Option Analysis
1 Replacement Options
1.1 General
As outlined in section 1 the range of bridge options to be considered are set out as follows:
· Simple open pedestrian bridge with ramps and stairs to the South.
· As above but with an enclosed bridge structure
· More attractive options
The design criteria utilised are set out in section 3 and the key constraints influencing bridge design are set out in section 2.
In particular it has been assumed that Central Way will have to remain open throughout with only occasional overnight closures for the installation of bridge elements.
The existing bridge has a minimum headroom above Central Way of 5.68m. As the minimum clearance for any new structure will be 5.7m and as there is a requirement for the new structure to tie into the existing shopping centre the construction depth for any option will be limited. The construction depth is defined as the depth from the walkway surface of the bridge to the lowest point on the bridge deck soffit. This limitation on depth therefore limits the choice of bridge form to through type structures such as trusses or cable stayed options.
As the existing pier on the North side of Central Way does not meet minimum criterion in terms of setback from the carriageway it would be preferable to locate the new pier behind the service road.
We recognise that this may be out with land in the control of Campsies Centre Cumbernauld Ltd.
Notwithstanding this point re-locating the pier will minimise the disruption to traffic flow on Central Way during construction and should avoid the possible need to demolish the existing foundations.
Utilising a narrower bridge deck width than the existing structure will provide for a clear gap between the footbridge and the East facing wall of the existing shopping centre which will be of benefit in the maintenance
of the structure but will require that the opening into the shopping centre be made good. If an open footbridge is selected then a canopy structure may be required to prevent the ingress of water into the building.
Given the requirement to minimise disruption to Central Way, all of the options considered utilise steel as it is lighter and provides for easier installation while also maximising offsite pre-fabrication.
On the basis of the foregoing the following replacement options have been considered:
· Modified Warren Truss
· Vierendeel Truss
· Vierendeel Girder
· Bowstring Arch
· Asymmetric Cable Stayed
· Enclosed Elliptical Truss
1.2 Option I – Modified Warren Truss
1.2.1 Description
This option comprises single span fabricated steel warren truss bridge spanning between two reinforced concrete leaf piers. To the North access to the shopping centre is provided by a 7m extension to the deck, access to the ramps and stairs to the South is via a 4m cantilever deck section. The truss comprises four square hollow section chords (two top chords and two bottom chords). The top chords are braced in plan by transverse square hollow sections. The bridge deck comprises rectangular hollow sections spanning transversely between the bottom chords on the deck edge and supporting a steel deck plate. The deck plate is surfaced with a proprietary non slip surfacing. The deck edge members are connected to the top chords by inclined square hollow section bracing.
The parapets are 1400mm high and comprise hollow steel sections with vertical infill members. A handrail is fixed to the parapet at 925mm above deck level
The bridge will be fully fixed to the South pier and will be supported on the North pier with guided bearings allowing longitudinal movement of the structure.
Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise rectangular hollow longitudinal sections with transverse sections and a steel deck plate. The ramps and stairs will also be treated with a proprietary non slip surfacing. The ramps and stairs are supported on steel hollow section columns and reinforced concrete pad foundations.
Figure 1 - Modified Warren Truss Footbridge
1.2.3 Construction
Space at the site is limited and areas would have to be identified for the contractors compound and for material laydown areas. The footbridge will be delivered to site in two sections and could be assembled in the South verge of Central Way marked assembly location on location plan A – Appendix A. Prior to the delivery of the bridge the piers will be constructed using conventional techniques for reinforced concrete construction. The bridge superstructure will be assembled into one piece and the parapets installed and surfacing applied prior to installation. Once assembled, the bridge will be lifted onto the piers during an overnight closure of Central Way. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work other than the installation of the bridge should not require closure or lane restrictions on Central Way although construction of the north pier foundation will involve some disruption to the service road and access opening into the existing shopping centre.
