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/306189
As part of a plan to revitalize riverfront commercial development, the city of Laredo, Tex., required a solution to provide easier access to the area. To reach the riverfront, commuters had to negotiate a route of heavily congested, narrow, streets through business and residential areas. The i d e a w a s p r o p o s e d t o b u i l d a n elevated connector from Santa Ursula Avenue, an access road from I-35, to the riverfront and thereby avoid the congestion of the city streets. The Santa Ursula Connector was proposed to be 31 ft wide and 1155 ft long. The bridge would be required to connect the higher elevation Santa Ursula Avenue to the flood plain. It required spanning over federal property between two international bridges on the Texas-Mexico border. Because the majority of the bridge is below the design high water level by as much as 25 ft, it was necessary to design the bridge to withstand the flood forces of the Rio Grande River. The engineer's first task consisted of a feasibility study. The study was followed by the layout and design for the bridge, and coordination with the city of Laredo, Webb County, local utility companies, and federal agencies i n c l u d i n g t h e G e n e r a l S e r v i c e s Administration, Customs and Border P ro t e c t i o n , a n d t h e I n t e r n a t i o n a l B o u n d a r y a n d Wa t e r C o m m i s s i o n (IBWC). T h e I B W C re g u l a t e s c o n s t r u c t i o n within the floodplain; in order to gain their project approval, the impact to the river had to be minimized. T h e re f o re , t h e b r i d g e re q u i re d a shallow superstructure to reduce the impact on the river during a flood event. Additionally, a sharp horizontal curve was required to allow space for a future "up ramp" connector to be built. Finally, methods were needed to resist forces from a flood. Design Challenges The project presented several design challenges. The requirement for a future "up ramp" adjacent to the Santa Ursula Connector down ramp, necessitated a very sharp horizontal curve with a radius equal to 340.5 ft. It was desired to expedite construction within the federal property. Security within federal property (especially areas at border crossings) is extremely stringent. Workers required being on federal property needed to go through a lengthy federal security background check. Completing construction in secure areas as quickly as possible was a critical aspect of the design. Finally, due to the bridge's location, an aesthetically pleasing structure was necessary. Design Solutions T h e T e x a s D e p a r t m e n t o f Transportation's (TxDOT's) 15-in.-deep slab beam was selected to provide the minimum superstructure depth. The slab beam has a rectangular cross section and is available in both 4 and 5 ft widths. The typical cross section was framed with four, 5-ft-wide slabs and two, 4-ft-wide slabs. The slab beam provides a superstructure total depth of just 20 in. including a 5-in. deep cast-in-place composite concrete deck. Because the beams are abutted side-by- side, debris could not become lodged under the bridge during high water. profile SANTA URSULA CONNECTOR / LAREDO, TEXAS br IDgE DESIgN ENgINEEr: Structural Engineering Associates Inc., San Antonio, Tex. prIME CoNTrACTor: Concho Construction Co. Inc., Garland, Tex. prECASTEr: Bexar Concrete Works Inc. Ltd., San Antonio, Tex. AWArDS: 2010 PCI Bridge Design Award, Best Bridge with Main Span up to 75 ft; 2011 Texas CEC Engineering Excellence Awards, Silver Medal Winner, Category C: Structural Systems by David T. Covarrubias, David A. Rocha, and Jesse S. Covarrubias, Structural Engineering Associates Inc. The Santa Ursula Connector The completed Santa Ursula Connector in Laredo, Tex., curves downward towards the Rio Grande River front. All photos and drawing: Structural Engineering Associates Inc. 38 | ASPIRE , Fall 2011 Book_Fall11.indb 38 9/29/11 11:59 AM