SUSPENDED FLOORS : concrete
posttensioning:
- performed on site
- tension the steel tendons after the concrete is cast
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : concrete
precast concrete systems:
-
precast slabs, beams, tees, one-way spanning units
cast and cured in a factory / plant
transported as components to site
set in place on site
-
prestressed for greater structural efficiency
-
supported by
walls: insitu concrete / precast concrete / masonry loadbearing
frames: steel / insitu concrete / precast concrete
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : concrete
-
advantages
less depth
reduced weight
longer spans
-
disadvantages
modular nature not suitable for irregular building shapes
generally standard sizes
all service opening must be predetermined
cannot chase or core into slab
(unless first analysed by engineer although still risky)
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : concrete
hollow core slabs:
- one-way spanning units
- placed on in-situ or precast concrete beams /
steel beams / masonry
- cast in the factory with
- smooth finish for painting or floor finish
- delivered so site
- positioned with cranes
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : concrete
-
precast ribs with infill blocks:
ribs with blocks are one-way spanning units
supported on
in-situ or precast concrete beams
steel beams
masonry walls
infill blocks
made of concrete or clay
hollow or solid
cast in the factory
delivered to site
positioned without cranes
reinforced concrete topping cast over
forms part of the structure
CONSTRUCTION AND DETAILING 1
steel
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : steel
-
steel beams or joists to support
metal decking
precast concrete slabs
-
light-gauge steel joists
light, stable, span long distances
easily assembled
lightweight, non-combustible
floor structure
laid out at certain spacings
depending on loads of floor
spans:
150x40mm between 3m – 4,2m
300x65mm between 5.4m – 8m
-
covered with
rc concrete over metal decking
-
timber subflooring
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : steel
-
metal decking
corrugated
increase stiffness and spanning
acts as formwork for insitu concrete slab
decking panels secured with studs
welded or rivetted through to the beams
panels fastened to each other with screws / welds / seams
types
steel beam decking
open web steel joist decking
-
types of steel beam decking:
form decking
composite decking
cellular decking
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : steel
form decking:
- permanent formwork
for reinforced concrete slab
until slab can support itself and live load
composite decking:
- tensile reinforcement for concrete slab
bonded to it with embossed rib patterns
- composite action achieved
between slab and beams
welding shear studs through decking
onto beam below
cellular decking:
- welding corrugated sheet to flat sheet
- series of concealed spaces formed for
electrical and communication services through floor outlets
acoustic ceiling when filled with glass fibre
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : steel
open-web joist framing:
-
floor deck may span across joists
(spacing dependent on floor load)
-
floor deck comprised of
metal decking with concrete fill
precast concrete panels
plywood panels / timber planking
-
horizontal bridging
prevent lateral movement of joist chords
-
spaced between 3m – 6m
CONSTRUCTION AND DETAILING 1
timber
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
joists / beams to support
planking / decking
subflooring
-
joists / beams supported by
posts
loadbearing walls
-
wood joist floors
form a level platform with wood panel sheathing or sub-flooring
combustible: rely on flooring and ceiling for fire resistance
susceptible to decay and insects: drainage and ventilation
may be exposed (without ceiling) or concealed (with ceiling)
if exposed
susceptible to sound transmission
-
lacks concealed space for insulation, service reticulation
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
joists supported by
wood stud framing
wood or steel beams
loadbearing masonry
concrete wall
-
lateral support for joist ends
rim joist or header
additional support
double end joists
-
-
stiffness of joist framing is more important than its strength
deeper joists spread further apart better stiffness THAN
shallower joists closer together
-
spans dependent on loads, joist size, type of timber
depth: span/16 (rule of thumb)
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
spacing: approx. 400mm centres
spacing cavities for piping, services, insulation
-
sizes and span ranges: approx. 3 – 5m
150x50mm up to 3m
200x50mm between 2.4m – 3.6m
250x50mm between 3m – 4.2m
300x50mm between 3.6m– 5.4m
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
finish to floor structure
floor finish applied over timber plank sub-flooring
underside may be left exposed
ceiling applied directly to joist or suspended below
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
types:
subflooring
decking
timber subflooring:
- closely spaced joists with timber panel subflooring over
- joists
openings or cuts may be made for services
supported on load-bearing walls
wrapped in dpm, built into the corbelled brickwork wall
fixed to steel brackets bolted onto concrete walls
bridging between joists
-
stiffen the floor construction
metal or timber cross bracing OR
full depth blocking at 2.4m intervals
omitted if joist ends are supported laterally against rotation and
edges restrained by subflooring
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
-
subflooring
material that spans across floor joists
working platform during construction
provides a base for finish flooring
typically plywood
underlay and screeds
-
impact (load) resistance
sound insulation
smooth surface
for flooring material
decking:
- types:
timber planking
plywood
stressed-skin panels
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : timber
timber planking:
timber planks laid across spaced joists
laid perpendicular to joists / beams
-
solid or laminated strip flooring
plywood:
laid perpendicular to joists / beams
30mm thick over 1.2m span
laid continuously over 2 spans
tongue and groove edges
stressed-skin panels:
- plywood facing bonded to distribute loads
with adhesive
-
to solid timber stringers and cross bracings
integrated into single component with:
thermal insulation, vapour retarder, interior finish
CONSTRUCTION AND DETAILING 1
SUSPENDED FLOORS : sources
1. Ching, F.D.K (2008) Building Construction Illustrated 4th edition, John Wiley & Sons:
New York.
Chapter 4: Floor Systems
Chapter 5: Wall Systems
2. Grobbelaar, A (2010) Building Construction & Graphic Standards, Anglo-Rand
Publications: South Africa.
CONSTRUCTION AND DETAILING 1