1.3 Option II – Vierendeel Truss
Description
This option is a single span fabricated steel vierendeel truss bridge comprising four circular hollow section chords (two top chords and two bottom chords). The bridge deck comprises rectangular hollow sections spanning transversely between the bottom chords on the deck edge and supporting a steel deck plate which is surfaced with a proprietary non slip surfacing. The deck edge members are connected to the top chords by circular hollow section bracing inclined radially in elevation. In addition the top chord is curved in elevation and plan to give an open appearance to the structure
The parapets are 1400mm high and comprise hollow steel sections with vertical infill members. A handrail is fixed to the parapet at 925mm above deck level
The bridge is supported on two reinforced concrete leaf piers with a rebated feature finish. The bridge will be fully fixed to the South pier and will be supported on the North pier with guided bearings allowing longitudinal movement of the bridge.
The link between the new structure and the existing shopping centre to the North is provided by a 7m deck section spanning between the North pier and the Southern wall of the centre. The bottom chord of the bridge deck would continue over this length to provide uniformity of appearance and detailing. Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise rectangular hollow longitudinal sections with transverse hollow sections supporting a steel deck plate. The ramps and stairs will also be treated with a proprietary non slip surfacing. The ramp and stairs are supported on steel hollow section columns and reinforced concrete pad foundations.
1.3.1 Construction
Space at the site is limited and areas would have to be identified for the contractors compound and for material laydown. The footbridge will be delivered to site in two sections and could be assembled in the South verge of Central Way marked assembly location on location plan A – Appendix A. Prior to the delivery of the bridge the piers will be constructed using conventional techniques for reinforced concrete construction. The bridge superstructure will be assembled into one piece and the parapets installed and surfacing applied prior to installation. Once assembled, the bridge will be lifted onto the piers during an overnight closure of Central Way. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work other than the installation of the bridge should not require closure or lane restrictions on Central Way although construction of the north pier foundation will involve some disruption to the service road and access opening into the existing shopping centre.
1.4 Option III – Vierendeel Girder
Description
This option comprises a traditional single span fabricated steel vierendeel girder. The main span consists of four square hollow section chords (two top chords and two bottom chords). The bridge deck comprises rectangular hollow sections spanning transversely between the bottom chords on the deck edge and supporting a steel deck plate which is surfaced with a proprietary non slip surfacing. The top and bottom chords are connected by vertical hollow section members.
The parapets are 1400mm high and comprise hollow steel sections with vertical infill members. A handrail is fixed to the parapet at 925mm above deck level
The bridge is supported on two reinforced concrete leaf piers with a rebated feature finish. The bridge will be fully fixed to the South pier and will be supported on the North pier with guided bearings allowing longitudinal movement of the bridge.
The vierendeel girder would continue beyond the north pier to form the link with the existing shopping centre and would cantilever beyond the south pier to connect with the ramp and stair. Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise rectangular hollow longitudinal sections with hollow sections spanning transversely between the girders supporting a steel deck plate. The ramps and stairs will also be treated with a proprietary non slip surfacing. The ramp and stairs are supported on steel hollow section columns and reinforced concrete pad foundations.
1.4.1 Construction
Space at the site is limited and areas would have to be identified for the contractors compound and for material laydown areas. The footbridge will be delivered to site in two sections and could be assembled in the South verge of Central Way marked assembly location on location plan A – Appendix A. Prior to the delivery of the bridge the piers will be constructed using conventional techniques for reinforced concrete construction. The bridge superstructure will be assembled into one piece and the parapets installed and surfacing applied prior to installation. Once assembled, the bridge will be lifted onto the piers during an overnight closure of Central Way. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work other than the installation of the bridge should not require closure or lane restrictions on Central Way although construction of the north pier foundation will involve some disruption to the service road and access opening into the existing shopping centre.
1.5 Option IV – Bowstring Arch
Description
This option comprises a single span fabricated steel bowstring arch bridge The two arch ribs comprise square hollow section bent in elevation and inclined in section at 7.5 degrees to the vertical. The two arch ribs are braced in plan by transverse square hollow sections at the hanger locations. The bridge deck comprises rectangular hollow sections spanning transversely between the bottom chords on the deck edge and supporting a steel deck plate which is surfaced with a proprietary non slip surfacing. The deck edge members are connected to the arch by inclined hanger bars with fork and pin connections. The bars are inclined in opposite directions on either side of the bridge. In addition to the hanger bars the arch is also connected to the edge members at the supports by a transverse hollow section trimmer beam.
