Reinforced Concrete Design Standards
reinforced-concrete-design encompasses the regulations for the quality, design, and construction of structures using reinforced concrete, as detailed in Chapter 26 of the 1935-miami-dade-building-code. These standards are intended to supplement general building code provisions, governing matters where specific regulations conflict with other code sections (p. 166).
Scope and Special Systems
The code covers all reinforced concrete structures erected under its provisions. Special systems of reinforced concrete that have been successfully used or proven by test, but whose design conflicts with or is not covered by standard provisions, may be presented to a “Board of Examiners for Special Construction” board-of-examiners-and-appeals. This board, composed of engineers, architects, and builders, has the power to investigate data and formulate rulings with the same force as the code (p. 166).
Materials and Testing
Materials must conform to standards set by the american-society-for-testing-materials (ASTM). Portland cement must meet Specification C9-30 (p. 171). Aggregates must be clean, durable, and free from excessive deleterious materials (p. 171). Metal reinforcement must conform to Specifications A 15-35 for billet-steel or A 16-35 for rail-steel (p. 171-172). The building-inspector has the authority to require tests for materials and completed structures. Load tests on completed structures involve applying a superimposed load equal to 1.5 times the live load plus 0.5 times the dead load for 24 hours (p. 170).
Concrete Quality and Stresses
Concrete quality is primarily determined by the water cement ratio. For average materials, specific water-cement ratios correspond to assumed 28-day compressive strengths, ranging from 1,500 to 3,000 psi (p. 172-173). When established by test, the water-cement ratio must yield a strength 15% higher than the minimum required (p. 173). Allowable unit stresses in concrete depend on the 28-day strength (). For instance, extreme fiber stress in compression () is limited to (p. 176). Allowable stresses in reinforcement include 20,000 psi for tension in intermediate grade billet steel (p. 177).
Construction Practices
Concrete mixing must ensure uniform distribution and color, with mixing continuing for at least one minute after all ingredients are added (p. 178). Concrete must be placed continuously and thoroughly compacted (p. 179). Curing requires keeping exposed surfaces moist for at least 7 days (p. 179). In cold weather, concrete must be maintained at at least 50°F for 72 hours (p. 179). Forms must be substantial and tight, and reinforcement must be free from rust and scale before placement (p. 180-181). Protective concrete cover for reinforcement ranges from 1 inch in slabs to 3 inches in footings (p. 181). Construction joints must be located to minimize strength impairment, with horizontal joints requiring cleaning and roughening of the hardened surface (p. 181).
Design Assumptions and Beam/Slab Design
Design is based on working stresses and safe loads, assuming steel takes all tensile stress (p. 181). The ratio of modulus of elasticity of steel to concrete () is defined as (p. 182). The code provides extensive notation for design variables, including span lengths, moments, and reinforcement areas (p. 182-184). Design loads must account for all dead and live loads, with wind loads allowing a 50% increase in allowable unit stresses (p. 184).
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, 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 Slab Design
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).
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).
Column Design
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).
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).
Footing Design
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).
Code Updates and Adoptions
Ordinance No. 3703 (1949) significantly updated the reinforced concrete provisions by formally adopting the aci-318-47 (Building Code Requirements for Reinforced Concrete) dated September 1947 from the american-concrete-institute (p. 336). This adoption standardized design practices with national engineering guidelines. Additionally, the ordinance specified detailed reinforcement requirements for concrete columns in masonry walls, requiring a minimum width of 12 inches, four 5/8-inch continuous vertical rods, and 1/4-inch ties spaced on 12-inch centers, explicitly prohibiting the use of U-Type Blocks for reinforced concrete columns (p. 336).
Related Documents
- 1935-Miami-Dade-Building-Code_p325_344
- 1935-Miami-Dade-Building-Code_p185_204
- 1935-Miami-Dade-Building-Code_p165_184
- 1935-Miami-Dade-Building-Code_p325_344
See also: 1935-Miami-Dade-Building-Code_p205_224