Straps and blocking

The X that keeps the wall square

The example house, as it stood after Columns, has walls, openings, headers and a column in the facade. All of that carries vertical load. Nothing yet stops the wall from racking sideways when the wind pushes it in its own plane.

The Strap and blocking tool places two things. The first is the X strap, the bracing: two diagonals between two upright members. The second is blocking, which restrains the studs at mid-height and is the subject of Bracing.

The strap goes in with two clicks: one upright member, then another in the same wall. Before the second click the ghost already shows the span and the angle, and turns red outside the range.

In the example house: an X strap in the front wall, between the left corner and the jamb of door P1.

Taking the wind from the wall to the ground

A wall of studs and tracks, on its own, is a pinned frame: pushed sideways, it turns into a parallelogram. What keeps it square is the bracing. In LSF it is a steel strap in X, fixed from foot to head on two upright members: the wind pushes the top, one of the diagonals goes into tension and carries the force down to the base.

A strap only works in tension. That is why it always goes in pairs, in X: when the wind turns, the other diagonal takes over. The compressed diagonal is a thin strap that buckles under almost nothing, and the analysis ignores it.

A tie, and the angle is a design decision

For the standard, the strap is a cold-formed steel member in tension. Its resistance comes from NBR 14762. NBR 16970-2 covers the LSF system as a whole and is the source of the wall drift limit.

What the standard does not say

The standard does not set the strap angle. The 30° to 60° range, with 45° as the target, comes from the market rule the app loads, decided by CalcSteel and declared as a source in the rule. Below 30° the strap lies too flat and stretches a lot for little force; above 60°, it stands too upright and barely holds the top.

The typical detail DT4 on the drawing sheet calls for a minimum strap of 38 x 0.84 mm, following CBCA, the Brazilian Steel Construction Center. The strap in the analysis model is the one in the Bracing role: RET 50x0.95.

One X per wall, close to 45°

• 45°: the target: span equal to the height
• 1.62 to 4.85 m: the span that fits on a 2.80 m wall
• 2 faces: one strap on each side of the wall
• 2 directions: X and Z of the house, always

1. Bracing in both directions of the house. Wind comes from any side. With no strap or truss diagonal in one of the directions, the engine reports a missing bracing error. Sheathing on its own does not count.
2. Aim for 45°. With the span equal to the height the strap works best: on a 2.80 m wall, a span close to 2.80 m. The accepted range goes from 1.62 m (60°) to 4.85 m (30°).
3. Keep solid wall for the strap. The strap does not cross a door or a window. When you place the openings, leave a solid stretch of at least 1.62 m in each facade. On a wall with many openings, use Ends, which looks for the stretch at each end.
4. Both faces and double stud: keep the defaults. A strap on one face only pulls the wall to one side. The double stud at the end receives the concentrated force the strap delivers. Uncheck them only with a reason. At an end that falls on a wall joint the double stud is not created: the joint stud is already there.
5. Anchor the foot of the double stud. The strap force reaches the ground through the end stud. The anchor bolt at that foot is designed and detailed separately.
6. Leave "Up to the beam", the default. Under a beam the studs are cut at it, and the strap stops at the bottom chord of the beam. With no beam over the stretch, as in the example house, the height is the wall height: the box shows "with 2.80 m up to the beam".

1. Open the view and arm the tool

Select the front wall and click Open view on its card, in the footer. On the LSF card, click the Strap and blocking button. The box opens next to the card. Leave the defaults: type X strap, placement Span, height Up to the beam, both checkboxes on. The tool works in the plan, in the wall view and in 3D; in the view the upright members are large and the angle is easy to read.

2. Click the first upright member

An upright member is any vertical member of the wall: stud, stud cut by the beam, door or window jamb, column chord. Click the stud at the left corner. It turns blue, and the members it can close a strap with inside the range light up in green.

Esc releases the first member without leaving the tool.

3. See a refusal before clicking

To understand the rule, move the cursor over the next stud, 0.60 m from the corner. The strap would stand almost upright, at 78°. Nothing lights up in green and the box says why, with the number that solves it: on the 2.80 m wall, the span goes from 1.62 m to 4.85 m.

4. Move the cursor to the door jamb

Move the cursor over the left jamb of door P1. The green ghost draws the whole X, from the foot of the corner to the head of the jamb and from the foot of the jamb to the head of the corner, and the box shows the measurement in green: 1.66 m · 59°.

59° is inside the range, but close to the upper limit. Between the corner and the door there is only 1.66 m of solid wall, and the strap does not cross the opening. To get close to 45° on the 2.80 m wall, the solid stretch would need to be close to 2.80 m: that is an architectural decision, made when the openings are placed (Doors and windows).

5. Click and check

Click the jamb. The strap appears in orange, in X, on both faces of the wall, and a double stud (the Back to back column from Columns, in red) appears next to the jamb. At the corner it is not created: the joint stud is already there. Strap and double stud are a single Ctrl+Z step.

Click one of the diagonals to select the strap. The footer becomes its inspector: name, wall, the saved span and angle, and the options you can change without deleting it.

6. Repeat in the other direction and look in 3D

The front strap braces the house in the direction of the facade. The perpendicular direction is still loose: do the same on a side wall, on a solid stretch of at least 1.62 m. As long as one direction has no strap or truss in the wall, the analysis reports an error (see What the engine does with it).

Esc releases the selection and closes the view. In 3D, the strap shows at the front, with the double stud in red and the lattice column further along.

The box and the strap footer

The box (applies to the next strap)

The footer (applies to the selected strap)

The refusals, and what to do

Every refusal shows before the click, in red in the box. Nothing is saved.

1. "The strap would fall outside 30° to 60°." Seen in step 3. Fix: pick the upright member that gives a span inside the range the message itself gives.
2. "The strap would cross a door or window in this wall." The diagonal does not cross an opening. Fix: a solid stretch between openings, or move the opening.
3. "No stretch of this wall fits the strap: there is not enough solid wall between openings." Shows in Ends mode. Fix: brace another wall in the same direction, or rethink the openings.
4. "It hits another strap in this wall." Two straps do not overlap. The message says which one. Fix: delete the other one or pick another pair.
5. "The two members must be in the same wall." The strap does not turn the corner. Fix: one strap on each wall.
6. Project in a market without a strap rule. The angle range and the section come from the project's market rule, and the box only places straps with it. Use a market that brings the rule, such as Brazil.

One tie per combination

Model. Each diagonal goes in as a tie (class FIT), fixed to the nodes of the two upright members. Where it crosses a stud along the way, there is no node: the strap only bears at its ends.

Tension only. The analysis is nonlinear: in each combination, the diagonal that would be in compression leaves the model, and the other one takes the wind alone. With the wind from the other side, the roles swap.

An approved strap is not finished bracing

The engine checks the strap as a member. The load path continues through the connections and the foundation, and that part is checked separately.

The path, in one line

Wall view → LSF card → Strap and blocking → X strap, Span → click the first upright member → cursor on the second (green ghost, measurement in the box) → click → strap footer → repeat in the other direction → calculate and read the FX and the displacements.

The example so far

• 1: X strap, F1-V6, in the front wall
• 1.66 m: of span, at 59°
• 2: faces, RET 50x0.95
• 1: double stud, at the jamb of P1