GB2328455A - Concrete slab shear reinforcement - Google Patents

Abstract

A reinforced or post-tensioned concrete slab is provided with radially-directed shear reinforcement at column/slab intersections. The reinforcement comprises spaced slices of Universal Beams, interconnected by horizontal bars welded to the slices. The slices may be arranged in a single plane, or with their sections arranged transversely of the bars.

Description

PREFABRICATED I-RAILING Refer to the first application no GB 9617635.9 of 21 August 1996 and second application no GB 9618198.7 of30 Aug. 1996.

This invention relates to prefabricated I railings for reinforced concrete flat-slab and prestressed slabs, using normal or high strength concrete. Prefabricated I railings are made of horizontal web reinforcement welded towards the centre ofthe web of vertical steel I section (slice ofUB).

This assembly of two horizontal bars and several vertical steels ofI section (slices of UB), are placed radially or as a cross in reinforced concrete or prestressed slabs within proximity of column. The result is an improvement of shear resistance of the slab against punching shear.

In modern building design flat slab arrangements are generally recognised as the most economical form of construction. Traditionally to resist punching shear, drops and column heads(capitals) are used Prefabricated I railings produce savings in costs of steel work and form work as the result of elimination of drop beams and/or capitals at support locations.

The optimum design of flat reinforced concrete or prestressed concrete slabs, is often compromised by their ability to resist punching stresses at column and other support locations. The main reinforcement, designed to resist the bending forces applied to the slab, is ideally positioned at the maximum permissible lever arm within the concrete member taking into account due regard to the cover requirements.

In practice , on two way spanning slabs , to avoid problems with cover the shear reinforcement can only be anchored over the lower of the two top layer of reinforcement which, as detailed in many international design codes, creates problems with the effective anchoring ofthe shear reinforcement.

With the introduction or prefabricated I-Railing all these criteria can be accomplished.

Prefabricated I-Railing can result to elimination of 'drop panels' and 'column heads' in construction of reinforced or prestressed concrete flat- slab panels of various thickness with long spans for residential and commercial buildings.

The term 'flat slab' means areinforced concrete slab with or without drops and supported, generally without beams, by columns with or without column heads. The slab may be solid or may have recesses formed on the soffit so that the soffit comprises a series of ribs in two directions.

The term 'prestressed slab' means a post tensioned concrete slab.

A 'drop' is a local thickening of the slab in the region of a column Where recesses are formed on the soffit, it will usually be necessary to make the slab solid in the region surrounding the column heads in order to provide adequate shear strength A column head or Capital' is a local enlargement of the top of the column providing support to the slab over a larger area than the column section alone. The head may be of uniform cross-section or may be tapered generally referred to as flared Cigh strength concrete is a concrete of constituents such as silica fUme, super plasticiser and very low water/cement ratio.

'Horizontal web steel as shear reinforcement' is an alternative form of web reinforcement.

Central bars bonded with surrounding normal or high strength concrete providing a strong core resisting the progress of shear crack into the compression zone acting as a dowel across the crack to resist the rotation about its tip.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying two pages of drawings which show prefabricated I-railing for reinforced concrete and prestressed concrete slabs One page of photocopy of a photograph shows a model for I-railing.

Two horizontal shear web reinforcements welded near the centre of the web of the steel l-section( 1). Slice of UB with web cross sectional area providing vertical shear steel ( 2). Holes providcd to nail the railing to formwork. ( 3) Layout of railings in proximity of columns for prestressed concrete slab (4). Slices of UB(S).

Two holes to nail the railinig to formwork (6) Section of railings to resist punching shear for prestressed slab (7). Slice of UB with web cross sectional area providing vertical shear sted.(8). Elevation of stud rail to resist punching shear for prestressed slab (9) Two horizontal shear web reinforcements welded near the centre ofthe web of the steel l-section(l0). Slice of UB with web cross sectional area providing vertical shear steel (11). Holes provided to nail the railing to formwork( 12). Two reinforcing bars. Slices ofUB(l4). Two holes to nail the railing to formwork (15) Elevation of railings to resist punching shear for reinforced slab (16). Section of railings to resist punching shear for reinforced slab (17).

Claims (1)

1) Prefabricated I-railing is made of two horizontal web reinforcement bars of designed size diameter welded towards the centre ofthe web of a slice of designed UB providing vertical shear steel area equivalent to the cross sectional area ofthe web ofI (UB).

2) Prefabricated I-railings as claimed in Claim 1. These railings are placed within proximity of the column radially or as a cross with the centre approximately coinciding with the centre ofthe column 3) Prefabricated I-railings as claimed in Claim 1 or Claim 2. used in slabs made of steel cage or prestressed tendons placed in normal strength or high strength concrete.

4) Prefabricated I-railings as claimed in Claim 1, Claim 2 or Claim 3, improve punching shear resistance ofthe concrete slab while resulting to more cost efficient installation of a slab column connection This is due to reduction in labour and elimination of expensive form work of slab drop, column capital, and cumbersome fabrication of stirrup cages.

5)Prefabricated I-railings as claimed in Claim 4, prevent brittle punching shear failure by providing the ductility that is particularly essential in seismic zones.

6)Prefabricated I-railings claimed in Claim 5, meets the criteria and recommendation of international Design Codes ofPractice.

7) Prefabricated I-railings as claimed in Claim 6. Where the flange of the I section is parallel to the horizontal web reinforcing bars. This railing is more desireable in prestressed concrete slabs because its flanges do not block access for the post-tensioning tendons.

8) Prefabricated I-railings as claimed in Claim 7. Where railing is placed within proximity of the column as a cross with the centre roughly coinciding with the centre ofthe column in the normal strength or high strength concrete slab.This layout of railing is preferable to raidial layout of railing because it does not block access for the post-tensioning tendons.

9) Prefabricated I-railings as claimed in Claim 6, by using central horizontal bars that act independently to improve resistance of the slab against punching shear results to offseting sone of I sections (slices of UB), such combination could ease congestion of I sections (slices of UB), improving detailing of reinforcement.

10) Prefabricated I-railiny as claimed in claim 9, improve fire resistance of the building since links or studs or I sections which form the vertical shear steel requirement, are vulnerable to high temperatures under fire exposure conditions, as they are placed close to the sXce.

Two horizontal central bars protected from fire by the surrounding concrete could provide a larger share of the shear resistance at elevated temperature than at normal room temperature.