GROUND LEVEL AND OVERHEAD STRUCTURES General

4.3.5.1 This subsection sets out the specific requirements for the design of ground level and overhead structures such as bridges, footbridges and similar components of building structures.

Bridge Deck Continuity

4.3.5.2 The type of trackform may determine the articulation arrangements of viaducts and bridges. However, deck continuity shall be adopted wherever possible, as it possesses other beneficial features, such as reductions in deck

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construction depths, pier bearing widths and foundations, in addition to a better running quality. Cantilever spans and half-joint arrangements will not be permitted in bridges or other structures.

4.3.5.3 Deck continuity in prestressed structures generally requires prestressing tendons located in the deck slab or in the top flange and top of web locations of box girders. Account shall be taken of the increased vulnerability of the tendons to corrosion due to water which may penetrate the deck through construction joints or cracks. Therefore, the number of vertical construction joints shall be minimised and, wherever possible, they shall be located away from zones subject to hogging moments. All such vertical construction joints shall be waterproofed in accordance with Cl.4.3.5.14.

Bridge Deck End Joints

4.3.5.4 Several design measures may be adopted to minimise the deleterious effect of water penetrating through the joints.

4.3.5.5 Fig. 4.3.5.F1 shows a typical arrangement at an abutment movement joint which shall, unless otherwise agreed by the Corporation, be adopted to avoid some of the problems caused by leakage and to provide access for inspection and maintenance. The abutment curtain wall is set back and the abutment shelf dropped as necessary to provide sufficient access at the back of the deck for inspection and maintenance. Short reinforced concrete corbels cantilevering from the deck and the curtain wall shall be provided to contain the movement joint. The deck waterproofing membrane shall be carried down the vertical/inclined edges of this joint and tucked into a drip groove formed under the corbel, providing rundown protection. The inclined edges allow hand access to the joint and assist in attaching or maintaining the membrane.

4.3.5.6 The abutment movement joint corbels soffit and the deck vertical edge shall be treated with a 2 coat 2 mm thick polymer modified cementitious waterproofing material to protect the prestressing anchorage recess. Similar waterproofing treatment shall be applied on all internal surfaces of the abutment gallery including the curtain wall, cheek walls and bearing shelf. A fall of at least 1 in 40 shall be provided to a substantial drain at the back of the abutment to prevent any rundown on the abutment face.

4.3.5.7 Structural Movement Joints (SMJ) above intermediate piers shall not include the inspection gallery. However, the joint must be accessible from within the deck voids for inspection purposes. Intermediate piers beneath SMJ shall be detailed such that the top surface of the pier shall be laid to fall towards the drainage hopper. Bearings shall be seated on reinforced concrete plinths and the top surface of the pier and the bearing plinths shall be treated with a 2 coat 2mm thick polymer-modified cementitious waterproofing material. Similarly, all piers incorporating deck-drainage down pipes shall be laid to falls and waterproofed as described above.

4.3.5.8 A drip groove, at least 25 mm deep and 25 mm wide, shall be provided at all SMJ in the superstructure soffit between the throat of the joint and the bearing top plate to prevent any moisture migration across the deck soffit, care in detailing shall be taken to ensure full concrete cover is maintained at the drip.

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Moisture migration might occur due to the failure of the drainage arrangements or excessive condensation within the access chamber.

Access to Bridge Voids

4.3.5.9 Access shall be provided into all structure voids whether in bridges, building structures, superstructures or substructures to the approval of the Corporation in accordance with Subsection 4.2.

4.3.5.10 In addition to the requirements in Subsection 4.2., there shall be access holes in each bridge span from the top slab and continuous access from abutment galleries along the deck within the voids, unless it can be shown to be impracticable. If this level of access is shown to be impracticable, then alternative access arrangements shall be proposed for the approval of the Corporation. Voids shall be provided with a permanent lighting and electricity supply for use during inspection and maintenance.

4.3.5.11 Covers shall be water-tight, lockable with a security key whilst maintaining the minimum clear opening specified above. Each opening shall be installed such that the opening is at least 100 mm above the adjacent concrete level, the actual level may be constrained by the trackform design.

4.3.5.12 Where access holes are required directly beneath the trackform, liaison with the trackform Designer shall be carried out. The underground openings shall not be located beneath a turnout slab or areas of floating trackform. The trackform Designers will specify the covers. However, the superstructure designer shall specify a temporary cover which shall prevent ingress of water and shall also be robust enough to prevent personnel and materials from falling into the void.

4.3.5.13 Where access holes are outside the trackform, the superstructure Designer shall specify the covers.

Precast Segmental Bridge Construction

4.3.5.14 To prevent water penetrating the joints between segments a liquid applied polymer waterproofing membrane shall be applied across the joints and to the deck surface extending at least 250 mm on both sides of each joint.

