Steelwork
4.8.4.6 Steelwork and concrete incorporating welded reinforcing steel which supports railway, road or other significant cyclic loading shall be designed for fatigue in accordance with the provisions of HKSDM Chapter 10 and BS5400 Part 10. 4.8.4.7 For steelwork elements and welded reinforcing steel in concrete subject to
railway vehicle loading, a design stress spectrum for non-standard loading shall be derived and the steel details assessed in accordance with BS5400, Part 10, Cl. 9.3.3 to 9.3.5 inclusive, using the fatigue axle load and the axle spacings given in Subsection 4.4. The stress history for the element under consideration shall be derived using the 'Reservoir Method' described in BS5400, Part 10, Appendix F Example 4; the fatigue axle loads and the specific structure geometry under consideration. A simplified design stress spectrum shall be provided in a similar format to BS5400, Part 10 Fig.13 and subject to the approval of the Corporation.
4.8.5 DESIGN FOR FLOTATION
4.8.5.1 Underground structures shall be designed for flotation loads. If the net downward load (NDL) at Construction, Operational or Extreme stage is negative, the structures shall be positively held down by an acceptable means which provides the required restraining forces against uplift. The downward loads in Construction, Operation and Extreme stages are load combinations to be considered in assessing the safety against flotation. The construction combination shall include, but not be limited to, the main construction and the case of an excavation over the structure at a later date.
4.8.5.2 The NDL and Uplift are defined by the following equations: i) NDL = Σ Factored Downward Loads – Buoyancy ii) Uplift = - NDL (if NDL < 0)
In calculating the NDL, the characteristic downward loads shall be divided by appropriate factors of safety to reflect the uncertainty in the loads. Unless other factors can be shown to be more appropriate, the factors for downward loads in Table 4.8.5.T1 shall be used. The buoyancy shall be calculated according to Subsection 4.4.9.
4.8.5.3 Adequate factors of Safety against uplift at each of the Construction, Operational and Extreme stages shall be achieved in the design. The factor of Safety against uplift is defined by the following equation:
i) Factor of Safety = Σ Restraining Forces / Uplift
The Factors of Safety shall not be less than the factors of safety for restraining forces in Table 4.8.5.T2.
Section 4: Civil Engineering D/MTRC/NW/DSM/ST/408/A5 4.8 Structural Design
NWDSM-Section 4(4.8)-A5 April 2013
4.8/6
Table 4.8.5.T1 Factors of Safety for Downward Loads Stages Downward Loads
Construction Operational Extreme
Self weight 1.03 1.10 1.03
Superimposed DL 1.03 1.10 1.03
Backfill 1.20 1.30 1.03
Table 4.8.5.T2 Factors of Safety for Restraining Forces Stages
Restraining Forces
Against Uplift Construction Operational Extreme
Friction effects 1.75 3.00 1.03
Tension piles 2.00 3.00 1.03
4.8.5.4 In calculating the restraining forces, consideration shall be given to the possibility of the introduction of sliding interfaces during construction. These include, inter alia, the following:
i) waterproof membrane; ii) temporary works interface; and
iii) a bentonite layer on the face of diaphragm walls.
4.8.5.5 An allowance shall be made for the possible reductions in both of the following:
i) average depth of backfill of 0.5m over the roof due to future construction; and
ii) the top 2m high portion of diaphragm wall immediately below ground surface due to future utility work.
Anchors Resisting Structure Flotation Forces
4.8.5.6 The use of permanent anchors to resist flotation forces will generally not be permitted. However, where the base slab of the structure is founded on rock it may be economic to use permanent rock anchors or an adequate mechanical key into sound rock to resist a portion of the flotation forces. Where other methods are shown to be impractical, rock anchors may be allowed provided they comply with the criteria given in Cl. 4.8.5.7 to Cl.4.8.5.8 inclusive. For structures with developments above, Buildings Department’s approval will also be required.
Section 4: Civil Engineering D/MTRC/NW/DSM/ST/408/A5 4.8 Structural Design
NWDSM-Section 4(4.8)-A5 April 2013
4.8/7
4.8.5.7 For Cl. 4.8.5.6 to 4.8.5.9 rock shall be defined as material which may only be excavated using robust mechanical means or drill and blast methods. For the purposes of the NWDSM rock shall normally mean Grade I, II or III material as defined in Geoguide 3.
4.8.5.8 If approval by the Corporation is given for rock anchors, they shall not protrude beyond the plan area of the structure. The detailing of anchors and surrounding structures shall include provisions for access for long term load monitoring, corrosion monitoring and replacement. The design of anchors shall be in accordance with Subsection 4.6 and the design shall provide sufficient redundancy in the provision of anchors to enable anchor replacement to be safety and efficiently undertaken.
4.8.5.9 Where there are both economic and technical justifications for permanent rock anchors, full details of proposals shall be submitted for the approval of the Corporation. In addition a detailed materials and workmanship specification and method of measurement for such work shall be submitted. The specification shall include, inter alia, the highest standard of corrosion protection for both tendons and anchorages; provision for testing during construction and during the Design Life of every anchor by lifting points and adequate clearance; and the requirement for Contractors to provide full suppliers details of the anchors, such as assumptions, forces, serial numbers and type. The design shall also provide sufficient redundancy in the provision of anchors to enable anchor replacement to be safely and efficiently undertaken