This post is the second article in the series on concrete bridges. It aims to address the primary considerations and processes involved in the selection of a concrete bridge. In the last post, we submitted that common bridge parameters such as, typical spans, clearance requirements, alignment and aesthetics would start to suggest which bridge type is suitable and appropriate. However, making a final choice requires a lot more consideration, and depends on other important parameters, many of which correlates between construction program, cost and risk.
In contrast to other conventional structures, for bridges, it is the construction method that determines the actual design solution. This is based on the fact that, no bridge can really be designed without prior knowledge of how it will be built. The construction method has a large influence on the forces in the bridge as well as many of it details. Construction method in turn is influenced by other factors such as site constraints, with the overall program and speed of construction playing a key part in the break down and cost. Quite often it is the optimum solution that get selected however, a skillful team would always ensure that the same scheme offers the best value to the owner.
Constructing a Bridge
Before the factors affecting the selection of a Bridge type can be discussed, an understanding of the process of constructing a typical bridge is required. The construction of a bridge can be split into three broad stages. Casting, Transportation and Erection.
Casting relates to the types of form work that will be used and falsework for the bridge components. It also relates to the choice of the bridge type. Casting must consider the extent of the landscape available for use (Figure 1). Transportation on the other hand relates to how pre-cast options would be moved from the casting area to the erection area. This would involve the use of pulley systems, rail systems, straddle carriers, wheeled booties etc. Erection is the use of mobile cranes and falsework lift, to fix and support the precast unit in place (figure 2). These falsework are in form of scaffolds, steel beams, girders or trusses that need to keep launching as the construction itself evolves. This movement can be via a dismantling and re-erection process, or by using gantry systems that travel forward. The bridge and all its falsework must remain stable during all these operations, which will often need several other items of temporary works to ensure that stability is not compromised or that the permanent works are not adversely affected.
Access for all these items of construction plant is important for both buildability and the speed of construction, with the ease and degree of access being crucial to the way in which the project is programmed. The programme will also need to take account of utility diversions or installations, and any traffic management, which can be either on the bridge, over it or underneath it. The particular supply of materials, labour and plant will vary from site to site, and in relation to the site’s remoteness.
Selection of a Bridge Type
As explained in the introductory part of this post, the factors influencing the choice of bridge type are all connected to the available method of construction. This factors can be viewed from three holistic angles vis a vis:
Many of the decisions regarding the type of bridge deck to use, is driven by issues related to construction programme. In every construction project, the need to increase speed and simplicity of the construction process cannot be overstated. Faster construction will often lead to a reduction in the duration of overheads, which in turn leads to reduction in the overall cost of the project. Hence, any step taken to increase speed will generate savings and ultimately lead to a increase in the overall operational benefits. Alternatively, faster construction method may be borne out of the desire to minimize disruption to traffic. In extreme scenarios, more expensive construction method might be adopted in order to realise this benefits. For example, within the railway environment where cost of obtaining possessions is very high.
The design decision seriously needs to consider buildability. Thus, the total involvement of the design team will yield significant cost benefits. The resulting details will reflect the contractor’s requirements, while safety issues can be better recognised and the cost implications of decisions can be readily assessed. Sufficient time must be allowed for designers, contractors, sub-contractors and suppliers to adequately design, plan and execute the scheme in relation to any restrictions. The owner will be an essential member of this process and will have an active part in the discussions, resolving any issues with third parties. Simplicity is a key element for efficiency – simple and standardised detailing should also allow for easier and more rapid construction.
Road closures, diversions & railway possessions are serious programme issues, can be a very expensive venture and could significantly affect the speed of construction. On railway or water-way, the usual decision is to close the route for the duration of the works, whilst on a highway, it’s common to divert the traffic. Such activities must be planned ahead of time, and so the role of the owner is very crucial. Railway possessions almost always will require months of notice whereas an highway diversion could take only weeks as long as there’s no serious disruption to traffic. The phasing of the works must as well suit these diversions or closures at a very early stage, else several construction methods might become impractical. For example, a single box girder across a carriage way is impossible. Thus if the highway is to be built in two phases, then alternative beam section would be favoured.
