DETAILED DESIGN

8.10.1 Redistribution of Moments (only applicable for structures designed to SS CP 65)

Redistribution of moments is applicable only for structures designed to SS CP 65 at the ultimate limit state. It is subject to the stipulation of Clause 8.9.3 above. Clause 3.2.2 of Part 1 of SS CP 65 applies except that redistribution is limited to 10% and the value given in condition 3 shall read 90%.

8.10.2 Design Moments

In analysis of continuous and rigid frame members, distances to the geometric centers of members shall be used for the determination of moments. Where members are integral with (i.e. monolithic with) their supports, the design support moment may be taken at the face of the support. Where members are not designed integrally with their supports, the moments at supports shall be taken as the centre-line peak moments but may be duly reduced to allow for the effects of the support widths. However this reduction shall not exceed 10% of the centre-line peak moment.

Where haunches are provided on the compressive face, the portion of haunch defined by a slope of 1(perpendicular to the member axis) : 3(parallel to the member axis) shall be considered effective for determining section capacities. To maximise the space available inside the structure for services etc. square and rectangular haunches shall not be used.

8.10.3 Bottom Loaded Structural Elements

Further to SS CP 65: Part 1 Clause 3.4.5.11, where a load is applied at the bottom of any structural element (beam, slab etc) additional reinforcement shall be provided to carry the full load to the far face of the section where it shall be positively anchored by steel plates or hooks.

8.10.4 Internal facing of Diaphragm and Secant Pile Walls

Irrespective of whether or not composite action is assumed, the internal facing (see Clause 8.16.1(f)) of diaphragm walls, secant piled walls and the like shall be designed for the full hydrostatic loads.

When composite action is assumed reinforcement ties anchored into the remote faces of the two walls shall be provided to prevent interface separation and slippage under full hydrostatic pressures. The amount of reinforcement ties required to prevent separation shall be additional

8.10.5 Fixings for E&M Equipment

Fixings in the form of cast-in sockets or formed or drilled holes for expanding bolts or sockets shall be positioned and sized to suit the requirements of the System-wide Contractors.

The tolerance on fixing positions shall be ± 5mm with the spacing between any two adjacent fixings not differing by more than 6mm from the intended value.

To warn against impending failure, fixings shall be installed deep enough to ensure that brittle failure of the base material or pullout does not occur before yield of the fixing.

8.10.6 Post Fixed Reinforcement

To satisfy ductility requirements of reinforced concrete design post fixed reinforcement shall be installed deep enough to ensure that brittle failure of the base material does not occur before pullout or yield of the reinforcement fixing, irrespective of the magnitude of load.

8.10.7 Connections between Bored Tunnels / Cut-and-Cover Structures Where bored tunnels are connected to cut and cover structures, the connection shall be designed so that completion of the joint is carried out by the contractor for the cut and cover structure. Design of the joint shall consider the possibility of differential movement, during backfilling or subsequently. Unless it can be shown that differential movement of the bored tunnel and cut and cover structures will be sufficiently small not to cause overstressing with a rigid joint, the joint shall be designed to permit an appropriate degree of articulation. Particular attention shall be paid to the waterproofing detail, to ensure that the watertightness of the joint is not in any way inferior to that of the standard joint between precast tunnel segments.

8.10.8 Pile Foundations and Deep Foundation Elements Refer to Chapters 6 and 9.

8.10.9 Torsion (only applicable for structures designed to SS CP 65) Notwithstanding SS CP 65: Part 1 Clause 3.4.5.13 and subject to the prior acceptance of the Engineer, the torsional stiffness of a member may be ignored in the analysis and torsion disregarded in the design for that member only when each of the following requirements are satisfied:

(a) It can be demonstrated that the torsional strength of the member is not required to achieve equilibrium of the structure.

(b) It can be established through sound engineering judgement that the structural system is self-evidently not one in which significant torsional effects will tend to occur.

