Associated Problems

Bridge drainage is important for structural loads, end user satisfaction and stain control. An accumulation of water in a certain area of the bridge can add to the dead loads and could put the foundations and supports under a great amount of pressure.

Pools of water can accumulate and may make it difficult for the end user to cross. Stains and water damage to materials can also be a problem here. Therefore it is essential both by regulation and by good design that the drainage strategy for the bridge is

Solution

The idea for an ‘in-built’ drainage strategy came from the use of a drain pipe as part of the formwork for our model. The idea that the ‘undercarriage’ of the bridge could hold and channel water as part of its structure rather than a disapproving piped structure fixed underneath.

At the Mithras House side, the topography is such that a water spout could be used to drain down the existing road and into the gulley. However, the Preston Barracks side is far higher up off the ground to the point where a spout could cause water to fall rapidly and splash around lifting dirt and mud and producing a poor aesthetic.

In turn, this system will need to be piped and this can easily be achieved using a branch over to the supports for the ramp. This will then drain into either an existing system or otherwise the new drainage system within the Preston Barracks development.

Image Reference: http://www.secpinc.com/products/fiberglass-frp-drainage-systems

On Site Constraints

The onsite constraints involve the active Lewes Road which is present to large amounts of traffic and pedestrian activity; it is also a major bus route. Section 4.0 of The Design Manual for Roads and Bridges, requests that all construction activities near to an existing highway must not cause disruption to the traffic. However, in any case, this road is going to require closing in order to complete the connections to the underside of the bridge.

Proposed Solution

Many highways projects involving bridge construction make use of night time/holidays where traffic will be significantly reduced; a road closure would not affect the local residents as much as if it were to proceed within the day time.

The foundations and supports however have been designed as to be constructed far away from the road so that a closure may not be required. However the highways act 1980 requires suitable hoarding around any construction site near to an existing carriageway. Therefore, when the piers are constructed, suitable hoarding and provision for protection against the public realm must be enforced.

Foundations

The foundations will be cast pad foundations that protrude from the existing ground level. These will be cast in situ with suitable falsework and left to cure subject to engineer’s requirements (usually 48hrs). These will be cast in 3 locations as displayed. If possible, the ground should be graded as to incorporate the pad foundations to allow a flush presentation. Excavators and concrete trucks can access these areas through the existing Lewes road.

Supports

The supports will be made of stainless steel or a steel alloy as with the helical design of the bridge. These will use a very similar methodology to that of the M8 bridge whereas they will connect on a pivot to make up loss of tolerance. These will be prefabricated and fixed to the pad foundation using concrete anchors once the concrete has set.

Bridge Sections

The sections will be delivered in quarters, the first half will be fitted within the Mithras house end and the second will be suspended and attached underneath via platform on the Lewes road. It is at this point of the construction process that we will require a road closure. The sections will be delivered by flatbed wagon.

Ramps and Stairs 

The ramps and stairs towards the Mithras house end will not require a great amount of engineering to install as the design intention was to use the majority of the topography before installing ramps ( as specified in the DMRB). Both stairs and ramp at this end will be prefabricated and fixed to the ground.

Towards the other end however there will be a greater height of which to comply which can lead to a requirement in great lengths of ramp. These will also be prefabricated but most likely bought in sections, again with a similar design process to the other side of the bridge, by fixing to relative foundations. The railings and buttresses will be made of similar bridge materials. And the ramp possibly of cast reinforced concrete with waterproofing membrane.

Here are some sketches and annotations showing our plan of construction methods  for the proposed bridge model. We followed the numerous steps in order to create a realistic model at a scale of 1:75 of the actual size. However there were a few changes to the methods of which we felt were needed in order to produce a better final model. These are shown in the Changes and amendments document.

M8 Harthill Pedestrian Bridge

The project involves building a footpath over the Lewes Road which, despite its speed limitations, can get quite busy throughout the day. Building this bridge over a road as important as this one can be quite a challenge and therefore I have decided to study this in detail.

The M8 Harthill Pedestrian Bridge, designed by Burohappold, had a very similar problem in the instance that it had to help the local residents cross the busy road safely and simply. It also uses a lightweight helical structure as per our design.

The bridge uses materials that maximise durability and minimise maintenance, something that would be also very apparent in our design. The construction of this bridge was completed in prefabricated sections which were then raised onto the piers from the road and fixed onto these supports.

If we take this piece of evidence and apply this to our design, we can see that this will be similar to how we can procure the construction of the footbridge. This will entail forming the supports before attaching the pier via a crane. This may entail a partial close of lewes road and deliver prefabricated sections to site.

Reference: http://www.burohappold.com/projects/m8-harthill-pedestrian-bridge/

Formation and Assembly of Detail Model

During the model making process, I was tasked to complete the detail model. I decided to do something that can show elements that haven’t been presented to the overall model, and in that case, this would be the staircase and bike ramp.

I started by sketching out the dimensions of the staircase as to the design manual for roads and bridges specification.

I decided that the easiest option for producing the detail would be to make it out of foam board. I measured out the stairs and scored the board before folding it into the desired position.

Then using a glue gun, I fixed it into the desired place.

I then undertook a similar process for the sides back and underneath of the stairs with Drew from Hazelwick Sixth Form. This formed the main stairs.

Then using pins and art straws, we fixed the railings to the side of the stairs and glue gunned the banister on. Finally, we added the bike ramp which was made from a model of an I beam. This was also fixed into place with a pin as to represent the concrete anchor which would be the fixing in real life.

The final detail was scaled at 1:10.

I hope to make annotations on the model as to show literally where this complies with DMRB guidance.