Having considered the programme issues, the next set of issues has to do with costing. Costing is another important consideration in the selection of a bridge type. The quantities of materials required for each plausible scheme must be known before-hand. This will appraise the design team at arriving at the best scheme. For most bridges, the decking seems to be the definitive factor influencing the material quantity. To have a feel of this material quantity, the effective thickness of the decking must be known. This is defined as the ratio of the total concrete volume and the surface area of the bridge deck. A number of charts have been produced in publications by institutions, that can be used in estimating this effective thickness. However, as a first step the bridge deck could be checked against historical data of similar bridges. Having determined the effective thickness, the required volume of casting can be determined as well as the required reinforcing steel bars and prestressing tendons estimated .
Once the quantities have been determined, it is now possible to estimate the cost of the bridge deck. The total cost should be the gross rates, i.e. it should incoporate the combined cost of material, labour and supplies.
However a difficulty may arise in the pricing of concrete bridge decks, with regards to hoisting of the deck concrete in place i.e. the combined cost of form work and falsework. This difficulty arises from the fact that there are several number of construction methods available, each of which is distinct enough to be priced differently. Thus, a combined formwork/falsework rate must be calculated using the breakdown of the principal construction stages. This again emphasizes the need to consider buildability at the programme level.
Having obtained an initial estimate of the material quantity in the bridge deck, including a pricing for the formwork/falsework. The design team will then be able to price a full range of options available for a scheme size. For a detailed pricing a rigorous programme and cost exercise would need to be carried out.
To make comparison between different schemes, the cost of the sub-structure is required. Though for most bridges, the variation in cost of constructing the sub-structure is usually not large enough. For example, even if the self-weight of a steel-composite deck is close to half that of concrete, the combined differences at foundation level (including finishes, eccentric traffic loads, lateral loads and substructure loads) are usually only 15-20%¹. As the substructure might represent about 30% of the total bridge cost, the effects of the reduced weight of the steel scheme are then only around 5%, which certainly needs to be accommodated but is not dominant².
Having considered programme and cost issues, the last but certainly not the least consideration falls under the umbrella of risk. The entire project team must assess the risk involved with choosing a particular scheme over the other. Of utmost importance is the need to ensure that the project falls under the health and safety regime of the project site and vicinity. Besides the usual factors associated with working with concrete, there will always be others related to the construction methods. For example, transportation and erection of heavy bridge components, stability of temporary works etc. Factory methods that form part of many techniques can significantly improve safety regime by shifting the works to a more regular and controlled series of operations, with a workforce who have become familiar with the process². For example, many construction mundane tasks could be removed through the use of mobile cranes or other heavy lifting equipment, resulting in a reduction on the need to work at height. Launching (a technique of construction bridges) in particular, require limited numbers of men to work at height, as the majority of activities take place behind an abutment (Figure 5).
To adress all of this issues the owner must make a health and safety assessment, considering the residual risks involved with, construction, operation, maintaince and possible demolition of the bridge in future. Overall, whatever decisions or assumptions are made on the project site, their adoption must never compromise safety.
In summary, the available construction method has shown to be the key in the selection and decision table for the most suitable bridge type. Firstly, the programme issues were identified and appraised, and then tied to issues of costing and finally the associated risk issues. Indeed before a bridge can be designed you must know before hand how it will be built, what challenges could be encountered, this will lead you to the possible costs benefits and the associated risks involved. The next post in the series will consider the different types of concrete bridges available.
Also: See Introduction to Concrete Bridges
1. Concrete Bridge Development Group (2005) Technical Guide No. 5: Fast Construction of Concrete Bridges, Camberley, UK: CBDG and The Concrete Society
2. Concrete Bridge Development Group (2014) ‘Concrete Bridge Design and Construction series No. 4 Types of concrete bridge ’, The Structural Engineer
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