Insudek Suspended Slab System
INSUDEK RIBBED SLAB
DESIGN METHODOLOGY
- Lipped channels 150/225/300mm wide x 60mm high with Prefixed reinforcement spanning between RC beams, structural steel beams and/or load bearing brickwork are spaced at 650/725/800 c/c – limiting steel fixing on site
- The slab is designed as simply supported or continuous ribs between supports.
- The concrete cover between channel reinforcement and the lipped channel is 20/25mm in accordance with the ruling design codes of practice. Insudek has an accredited two-hour fire rating achieved by using a 35mm concrete cover between the channel and reinforcement eliminating the requirement of a plaster finish to the soffit of the slab.
- Rebated high density, fire retardant solid Polystyrene void formers (non-structural) 400/500mm wide fit between the channels creating a permanent shutter and creating a flat soffit – blocks may be removed or left in place after construction offering excellent inbuilt insulation
- Typically, Y10 reinforcing bars are inserted/lapped along the entire length of each propping line forming cross ribs – stiffening the slab
Reference Codes:
- SANS10100: The Structural Use of Concrete
- SANS10400: Code of Practice for the Application of National Building Regulations
- SAN10160: South African Loading Code
- BS8110/EC2: Structural Use of Concrete (UK)
- +/- 60mm of Structural Concrete incorporating welded mesh fabric ref 100 is cast over the blocks & into the reinforced channels – forming a series of rigid cast in-situ R.C. beams
- A simple prop-bearer arrangement supports the erected components until the concrete is cured – eliminating the requirement of expensive full formwork
- Where Reinforced Concrete Beams are specified, hooked reinforcement in the channels protrudes beyond the length of the channel into the beams. Concrete to ribs, beams & topping are cast simultaneously – creating a sound monolithic structure.
- The composite action between the lipped channel and the concrete rib is a traditional design method utilized in concrete rib slab construction
- Up to 20% of the channel forming the rib can be used as part of the reinforcement by virtue of the bond developed between the channel and the concrete, particularly in the area around the channel lips
- In instances where the soffit of the slab is not plastered, the basic design takes account of the following: The metal sections (lip channel) is treated as only functional to the system and plays no structural role in the event of a fire.
WHEN DESIGNING ONE WAY SPANNING RIBBED SLABS
THE FOLLOWING STEPS SHALL BE APPLIED
STEP 1:
DEAD LOAD CALCULATIONS
This will include the self-weight of the slabs and ribs. Characteristic superimposed loads due to screed, tiles etc. should be included.
Typical Loading Assumptions:
• Floor Finishes (Tiles + Screeding): 1.5 kN/m2
• Ceiling and Services: 0.5 kN/m2
• Dry Partition/Gypsum Walling: 1.0 kN/m2
• 115mm Brick Partition Walling: 2.5 kN/m2
• 230mm Brick Partition Walling: 5.0 kN/m2
STEP 2:
LIVE LOAD CALCULATIONS
The live load might not be given in calculations. Use the necessary code of practice; SAN10160: Part 2 (TABLE 1) South African Loading Code
STEP 3:
ULTIMATE DESIGN LOADS
The ultimate design load is calculated from: @ULS, n = 1.2 Dead load + 1.6 Live load
STEP 4:
DESIGN DATA
Before proceeding with calculations of moments, it is imperative to list down all known design data: strength of concrete, strength of steel used, assumed diameter of main steel, cover. Effective depth shall be calculated.
STEP 5:
BENDING MOMENT AT MID-SPAN
The bending moment at mid-span shall be calculated using formulae or using coefficients in SANS10160.
STEP 6:
CALCULATION OF REINFORCING STEEL AT MID-SPAN
(DESIGN AT T-SECTION)
(DESIGN AT T-SECTION)
Once M is calculated, then the following parameters need to be calculated:
K, z = 0.95d (maximum), As and As min
STEP 7:
DEFLECTION CHECK
At mid-span, Service stress fs need to be calculated. M/bd2, Modification ratio, Basic l/d shall be known, Permissible l/d calculated and Actual l/d calculated. For deflection criteria to be satisfied, the permissible l/d ratio shall be greater than the actual l/d ratio.
STEP 8:
SHEAR CHECK
Maximum shear from the support center-line shall be determined and the shear stress calculated using: Shear stress = V/(bxd) < 5Mpa or formula 8sqrt (fcu) 100As/bd. From table 6 (SANS 10100-01) vc shall be determined.
STEP 9:
TOPPING REINFORCEMENT
Minimum steel shall be provided in the topping = 0.13%bh\
SIZING OF SLABS & RIBS
RIB SECTION – CENTRE TO CENTRE
150mm – 650mm
225mm – 725mm
300mm – 800mm
150mm – 650mm
225mm – 725mm
300mm – 800mm
Ribs’ spacing should be less or equal to 1.5m and their depth below the flange should not be greater than four times their width.
STRUCTURAL TOPPING:
50mm or 1/10th of the clear distance between ribs, whichever is greater, for slabs without permanent structural blocks.
Reinforcement in structural topping shall constitute of a wire mesh.
Download a Typical Insudek Suspended Slab Layout + Drawings
