Ribbed slabs are made up of wide band beams running between columns with equal depth narrow ribs spanning the orthogonal direction. A thick top slab completes the system. The term “ribbed slab” in this sub -clause refers to insitu slabs constructed in one of the following ways. a) Where topping is considered to contribute to structural strength 1) as a series of concrete ribs cast in-situ between blocks which remain part of the completed structure; the tops of the ribs are connected by a topping of concrete of the same strength as that used in the ribs; 2) as a series of concrete ribs with topping cast on forms which may be removed after the concrete has set; 3) with a continuous top and bottom face but containing voids of rectangular, oval or other shape. b) Where topping is not considered to contribute to structural strength: as a series of concrete ribs cast in-situ between blocks which remain part of the completed structure; the tops of the ribs may be connected by a topping of concrete (not necessarily of the same strength as that used in the ribs).
Providing ribs to the soffit of the floor slab can reduce the quantity of concrete and reinforcement, and thus the weight of the floor. The deeper, stiffer floor permits longer spans to be used. Formwork complexity can be minimized by the use of standard modular, re-usable formwork. When flying form panels are used, the ribs should be positioned away from the column lines. Ribbed slab floors are very adaptable for accommodating a range of service openings. Economic in the range 8 to 12 m.The saving of materials tends to be offset by some complication in formwork. The advent of expanded polystyrene moulds has made the choice of trough profile infinite and largely superseded the use of standard T moulds. Ribs should be at least 125 mm wide to suit reinforcement detailing.
ADVANTAGES
• Medium to long spans • Lightweight • Holes in topping easily accommodated • Large holes can be accommodated • Profile may be expressed architecturally, or used for heat transfer in passive cooling Electrical and mechanical installations can be placed between voids Good resistance to vibrations •
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DISADVANTAGES
• Higher formwork costs than for other slab systems • Slightly greater floor thicknesses • Lower span Only moderate and uniformly distributed load can be accommodated •
SINGLE AND DOUBLE TEE SLABS
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Combination beam and slab Spans up to 120’-0" Typical width = 8’-0" Typical depths of 36" and 48" Designation = 8ST36+2 (8 = width in feet, 24 = depth, +2 = 2" topping)
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Combination beam and slab Spans up to 100’-0" Typical width = 8’-0" Depths of 12", 18", 24" and 32" Designation = 8DT24+2 (8 = width in feet, 24 = depth, +2 = 2" topping)
DOUBLE TEE SLABS
Introducing voids to the soffit reduces dead weight and these deeper, stiffer floors permit longer spans which are economic for spans between 9 and 14 m. The saving of materials tends to be offset by complication in site operations. Standard moulds are 225, 325 and 425 mm deep and are used to make ribs 125 mm wide on a 900 mm grid. Toppings are between 50 and 150 mm thick. The chart and data assume surrounding and supporting down stand beams, which should be subject to separate consideration, and solid margins. Both waffles and down stand beams complicate formwork. ADVANTAGES
• Medium to long spans • Lightweight • Profiles may be expressed architecturally, or used for heat transfer DISADVANTAGES
• Higher formwork costs than for other slab systems • Slightly deeper members result in greater
POST-TENSIONED WAFFLE/JOIST SLAB CONSTRICTION
Where design requirements demand more reinforcement that is generally assigned to a typical interior waffle stem, solid strips along the lines of supports is used to accommodate the excess of reinforcement.
With larger loads and longer spans, such as is common in department stores a heavier solid slab band between the supports accommodates the overage of reinforcement from the individual waffle stems in each direction
ANALYSIS OF A LONG-SPAN WAFFLE SLAB WITH STOUT SOLID SLAB BANDS ALONG THE LINES OF SUPPORTS
HOLLOW CORE SLAB Introduction Hollow core slabs are precast, pre-stressed concrete elements that are generally used for flooring. Some of their advantages are as follows: long spans, no propping; flexible in design; fast construction; light weight structures. The slabs have longitudinal cores running through them, the primary purpose of the cores being to decrease the weight, and material within the floor, yet maintain maximal strength. To further increase the strength, the slabs are reinforced with steel strand, running longitudinally. Hollow core slabs derive their name from the voids or cores which run through the units. The cores can function as service ducts and significantly reduce the self-weight of the slabs, maximising structural efficiency. The cores also have a benefit in sustainability terms in reducing the volume of material used. Units are generally available in standard 1200mm widths and in depths from 110mm to 400mm. There is total freedom in length of units and splays and notches can readily be accommodated. Hollowcore slabs have excellent span capabilities, achieving a capacity of 2.5 kN/m2 over a 16m span. The long-span capability is ideal for offices, retail or car park developments. Units are installed with or without a structural s creed, depending on requirements. Slabs arrive on-site with a smooth pre-finished soffit. In car parks and other open structures, pre-finished soffits offer a maintenance free solution. Prestressed units will have an upward camber dependent upon the span, level of prestress, etc. This will be reduced when screeds/toppings or other dead loads are applied.
