3.1 Temporary Structures and Shoring

Concrete Formwork Design

Concrete formwork is a temporary structural system designed to contain and support freshly placed concrete until it cures and develops sufficient structural strength to support itself and other construction loads. Because formwork is a temporary structure, its design focuses on safety, constructability, and economy.

Formwork design must account for both vertical loads (gravity) and lateral pressures (fluid pressure from the fresh concrete mix).

Design Loads

According to ACI 347 (Guide to Formwork for Concrete), formwork design loads are categorized into vertical loads and lateral loads:

1. Vertical Dead Loads (D): The dead load includes the weight of the forms themselves and the weight of the reinforced concrete. Normal-weight concrete (including reinforcing steel) is assumed to weigh 150 pcf (24 kN/m³). The weight of the formwork itself is typically estimated between 10 to 15 psf (0.5 to 0.7 kPa).
2. Vertical Live Loads (L): Live loads represent the weight of workers, construction equipment, runways, and concrete accumulation. ACI 347 recommends a minimum design live load of:
   • 50 psf (2.4 kPa) for workers and hand tools.
   • 75 psf (3.6 kPa) if motorized carts or buggies are used to place concrete.
3. Minimum Combined Combined Vertical Design Load (U): To ensure safety against unexpected construction impacts, the total vertical design load must not be less than:
   • 100 psf (4.8 kPa) when using hand tools.
   • 125 psf (6.0 kPa) when using motorized carts.

Lateral Pressure of Fresh Concrete

Freshly placed concrete acts as a fluid, exerting lateral pressure against vertical forms. As concrete begins to set, this lateral pressure decreases from full hydrostatic pressure to a stabilized lower value. The maximum lateral pressure (P_max) is highly sensitive to the rate of concrete placement (R, measured in ft/hr), the concrete temperature (T, measured in °F), the placement height (h, in ft), and the mix chemistry.

Under ACI 347-14, the design lateral pressure for normal-weight concrete containing Type I cement without admixtures, compacted with internal vibrators to a depth of 4 feet or less, is determined by empirical formulas based on member geometry:

Column Forms

Columns are vertical structural elements with plan dimensions less than 6 feet in both directions. The maximum lateral pressure is:

P_max = C_C * C_W * [150 + 9000 * R / T]

This formula is subject to the following design boundaries:

• Minimum Limit: P_min = 600 * C_W psf
• Maximum (Hydrostatic) Limit: P_max = 150 * h psf (where h is the total head of concrete above the point of interest).

Wall Forms

Walls are vertical structural elements with plan dimensions of 6 feet or more in at least one direction. The formulas are split based on the rate of placement:

• Slow Rate of Placement (R <= 7 ft/hr) and placement height h <= 14 ft: P_max = C_C * C_W * [150 + 9000 * R / T] Subject to bounds: P_min = 600 * C_W psf and P_max = 150 * h psf.

• Fast Rate of Placement (R > 7 ft/hr, or R <= 7 ft/hr and h > 14 ft): P_max = C_C * C_W * [150 + 43400 / T + 2800 * R / T] Subject to bounds: P_min = 600 * C_W psf and P_max = 150 * h psf.

Coefficient Definitions

• Chemistry Coefficient (C_C): Accounts for concrete chemistry and setting retarders:
  • Plain concrete (Type I cement, no retarders, no fly ash/slag): C_C = 1.0
  • Retarding admixtures: C_C = 1.2
  • Fly ash blends (less than 40% fly ash): C_C = 1.2 to 1.4 depending on percentages.
• Unit Weight Coefficient (C_W): Accounts for concrete density:
  • For density less than 140 pcf: C_W = 0.5 * [1 + w / 145] >= 0.80
  • For density between 140 and 150 pcf: C_W = 1.0
  • For density greater than 150 pcf: C_W = w / 145 (where w is concrete unit weight in pcf).

Worked Example: Formwork Pressure Calculation

Problem: Wall forms are being designed for a concrete foundation wall with a pour height of 10 ft. The placement rate is 5 ft/hr. The concrete temperature is 70°F. The concrete has a density of 145 pcf and contains no retarders or fly ash. What is the maximum lateral design pressure?

Step 1 — Identify the parameters:

• Pour height: h = 10 ft
• Placement rate: R = 5 ft/hr (Note: R <= 7 ft/hr and h <= 14 ft)
• Temperature: T = 70°F
• Density: 145 pcf, so C_W = 1.0
• Plain concrete: C_C = 1.0

Step 2 — Select the appropriate formula: Since R = 5 ft/hr is less than or equal to 7 ft/hr, and the height h = 10 ft is less than or equal to 14 ft, we use the slow rate wall formula: P_max = C_C * C_W * [150 + 9000 * R / T]

Step 3 — Calculate the lateral pressure: P_max = (1.0) * (1.0) * [150 + 9000 * 5 / 70] P_max = 150 + 642.86 = 792.86 psf

Step 4 — Verify against the design bounds:

• Minimum limit: P_min = 600 * C_W = 600 * 1.0 = 600 psf
• Hydrostatic maximum limit: P_max_hydro = 150 * h = 150 * 10 = 1500 psf

Our calculated pressure of 793 psf falls within the range: 600 psf <= 793 psf <= 1500 psf.

Result: The formwork must be designed for a maximum lateral pressure of 793 psf.