The parapets are 1400mm high and comprise hollow steel sections with vertical infill members. A handrail is fixed to the parapet at 925mm above deck level
The bridge is supported on two reinforced concrete leaf piers with a rebated feature finish. The bridge will be fully fixed to the South pier and will be supported on the North pier on guided bearings allowing longitudinal movement of the bridge.
Figure 3 – Bowstring arch footbridge
Access to the existing shopping centre to the North of the bridge is provided by a 7m extension of the deck spanning between the North pier and the south wall of the shopping centre. The bottom chord would continue over this length to provide uniformity of appearance and detailing. Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise longitudinal rectangular hollow sections connected transversely by hollow sections and a steel deck plate. The ramps and stairs will be treated with a proprietary non slip surfacing. The ramps and stairs are supported on steel hollow section columns and reinforced concrete pad foundations.
1.5.1 Construction
Space at the site is limited and areas would have to be identified for the contractors compound and for material laydown areas. The footbridge will be delivered to site in two sections and could be assembled in the South verge of Central Way marked assembly location on location plan A – Appendix A. Prior to the delivery of the bridge the piers will be constructed using conventional techniques for reinforced concrete construction. The bridge superstructure will be assembled into one piece and the parapets installed and surfacing applied prior to installation. Temporary members would also be required to stabilise the bridge prior to erection. Once assembled, the bridge will be lifted onto the piers during an overnight closure of Central Way. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work Other than the installation of the bridge should not require closure or lane restrictions on Central Way although construction of the north pier foundation will involve some disruption to the service road and access opening into the existing shopping centre.
1.6 Option V –Asymmetric Cable Stayed
1.6.1 Description
This option comprises an asymmetric cable stayed bridge with an “A” frame pylon. The pylon comprises two square steel hollow sections which extend to a height of 16.5m above deck level and are connected below deck level by a square hollow section member which also supports the deck. Below deck the two pylons legs turn in on each other and are fixed to a reinforced concrete plinth which is supported on a spread foundation. The deck comprises two longitudinal rectangular hollow section edge members on the deck edge connected transversely by rectangular hollow sections which support a steel deck plate. In addition to being supported at the pylon the deck is supported by a pier at the south end of the bridge, by the existing shopping centre structure at the north end and by cables emanating from the pylon. The cables could either comprise preformed steel strands or steel bars. At the cable supports the deck transverse member is a larger rectangular hollow section. The pylon is restrained by cables connecting the pylon to the south pier. This pier provides a tie down for the pylon and will be sufficient to resist uplift. The bridge deck will be surfaced by a proprietary non slip surfacing
The parapets on the structure are 1400mm high and comprise hollow steel sections vertical member infill. A handrail is fixed to the parapet at 925mm above deck level
Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise longitudinal rectangular hollow sections connected transversely by hollow sections and a steel deck plate. The ramps and stairs will also be treated with a proprietary non slip surfacing. The ramps and stairs are supported on steel hollow section columns and reinforced concrete pad foundations.
An advantage of this option is that it avoids the need for a pier to the north of central way although as a consequence the loads transferred to the existing building could be greater than the present situation.
1.6.2 Construction
Construction on site will begin with the pier and pylon foundation to the South of Central Way. Once complete the bridge structure will be delivered to site and stored on the South verge of Central Way marked assembly location on location plan A – Appendix A. The pylon will be erected in a single piece and fixed to its foundation. The first section of the deck will be lifted into place and supported on a temporary trestle located in the Westbound carriageway of Central Way prior to the cables being installed and stressed. The remaining sections of the deck will be erected and tied to the pylon using the same method of temporarily propping from the carriageway. During the erection sequence the traffic flow on Central Way will be reduced. It is anticipated however that the erection of the bridge would be completed in a short time period thereby limiting the disruption to traffic. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work other than the installation of the bridge should not require closure or lane restrictions on Central Way.
