10.1 Formwork Design

Introduction to Formwork Design

For the PE Construction exam, formwork design is a critical competency. Formwork represents a significant portion of the cost of concrete construction (often 35% to 60%) and is essential for both the safety of the workforce and the quality of the finished structure. Formwork must support the dead load of the freshly placed, unhardened concrete, live loads from personnel and equipment, and environmental loads such as wind.

The design of concrete formwork is governed primarily by ACI 347: Guide to Formwork for Concrete. This standard provides the empirical formulas necessary to calculate lateral concrete pressure, which is the primary load that wall and column forms must resist. Inadequate formwork can lead to catastrophic blowouts, resulting in severe safety hazards, financial loss, and project delays.

Formwork Components and Load Paths

Understanding the anatomy of a wall or column form is fundamental. The load path in typical wall formwork is as follows:

1. Sheathing: The surface directly in contact with the concrete (e.g., plywood, steel, or fiberglass). It transfers the lateral concrete pressure to the studs.
2. Studs: Vertical framing members that support the sheathing and transfer the load to the wales.
3. Wales (Walers): Horizontal framing members that support the studs and transfer the load to the form ties.
4. Form Ties: Tensile members that pass through the concrete section to connect opposing formwork panels, resisting the outward lateral pressure.

Each component acts as a structural beam (often modeled as a continuous beam over multiple supports) and must be checked for three distinct failure modes: Bending (Flexure), Rolling Shear (or Horizontal Shear), and Deflection.

Lateral Concrete Pressure (ACI 347)

Freshly placed concrete behaves as a fluid initially, exerting hydrostatic pressure. As it begins to set, it develops shear strength, and the lateral pressure drops below the fluid pressure. The maximum lateral pressure, $P$, depends on the unit weight of the concrete ($w$), the rate of placement ($R$), and the concrete temperature ($T$).

The base hydrostatic pressure formula is: $P = w \cdot h$ where $h$ is the depth of fluid concrete.

For columns and walls with specific pour rates and temperatures, ACI 347 provides modified equations to calculate the maximum design pressure ($P_{max}$).

For Columns:

$P_{max} = C_W \cdot C_C \left[ 150 + \frac{9000 \cdot R}{T} \right]$

For Walls (Pour rate $R < 7$ ft/hr and placement height $\le 14$ ft):

$P_{max} = C_W \cdot C_C \left[ 150 + \frac{9000 \cdot R}{T} \right]$

For Walls (Pour rate $R < 7$ ft/hr but placement height $> 14$ ft, or $7 \le R \le 15$ ft/hr):

$P_{max} = C_W \cdot C_C \left[ 150 + \frac{43,400}{T} + \frac{2800 \cdot R}{T} \right]$

Where:

• $P_{max}$ = maximum lateral pressure (psf)
• $R$ = rate of placement (ft/hr)
• $T$ = temperature of concrete during placement ((^{\circ})F)
• $C_W$ = unit weight coefficient (1.0 for normal weight concrete)
• $C_C$ = chemistry coefficient (1.0 for standard cements without retarders)

Important Exam Trap: The calculated $P_{max}$ must never be less than $600 C_W$ psf (minimum), and it must never exceed the hydrostatic pressure $w \cdot h$. Always check these boundaries!

Structural Checks for Members

Once the uniform lateral pressure $P$ is determined, you must design the sheathing, studs, and wales. For a typical continuous beam over three or more spans, the structural mechanics formulas are:

Bending Moment (Flexure): The maximum bending moment $M$ for a continuous beam is generally $M = \frac{w \cdot L^2}{10}$, where $w$ is the line load (lb/in or lb/ft) and $L$ is the span between supports. The allowable span based on bending is: $L = \sqrt{\frac{10 \cdot F_b \cdot S}{w}}$ where $F_b$ is the allowable bending stress and $S$ is the section modulus.

Shear: The maximum shear $V$ is generally $V = \frac{w \cdot L}{2}$. The allowable span based on shear is: $L = \frac{2 \cdot F_v \cdot A}{w} + 2d$ where $F_v$ is the allowable shear stress, $A$ is the cross-sectional area, and $d$ is the depth of the member (accounting for loads within distance $d$ of the support).

Deflection: The maximum deflection $\Delta$ is generally $\Delta = \frac{w \cdot L^4}{145 \cdot E \cdot I}$. The allowable span based on a deflection limit of $L/360$ is: $L = \sqrt[3]{\frac{145 \cdot E \cdot I}{360 \cdot w}}$

In formwork design, you calculate the allowable span $L$ for all three conditions (bending, shear, deflection) and control the design using the smallest calculated span.

Worked Design Calculation

Scenario: A 12-ft high concrete wall is being poured at a rate of 5 ft/hr. The concrete temperature is 60(^{\circ})F. The concrete is normal weight (150 pcf) with standard Type I cement ($C_W = 1.0, C_C = 1.0$). Calculate the maximum lateral design pressure.

Step 1: Check the parameters. Wall height $h = 12$ ft. Rate $R = 5$ ft/hr. Since $R < 7$ and $h \le 14$, use the first wall equation.

Step 2: Apply the ACI 347 formula. $P_{max} = 1.0 \cdot 1.0 \left[ 150 + \frac{9000 \cdot 5}{60} \right]$ $P_{max} = 150 + 750 = 900$ psf.

Step 3: Check boundaries. Minimum pressure = $600 \cdot 1.0 = 600$ psf. ($900 > 600$, OK) Maximum hydrostatic pressure = $w \cdot h = 150 \cdot 12 = 1800$ psf. ($900 \le 1800$, OK)

Conclusion: The design lateral pressure is 900 psf.

If the sheathing transfers this load to studs spaced 16 inches (1.33 ft) on center, the line load on each stud would be $w = P \cdot s = 900 \cdot 1.33 = 1200$ lb/ft.