External (Unbonded) Tendons in Bridges

4.3.5.15 In view of the problems which can be caused by the corrosion of tendons, the design of prestressed concrete bridge decks which use external (unbonded) tendons shall allow for the tendons to be fully accessible for inspection, maintenance and replacement. The design shall allow for all these three operations to be carried out without affecting the safety of the structures, in addition if practicable design shall allow these operations to be carried out without causing disruption to railway operations either. Consideration shall be given to individual strand replacement systems.

4.3.5.16 Bridges designed to these principles shall be of box or beam and slab construction using external prestressing tendons located between the webs.

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The tendons shall typically be deflected or anchored at the soffits of cross beam diaphragms and the tops of pier diaphragms beneath the deck slab. 4.3.5.17 If the anchorage of non-grouted tendons should fail, the complete

prestressing effect and any ultimate strength is lost. This is not the case with bonded tendons. Therefore consideration to the provision of tendon redundancy in respect of this concern shall be given in tendon design.

4.3.5.18 The corrosion protection system to external tendons shall adopt best current practice and be subject to the approval of the Corporation. The protection shall consist of a multi-layered system of sheaths and corrosion inhibiting fillers. The external sheath shall be non-metallic and provide robust abrasion protection, impact protection and UV protection if exposed to daylight.

4.3.5.19 External prestressing is not covered by the current issue of BS 5400, Part 4. Where it is considered that external tendon methods would offer substantial whole-life cost-benefits, full details shall be submitted of methods and design standards to be adopted to the Corporation for its Approval, before proceeding with the detailed design. Unless otherwise approved by the Corporation, the requirements of the UK Highways Agency BD 58/94: “The Design of Bridges and Concrete Structures with External and Unbonded Prestressing”, shall be adopted.

Stability of Single Track Bridges

4.3.5.20 The use of holding-down or stability bars shall not be used as a method of ensuring bridge deck stability, unless it can be shown to the satisfaction of the Corporation that there is no other feasible alternative.

4.3.5.21 If the use of stability bars is accepted, all components shall be dimensioned and located such that the full range of viaduct movement can be accommodated without overstressing. The ducts which have stability bars shall be fabricated from grease tight stainless steel grade 316S33 to BS 970 or EN Grade 1.4436 and provided with grease-tight plugs. The design and/or tender and/or contract documents as appropriate shall indicate a maximum level for the grease to prevent overtopping of the end due to thermal expansion. Access shall be provided for inspection, maintenance and replacement of the stability bars.

Bearings

4.3.5.22 The general provisions of Chapter 9 of the HKSDM shall apply to bearings for all railway and highway bridges, pedestrian bridges and other structures, as appropriate. In addition, it shall be ensured that bearings have adequate allowance for the full range of bridge or viaduct movements. The drawings shall include full details regarding the presetting of bearings to take account of the average ambient temperature at the time of installation and allowances for shrinkage, creep, application of prestress (if any) and rotation due to live load. Screens to conceal the bearings shall not be used. The design shall allow bearings to be inspected, maintained and replaced in a safe manner, and where possible drawings shall indicate the intended jacking position for bearing replacement.

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4.3.5.23 Whenever possible elastomeric bearings shall be specified. However, all specified bearings must be suitable for the applied loads and compatible with the structures articulation and movement limits.

4.3.5.24 Where elastomeric bearings are proven to be unsuitable and mechanical bearings are specified, bearings and guides shall be fixed at the top and bottom by bolts. Where possible, bearings shall be fixed in such a manner as to facilitate removal. The use of permanent seating plates cast or bolted to the concrete structure is preferred. Refer to Subsection 4.2 for bearing stray current mitigation requirements. For precast bridge deck where upper plinths are used between the bearing top plate and the deck, the horizontal forces shall be resisted by a positive fixing into the structure and not by friction. 4.3.5.25 The possibility of bearing uplift shall be avoided wherever possible. However,

if this proves to be unavoidable and is demonstrated to be as such to the Corporation, then notwithstanding the requirements of BS 5400, Section 9.1, the following criteria shall apply:

i) Bearings capable of resisting uplift forces

The bearing schedule shall clearly indicate uplift design load effects at SLS and ULS.

ii) Bearings not capable of resisting uplift forces

At SLS uplift is not permitted. At ULS uplift is permitted provided that the following is complied with:

a) the bearings are designed to allow uplift to occur;

b) the uplift of a bearing from its mountings, or separation of the bearing component parts, must not be such that they do not return to their designed positions for SLS loading; and

c) the loss of support due to the unloading of a bearing is considered in the ULS design of other structural elements. 4.3.5.26 Where rails are continuous over discontinuities in the support to the track, the

bridge structure (bridge deck, bearings and substructure) and the track jointly resist the longitudinal forces due to traction, braking or thermal effects. The effects resulting from the combined response of the bridge structure and the track shall be taken into account for the design of the bridge structure, bearings, the substructure and for checking load effects in the rails.