1.7 Option - VI Elliptical Enclosed Footbridge
Description
This option comprises an enclosed elliptical structure. The bridge comprises five circular hollow longitudinal sections braced by smaller circular hollow sections inclined to the vertical. The cross section of the deck forms an ellipse and will be covered with polycarbonate sheeting manufactured to the same profile. The bridge deck comprises two of the five longitudinal sections connected transversely by rectangular hollow sections and a steel deck plate. The deck plate will be treated with a proprietary non slip surfacing.
The span is supported on two concrete leaf piers. The bridge will be fixed to the southern pier and supported on the northern pier on guided bearings to allow for longitudinal movement of the span. The enclosed bridge structure continues beyond the north pier to tie into the existing shopping centre building.
The parapets on the bridge are 1400mm high and comprise hollow steel sections with vertical infill members. A handrail is fixed to the parapet at 925mm above deck level. Lighting would also be provided within the bridge structure.
Access to the South of the bridge is provided by a ramp structure to the West and stairs to the East. The ramp and stairs comprise rectangular hollow longitudinal sections with transverse hollow sections supporting a steel deck plate. The ramps and stairs will also be treated with a proprietary non slip surfacing. The ramp and stairs are supported on steel hollow section columns and concrete pad foundations.
Figure 6 – Enclosed Elliptical Footbridge
1.7.1 Construction
Space at the site is limited and areas would have to be identified for the contractors compound and for material laydown areas. The footbridge will be delivered to site in two sections and could be assembled in the South verge of Central Way marked assembly location on location plan A – Appendix A. Prior to the delivery of the bridge the piers will be constructed using conventional techniques for reinforced concrete construction. The bridge superstructure will be assembled into one piece and the parapets and the enclosure panels installed and the surfacing applied prior to erection on the piers. Once assembled, the bridge will be lifted onto the piers during an overnight closure of Central Way. Construction of the ramp and stairs could proceed in parallel with construction of the bridge depending on the availability of space. Construction of the elements of work other than the installation of the bridge should not require closure or lane restrictions on Central Way although construction of the north pier foundation will involve some disruption to the service road and access opening into the existing shopping centre.
1.8 Common Aspects
It is considered that the on site construction program for a replacement bridge will be approximately 20 weeks. The structure fabrication program would be approximately 16 weeks.
Maintenance activities throughout the life of all of the structure will include:
· General inspections
· Cleaning
· Bearing Replacement
· Repainting
· Resurfacing
· Concrete Repairs
Steelwork above deck level will be inspected and maintained from deck level. Inspection/ maintenance to be undertaken below deck level would be conducted using an access platform and traffic restrictions on Central Way.
All vertical and horizontal restraint bearings would be replaceable. Provision on the bearing shelves and in the diaphragms will be made for jacking of the bridge for possible future bearing replacement.
Repainting of the deck steelwork will be facilitated by conventional scaffold access. Lane closures would be required for elements of the deck over the carriageway.
1.9 Health & Safety
From and initial assessment the following health and safety risks and hazards have been identified. As these are similar for all options considered a generic list is outlined as follows
Wherever possible all risks would be eliminated or mitigated at detailed design stage and the Contractor would be advised of all residual risks prior to construction commencing.
· Working adjacent to live carriageways – new piers to be remote form carriageway to reduce risk
· Construction: steel construction adopted to minimise site based activities and maximise prefabrication of elements.
· Erection stability: steel superstructure shall be erected as to ensure stability at all times during the construction process. Where possible all pre assembly will take place prior to erection
· Excavations: spread foundations adopted thus minimising the amount of construction required.
· Manual handling – use of large elements to avoid need for manual handling
· Work at height: all relevant personnel to be suitably qualified and fall arrest systems employed.
· Fabricated elements to be finished in the fabricators yard where greater control of the working environment is possible.
· Maintenance: all steel surfaces to be accessible for regular maintenance and future painting.
