Structural Design Requirements: Beams, Slabs, Columns, and Footings
This document excerpt from the 1935-miami-dade-building-code details specific engineering requirements for reinforced concrete structures, focusing on moment calculations, reinforcement detailing, and load capacities.
Beams and Slabs
The code defines the effective depth of a beam as the distance from the centroid of tensile reinforcement to the top surface of the structural slab, excluding non-monolithic floor finishes (p. 185). For T-beams, the effective flange width is restricted to one-fourth of the span length for symmetrical beams, with stricter limits for single-sided flanges (p. 185). Ribbed floor construction requires ribs to be no more than 36 inches apart and at least 4 inches wide (p. 186).
Moment coefficients are provided for various support conditions:
- Freely supported beams use for positive moment (p. 187).
- Fully restrained continuous beams use for negative moment at interior supports and for positive moment in interior spans (p. 187-188).
Shear and diagonal tension must be calculated using specific formulas, with web reinforcement (stirrups or bent-up bars) required when shear stress exceeds concrete capacity (p. 189-190). Stirrup spacing is limited to or depending on shear stress levels (p. 190).
Flat Slabs
Flat slabs are defined as concrete slabs without beams or girders, reinforced in two or four directions (p. 193). The design divides panels into “column strips” and “middle strips” (p. 194). Moment distribution varies based on the presence of dropped panels and capital dimensions. For example, in interior panels without dropped panels, the column strip carries 46% of the negative moment and 22% of the positive moment (p. 195). These methods are central to flat-slab-design-methods.
Minimum slab thickness is calculated using formula (20), which accounts for span length, load, and column capital size (p. 196). Reinforcement ratios must not be less than 0.0025, and bar spacing is limited to 1.5 times the slab thickness (p. 197). Special provisions exist for wall panels, marginal beams, and openings in slabs (p. 198-199).
Columns
Columns are classified as short or long based on their unsupported length and radius of gyration. Short columns are limited to 11 times their least lateral dimension (p. 200).
- Spiral Columns: Permissible axial load is determined by 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).
These requirements fall under reinforced-concrete-column-design. Composite columns, encasing steel or cast-iron sections in concrete, 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 must be 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 the edges (p. 204). These structural calculations are part of structural-load-calculations and reinforced-concrete-design.
Related Concepts
- flat-slab-design-methods
- reinforced-concrete-column-design
- reinforced-concrete-design
- structural-load-calculations
- building-inspection-protocol