by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : June 2013
28 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES find yourself mentally pigeon-holing structural details and working out in your head how the bridge was built, and what you think of the design. Leaving aside the fact that this tends to annoy your partner, it does enable you to rapidly deduce what sort of market for bridges you are geographically in and how sophisticated their bridge designers are. Of course, you also need to read professional articles and attend conferences, but there is nothing quite like seeing your bridges "in the wild". Based on my own design and construct experience, and my "in the wild" observations, I have shown in Figure 2 a basic range of generic concrete bridge types. Of course, there are also steel bridges (especially in the UK and the US). e concepts depicted relate generally to what has happened in Australia, but from my own observations tell much about what you will see elsewhere in the world. When I worked at Main Roads in Western Australia, prior to the 1990s, most bridge design was done in house by the government. ere was a hierarchy in the Bridge Design Branch. Top of the pecking order were the elite engineers who did the big city bridges. Because these special bridges took around two years to design in those days, we only needed a few projects for the top engineers to work much of their careers on wonderfully inspiring structures. And in between the really big ones, they could practise on the smaller really interesting ones -- possibly even making them more interesting than they needed to be. It wasn t quite like designing cathedrals, but it came close. Second in the pecking order were the city bridge designers -- this is where I spent some time. Each city bridge was individually designed and you felt you were sort of an understudy to the main act. All were cast insitu, and in the bridge types shown we would have produced box girders, then later twin T-beams, and by the end of the 1980s, voided slabs were very widespread because of their ease of construction (especially compared to their forebear, the multi cell box). All of these bridges would now be Super Tees, or occasionally launched. at era came to a grinding halt in the 1990s. Moving to precast Still in the 1970s and 1980s, at the bottom of the pecking order were "rural bridges" -- built out in the sticks we "didn t care" so we put up the "cheapest solution", the NAASRA I-girder. Being at the bottom of the pecking order, perhaps not so much thought was given to innovation, although in Western Australia at least, continuity details were developed which made a much better bridge. Of course, we looked down our noses at Queensland, which used precast nearly everywhere, even "city bridges", and so obviously "didn t care" about anything, but perhaps they knew more than we did, because this was to be the forerunner to where we are today. e I-girder is messy to build because it is inherently unstable, having a very narrow compression zone at the top, which is why it is harder to get spans up to 30m. ere have been a number of construction collapses, including a number of fatalities. One also has to deck out the space between the girders with temporary formwork and possibly falsework, and the edge cantilever is difficult. ere are also many steel I-girder bridges, where the lighter transport weight over long hauls compensated for the extra cost of the steel, but nowadays steel finds it hard to compete over almost any distance against Super Tees. Of course, when we moved to the era of design and construct in the 1990s, construction contractors were very interested in the "cheapest solution", and right about then the Victorians invented the Super Tee. I had read about bulb Tees in magazines and they looked like a pretty good idea to me. e wide top flange is much better for buckling stability under the "wet concrete" load as the deck is cast. I recall them promoted as a French idea, but I don t think I have ever seen one driving around France. In fact in France, Holland, Belgium, Germany and Switzerland, I have observed in recent years that many of these "unobtrusive" bridges still seem to be cast insitu post-tensioned, like we used to do in city bridges in Perth in the 1970s and 1980s. I have looked for, but so far not seen, anything like a Super Tee and perhaps this reflects the dominance of the post- tensioning companies in bridge design in those countries. I see Super Tees as the next logical step beyond the idea of a bulb Tee, and it is a very large step. Structural efficiency of the Super Tee e Super Tee can really be thought of as a bulb Tee where the web is split down the middle, and put on the outside of the lower flange. is enables the lower flange to be stretched laterally and allows the upper flange to have a much wider "wing span". e result is that for the same deck thickness, you can use half the number of precast elements. e really brilliant thing about Super Tees structurally can be seen in Figure 2 -- it is that four precast elements have eight thin webs. Consider Table 1. Table 1. Effective thickness for bridge type. Bridge Type Effective thickness (m) NAASRA I-girder Bulb T Super Tee Voided slab Twin T Box Girder 0.451 0.533 0.492 0.896 0.654 0.553 e "effective thickness" is just the area of the section divided by the structural width -- the thickness as if all of the concrete was just cast as a slab. I have found this to be a useful quick guide to structural efficiency. Any value below 0.5m is going to be very competitive, all other things being equal. Of course, this cannot always be taken literally, because a twin Tee or box girder are both definitely more structurally efficient than voided slabs, but the simplification of the formwork and falsework made them competitive in the late-80s, until the Super Tee came on the scene. e Super Tee is not only much more structurally efficient, but also has numerous other advantages.