· Demolition: Mechanical Handling – Where possible the bridge deck can be removed as a single unit. For option V the splice joints can be unbolted in a reverse process to that adopted for erection.
· Cable/Hanger stressing – Where applicable Method statements to be prepared and temporary works utilised for handling jacks and other heavy equipment. Cables/hangers to be stressed using pre-determined sequence with measurement of jacking loads throughout.
1.10 Cost Estimate
The cost estimates for each option are set out below and are based on preliminary design calculations to establish principal member sizes.
The cost estimates have not made allowance for the following:
· Removal of existing link bridge including piers and the gazed pod,
· Relocation of existing services and
· Traffic Management.
· It is envisaged that the cost of the above elements will be very similar for all 5 options.
· All cost estimates include the following:
· Site clearance (after removal of the existing structure),
· Fabrication of the structure,
· Delivery to site, assembly and installation, and
· Construction of all concrete substructure elements.
· Finishes
| Item | Option I – Modified Warren Truss | Option II – Vierendeel Truss | Option III – Vierendeel Girder | Option IV Bowstring Arch | Option V - Asymmetric Cable Stayed | Option VI – Enclosed Elliptical Truss |
| Site Investigation (Preliminary) | £16,000.00 | £16,000.00 | £16,000.00 | £16,000.00 | £16,000.00 | £16,000.00 |
| Preliminaries 1 | £77,220.00 | £80,960.00 | £79,860.00 | £79,310.00 | £80,960.00 | £84,920.00 |
| Substructure | £36,000.00 | £36,000.00 | £36,000.00 | £36,000.00 | £36,000.00 | £36,000.00 |
| Fabrication | £218,000.00 | £230,000.00 | £226,000.00 | £224,500.00 | £230,000.00 | £246,000.00 |
| Delivery/ Installation | £52,000.00 | £57,000.00 | £56,000.00 | £55,000.00 | £57,000.00 | £59,000.00 |
| Finishes 2 | £5,000.00 | £5,000.00 | £5,000.00 | £5,000.00 | £5,000.00 | £5,000.00 |
| Professional Fees (including check) | £40,000.00 | £40,000.00 | £40,000.00 | £40,000.00 | £40,000.00 | £40,000.00 |
| Sub Total | £428,220.00 | £448,960.00 | £442,860.00 | £439,810.00 | £448,960.00 | £470,920.00 |
| Contingencies (25%) | £107,055.00 | £112,240.00 | £110,715.00 | £109,952.50 | £112,240.00 | £117,730.00 |
| TOTAL | £535,275.00 | £561,200.00 | £553,575.00 | £549,762.50 | £561,200.00 | £588,650.00 |
2 Summary of Replacement Options
2.1 Summary
Consideration of options for a replacement footbridge crossing Central Way has been based on the collation and review of relevant information including topographic survey data, ground conditions and details of existing and proposed developments.
As the site is well developed key constraints affecting the form of any new bridge have been identified and design criteria for a new bridge have been established.
On the basis of the foregoing and the clients brief six options have been identified and evaluated as described in Section 4. As the ramps and stairs at the South end of the proposed footbridge are common to all options and as these represent a reasonable proportion of the total project cost, differences between options are not as significant as might otherwise have been the case.
With the exception of Option V The cable stayed option, construction methods do not vary significantly between options and health and safety risks during construction will therefore be similar for all options.
An advantage of the cable stayed option over the others is that it avoids the need for a new pier support on the north side of Central Way in land outwith the control of Campsies Central Cumbernauld Ltd.
Option VI has amenity benefits from being an enclosed structure although this presents problems for maintenance and as the ramps and stairs are open the benefits of an enclosed link bridge are somewhat limited.
On the basis of the foregoing and notwithstanding the benefits of options V and VI outlined above the choice between options effectively comes down to aesthetics and capital cost.
