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Transcript of Ch14 Slabs
13.14.2 Lapped Splices Lapped splices in longitudinal reinforcement,
located in a region of tension or reversing stress, are to be confined by a minimum of two closed ties at each splice to inhibit the possibil ity of non-ductile failure at this point. The position of maximum moment under seismic load will be dependent upon the magnitude of the earthquake. (Figure 13.11).
The position of the splice should therefore be located at a position of known moment, perhaps in the middle third of the span , unless the designer is confident that the splice is sufficiently confined to safely locate it elsewhere in the span.
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Figure 13.11 Localities of Plastic Hinges when Stirrups are Required. Note: Plastic Hinges will Form when the Flexural Capacity Envelope and the Actual Moment Coincide
13.14.3 Detailing for Shear Shear type failures tend to be brittle. Also,
as mentioned above, maintaining a stable hysteretic response of plastic hinge regions requires that the compression bars be prevented from buckling. It must therefore be assumed that major spall ing of concrete cover will occur and that compression bars must rely solely upon transverse support provided by the ties. Limitations on maximum tie spacing are required to ensure that the effective buckling length the compression bars is not excessive and that concrete within the stirrup ties has reasonable confinement. Furthermore, due to the possible occurrence of the Bauschinger effect and the reduced tangent modulus of elasticity of the steel, a smaller effective length must be considered for bars subject to flexural compression, rather than compression alone. Appendix A of AS 3600 specifies a minimum area of shear reinforcement:
Asy ~ 0.5bw s/fsy.f (ie 50% greater than stipulated in the body of
the Code) with closed ties provided over a minimum distance of 20 from the face of the support. The first placed 50 mm from the support face, and the remainder spaced at 0.25do, Bdb, 24df or 300 mm, whichever is least.
Where: bw = width of web. s = centre to centre spacing of ties. fsy.f = yield strength of ties. o = overall depth of cross-section in the
plane of bending. do = the distance from the extreme
compression fibre of the concrete to the centroid of the outermost layer of tensile reinforcement, but not less than O.BO.
db = the diameter of the smallest longitudinal bar enclosed by the tie; and
df = the diameter of the bar forming the tie. Since tension in vertical tie legs acts
simultaneously to restrict longitudinal bar-buckling and to transfer shear force across diagonal cracks, it is considered that the tie areas are sufficient to satisfy both the requirements for bar buckling and those for shear resistance. See Figure 13.10.
(Note: These requirements do not preclude efficient fabrication techniques such as loose bar detai ling described elsewhere in this manual).
Suspended Slabs and Slab Systems
14.1.1 Purpose The term 'slab' is generally thought of as a
floor, although it is equally applicable to a roof or other member whose structural behaviour is the same as a slab.
The width and length of a slab are much greater than the depth.
Figure 14.1 shows various slab types. The slabs transfer the floor loads to the supporting beams, walls and columns and ultimately by footings to the foundations. The lowest level floor slab may transfer its load directly to the ground.
Some of the terms used with slab design are: Single span or multiple span slabs - determined
by the number of supports. One-way or two-way slabs (see AS 3600
Clause 1.6.3) - depends whether the slab is supported on two opposite sides or on all four sides.
Combinations of the above. Beam-and-slab systems - where the slab is
supported by the beams and becomes the flange of the T-beam, L-beam or band-beam.
Solid slab - supported by columns without the need for beams. Variations are flat plates in which the slab is of uniform thickness throughout, and flat slabs where drop panels thicken the slab for some distance around the column.
Ribbed slabs which consist of narrow beams or ribs at close centres and a very thin slab above. Ribbed slabs can be one-way or two-way (waffle slabs) in Figure 14.1.
Hollow core slabs are floor units, precast and prestressed.
Concrete soffit-slabs - where the positive moment reinforcement is included in the precast permanent-formwork soffit-slab.
Precast Tee-beams and extruded pretensioned beams.
14.1.2 Description of Method of Load Carrying by Slabs
Bending. Slabs carry the applied loads either as one-way or two-way bending. Once the magnitude of the bending moments is calculated (the analysis), the design of the cross-section is similar to that for beams. Therefore, as for beams, slabs must be capable of resisting both positive and negative bending moments.
Shear. Shear forces cause complex shear stress effects on slabs. Spandrel beams and torsion strips may be needed. For flat slabs and flat plates, punching shear around the column support requires careful attention by the designer.