Thicknesses of 4", 6", 8", 10" and 12" Spans up to 40’ -0" Standard panel width = 4’ -0" Typical designations = 4HC6 (4 = panel width in feet, HC = Hollow Core, 6 = slab thickness in inches) = 4HC6+2 (2 = 2" of concrete topping added)
VOIDED SLABS Introduction
A relatively new technology developed in Europe has taken the efficiency of cast-in-place flat plate slabs to new heights. Voided slabs have been used in the construction of office buildings in Switzerland, Germany, Austria and the United Kingdom, with floor spans up to 17 meters (~56 feet) and overall slab thicknesses up to 60 cm (~24 inches). These slabs are more efficient than traditional structural floor systems commonly used in the construction of office buildings in the United States. The main effect of the voided slab system is to decrease the overall weight by as much as 35% when compared to a solid slab of the s ame capacity. From a sustainability standpoint, the reduced slab weight also allows the quantity and dimensions of vertical bearing elements, such as columns, to be reduced by as much as 40%. Reduced dead weight also means a smaller deflection of the slab, and provides scope for potential savings in foundation design, including fewer piles and/or reduced length of piles. While the design lowers overall weight, the voided two-way slabs offer very high load-carrying capacity and considerable flexibility. From the developer’s and contractor’s viewpoint, this technology can offer other potential benefits, including direct and indirect cost savings due to reduced volume, lower transportation requirements and easier lifting.
Design Principle The concept centres on removing the non-working concrete dead load while maintaining biaxial strength throughout the slab. This is an essential feature found in the wings of birds. A hard shell with struts formed by multiple cavities, appropriately located, gives the bones a stability that is equivalent to solid bones. The result is a highly efficient structure that has less mass and requires less force to lift. The design principle of these slabs is based on industrially produced spherical hollow shells made from recycled plastic that are inserted into the positioning cage to create modules of several lengths, depending on the application. These cage modules are placed on the lower reinforcing mat, and the upper reinforcing mat is then placed on top of them. The voids in the slab displace non -working concrete with the aim of saving material where it is not required for structural reasons. The voided slab system has the same bearing capacity as conventional concrete solid slabs, and standard design and detailing techniques can be directly applied. However, research performed at a university in Germany has produced several numeric factors that have to be considered to reflect the presence of the void formers. This affects: Dead load Stiffness of the slab Maximum shear stress Also, the positioning cages have a compensating positive effect on the slab’s shear strength, which is impacted by the presence of the voids. In the vicinity of the column, the slab is designed to resist punching shear stresses using a solid cross-section, with additional shear reinforcement as required to maintain a flat soffit throughout the slab.
The shallow profile of the voided slabs is another attribute that offers the opportunity to reduce floor to floor heights. The implication is the potential addition of rentable floor space, or conversely a reduction in energy requirements along with cost savings to the structure and associated building systems such as cladding, elevators, fire protection systems, heating and air conditioning requirements. Earthquake resistance is another major benefit of this system. During an earthquake event, the accelerated mass of the building creates seismic forces that have to be absorbed by the vertical elements of the structure. The reduced dead weight results in lower force demands on the structure, with associated savings in detailing and constructability requirements. Voided slabs can also be coupled with post-tensioning to minimize dead load deflections further, while still maintaining the same light weight and biaxial attributes. One of the main benefits of post-tensioning is to obtain a slab that is "almost" free of cracks and deflection at the service load level. As a result, the slab is stiffer, since the full cross-section rigidity is available to resist the applied loads.
Biaxial capacity Larger spans without beams Larger open floor areas Lower floor to floor heights Earthquake resistance Resource efficiency