4.3.5.27 The combined response of the bridge structure and the track shall be assessed in accordance with the loading provisions in Clause 6.5.4 of BS EN 1991-2:2003 (Traffic Loads on Bridges).

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General

4.3.6.1 This subsection sets out the specific requirements for the Design of Structural Movement Joints (SMJ) for external below ground, overhead or ground level structures.

External Structural Elements Below Ground

4.3.6.2 All cast in situ concrete underground structures shall be without SMJ to limit possible ingress of water. Where this is not practicable particular attention shall be paid to SMJ in external walls, bases, and roof slabs below ground with respect to their watertightness and durability.

4.3.6.3 SMJ shall have a heavy duty external and internal water bar. Where shear capacity across the joint is required to resist loads and/or movements it shall generally be provided by dowels, shear dowels shall in all cases be a proprietary stainless steel product in accordance with grade 316S33 or 1.4436 (see BS EN 10088) . They shall be designed and fabricated so that both loads and movements can be accommodated in the orientations assumed in the design. However where shear movement across the joint is critical such as in slabs which support trackform, shear connection shall additionally be provided by a concrete key.

4.3.6.4 The movements shall be identified and a schedule submitted of predicted movements in accordance with Fig. 4.3.6.F1, for the approval of the Corporation.

4.3.6.5 SMJ in Immersed Tube Tunnels shall receive special consideration. Particular reference shall be made to the requirements of Subsection 4.3.4. Overhead and Ground Level Structures

4.3.6.6 Generally the number of SMJ shall be kept to an absolute minimum within the constraints defined in the NWDSM and those appropriate to the specific structure under consideration, i.e. above ground structures shall have a maximum length of structure between movement joints within the movement joint limitations.

4.3.6.7 Wherever practicable, SMJ shall be watertight proprietary systems. They shall be designed to withstand the applied loads and accommodate all the anticipated movements of the structure without developing unacceptable stresses within the joint or in other parts of the adjacent structure.

4.3.6.8 Where the SMJ is in a visibly sensitive area or subject to pedestrian/maintenance traffic, it shall be protected by an appropriate stainless steel cover plate. It shall also be ensured that the SMJ cover plates do not present a hazard to pedestrians or other users.

4.3.6.9 All SMJ, even if designed to be watertight, shall be assumed to leak. Therefore, all SMJ should be detailed such that:

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i) the amount of water falling directly onto the joint is minimised; ii) water is not allowed to pond on or flow over the joint;

iii) whatever water does fall onto the joint or its cover plate shall fall away from the joint;

iv) a robust gutter beneath the joint shall positively drain water from the joint to the main drainage system; and

v) Rail Movement Joint (RMJ) can be installed, inspected and maintained as required by Section 3 of the NWDSM. RMJ structural depth and support systems may be incorporated in the structure. 4.3.6.10 All SMJ components shall be easy to inspect and maintain. Parts liable to

wear shall be easy to replace. Where appropriate, SMJ shall be designed to the requirements for fire resistance and separation described in Subsection 4.2. In addition, SMJ for highway and pedestrian bridges shall be designed to Section 11 of the HKSDM.

4.3.6.11 SMJ in elements supporting rail tracks shall be designed to, but not be limited to, accommodate movements due to the following in the appropriate combinations (See Subsection 4.4.):

i) thermal variations;

ii) long term creep and shrinkage;

iii) railway live loads such as traction, braking and the like; iv) wind loads;

v) earthquakes;

vi) continuously welded rail forces; and vii) ground movements.

4.3.6.12 An adequate allowance shall be made for the longitudinal and transverse components of movement that may occur due to rotational movements in the supporting structure. Curved or skew structures shall be given special consideration.

4.3.6.13 The movements in the SMJ after installation of the trackform shall be limited to the range given in Table 4.3.6.T1. Larger longitudinal movements (see *) will be permitted subject to the prior approval of the Corporation. These larger joints may require modification to the supporting structure and SMJ.

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Table 4.3.6.T1 SMJ Movement Limits

Movement Range Longitudinal (parallel to rails) 150 mm *

Transverse (perpendicular to rails) 2 mm

Vertical 2 mm

Horizontal rotation (plan) 0.003 rads Vertical rotation (elevation) 0.003 rads

Axial rotation (twist) 0.001 rads

4.3.6.14 Reference shall be made to the permanent way alignment design to ensure that there are no SMJ positioned beneath turnout slabs or horizontal curves of radii less than 750 m.

4.3.6.15 The movements shall be identified and a schedule shall be submitted of predicted movements to the Corporation for approval in accordance with Fig. 4.3.6.F1.

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