NEW TECHNOLOGY Precast, Prestressed Pedestrian Bridge World's First Reactive Powder Concrete Structure Spanning 797 ft ( 60 m), this precast prestressed pedestrian/bikeway bridge in Sherbrooke, Quebec, is a post-tensioned open-web space truss containing no conventional steel reinforcement. Made up of six prefabricated match-cast segments, it was manufactured using Reactive Powder Concrete (RPC), a cement-based material proportioned with sand, cement, and powders of silica fume (microsilica), and quartz, with fine steel fibers added to enhance ductility. In the top and bottom chord members, the RPC has a compressive strength of 29,000 psi (200 MPa). For the web member Pierre Y. Blais, P. Eng. diagonals, RPC was confined in stainless steel tubes, attaining Senior Partner Le Groupe Teknika improved ductility and a compressive strength of 50, 000 psi (350 Sherbrooke, Quebec MPa). An extensive program monitoring bridge deflections and forces Canada in the prestressing tendons has been implemented to provide information on long-term performance of RPC. he Sherbrooke pedestrian/bike­ segments that were assembled on site way bridge (see Fig. 1), erected using internal and external post-ten­ Tin July 1997, is the world's first sioning. The deck and top and bottom major structure to be built with Reac­ chords are made of RPC with a com­ tive Powder Concrete (RPC). Besides pressive strength of 29,000 psi (200 using this new ultra-high-strength ma­ MPa). For the diagonal web members, terial, the Sherbrooke Footbridge (as it the RPC is confined in stainless steel is also called) encompasses several tubes and can withstand 50,000 psi Marco Couture, P. Eng. other innovations - confinement of (3 50 MPa) in compression. The 10 ft the concrete, the total absence of steel (3 m) deep truss spans 197 ft (60 m) Operations Manager Beton Bolduc bar reinforcement, and pioneering across the Magog River (in dowtown Ste.-Marie, Beauce, Quebec practices in the design and detailing of Sherbrooke) in a circular arch with a Canada precast/prestressed concrete. radius of 1070 ft (326 m) , carrying The bridge superstructure is a post­ both pedestrian and bicycle traffic. tensioned open-web space truss com­ The City of Sherbrooke, Quebec, posed of six prefabricated match-cast wanted to demonstrate its vision by 60 PCI JOURNAL Fig . 1. The Sherbrooke Footbridge in Sherbrooke, Quebec, spans 197ft (60 m) across the Magog River with a precast truss made of reactive powder concrete. committing to the construction of a to­ precasting operations, and erection of standing mechanical properties of RPC. tally new kind of bridge, in effect a the structure. Also discussed is the in­ In order to obtain a minimum length­ technological window located close to strumentation and monitoring pro­ to-depth ratio of 197 to 10 ft (60 to 3 City Hall. Other objectives of the City gram as well as the potential of using m), the design team combined the new were to: RPC in precast/prestressed concrete RPC technology with concepts of pre­ • Connect the American bikeway sys­ applications. casting and post-tensioning. Using an tem with the Quebec-St. Lawrence open-web truss with diagonals of con­ South Shore system. fined RPC, concrete volume can be re­ • Emphasize the aesthetic elegance of STRUCTURAL CONCEPT duced without saclificing rigidity. the prefabricated three-dimensional The Sherbrooke Footbridge was de­ With the very high compressive truss by comparing it with the signed to take advantage of the out- strength of RPC, it is possible to de- nearby old-fashioned steel truss bridge (see Fig. 2). • Take advantage of the long-term durability anticipated for this presti­ gious innovative solution that will require only very low maintenance. • Demonstrate that RPC will be feasi­ ble for use in rehabilitation as well as construction of the municipal in­ frastructure. • Provide a real world research plat­ form for the University of Sher­ brooke research group and Concrete Canada. The bridge was erected at a total cost of $425 ,000 (U.S.) and the exten­ sive monitoring program was imple­ mented at a cost of $70,000 (U.S.). This article discusses the structural concept and design features of the Fig. 2. Pedestrian bridge before railing was insta lled. The new precast RPC footbridge project, new materials technology, contrasts sharply with the old steel truss crossing on the ri ght. September-October 1999 61 ()) 1\) ----------, I 0 __ !.!!909 MAGOG RIVER 0 •' .