ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306198
The transition pier showing the ear walls to hide the ends of the precast, prestressed concrete beams. Photo: Dean Van Landuyt, TxDOT. For more information on this or other projects, visit www.aspirebridge.org. A E S T H E T I C S C O M M E N T A R Y by Frederick Gottemoeller "Having an attitude" is usually considered a negative. But, if designers want to accomplish something worthwhile, they have to "have an attitude" toward the features of their bridge. It's another way for saying that they have to have a vision of what they want to accomplish, not just for the technical features, but for the aesthetic features as well. For girder bridges, and particularly for concrete box girder bridges, a key decision is the relationship of the pier to the girder. Does the designer want them to be seen as separate elements, with the girder foating above the pier? Or does the designer want them to be seen as a single monolithic shape, with the pier blending into the girder? Actually, that decision should be made not on visual grounds, but on structural grounds. If the bridge is designed with bearings at the piers, then that fact should be evident, and the pier top should be attenuated to demonstrate the presence and role of the bearings. If the bridge is designed for the girder to act monolithically with the piers, the girder and piers should physically blend together. An excellent aesthetic result can be accomplished with either approach. Designs that fail aesthetically often do so because of an attempt to make one approach look like the other. The box girder bridge at Matagorda is an excellent example of blending girder and piers together. The girder and pier are shaped similarly in a simple but sophisticated way. The planes of the girder sofft turn and become the planes of the pier shaft. The obvious similarities between the girder and the pier ensure that the bridge is perceived as a sin- gle integrated entity. At the same time the recess between the pier halves, perceived as a dark vertical line in the daytime and as a lighted vertical line at night, punctuate the bridge and give it an additional level of interest. All of this is accomplished with the shapes and sizes of the structural elements themselves, the elements that have to be there anyway. Nothing (except the lighting) is added solely for aesthetic effect. with the projected path, the channel-side travelers were inadvertently removed. While somewhat mitigating the wind effects, it further unbalanced the longitudinal moment on the main pier. The bridge was left with a 160-ft-long cantilever supporting a 100-kip traveler and a 152.5-ft-long cantilever with no traveler. Immediately upon discovery, the channel-side cantilever was counter-weighted with timber mats and concrete blocks. While the longitudinal moment was brought back within design limits, a larger issue remained. The first severe hurricane in 50 years was heading toward Matagorda and the bridge was in full cantilever, supported only by a column with minimal torsional strength. While meeting the AASHTO LRFD Bridge Design Specifications requirements for wind, engineers were concerned that an absence of unbalanced wind loading requirements in the specifications left the structure vulnerable. An analysis of the structure based on eccentric wind loading conditions established by ASCE 7 revealed that the pier could experience torsional moments equal to about twice the cracking moment. A cable guying plan was enacted that would reduce torsion to nearly half the cracking moment. The end of the side span cantilever was secured to the ear walls cast on to the transition bent. Fortunately for the bridge, the hurricane turned north shortly before landfall, leaving the bridge on the "good side" of the storm. The maximum sustained 1 minute wind speeds were only 58 mph and the bridge suffered no damage. Lighting Engineers wanted to produce a sliver of blue light on either side of the channel to make the bridge unique to Matagorda and recognizable by those using the waterway. After much consideration, including photometric studies, a series of individual induction florescent fixtures was located up the sides of each of the four openings. These fixtures were chosen because of their long life (more than 20 years) and light intensity. Unfortunately, they can only produce white light. There was a fear that because the lights were so bright and only located 4 ft from the center web, the light would reflect mostly white hot. However, this was not the case, helped in part by the shade of blue paint chosen. The plans required that four large test panels of varying shades of blue temporarily be affixed before painting. A dark color was chosen for its ability to reflect a variegated light-blue/white fusion. The 46 fixtures and accompanying electrical materials cost $75,000—a small percentage of the $20 million project cost. The bridge was opened to traffic in the summer of 2009 and the old swing bridge dismantled shortly thereafter. The frequent traffic stops that provided for the passage of barges and pleasure craft are now a thing of the past. The future of the town and peninsula will undoubtedly be influenced by the improved accessibility and distinctive architecture of the segmental bridge. ___________ Dean Van Landuyt is a senior design engineer for the Bridge Division of the Texas Department of Transportation, Austin, Tex. ASPIRE , Winter 2010 | 23 ASPIRE_Winter10.indb 23 12/18/09 2:17:50 PM