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Feasibility study (aesthetics, bearings and maintenance)
The aim of this assignment is to implement a design for a footbridge over Fore St which will provide direct entry into the new building above street level that is to be designed by the Building groups in which we will be working in conjunction with to provide appropriate calculations for the structural elements of said design.
After extensive research the design for the bridge has been a subject of much discussion and the final design has yet to come to fruition as this will happen over the course of the year resulting in the final presentation by the group in semester two. Whilst researching the design of the proposed bridge images of the Bishops Stortford Goods Yard Footbridge were looked at and it was decided as a group that we would like to have some similarities between our design and this quite unique structure. We discussed the idea of partaking in case study of this structure to obtain a more in depth ideas as to the design and cost elements of a structure of this type with this report focussing on the design life, shape, aesthetics and maintenance requirements of the proposed new bridge also to be analysed is a brief method of maintenance with regards to bearing plates/joint replacements Painting etc. and how this might affect its use.
In April 2007 East Herts. Council's Development Control committee gave the green light to a new £1.5m replacement. The Council was concerned about any disruption caused by the project thus installing a temporary crossing when the old bridge was to be closed. To save time, Construction firm Birse Civils agreed that most of the work would be carried out off site - meaning the new bridge could be slotted into place in just four months. (www.eastherts.gov.uk).
This procedure would benefit us in our bridge design/construction as only the foundations for the piers would need to be built in-situ thus avoiding the disruption it would cause by constructing the bridge deck itself in-situ.
The design for the Goods Yard Bridge was drawn by Chetwoods Architects for the East Hertfordshire District Council. Structural Engineers Gifford (www.gifford.uk) had to immediately address the very difficult issue of how long ramps could be be accommodated in a confined site, which would eventually be achieved with a light and transparent structure which still had to adhere to the maximum permitted gradient of 1-in-20. The bridge structure itself comprises a simple lightweight steel tubular spine beam, gently curved in plan, which is supported on steel cantilever arms which were formed by way of piled foundations on the river banks that was manufactured by Rowecord Engineering Ltd (http://www.rowecord.com), who specialise in the design, fabrication and erection of steel framed structures. The steel deck is fixed to its supporting cantilever arms by bolting so as to avoid the need for bearings where they would be difficult to inspect out over the river. This type of design may differ from the proposed design we are using, as a bearing pads may offer more appropriate and cost effective solution for the type of bridge to be designed.
Aesthetics for design
Bridges tend to be dominant features in both rural and urban contexts and usually are expected to have a minimum of 120-year life. Although some smaller steel structures may have an estimated design life of approximately 60 years.
Therefore, although function and utility are paramount, aesthetic considerations should also weigh heavily in their design and execution. Sadly this is by no means always the case.
Apart from personal taste there are some specific aesthetic factors that may need to be considered in the design of bridges and these have to satisfy several different points of view, for example, that of motorists, passengers on buses and trains, pedestrians and cyclists, as well as those living and working in the vicinity of the bridge. As a minimum the following need to be investigated External appearance of the bridge and it whether it blends into to existing environment, the internal view of the bride while crossing and the Sequential view which is the changing view of the bridge on approach (www.corusconstruction.com). The design of our bridge is such that we have considered that it does fit with the local surroundings. Although this is an established part of the town redevelopment in recent years to the area around the docks and of the new waterfront building means the proposed contemporary design for our bridge will not only meet the aesthetic requirements it could also help to enhance the appearance of the area.
It is generally recognised that the aesthetic of bridges with an odd number of spans is superior to that of bridges with an even number of and that the proportion of those spans also has a considerable influence on the bridge's appearance. A larger central span is generally more pleasing to the eye, which will not affect our design as there will be no central pier as the location and type of design of our bridge is such that a central pier is not only not required it would not be feasible as there it would block access along the major route that is Fore street.
Bridge bearings considerations
Bridges are subject to horizontal forces resulting from wind, and the braking of road vehicles or trains passing over the deck. They are also subject to movement due to thermal expansion and contraction caused by change in ambient temperature. These forces and movements are absorbed and controlled by supporting the bridge on a system of appropriate fixed, rotating, elastomeric and/or sliding bearings.
Being a footbridge traffic on the deck will not affect our design however the position of the location of our bridge being adjacent to the docks does make for a windy, cold environment particularly in the winter months, all factors that need to be considered in the design phase and indeed when partaking in a feasibility study.