Concept Sketch

Our group discussed that due the size of our bridge, from an engineering and feasibility point of view, a beam bridge would be most suitable. However, beam bridges are not ‘fun’ bridges and we wanted to do something that took a step up from that. Jenny showed us some great images of a helical bridge and we decided that it would be good for us to do something similar.

Once we agreed on the bridge, we discussed possible materials we could use. Once we made a list of these materials, I formulated a concept sketch so we could all get a ‘feel’ for how this will work when it comes to the modeling process.

The only amendment since sketching this drawing would be the notable 1:10 gradient which we now agree will be 1:20 or 1:15 with intermediate platforms.

Introduction

The bridge needs to be efficiently coordinated to meet with the various codes of practice listed by the government. In the instance of this footbridge, I have studied British Standards and The Design Manual for Roads and Bridges. Within this blog, I hope to list, in brief, the various parts of these standards that meet the environment that we are designing our bridge to.

Our intention as a group is to give each other various headings to talk about during our presentation, it also gives us an idea of how to split our poster.

The standards mainly used to generate this list include:

TD 27/05 – Cross sections and headroom – Available via CIS

BD 74/00 and BS 8004 – Code of Practice for Foundations – Available via CIS

BD 29/04 – Design Criteria for Footbridges – http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol2/section2/bd2904.pdf

Bridge Foundations (BD 74/00 and BS 8004)

• Abutments are defined as substructure
• If the soil is found to be non-cohesive, bridge piers will need an element of reinforcement/safety put in place to avoid further settlement
• Important that in the excavation of these foundations that elements of water control is put in place. Piers built in water logged ground can rise and dimensionally change the bridge design
• Loads to be distributed evenly to all foundations
• Eccentric loading of foundations due to any cause other than wind pressures should be fully investigated
• Excavation near to existing structures can reduce the stability of their foundations
• Foundations need to resist against lateral forces such as wind

General Principles (Section 2 of BD 29/04)

• Design against vandalism
• High scrap value materials not to be used or to be secured efficiently so that no damage is done to the structure
• Consideration into visually impared users as well as mobility impared
• User groups are key to the bridge design, orientate to facilitate the main user

Layout (Section 3 of BD 29/04)

• Maximise topography use before including ramps
• People walking towards the bridge should be facing traffic where is provisional
• Limit access from the road
• Access stairs and ramps are the most environmentally damaging and should be avoided as much as possible
• Suitable guard rails should be installed to encourage people to use the structure
• Use existing foliage to minimise visual impact of the road
• Accumulation of rubbish underneath stairs needs to be considered
• The user should never be concerned for their personal safety at any time
• Aesthetically pleasing
• Visually pleasing from all viewpoints

Bridge Supports (Section 4 of BD 29/04)

• Foundations to be designed as to cause minimum delay to traffic during construction

Design Standards (Section 5 of BD 29/04)

• Minimum thickness of steel components is 6mm

Dimensional Standards (Section 6 of BD 29/04)

• The horizontal clearance from the edge of the carriageway to the bridge supports shall be a minimum of 4.5m
• Ramps and footbridge should not be less than 2m wide
• Ramps can be continuous at 1:20 level
• Gradients of the bridge should be no steeper than that of an access ramp
• Stairs to comply with BS 5395
  • No more than 13 stairs in a single flight
  • A maximum of 3 successive flights may be used in a line
  • Risers and tread of each stair to be uniform
  • Risers should not vary in height
  • No more than 150mm for a riser
  • Tread width no less than 300mm and no more than 350mm
  • Landing lengths shall not be less than 2m
• Riser should be sealed or perforated
• No steeper than 1:20 for a ramp unless agreed with local authority, no steeper than 1:15 preferable, definitely no steeper than 1:12
• Cycle paths should not ‘link up’ with the ramp
• Landings for ramps at 1:20 should be a maximum rise of 2.5m
• Ramps steeper than 1:20 should have a max rise of 650mm
• Landings should not be less than 2m

Parapets (Section 7 of BD 29/04)

• Parapets are required on all bridges
• Handrails should be provided for stairs and should be no less than 900mm and no greater than 1000mm
• Handrails to have a diamater of 50mm
• Handrails to have a different colour to the parapet as to help the visually impared

Enclosed Footbridges and Clearance Gauge (Section 8 of BD 29/04)

• Where it is likely that things can be thrown from the bridge, this should be enclosed
• If very windy or unsafe, bridge should be enclosed
• Headroom inside enclosure
  • Pedestrian only 2.3m
  • Pedestrian and Cyclist 2.4m
  • Equestrian 2.7m
  • Equestrian (mounted) 3.7m
• Primary structural elemetns should not penetrate the enclosure

Drainage (Section 9 of BD 29/04)

• Drainage is required
• It should not spill onto the users below

Surfaces (Section 10 of BD 29/04)

• Materials on surface need to be slip and corrosive resistant.
• Decks should be waterproofed
• No gaps larger than 12mm in walkway

Lighting (Section 11 of BD 29/04)

• Footbridges should be illuminated
• Should adopt onto existing lighting supply, consider parapet lighting, parapets cannot be used as cable ducts

Requirements for Combined Use by Pedestrians and Cyclists or Equestrians (Section 12 of BD 29/04)

• Paths can be shared or segregated dependent on use
• Segregation widths
  • Unsegregated – 2m
  • Segregated by railing – 1.95m
  • Segregated by line – 1.5m
  • Segregated by kerb – 1.75m
• Minimum footbridge width is 3.5m
• Should be able to see through the bridge whereas curved bridges could cause crash incidents
• Suitable signs warning of cycle shared footbridge to be included

Cross sections and headroom (Table 6-1 of TD 27/05)

• Minimum crossing level is 5.7m+Sag