(c) Links provided to resist shear are closed type complying with shape code 79 of BS 4466 or otherwise detailed so that they are fully developed around the circumference of the member.

8.11 DETAILING

Detailing of all structures shall reflect best practice.

8.11.1 Slabs and Walls

Minimum reinforcement shall be provided on each face in each direction in all walls and slabs to distribute and limit cracking arising from shrinkage, early thermal and temperature effects in accordance with BD 28/87 and SS CP 65: Part 1 Clause 3.9.4.19 for internal elements of structures designed to BS 5400 and SS CP 65 respectively and shall be not less than that required by Clause 8.4.2 above for members exposed to earth and/or ground water.

This reinforcement shall be placed as close to the surface as is consistent with cover requirements and be distributed evenly around the perimeter of the section.

Generally, pairing of bars and more than one layer of bars are not preferred. It is preferred that smaller diameter bars in any direction are placed at closer intervals to prevent early thermal and shrinkage cracks. The minimum diameter of reinforcement bars shall be 13mm.

For the reinforcement layers nearest the concrete faces, the spacing of reinforcement bars shall be no greater than 200mm and the clear distance between bars shall not be less than 100mm, except at laps where it shall not be less than 75mm.

8.11.2 Columns / Piers

The spacing of links and legs of links shall not exceed 300mm or 0.75 times the effective depth of the section whichever is lesser.

The links shall be of grade 460 steel and the minimum bar diameter shall be 10mm, for column dimensions exceeding 500mm x 500mm or 600mm dia.

8.11.3 Beams (only applicable for structures designed to BS 5400)

The area of nominal tension reinforcement shall not be less than 0.25%

of bd when using grade 460 reinforcement where:

(a) b is the average breadth of the section excluding the compression flange for T, I and box section; and

(b) d is the effective depth to tension reinforcement.

For beams where the depth of the side face exceeds 600mm, longitudinal reinforcement with the minimum area of at least 0.12% of bd shall be provided on each face with spacing not exceeding 300mm vertically and the diameter of the bars shall be not less than 16mm. b is the average breadth of the web and d is the effective depth to tension reinforcement.

The link requirement outlined in 8.11.2 above shall also be applicable to compression reinforcement in beams. Where beams are designed to resist torsion, the spacing of links shall not exceed 200mm.

8.11.4 Corner Details

Corner joints of large structural members shall be carefully detailed, particularly in the case where moments tend to “open” them. Specialist literature shall be consulted and the following minimum requirements observed:

(a) For corners subject to moments tending to “close” the corner, the bend radius of the main tension bars shall be increased to cater for the high bearing stresses induced. For this purpose it shall be assumed that the bars remain fully stressed through the corner detail. For heavily reinforced members a longitudinal bar within the bend should be provided or an orthogonal grid of ties for crack control as necessary.

(b) For corners subject to moments tending to “open” the corner, adequately anchored transverse ties to carry the diagonal tension required for joint equilibrium shall be provided when the amount of tension steel exceeds 1% in either of the adjoining members.

8.11.5 Construction Joints

The design and detailing shall be such that the number of construction joints will be as few as practicable. The suggested location of construction joints shall be indicated on the design drawings.

8.11.6 Slab to Wall Connections

Particular attention shall be paid to the practicalities of the design and detailing of the slab to wall connections and the means by which the integrity of the construction joints at these connections will be assured.

Where possible, slab to wall connections, including slab connections to embedded walls such as diaphragm walls etc, shall be designed and detailed to be monolithic providing full continuity of bending moments, and reinforcement shall be provided (using couplers where necessary) to meet the requirements of the code.

Departures from monolithic connections shall only be considered with the acceptance of the Engineer. In all cases, however, the detailing shall reflect the boundary conditions assumed in design and shall be such as to ensure water-tightness and durability.

8.11.7 Detailing of Shear Links

Where shear links are provided, they shall enclose all tension reinforcement.