3305.3.2 Design of Concrete Formwork

Design of formwork, including but not limited to forms, shores, and shoring foundations, shall comply with ACI 318, Section 6.1.5, and the requirements of Sections 3305.3.2.1 through 3305.3.2.8 of this code.

Site-specific formwork design drawings prepared by a registered design professional shall be required in the following cases:

1. For concrete formwork in a structure classified as a major building; or
2. Wherever the shore or form height exceeds 14 feet (4267 mm); or
3. Wherever the total vertical load on the forms exceeds 150 pounds per square foot (732 kg/m²); or
4. Wherever power buggies are used; or
5. Wherever multi-stage shores are used; or
6. Wherever the slab thicknesses or beam heights equal or exceed 10 inches; or
7. Wherever there are concentrated loads exceeding 2000 pounds (907 kg) imposed on the formwork; or
8. Wherever there are loads imposed on existing structures in accordance with Section 3305.3.1.2.1.

Exception: Design drawings prepared by a registered design professional are not required for formwork installed in conjunction with slabs supported directly on grade or footings where such slab or footing does not impart any load on an adjacent structure.

Vertical loads shall include the total dead and live loads. Dead load shall include the weight of the formwork plus the weight of the reinforcement and fresh concrete. Live load shall allow for the weight of the workers and equipment, with allowance for impact, but in no case shall be less than 20 pounds per square foot (98 kg/m²).

Design of forms, ties and bracing shall satisfy the minimum lateral pressures of fresh concrete specified in Table 3305.3.2.3.

TABLE 3305.3.2.3 MINIMUM LATERAL PRESSURES TO BE ASSUMED FOR FRESH CONCRETE WEIGHING 150 POUNDS PER CUBIC FOOT ^{a, b, c}

TYPE OF WORK	MINIMUM LATERAL PRESSURE ASSUMED (PSF)	LIMITATIONS
Columns: Ordinary work with normal internal vibration	p = 150 + (9000R/T)	Maximum 3,000 psf or 150h, whichever is less
Walls: Rate of placement at 7 feet per hour or less	p = 150 + (9000R/T)	Maximum 2,000 psf or 150h, whichever is less
Walls: Rate of placement at greater than 7 feet per hour	p = 150 + (43400/T) + (2800R/T)	Maximum 2,000 psf or 150h, whichever is less
Slabs	p = 150h	None

For SI: 1 inch = 25.4 mm, 1 foot per second = 0.305 m/s, 1 pound per cubic foot = 16.02 kg/m3, 1 pound per square foot = 4.882 kg/m2, °C = (°F-32)/1.8.

where:

R = rate of placement, feet per hour.

T = temperature of concrete in the forms, °F.

h = height of fresh concrete above point considered, feet.

a. Allowances for change in lateral pressure shall be made for concrete weighing other than 150 pcf; for concrete containing pozzolanic additions or cements other than Type I, for concrete having slumps greater than 6 inches, or for concrete consolidated by revibration or external vibration of forms.

b. Where retarding admixtures are employed under hot weather conditions, an effective value of temperature less than that of the concrete in the forms shall be used in the above formula.

c. If retarding admixtures are used in cold weather, the lateral pressure may be assumed as that exerted by a fluid weighing 150 pcf.

Braces and shores shall be designed to resist all external lateral loads, including, but not limited to, wind, cable tensions, inclined supports, dumping of concrete, and starting and stopping of equipment. In no case shall the assumed value of lateral load due to wind, dumping of concrete, and equipment acting in any direction at each floorline be less than 100 plf applied along the edge or 2 percent of total dead load of the floor, whichever is greater. Except for foundation walls that are poured against a rigid backing, wall forms shall be designed for a minimum lateral load of 10 pounds per square foot (49 kg/m²), and bracing for wall forms shall be designed for a lateral load of at least 100 pounds per linear foot (148.8 kilograms per linear meter) of wall, applied at the top. The lateral load acting on walls greater than 14 feet (4267 mm) high shall be determined by analysis of conditions applicable to the site and building.

The formwork shall be designed for any special conditions of construction likely to occur, such as asymmetrical placement of concrete, impact of machine-delivered concrete, uplift and concentrated loads.

Shoring and bracing shall comply with Sections 3305.3.2.6.1 through 3305.3.2.6.4.

When patented or commercial devices that are not susceptible to design are used for shoring, bracing, or splicing, they shall be approved by the commissioner.

Splices shall develop the full strength of the spliced members.

Where shore height exceeds 10 feet (3048 mm), or when necessary to provide structural stability, diagonal bracing shall be provided. Struts, anchored into masonry or to panel joints of adjacent braced bays may be used to prevent buckling of individual members not supported by the diagonal bracing, but bracing an entire tier of shores with struts without diagonal bracing shall be prohibited unless the system can be demonstrated to be braced by other rigid construction.

The unbraced length of shores shall not exceed the maximum length determined in accordance with the requirements of this code for the structural material used.

Foundations for shores more than 10 feet (3048 mm) high and supported on the ground shall be designed by a registered design professional.

Formwork shall be so constructed that vertical adjustments can be made to compensate for take-up and settlements. Wedges, jacks or other positive means shall be provided for this purpose.