This week we were presented a problem by consulting engineering firm AECOM, who employ over 100,000 individuals from around the world. We were visited by Mike Hackney and Chris Curley, both of whom work in the Glasgow office as part of the bridges team at AECOM.
As a group we were given the task of tackling the initial stages of conceptual design for a bridge crossing a motorway which was going through the process of dualing to increase capacity.
This was a particularly interesting problem as during all of my studies so far at Strathclyde, the focus has been on large scale bridge projects with grand designs and relatively large spans. This was my first exposure to the fact that although in many ways much simpler, the work of designing a common motorway fly-over requires just as much thought and technical input.
We were provided with a set of drawings detailing the plan and elevations of the proposed new roadways, and were asked to provide conceptual elevations and sections of our proposed bridge structure. These conceptual designs were to be restrained by a client brief, which imposed a number of restrictions and pre-requisites of any proposal.
The first task was to evaluate the spans we would be dealing with. Looking at the plan drawings we established that the road held aloft by the new bridge was to be 9m wide, that the span of the bridge would be a total of 46.6m including minimum clearances on either side of the verges as set by the client, and that we had space between the new North and Southbound lanes for an intermediate support.
With these factors in mind we decided that the most economical solution would be to reduce the maximum span of the bridge by taking the opportunity presented and introduce an intermediate support structure. This would allow for a 2 span continuous beam structure, which we knew would allow for a better span:depth ratio in whichever material we should choose.
Continuing on the theme of creating an economical design as specified by the client, we decided to employ integral reinforced soil bank-seat abutments for our end supports. These had several advantages: they would remove any reliance on an engineered soil slope, which had a maximum slope of 1:2 which would actually increase our span length; they were cheaper than a shear solid concrete abutment as they used less material; and they allowed for a bearing-less design, something which would improve drive quality across the bridge and reduce maintenance of the substructure.
Further economising could be made through introduction of spread foot foundations rather than piles. It was determined that this would be an allowable solution based on the shallow depth of the basalt bedrock under the site.
With these considerations set, the only thing left was our main span material. Initially, based on the larger of our 2 spans following the introduction of an intermediate support, we looked at span:depth ratios for a number of solutions. This was an important consideration given the restraint of a minimum headroom on the lower road surface. After applying this simple technique we were able to cut our available options further by assessing which materials would increase buildability of the final design. It was decided in the group that the most easily constructed option, which would also maintain quality control, would be a number of pre-fabricated beams craned into position before placing atop these a number of pre-cast concrete slabs, which would carry the road and services. With these considerations made, it became clear that the solution offering the fewest crane movements, and therefore safer construction, was a fabricated steel plate and web beam.
With our finalised conceptual design chosen, I was then given the opportunity to present our ideas to the room at large and justify the choices that had been made.
Our Final justifications for choosing to utilise a pair of steel beams with discrete columns on pad footings as intermediary supports and full height integral supports at the abutments were as follows:
- An allowable span:depth ratio
- Good quality control over prefabricated steel and concrete elements
- fewer crane movements than other options, increasing safety and reducing closure time of the road below
- smaller spans leading to less material used
- open abutments and columns meaning clearer sight lines for drivers
Over all this was a highly enjoyable exercise which opened up my thought process during the design of bridges, to the reality that even at an early stage in the design process, keeping a thought aimed on achieving client goals and priorities is always necessary.