· /o--- ' ' FRONTENAC STREET '' I PLAN '' '' 10000 15000 --~---- -· -·-·---- ------~~---------------------- · ---~----~1~0000~----- i 29867 29867 WALlNOJNORTH 1PIER N0,2 NORTH 1 PIER N0.1 NORTH PIER N0.1 SOUT H PIER N0.2 SOUTH ANOCE NT ERLINE OF FOOT BAIOGE c.... ~ ELEVATION zJJ f:. Fi g. 3. Pl an and elevation of the Sherbrooke Footbridge. Dimensions are in milimeters. sign a relatively lightweight pre­ stressed structure, have it plant fabri­ cated and assembled on site. 3300 mm (11 ft.) To bridge users, there is also the ~--r-30 mm upper slob of RPC (1-¥16 in) added benefit of enhanced comfort due to low vibration. Because the truss is lightweight and has high overall rigidity, the structure exhibits a first Eigen frequency of 2.5 Hz, even though the live load is within the range of the dead load. ' Oiogonols ---\ In this type of structure, the bend­ (RPC Confined in ' ing moment due to dead and live stainless steel tubes) loads produces compression in the top chord (comprised of upper beams and slab) and tension in the bottom chords of the truss, where the tension is counterbalanced by post-tensioning. Twin RPC bottom chords The relevant shear force results in ten­ sion/compression in the diagonals; here, also, post-tensioning can coun­ terbalance the tension. All other sec­ ondary tensile effects are carried di­ Fig. 4. Cross section of bridge truss, showing interacting stru ctural elements and rectly by the RPC. principal dimensions. Since this is the first major structure built using RPC, the full potential of ture projects, given the feedback from has a 15 in. (381 mm) thick end di­ the material was not exploited, both experience already gained with the aphragm. By using a truss depth of 10 for safety reasons and for practicalities Sherbrooke Footbridge, it will be pos­ ft (3.3 m) , the segments were of a size of construction. The choice was made sible to use even smaller structural ele­ readily transportable by standard drop­ to avoid any tension in the diagonal ments that take maximum advantage of deck trailer. members at service limit state (SLS) the performance capabilities of RPC. A cross section of the bridge truss and ultimate limit state (ULS) and to (see Fig. 4) shows the configuration have no tension in the lower chords and principal dimensions of the inter­ (beams) at service limit state. DESIGN FEATURES connecting structural elements. The The dimensions of both the top and Fig. 3 shows the plan and elevation bridge elevation (see Fig. 5) shows the of the Sherbrooke Footbridge. This bottom chords were dictated by the arrangement of the internal and exter­ space needed for crossing of two post­ three-dimensional prestressed concrete nal post-tensioning tendons. tensioning anchor heads. The size of truss has six prefabricated segments the lower beams was selected so that creating an arch with a radius of 1070 all the tendons could be placed in the ft (326 m). Each segment is 33 ft (10 Truss Chords deviators, and also to provide enough m) long, giving a total span of 197 ft The top chord of the truss (see Fig. space to connect the diagonals. In fu- (60 m). Each of the two end segments 4) is composed of the deck slab and Fig. 5. No. of P. T. Tendons Schematic showing No. of Strands arrangement of longitudinal Diameter of Strands (mm) post-tensioning tendons. & Note: Asterisks represent 2-4-13 external tendons. 4-7-13 60.0 m (197 ft.) September-October 1999 63 transverse stiffening elements embed­ longitudinal post-tensioning tendons. encapsulated and have neither a ded into two longitudinal ribs that are Note that internal tendons were in­ bearing plate nor local zone spirals. 8 x 12 in. (200 x 300 mm) in cross stalled in these lower beams to pro­ The anchor head is in direct contact section. The deck must support an 81 vide continuity between the segments with the RPC, which can withstand 2 psf (3.9 kN/m ) live load, and deicing and to avoid any tensile stress in these the high compressive stresses be­ and snow removal equipment weigh­ beams at SLS. neath the anchor head. ing 2.9 tons (2.6 tonnes). A significant concern during design The deck was designed as being was how to connect the precast diago­ embedded in the top beams and able Truss Diagonals nal members to the top and bottom to withstand a flexural tensile stress The truss diagonals, which slope in slab without passive reinforcement of 1740 psi (12 MPa) at ULS. This two directions, are made of RPC con­ (i.e., mild reinforcing bars). It was de­ constraint defined the amount of fined in 6 in. (150 mm) diameter stain­ cided to cross the tendons of the diag­ transverse prestressing to be applied, less steel tubes with a wall thickness onals in the top beams (see Fig. 6) so 1 1 i.e., one / 2 in. (12 mm) greased­ of h6 in. (2 mm). These diagonals are that shear forces are transmitted di­ sheathed monostrand every 4 ft 1 in. 10 112 ft (3.2 m) long, and inclined in rectly from one diagonal to the other. (1.25 m). Note that shear and punch­ two directions, i.e., at 41 degrees to Since the diagonals meet the bottom ing force was resisted directly by the the longitudinal direction and 14 de­ chord of the truss at joints between the RPC.