8 Conclusions 8.1 Summary 8.2 Recommendations 8.3 Ongoing study
The comparison has been made comparing the actions under BSALL and rail loading.
1. For the purpose of this report, the study has been limited to cases in which either one or two rail tracks are provided with footway loading, and with or without reduced highway applied to one lane in each direction. This selection is judged sufficient, within the present study, to justify whether or not a multi modal scheme for the bridge is feasible or not.
2. Whilst all of the proposed options are geometrically viable, only two of the options (2 and 6) can be classified as complying fully with the ideal of a single carriageway width of 6.0 m minimum. These are the single track option of DRG 021 and the twin track option of DRG 061. The latter option does, however, infringe on the outer air vent spaces and requires a reduction in width locally at the main towers.
3. Under dead plus live load combinations, the cable tension for all the options is less when compared to the BSALL base case, except for Option 6 which shows a nominal increase of 2%.
4. The bridge girders are sensitive to the short loaded lengths associated with train loading. The single track options are generally acceptable, but the twin track options with traffic indicate larger increases in girder moments. However, twin track options appear viable with shorter train lengths operating in conjunction with the reduced traffic configuration.
5. For the options studied, only the twin track options show any increase in load for the approach viaducts and even in these cases less than 5% increase is projected. It is likely therefore, that at worst only minor strengthening work will be required to the approach viaduct for the options presented.
6. Modification will have to be made to the movement joints, in particular those located at the main towers. Evidence exists (Tagus Bridge) that indicates a solution will be feasible in the space available. The joint would require development and testing in the normal way - but is entirely feasible.
1. All of the Options 1 to 6 presented have a positive potential and should be studied further.
2. The performance of the bridge is sensitive to the intensity of railway/ tram loading that is to be accommodated. Early consideration should, therefore, be given to specifying the type and characteristics of the light rail / tram vehicles that are anticipated to be employed on the bridge. It is tentatively recommended that an upper load intensity of 20 kN/m length, be considered. This is broadly consistent with many of the current light rail specifications in the UK. Lighter loads may be applicable for tram type loading.
3. The performance of the bridge is sensitive to the length of railway/ tram loading that is to be accommodated. Early consideration should, therefore, be given to specifying the type and characteristics of the light rail / tram vehicles that are anticipated to be employed on the bridge. It is tentatively recommended that a 50 m length, similar to that proposed for the Edinburgh Tram system, and others in the UK, be considered.
4. The present study is limited to a comparison of the effects of actions on the bridge under proposed changes of loading relative to the effects of actions under the current BSALL loading. An early understanding of the actual capacities of the bridge elements is therefore necessary.
5. A review of the requirement for 6.0 m width carriageway should be made accepting a possibility of narrow carriageways. This may unlock the other alternative options presented.
6. Additional studies should also investigate the possibility of using unrestricted traffic or twin lanes in a breakdown condition in combination with the various rail scenarios.
The following topics should be considered for review in subsequent studies:
1. Obtain as built / modified bridge details.
2. Review assessment reports for current state of bridge and the assessment loading when available to indentify the critical members based on actual rather than relative capacity.
3. Draw up a project specific design statement to cover rail and road loading and ongoing assessment methodology.
4. Further study of the rail joints to be carried out. In particular, accommodation in the space available.
5. Further study of aerodynamic behaviour.