Terminology

• Longitudinal bars: main vertical column bars or horizontal beam bars
• Ties / spirals: transverse reinforcement confining column bars
• Stirrups: transverse beam reinforcement for shear
• Drop panel / column capital: thickened slab or flared column top in flat-plate systems
• Flat plate / flat slab: two-way slabs on columns without beams
• One-way joist / pan joist / waffle slab: ribbed floor systems formed with pans or domes
• Post-tensioning: tensioned tendons in slabs and beams to reduce depth and cracking
• Special moment frame: ductile concrete frame detailing for high seismic regions
• Reshoring: supports left under new floors until concrete gains strength

Standards

Ratings

• Design per ACI 318-19 as referenced by the IBC; seismic detailing by frame category
• Fire resistance from member size and cover, per ACI 216.1 or tested designs
• Higher-strength concrete in columns reduces column size in tall buildings

Test standards

• ACI 318: structural concrete
• ACI 301: specifications for structural concrete
• ACI 117: tolerances
• ACI 216.1: fire resistance of concrete members
• ASTM A615 / A706: reinforcing bars
• PTI specifications: post-tensioned concrete

Materials

• Concrete: normal-weight; high-strength mixes for columns; lightweight for slabs
• Reinforcing: ASTM A615 bars; A706 where welding or seismic ductility applies
• Post-tensioning tendons: unbonded monostrand in slabs; bonded in some beams
• Formwork: flying tables, column forms, pan and dome forms

Detailing

• Keep column sizes and grid regular for reusable formwork
• Coordinate beam depths with ceiling and MEP; flat plates minimize floor-to-floor height
• Check reinforcing congestion at beam-column joints
• Plan slab penetrations early; post-tensioned slabs restrict coring
• Allow for long-term creep and shrinkage at facade connections

Uses

Specific uses

• Residential towers and hotels (flat plate)
• Parking structures
• Hospitals and laboratories needing stiff, vibration-resistant floors
• Podium levels below wood-framed buildings
• High-seismic and high-wind buildings with concrete frames or cores

Pros and cons

Pros

• Inherent fire resistance
• Stiff, quiet, low-vibration floors
• Thin flat-plate floors reduce building height
• Monolithic, durable, and moldable

Cons

• Slower schedule with forming, curing, and reshoring
• Heavy, increasing foundation and seismic loads
• High embodied carbon unless mixes are optimized
• Difficult to modify after construction, especially with post-tensioning

Manufacturing and cost

Construction

• Forms set; reinforcing tied and inspected; concrete placed and cured
• Floors cycle every few days with reshoring

Main manufacturers and suppliers

Formwork and accessories:

• PERI USA
• Doka USA
• EFCO
• Dayton Superior

Trade associations: American Concrete Institute and Concrete Reinforcing Steel Institute.

Typical costs

• Ready-mixed concrete alone: about $110 to $200 per cubic yard
• Formwork, reinforcing, and labor usually exceed the concrete material cost

Sources

• ACI: concrete standards, https://www.concrete.org/
• PCI: designer's notebook on fire resistance (ACI 216.1), https://www.pci.org/PCI_Docs/Publications/Ascent Magazine/2014/Summer/Designers-Notebook-Fire-Resistance.pdf
• UpCodes: 2024 IBC Chapter 19 concrete, https://up.codes/viewer/department-of-defense/ibc-2024/chapter/19/concrete
• Lawn Love: concrete cost, https://lawnlove.com/blog/concrete-cost/
• Latest Cost: concrete footing cost per cubic yard, https://latestcost.com/concrete-footing-cost-per-cubic-yard/
• Wikimedia Commons: Concrete designers' manual; tables and diagrams for the design of reinforced concrete structures (1921) (14765089492).jpg, by Hool and Whitney (Internet Archive Book Images) (No restrictions), https://commons.wikimedia.org/wiki/File:Concrete_designers'manual;tables_and_diagrams_for_the_design_of_reinforced_concrete_structures(1921)(14765089492).jpg