10.2 Falsework and Scaffolding

Introduction to Falsework and Scaffolding

In construction, temporary structures support the project before it is self-supporting. While formwork molds the concrete, falsework is the temporary structural system designed to support the dead load of forms, reinforcing steel, fresh concrete, and construction live loads. By contrast, scaffolding is an independent, temporary platform system engineered to provide safe access for workers, tools, and materials. Because falsework and scaffolding carry significant loads, engineering failures are often catastrophic. Consequently, they are heavily regulated by codes such as ASCE 37 (Design Loads on Structures During Construction), ACI 347 (Guide to Formwork for Concrete), and OSHA 1926 Subpart L (Scaffolds).

Structural Design of Falsework Systems

Load Paths and Structural Mechanics

Designing falsework requires tracing loads through an uninterrupted path to a solid foundation:

$$\text{Slab/Deck} \rightarrow \text{Plywood Sheathing} \rightarrow \text{Joists} \rightarrow \text{Stringers} \rightarrow \text{Vertical Shores} \rightarrow \text{Mudsills} \rightarrow \text{Subgrade/Soil}$$

According to ASCE 37, the design vertical load ($W$) is $W = D + L$, where $D$ represents the dead load, including concrete ($150\text{ pcf}$), reinforcing steel, and formwork self-weight ($5\text{ to }15\text{ psf}$). $L$ is the construction live load, which is a minimum of $20\text{ psf}$ for light-duty operations (hand placement) and $50\text{ psf}$ for heavy-duty operations (motorized carts). Concrete discharge via pump or bucket requires additional impact allowances. In addition, falsework must resist lateral forces. ACI 347 dictates bracing to resist a minimum horizontal load equal to $2\%$ of the total dead load or $100\text{ lb/ft}$ applied at the edge of the deck, whichever is greater. Diagonal cross-bracing is the primary method used to prevent racking.

Compression Member Stability and Buckling

Vertical shores are critical compression members. Their capacity is governed by elastic or inelastic buckling. The critical buckling capacity ($P_{cr}$) of a long compression member is defined by the Euler Buckling formula:

$$P_{cr} = \frac{\pi^2 E I}{(K L)^2}$$

where $E$ is the Modulus of Elasticity ($\text{psi}$), $I$ is the minimum Moment of Inertia ($\text{in}^4$), $L$ is the unbraced length ($\text{in}$), and $K$ is the effective length factor. The factor $K$ depends on end connections: pinned-pinned (typical wood shores with base plates): $K = 1.0$; fixed-pinned: $K = 0.7$; and fixed-free: $K = 2.0$. To maximize capacity, horizontal lacing is used to reduce the unbraced length ($L$). If a $16\text{-ft}$ shore is braced at its midpoint in both directions, $L$ is cut to $8\text{-ft}$, increasing its buckling load by a factor of four.

Worked Engineering Example: Shore Stability

Problem Scenario: A contractor is pouring a $9\text{-inch}$ thick reinforced concrete slab ($\gamma = 150\text{ pcf}$). The formwork dead weight is $8\text{ psf}$, and the design live load is $50\text{ psf}$. The slab is supported by nominal $4\times4$ timber shores (actual dimensions $3.5\text{ in} \times 3.5\text{ in}$) arranged in a $4.0\text{-ft}$ square grid. The shores are unbraced over their entire $12\text{-ft}$ height, with pinned-pinned ends ($K = 1.0$). Modulus of Elasticity $E = 1.6 \times 10^6\text{ psi}$. Using a Factor of Safety of 3.0 against Euler buckling, determine if the shores are structurally adequate.

Step 1: Calculate design load.

• Concrete Dead Load: $\text{DL}_{slab} = \frac{9}{12} \times 150 = 112.5\text{ psf}$
• Formwork Dead Load: $\text{DL}_{forms} = 8\text{ psf}$
• Live Load: $\text{LL} = 50\text{ psf}$
• Total Load: $w = 112.5 + 8 + 50 = 170.5\text{ psf}$

Step 2: Calculate axial load on a shore.

• Tributary Area: $A_{trib} = 4 \times 4 = 16\text{ sq ft}$
• Axial Load ($P$): $P = 170.5 \times 16 = 2,728\text{ lbs}$

Step 3: Calculate Moment of Inertia ($I$).

• $I = \frac{b d^3}{12} = \frac{3.5 \times (3.5)^3}{12} \approx 12.51\text{ in}^4$

Step 4: Calculate Euler buckling load ($P_{cr}$).

• Unbraced Length ($L$): $L = 12 \times 12 = 144\text{ inches}$
• Critical Buckling Load:

$$P_{cr} = \frac{\pi^2 \times (1.6 \times 10^6) \times 12.51}{144^2} \approx 9,524\text{ lbs}$$

Step 5: Determine allowable buckling load.

• $P_{allow} = \frac{P_{cr}}{\text{FS}} = \frac{9,524}{3.0} \approx 3,175\text{ lbs}$

Conclusion: The shore is adequate since the design load ($2,728\text{ lbs}$) is less than the allowable buckling load ($3,175\text{ lbs}$).

Scaffolding Systems and Safety Regulations (OSHA 1926 Subpart L)

Scaffold Load Ratings and Safety Factors

Under OSHA 1926.451, scaffolds are classified by uniform capacity: Light Duty ($25\text{ psf}$ for workers and hand tools), Medium Duty ($50\text{ psf}$ for bricklaying or concrete finishing), and Heavy Duty ($75\text{ psf}$ for stone masonry or stockpiled materials). Scaffold components must support their self-weight and at least four times the maximum intended load, representing a $4:1$ safety factor, while suspension cables require a $6:1$ safety factor.

Mandatory Erection and Safety Protocols

OSHA guidelines mitigate hazards: platforms must be fully planked with scaffold-grade lumber, with gaps under $1\text{ inch}$. Planks must extend past end supports by $6\text{ to }12\text{ inches}$ unless cleated. Fall protection is mandatory at heights of $10\text{ feet}$ or more; top rails must be $38\text{ to }45\text{ inches}$ high and support $200\text{ lbs}$ of force. Scaffold legs must rest on adjustable base plates and mudsills, never on loose bricks or barrels. Finally, supported scaffolds with a height-to-base-width ratio exceeding $4:1$ must be secured to the structure with ties or guys.

Critical Field Traps in Falsework and Scaffolding

Common field traps include placing mudsills on uncompacted or frozen ground, which thaws or saturates, causing differential settlement and progressive collapse. Another trap is exceeding the slenderness ratio limit of $L/d \le 50$ for timber shores, often by piecing timbers together with scab joints that act as hinges. Finally, omitting diagonal bracing leaves the temporary structure with no shear rigidity, allowing it to rack and collapse parallelogram-style under wind or concrete discharge surge loads.