Lateral forces must be transferred to suitable supporting structures in such a way that stability is maintained at all times whilst no undue forces are induced in the bridge deck or supporting structure. For beam type bridges, usually one support has fixed bearings, to locate the bridge in position in the longitudinal direction (along the deck) while other supports have bearings that allow movement in the longitudinal direction to accommodate thermal expansion/contraction of the deck (www.corusconstruction.com). The ideal location for the fixed bearing would be at the end adjoining the connection into the new building as this would lead to less disruption when maintaining.
There are three main types of bearing used for bridges these being, Elastomeric bearing which allow movement through vertical loads and also allows rotation, Plane sliding which does not allow rotational movement in the longitudinal or transverse direction only the vertical, and roller bearings which mainly allow for large longitudinal movement (www.childs-ceng.demon.co.uk). The system needed would have to be thoroughly investigated at design stage however a suggestion would be to use the plane sliding bearing because the bridge should not encounter much rotational movement as it would not be under extreme load. Whatever system is used in design need to conform to the following specifications;
Design Standards for Bearings
i. BS 5400: Part 2: Specification for Loads
ii. BS 5400: Part 9: Code of Practice for Design of Bridge Bearings
Design Manual for Roads and Bridges
i. BD37: Loads for Highway Bridges
ii. BA42: The Design of Integral Bridges
iii. BD20: Bridge Bearings, Use of BS 5400 Part 9
Many bridges span over water and/or are subject to the effects of de-icing salts. This can lead to severe corrosion of the steelwork unless adequate protection is applied. The ease of future maintenance should be considered at the initial design stage.
Corrosion protection can be provided in the form of a conventional multi-coat paint system, high-build coating system, by the use of weathering steel and/or by enclosure to reduce contact with atmospheric contaminants. As mentioned most bridges are designed to last for at least 120 years, therefore, adequate provision should be made for the inspection and maintenance of the finishes. Access is highly important in this respect as corrosion often occurs in hidden locations (such as around the bearings when water penetrates through expansion and movement joints), access to the bearings should not be an issue as all fixings will off the road and access will be gained from the pedestrian walkway, part of which could be sectioned off at the time maintenance needed to be performed. Some bridges need almost constant maintaining with the Forth Rail Bridge is a good example of this whereby constant painting has to be undertaken to halt the onset of corrosion. Basic painting systems are based on the use of conventional blast primers, primers, undercoats and finishes.
For highway bridges, the systems specified by the UK Highways Agency involve the application of up to six coats to produce a total dry film thickness of 200-300 microns following appropriate surface preparation. Selection of a suitable paint system is determined by the environment in which the bridge is situated (marine or inland) and the accessibility of the component in the bridge (ready access or difficult access).
High-build painting systems, based on 2-pack, chemically cured resins have been developed mainly for steel protection in the severe environment of the North Sea oil and gas extraction structures but this technology may also be applied to the protection of steel bridges. There are two main types of these paints which are
Elastomeric Urethane Coatings Which can be applied to film thicknesses of 1000 microns in a single coat. Specialist equipment is required and controlled shop conditions are needed to achieve a satisfactory coating. The life to first maintenance is anticipated to be well in excess of 30 years.
Glass Flake Epoxy Coatings which are capable of similar coating thicknesses to the elastomeric urethane types and produce extremely tough, abrasion resistant, durable coatings (www.corusconstruction.com). Both of these types of coating would offer a solution to the maintenance issue of road closures while renewing paintwork however these tend to be a more expensive option
Weathering steel is an alternative that contains up to 3% of alloying elements, which, on exposure to air, form an adherent protective oxide coating.
During the early part of their life, weather resistant steels corrode in a similar manner and at a similar rate to mild steels. As the protective layer develops, however, the corrosion rate falls to a low level and depending on the environment within which the steel is placed, a brown to almost black, stable oxide patina is produced after about two years. Corrosion products from the weathering steel may stain adjacent surfaces such as concrete piers and abutments, therefore, suitable drainage should be provided. The thickness of plates within the structure should be increased by 0.5 to 2mm depending on the degree of exposure to corrosive environments to allow for loss of material. Weathering steels are not recommended for highway bridges in certain more severe corrosive environments.