Structural Framing and Member Design

Formwork design treats the wood or steel framing as structural beams and columns:

• Sheathing: The vertical skin in direct contact with concrete. It behaves as a continuous beam over joists.
• Joists: Vertical studs supporting the sheathing, acting as beams spanning between horizontal wales.
• Wales: Horizontal double-beams supporting the joists, spanning between form ties.
• Form Ties: Tensile members holding opposing wales together to resist lateral pressure.

Formwork components are designed checking three key structural limits: bending stress, shear stress (horizontal rolling shear for wood), and deflection limits (typically restricted to L / 360 of the span or 1/8 inch maximum to prevent structural bulging and concrete cosmetic blemishes).

Scaffolding Systems

Scaffolding refers to any temporary elevated work platform and its supporting structures used to support workers, materials, and equipment. Scaffolds must be designed by a qualified person and erected, moved, or modified under the supervision of a competent person.

Safety Factors and Load Ratings

OSHA Standard 1926.451(a)(1) enforces strict structural limits for scaffolding design:

• General Safety Factor: Each scaffold and scaffold component must support, without failure, its own weight and at least 4 times the maximum intended load. This is a 4:1 safety factor.
• Suspension Ropes: Cables and ropes used for suspension scaffolds must support at least 6 times the maximum intended load (a 6:1 safety factor).

Scaffolds are classified by their maximum allowable design load:

• Light-duty scaffold: Maximum design load of 25 psf (1.2 kPa). Sized for workers and light hand tools.
• Medium-duty scaffold: Maximum design load of 50 psf (2.4 kPa). Sized for bricklayers, plasterers, and concrete workers carrying moderate materials.
• Heavy-duty scaffold: Maximum design load of 75 psf (3.6 kPa). Sized for stone masons and heavy stored materials.

Planking and Setup Requirements

• Plank Grade: Wood planks must be nominal 2x10 inch scaffold-grade lumber or equivalent. Deflection under full load is limited to L / 60 of the span.
• Plank Overhang: Planks must extend over their end supports by at least 6 inches (to prevent slipping) but not more than 12 inches (to prevent a cantilever tipping hazard), unless they are cleated or hooked.
• Lapping Planks: Where planks are lapped, the lap must occur over support members and must be at least 12 inches.
• Stability Ties: If a scaffold's height-to-base ratio exceeds 4:1, it must be tied, guyed, or braced to prevent tipping.

Cofferdams and Hydrologic Safety

A cofferdam is a temporary, watertight enclosure constructed in a water body or saturated soil to exclude water and soil from an excavation, creating a dry environment for permanent construction (such as bridge piers or pump stations).

Failure Modes: Piping and Boiling

When a cofferdam is dewatered, an upward hydraulic gradient is created inside the enclosure. Water flows down the outside of the sheet piles and up through the bottom of the excavation. If this upward seepage force exceeds the buoyant weight of the soil, the soil loses all shear strength, behaving like a liquid. This condition is called piping or boiling (the quicksand condition).

It occurs when the hydraulic gradient matches the critical hydraulic gradient (i_cr):

i_cr = (G_s - 1) / (1 + e)

where G_s is the specific gravity of the soil solids (~2.65 to 2.70) and e is the soil void ratio. Typically, i_cr is approximately 1.0. To prevent boiling, cofferdams require a minimum factor of safety against piping of 1.5 to 2.0.

Tremie Concrete Seals

To prevent water from entering and soils from boiling at the excavation bottom, a tremie concrete seal is placed underwater. This slab acts as a heavy gravity plug. Once it cures, the water inside the cofferdam is pumped out (dewatered). The required thickness (t) of the tremie seal to resist buoyant uplift forces is calculated as:

t = FS * h_w * γ_w / γ_c

Where:

• t = thickness of the tremie seal (ft)
• FS = factor of safety against flotation (typically 1.1 to 1.2)
• h_w = height of water head measured from the bottom of the seal to the water table (ft)
• γ_w = unit weight of water (62.4 pcf)
• γ_c = unit weight of concrete (typically 150 pcf for plain concrete)

Excavation Support Systems

Excavations are among the most hazardous construction activities. Deep trenches require structural support to prevent soil collapse.

Trench Shields vs. Shoring

• Trench Shields (Trench Boxes): These are heavy steel or aluminum boxes placed in a trench. They do not prevent soil collapse; rather, they shield workers from injury if a collapse occurs. The soil outside the shield is allowed to slough.
• Shoring: This is an active support system (e.g., timber shoring, hydraulic shores, sheet piles) that exerts lateral pressure against the trench face to prevent soil collapse.

Braced excavations and Peck's Apparent Earth Pressure Envelopes

Because braced excavations use rigid struts, the soil cannot rotate about the base of the cut. Consequently, the lateral earth pressure distribution is not triangular (as assumed in Rankine or Coulomb theories). Instead, designers use apparent earth pressure envelopes developed by Ralph Peck (1969):

• Sand: Rectangular envelope: P_a = 0.65 * K_a * γ * H (where K_a = tan²(45 - φ/2))
• Soft-to-Medium Clay (when stability number N_b = γ * H / c > 4): P_a = γ * H * [1 - 4 * c / (γ * H)] (or a minimum of 0.3 * γ * H)
• Stiff Clay (when stability number N_b = γ * H / c <= 4): Trapezoidal envelope varying between 0.2 * γ * H and 0.4 * γ * H.