side, and at the edge up to the wall. The sum is always the load per area times the area. There are two load cases, dead load and intermediate floor live load.

With joists at 0.60 m and the first at 0.60 m from the wall, the middle one takes a 0.60 m strip and the edge one takes 0.90 m. Web crippling (NBR 14762, Annex D, γ = 1.35): the web of the rim joist is checked over each stud; in the double box both webs count. The web of the floor joist is only checked with the single rim, with half the flange as bearing.

What is left to you

The path, in one line

Plan → LSF card → Floor → cursor over the room → click (draft) → check direction, governing wall and rings → X, Z or G if needed → Enter → repeat in the other rooms → check the wall view and the 3D.

The example so far

• 3: floors, one per room
• 25: floor joists: 11 + 7 + 7
• 4.50 m: the largest span, in PS1
• 92 mm: the rim joist box, in a stick-built project