Sunday, April 5, 2015

Highway Bridge 30

The location of this span is near the Oregon Convention center.  The span’s function is a highway to carry traffic towards Interstate 5 towards North to Seattle.  It is highway 30.  
The superstructure is steel girder with varying depth to span ratios.  The smallest span crosses a two lane road and pedestrian sidewalk.  It is approximately 30’.  The Steel girder for this section of the span is smaller than the previous span.  Thus it has a smaller depth girder.  The smaller girder sits on the bent cap next to the larger depth girder.  There are two different thicknesses for the bent cap.  The bent cap rises to meet the smaller girder to create a level transition. 
The superstructure depth to span ratio is bulky.  The proportion is not ideal.  The span of 30’ has nearly the same girder depth as the previous girder which seems to span 90 or more feet.  This might be because the much smaller girder would require another bent cap.  Further study is needed to find the right depth to span the 30’.  It does however match the next girder’s depth.  Perhaps the designer decided to match at least one girder to create a smoother transition and to make the bent cap seat the same for the right bent cap.  One option is to make the girder the same size as the previous girder.  However, the need for clearance might have motivated the designer to narrow the depth of the girder as much as can be done.  Most like the depth at this section was to match the next girder and to make ease of construction and detailing a priority instead of structurally expressing the span to depth ratio of the girder.  The designer could have celebrated this section of span as a gateway to the city with a smaller girder and creating haunches to express the gateway towards the city.  As is, the column land haphazardly on the pedestrian sidewalk and the other one extending to another level below.              
The bent cap is connected to circular columns.  The bent cap cantilevers beyond the circular column on both ends.  The cantilever part of the bent cap could have been drop to express the structural proportions.  The deck protrudes beyond the steel girders creating shadows to differentiate the different levels.  The effect is to reduce the mass of the structure and expresses each part of the bridge structure.  Underneath the girder there appears to be diagonal cross bracings which look to be approximately the same depth as the girder but flush with the bottom of the girder.  The railing is concrete which makes the bridge look bulky.  An option is to use metal lattice railing.  But the thin concrete railing is much smaller than the steel girder that one hardly notices it.  Perhaps the concrete railing provides more of a visual stability for the automobiles which could crash into the railing and fall onto the highway below it.  Some of the support is shew because there is no room for the column to land due to serval lanes coming together at various angles which intersect.   
Overall the design is not well thought out and the transitions clumsy.  As is typical of most highway bridges, the way it is designed, priority is given to practicality and utilitarianism.  There were opportunities to express the structural proportions in the depth to span ratio and create a unique gate way for the traffic heading toward the city center.              

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