Fabric reinforcement in slabs
(a) General. Two-directional reinforcement can be factory welded and fabricated into sheets to help speed fixing and achieve economy in construction costs. BS 4466: 1981 defines three types of fabric:
(i) designated (standard mesh) fabric section — stock sheet sizes are 4·82·4 m; these can be reduced by cutting to suit. Wire sizes range up to 12 mm with standard 100/200 mm meshes. Peripheral wires are welded at 12pitch from the edge of the sheet
(ii) scheduled (non-standard) fabric — wire sizes (maximum 12 mm) and sheet sizes can be varied. Wire pitches must remain constant but may be non-standard. Wire projections at edges may vary.
(iii) detailed (purpose-made) fabric — these sheets can be specified using standard reinforcing bars. These bars can be set at varying pitches and edge projections. Sheet sizes can vary with due consideration given to handling and transportation.
(b) Suspended solid floor construction. For clarity on plan it is recom-mended that the top sheets of fabric be drawn separately from the bottom sheets, preferably on the same drawing. Fabric is identified as a chain double-dashed line. Fabric detailing on plan. Each individual sheet is given a mark number and related on the plan to the concrete outline.
Indicate the direction of the main reinforcement and its layer notation.
Wherever multiple sheets of identical marks occur they can be combined as shown.
Areas of reinforcement can be increased by double ‘layering’. Structural mesh type ‘B’ is often specified for suspended slabs, possibly with the addition of loose bars. With reasonable production runs, consideration should be given to specifying ‘purpose made’ fabric. For each fabric mark indicate its reinforcement in a table alongside the plan. Minimum reinforcement requirements are shown in laps in fabric. The need for laps should be kept to a minimum and, where required, should be located away from regions of high tensile force. Allow sufficient clearance to accommodate any ‘multilayering’
of sheets at laps, reducing these occurrences where possible by ‘staggering’
sheets.
Voided-slab construction
A nominal designated fabric is normally placed within the topping of trough and waffle-type floors. The extent of the fabric is shown by a diagonal on the plan of the reinforcement drawing and the fabric type scheduled as gross area in m2by adding a suitable percentage to the net area of the floor to allow for laps. For ordering purposes, the contractor should translate this gross area into the quantity of sheets required to suit this method of working.
Ground-slab construction
The presence of fabric reinforcement can be indicated by a sketch and a prominent note on the drawing. This can be the general-arrangement drawing (in straightforward cases). The note should include the type of fabric, location within the depth of slab and minimum lap requirements. A typical section to clarify this construction should be included. The fabric type is scheduled as a gross area by adding a suitable percentage to the net area of slab to allow for laps. For ordering purposes, the contractor should translate this gross area into the quantity of sheets required to suit his or her method of working.
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Beam and slab arrangement
In typical steel beam-slab composite constructions, connectors are used between the concrete slab and steel beams. Several types of connectors are used for this type of construction. Sometimes steel beams are encased in concrete and the bressumers, as they are known, are monolithic with concrete slabs. A brief summary is shown on Sheet No. II.17.
In reinforced concrete building construction, every floor generally has a beam/slab arrangement and consists of fixed or continuous one-way or two-way slabs supported by main and secondary beams. Sheet No. II.18(a) shows such an arrangement. The usual arrangement of a slab and beam floor consists of slabs supported on cross-beams or secondary beams parallel to the longer side and with main reinforcement parallel to the shorter side. The secondary beams in turn are supported on main beams or girders extending from column to column. Part of the reinforcement in the continuous slabs is bent up over the support, or straight bars with bond lengths are placed over the support to give negative bending moments. In large slabs, separate reinforcement over the support may be necessary. This is also demonstrated in Section I. A typical one-way continuous slab/beam arrangement is given in Sheet No. II.18(b) for the general arrangement given in II.18(a).
A flat slab, as discussed earlier, if supported directly on and built monolithically with columns, may differ from a two-way slab in that it is not supported on beams. The slab may be of uniform thickness supported on simple columns. Generally the slab around the columns is thickened in order to provide columns with flared heads, known as drops. The drop stiffens the slab over the column and reduces the shear stress and the reinforcement.
Codes also recommend the distribution of bending moments between column strips and middle strips as shown in Section I. A great deal of research has been carried out on flat slabs without drops. Flat slabs without column drops and with drops are respectively detailed on Sheet Nos II.19 and II.20.
Continuous slabs with mesh fabric are given on Sheet No. II.21. Ribbed slab panel with reinforcement details are given in II.21(iii).
Sheet No. II.22 gives a reinforcement layout for a simple panel under missile impact. In Section IV, additional structural detailing is demonstrated for beam/slab column arrangements.