Dade County Building Code (1935)

structural-load-calculations

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descriptionStructural load calculations and reinforced concrete design methods for beams, slabs, columns, and footings.

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Structural Load Calculations

Structural Load Calculations refers to the engineering principles and specific values used to determine the strength requirements for buildings and their components. According to the 1935-miami-dade-building-code, buildings must be designed to support estimated or actual imposed dead and live loads without exceeding specified stresses (p. 140). The design process must admit of rational analysis based on well-established principles of mechanics (p. 140). Beyond basic load definitions, the code provides detailed methods for calculating moments, shear, and reinforcement for reinforced concrete structures, including reinforced-concrete-design and flat-slab-design-methods.

Types of Loads

Dead Load

Dead load includes the weight of walls and permanent stationary construction that becomes part of the building (p. 140).

Live Load

Live load encompasses all loads except dead loads. Minimum unit live loads are specified by occupancy type:

  • Apartments and Dwellings: 40 psf (p. 141)
  • Offices: 50 psf (p. 141)
  • Schools (Class Rooms): 40 psf (p. 142)
  • Stores (Retail): 75 psf (p. 142)
  • Warehouses (Heavy Storage): 250 psf minimum (p. 142)
  • Manufacturing (Heavy): 125 psf minimum (p. 142)

Special considerations include designing office floors for a concentrated load of 2,000 pounds on a 2.5-square-foot space (p. 140). Corridors and stairs in dwellings require 40 psf, while public corridors may require up to 100 psf (p. 140-141).

Roof Loads

Roofs with a rise of 4 inches or less per foot must be designed for a vertical live load of 30 psf (p. 142). Steeper roofs require calculation based on accepted engineering principles (p. 142).

Wind Pressure

Wind pressure design values depend on building height:

  • Up to 25 feet: 25 psf (p. 143)
  • Up to 75 feet: 35 psf (p. 143)
  • Over 75 feet: 45 psf (p. 143)

Exposed structures like signs require 50 psf (p. 143). The overturning moment from wind must not exceed 50% of the dead load resisting moment (p. 143). Allowable unit stresses may increase by 33 1/3% when combined with wind loads (p. 143).

Load Reductions

Reductions in assumed live loads are permitted for columns, piers, walls, and foundations based on tributary floor area and building type (p. 142).

  • Girders: Up to 15% reduction for tributary areas of 300 sq. ft. or more (p. 142).
  • Columns (Warehouses): Up to 20% reduction for floors five or more below the roof (p. 143).
  • Columns (Other Buildings): Up to 50% reduction for floors seven or more below the roof (p. 143).

No reduction is allowed for slabs, joists, or beams (p. 142).

Foundation Design

Footing base areas are determined by dividing the total load by the allowable soil load. The area of footings for other parts of the building is determined based on dead loads only, using the soil pressure derived from the footing with the highest live-to-total load ratio (p. 143). Retaining walls must account for earth and water pressure (p. 144).

Posting Requirements

Owners of commercial and industrial buildings must conspicuously post designed live loads and maximum seating capacities using durable metal signs (p. 144). Occupancy permits are contingent upon the installation of these signs (p. 144).

Reinforced Concrete Structural Design

The code specifies detailed calculations for reinforced concrete members, integrating load principles with material strengths.

Beams and Slabs

The effective depth of a beam is defined as the distance from the centroid of tensile reinforcement to the top surface of the structural slab (p. 185). For T-beams, effective flange widths are restricted to one-fourth of the span length for symmetrical beams (p. 185). Ribbed floor construction requires ribs no more than 36 inches apart (p. 186).

Moment coefficients vary by support conditions:

  • Freely supported beams use for positive moment (p. 187).
  • Fully restrained continuous beams use for negative moment at interior supports (p. 187-188).

Shear and diagonal tension must be calculated, with web reinforcement (stirrups) required when shear stress exceeds concrete capacity (p. 189-190). Stirrup spacing is limited to or depending on stress levels (p. 190).

Flat Slabs

flat-slab-design-methods are detailed for slabs without beams. Panels are divided into “column strips” and “middle strips” (p. 194). Moment distribution depends on dropped panels and capital dimensions; for interior panels without dropped panels, column strips carry 46% of negative moment and 22% of positive moment (p. 195). Minimum slab thickness is calculated using formula (20), accounting for span and load (p. 196). Reinforcement ratios must not be less than 0.0025 (p. 197).

Columns

reinforced-concrete-column-design distinguishes between short and long columns. Short columns are limited to 11 times their least lateral dimension (p. 200).

  • Spiral Columns: Permissible axial load uses formula (22), with minimum longitudinal reinforcement of 0.01 of the core area (p. 200-201).
  • Tied Columns: Permissible load uses formula (23), with minimum longitudinal reinforcement of 0.005 of the total area (p. 201).

Composite columns encasing steel or cast-iron sections have specific stress limits for the metal components (p. 202-203). Long columns require reduced permissible loads based on slenderness ratios (p. 204).

Footings

Footings are proportioned based on total column load plus footing weight, though upward reactions for moment/shear calculations exclude footing weight (p. 204). Critical sections for bending are at the face of the column or pedestal (p. 204). Sloped or stepped footings are permitted if shear requirements are met and minimum thicknesses are maintained at edges (p. 204).

See also: 1935-Miami-Dade-Building-Code_p205_224

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