Torsion . Torsional forces on slabs are treated with the shear stress analysis. The most critical torsion effects occur at spandrels (edge beams) which can require closed-ties to be used as the fitment. Torsion steel may also be required in the corners of slabs on walls. This is to resist stresses caused by the slab trying to lift itself off its supports. Without torsion steel here, slabs can crack diagonally across the corner.
14.1.3 Slab Reinforcement - the Meaning of "Grids" Suspended slab reinforcement is placed in one
or two grids of steel - one always near the bottom surface and one, if needed, near the top surface.
Each grid consists of two layers, usually at right angles. Thus there are four layers of steel.
In AS 3600, for strength purposes the term positive moment reinforcement refers to the bottom steel, and negative moment reinforcement means the top steel.
Slabs may also be post-tensioned in one or two directions, but this is outside the scope of this Handbook.
14:1 Reinforcement Detailing Handbook
Note: Slab may span one-way or two-way
BEAM AND SLAB
Drop panels typically one-third of span. Slab spans two-way
Note: Ribs may be two-way (known as 'waffle' slab)
Figure 14.1 Slabs and Slab Systems used in Buildings
14:2 Reinforcement Detailing Handbook
Note: Slab spans one-way
BAND BEAM AND SLAB
Note: Slab spans two-way
14.2 AS 3600 REQUIREMENTS (Clauses 9.1, 9.2 and 9.4) Most of AS 3600 Clause 9 refers to
reinforcement. Slab reinforcement detailing is generally controlled by "deemed-to-comply" rules. The amount of flexural steel is calculated similarly to beams, and is then evenly distributed across the width of the slab. Slab steel areas are therefore stated as area per unit width (mm2/m). See Chapter 4 for values of steel areas. See Clause 9.5.3 for calculation of numbers of bars in slabs.
14.2.1 Minimum Steel for Bending Strength (AS 3600 Clause 9.1.1) Table 14.1 gives the minimum steel ratio
Ast/bd for mesh and bar to be evenly-distributed in each direction as the bottom grid of reinforcement.
AS 3600 Clause 9.4.1 gives the maximum spacing of reinforcement for crack control due to flexure as the lesser of 2.00 or 300 mm.
Table 14.1 Minimum tensile reinforcement for strength fsy = 500 MPa
Bar or mesh area, Ast Slab support condition (mm2/m) Supported by columns 0.24 (0/d)2fctf/fsy Supported by beams or walls 0.19 (D/d) 2 fctf/ fsy
Example 14.1 A 150 mm thick two-way slab is supported by walls with a concrete characteristic strength of 32 MPa. Allowing for cover of 20 mm plus 5 mm for bar thickness, the effective 'd' would be 125 mm. Thus, the minimum mesh area would be 235 mm2/m (L8 @ 200 or SL82) and for steel 235 mm2/m (N12 @ 400).
14.2.2 Special Requirements for Two-Way Flat Slabs and Flat Plates (AS 3600 Clause 9.1.2) At least 25% of the design total negative
moment MUST be resisted by reinforcement and/or tendons within a width of (bcol + 2tslab) for flat plates and the drop panel width plus the width of the column centred over the supporting columns. (AS 3600 Clause 9.1.2).
Oetailers must check that adequate room is left for concrete placement. Tendons can be concentrated within this strip whilst reinforcement remains uniformly distributed.
Normal beams, band-beams and their associated slabs are not required to comply with this rule.
Also, with draped post tensioned cables which are not directly over a column, these can induce local bending and shear in slabs at columns which the designer must consider.
14.2.3 Detailing of Tensile Reinforcement in Slabs (AS 3600 Clause 9.1.3) Of all parts of a building, slab detailing is the
most time-consuming. This is caused more by the complexity of the shape and layout of slabs than by the detailing requirements of the Standard.
AS 3600 divides detailing of slab systems into two major procedures.
(a) Where the bending moment envelope is calculated. It is compiled from the "worst case" situations of bending, and is not just the bending moment diagram for just one worst-case loading arrangement. In fact, the envelope can contain segments which come from normally quite incompatible loading arrangements. Only calculations can define it.
(b) Where the bending moment envelope has not been calculated. Appropriate "deemed-to-comply" methods can be used. These are based on slab type, support c