2.2 Controlled Convergence Assessment
As a further check on the assessment the Controlled Convergence Method is used. where an option is chosen as a ‘datum’ and the other options are compared with the datum. An option is awarded a ‘+’ (better than the datum), ‘-’ (less than the datum) or ‘s’ (same as the datum) see (MacLeod I A and Hartvig S (1999))
Assessment Criteria
· Aesthetics: How well the bridge matches with the surroundings and how well it would be received by the public
· Size and Complexity: The overall size of the bridge and how complex the design is.
· Carbon Footprint: the amount of greenhouse gas emissions produced by the bridge’s construction and materials.
· Disruption to surrounding areas: The amount of disruption that will be caused due to the construction including, road closures, reduction of traffic and closure of business.
· Pier Position: The number and position of piers within the design
· Maintenance (Cost and Ease): the cost of essential maintenance that will need to be carried out on the bridge after construction and how easily it can be carried out.
· Total Production Cost: The cost for the fabrication and construction of the bridge. However, the cost estimate shown previously shows that the production cost for each option is very similar. Because of this the Total Production Cost is omitted from the assessment matrix.
Phase 1:
Table 1 Evaluation matrix for bridge options -Phase 1
| Criterion | Option I | Option II | Option III | Option IV | Option V | Option VI |
| Modified Warren Truss | Vierendeel Truss | Vierendeel Girder | Bowstring Arch | Asymmetric Cable Stayed | Enclosed Elliptical Truss | |
| Aesthetics | - | - | - | - | Datum | s |
| Size and Complexity | + | + | + | s | Datum | s |
| Carbon Footprint | - | - | - | - | Datum | - |
| Disruption to Surrounding Areas (Road Closures etc.) | - | - | - | - | Datum | - |
| Pier Position | - | - | - | - | Datum | - |
| Maintenance Costs | s | s | s | s | Datum | s |
| Maintenance Ease | + | + | + | + | Datum | - |
| total '+' | 2 | 2 | 2 | 1 | 0 | 0 |
| total '-' | 4 | 4 | 4 | 4 | 0 | 4 |
| total 's' | 1 | 1 | 1 | 2 | 0 | 3 |
Further Justification:
· Size and Complexity: Options I, II & III are very simple designs compared to the datum option, these options were given a ‘+’.
· Carbon Footprint: Most of the designs are similar to one another, meaning that the carbon footprint of each option is hard to compare. However, as Option V has only one pier needed, this will reduce the concrete needed in construction, meaning that less carbon will be present in this option.
· Pier Position: Option V is the only option to eliminate the North pier, because of this, all other options were given a ‘-’.
· Maintenance Ease: As options I – IV are more straightforward than option V, maintenance may be easier to execute on these options. As option VI is enclosed, this will make it harder for maintenance.
Observations:
· This matrix shows that all options investigated are quite similar and each option has their own benefits.
· As each option was given an ‘s’ for maintenance cost, which did not aid the evaluation, therefore this criterion was deleted from the list.
· We can see that Option VI is the clear loser, as it has no advantages over the datum option. Option VI can be eliminated from the analysis.
· Options I-IV have 1-2 advantages, however in each case, these advantages are outweighed by the negatives.
| Criterion | Option I | Option II | Option III | Option IV | Option V |
| Modified Warren Truss | Vierendeel Truss | Vierendeel Girder | Bowstring Arch | Asymmetrical Cable Stayed | |
| Aesthetic (Symmetry and Balance) | - | - | - | - | Datum |
| Size and Complexity | + | + | + | + | Datum |
| Carbon Footprint: | - | - | - | - | Datum |
| Disruption to Surrounding Areas (Road Closures etc.) | - | - | - | - | Datum |
| Pier Position | - | - | - | - | Datum |
| Maintenance Ease | + | + | + | s | Datum |
| total '+' | 2 | 2 | 2 | 1 | 0 |
| total '-' | 4 | 4 | 4 | 4 | 0 |
| total 's' | 0 | 0 | 0 | 1 | 0 |
Table 2 Modified matrix for bridge option evaluation - Option V1 Maintenance cost removed
The Controlled convergence analysis confirms that suitability of Option V as the preferred design solution