Joist Floor Systems View of Joist Slab from Below
Joist Floor Systems • Definition: The type of slab is also called a ribbed slab. It consists of a floor slab, usually 5-10cm thick, supported by reinforced concrete ribs. The ribs are usually tapered and uniformly spaced at distances that do not exceed 75cm. The ribs are supported on girders that rest on columns. In some ribbed slabs, the space between ribs may be filled with permanent fillers to provide a horizontal ceiling (ACI 8.13.5).
Joist Floor Systems • Laying Out Pan Joist Floors (cont.) • Typically no stirrups are used in joists • Reducing Forming Costs: • Use constant joist depth for entire floor • Use same depth for joists and beams (not always possible)
Joist Floor Systems • Distribution Ribs • Placed perpendicular to joists* • Spans < 6m.: None • Spans 6-9m: Provided a midspan • Spans > 9m: Provided at third-points • At least one continuous #12mm bar is provided at top and bottom of distribution rib. *Note: not required by ACI Code, but typically used in construction
Member Depth • ACI provides minimum member depth and slab thickness requirements that can be used without a deflection calculation (Sec. 9.5) • Useful for selecting preliminary member sizes • The most economical framing results if joists and supporting beams have the same depth. This will involve beams considerably wider than columns. Such a system is referred to as joist-band system.
Member Depth • ACI 318 - Table 9.5a: • Min. thickness, h • For beams with one end continuous: L/18.5 • For beams with both ends continuous: L/21 • L is span length in meters • Table 9.5a usually gives a depth too shallow for design, but should be checked as a minimum. • Building codes give minimum fire resistance rating: • 1 hour fire rating: 2cm cover, 7.5-9cm slab thickness • 2 hour fire rating: 2.5cm cover, 12cm slab thickness
Member Depth ACI 318-99: Table 9.5a
Block sizes and dimensions, cm Ytong blocks 30 50,55,60 variable Block unit weight=12kN/m3 Light weight concrete unit weight=6kN/m3 Ytong unit weight=5kN/m3
Example Given: fy = 420MPa for all steel and f’c = 24MPa. Floor height = 3.4m c/c of slab Superimposed dead (plaster 15mm: 23kN/m3 , tiles 10mm: 27kN/m3 , mortar 20mm: 23kN/m3 , filling (gravel) 100mm: 18kN/m3 ) Live load = 2.5kN/m3 residential building Perimeter walls are masonry of (masonry 50mm: 27kN/m3 , concrete 130mm: 25kN/m3 , insulation 20mm: 0.3kN/m3 , block 100mm: 12kN/m3 , plaster 15mm: 23kN/m3 ) Design the floor system shown next page as one way joist system
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