10.4 Bracing and Anchorage
Key Takeaways
- Temporary bracing is critical for structures during construction before permanent connections and diaphragm action are established.
- Masonry walls under construction act as cantilevers and are highly susceptible to wind-induced overturning; they must be braced until the roof structure is tied in.
- Steel erection relies on temporary wire rope bracing to plumb the structure and resist wind and erection loads.
- Anchorage of formwork must resist uplift, lateral pressure, and hydrostatic forces, especially in single-sided wall forms.
The Need for Temporary Bracing
A building is most vulnerable to collapse during the construction phase. Permanent structures are designed to be stable once entirely completed—relying on roof diaphragms, shear walls, and moment connections to resist lateral forces. However, in the intermediate stages of construction, these systems are incomplete.
Temporary bracing provides the interim lateral stability required to resist environmental forces (wind, seismic) and construction loads (equipment impact, material storage) until the permanent stabilizing elements are installed.
Masonry Wall Bracing
Unreinforced or partially grouted masonry walls are particularly hazardous during construction. A freestanding masonry wall acts as a cantilever fixed at the base. It has high compressive strength but very low tensile strength, making it highly susceptible to overturning from wind loads.
The Standard Practice for Bracing Masonry Walls Under Construction provides guidelines:
- Restricted Zone: An area on both sides of a wall under construction where access is restricted due to the hazard of collapse. The zone width is typically equal to the wall height plus 4 feet.
- Initial Period: The period up to 24 hours after the wall is built. The mortar is still wet, and the wall has minimal strength.
- Intermediate Period: The period from 24 hours until the wall is incorporated into the permanent structure.
Walls over 8 feet in height generally require temporary bracing unless structurally analyzed to be safe without it. Braces are typically installed at an angle (e.g., 45 to 60 degrees) and anchored to the slab or ground.
Worked Wind Load Bracing Check
Scenario: A freestanding 12-ft high concrete masonry unit (CMU) wall is under construction. The design wind pressure is 15 psf. The wall is braced by diagonal pipe struts attached to the wall at a height of 8 ft and anchored to the floor slab at an angle of 45 degrees. The braces are spaced 10 ft apart along the wall. Calculate the axial force in one diagonal brace.
Step 1: Determine the total wind force tributary to one brace. Tributary width = brace spacing = 10 ft. Total wind force $F_w$ on the tributary area = Pressure $\cdot$ Area Area = Height $\cdot$ Width = 12 ft $\cdot$ 10 ft = 120 sq ft. $F_w = 15$ psf $\cdot$ 120 sq ft = 1800 lbs.
Step 2: Determine the moment about the base of the wall. The wind acts as a uniform load. The resultant force acts at the centroid, which is mid-height (12 ft / 2 = 6 ft). Overturning Moment $M_o = F_w \cdot$ distance = $1800$ lbs $\cdot$ 6 ft = 10,800 lb-ft.
Step 3: Calculate the horizontal reaction at the brace connection point. To maintain equilibrium, the overturning moment must be resisted by the brace at a height of 8 ft. (Assuming the base acts as a pin for simplification of overturning resistance). Horizontal force in brace $H_b = M_o / \text{height of brace connection}$ $H_b = 10,800$ lb-ft / 8 ft = 1350 lbs.
Step 4: Resolve the horizontal force into the axial brace force. The brace is at a 45-degree angle. The axial force $P_{brace} = H_b / \cos(45^\circ) = 1350 / 0.707 = 1909$ lbs.
Conclusion: Each temporary pipe brace must be designed to safely resist an axial compression or tension load of 1,909 lbs.
Steel Erection Bracing
In structural steel construction, temporary bracing is used to plumb the structure (align columns vertically) and stabilize the frame against wind and erection loads.
Wire rope (cable) with turnbuckles is the most common temporary bracing material. Columns are erected, and cables are placed diagonally across bays to create a truss-like stiffness. OSHA Subpart R requires that temporary bracing be designed by a competent person and remain in place until the permanent structural stability elements (such as metal decking, shear connections, or permanent cross-bracing) are installed and fully secured.
Formwork Anchorage
Anchorage refers to securing temporary structures to the ground or existing concrete.
Single-Sided Wall Forms: Unlike typical walls where opposing forms are tied together with internal form ties to resist the outward concrete pressure, single-sided forms (used when pouring against earth or an existing wall) cannot use internal ties. The enormous lateral pressure of the concrete must be resisted entirely by external bracing anchored to the foundation or floor slab. These anchors are subject to massive shear and pull-out forces and require precise engineering.
Uplift: Forms for battered (sloped) walls or forms with steps are subject to uplift pressures from the fluid concrete. Anchorage must be designed to hold the forms down, or the concrete will physically lift the formwork off the ground.
When designing the temporary bracing for a single-sided concrete wall form, why is external anchorage critical compared to a standard two-sided wall form?
A temporary brace is attached to a masonry wall at a height of 10 feet. It must resist a horizontal wind reaction force of 2,000 lbs. If the brace is installed at an angle of 60 degrees relative to the horizontal ground, what is the approximate axial load in the brace?