Two Pedestrian Bridges

Recent advances in construction materials technology are allowing Ultra-High Performance Concrete (UHPC) to conquer new frontiers in the construction industry. UHPC has been in use around the world for two decades, and over the past five or six years has been gaining acceptance in North America. There have been a few pioneering projects in Canada. DIALOG was one of the first Canadian engineering firms to use the material and two pedestrian bridge projects in Calgary are showcased here. The first was completed in 2007 and the second in 2010.

The two bridges were realized through close collaboration between the City of Calgary – Transportation Infrastructure, the architects and structural engineers of DIALOG, and the UHPC material supplier and pre-caster Lafarge North America. It was the city’s vision to use advanced materials in the construction of two pedestrian bridges to minimize maintenance requirements. This resulted in aesthetically appealing and economical bridges, with life cycle costs taken into account. The bridge cross section was designed so that it could be altered or tailored for use in future projects.

The U.S. Department of Transportation describes UHPC as a material that tends to have compressive strength over 150 MPa, internal fibre reinforcement (for ductile behaviour), and a high binder (cement) content. It also has special aggregates (no coarse aggregate, and fine sand between 150 to 500 micrometers), resulting in low porosity and consequently high durability. UHPC typically has a very low water content and high-range water-reducing admixtures to achieve good workability.

Strength and ductility

Compared to UHPC’s very high compressive strength of up to 200 MPa (about half the yield strength of reinforcing steel), High Performance Concrete (HPC) has a strength below 50 MPa (special HPCs can reach 100 MPa). HPC has a low tensile strength that cannot be used in design, while UHPC can be designed for up to 8 MPa in direct tension and, depending on the concrete member shape and loading, up to 30 MPa (in flexural tension). In addition to its high strength characteristics, UHPC has excellent durability and ductility.

It is a well known fact that concrete cracks (for many reasons), and in design the behaviour before and after cracking must be taken into account. Under an increasing load, new hairline cracks form, and older ones grow and propagate until a localised, predominant crack forms at or near the member’s capacity load. Steel reinforcement is used to carry the tensile stresses across the cracks and maintain the members’ load carrying capacity. In UHPC members, fibres play a similar role to reinforcing bars, but their even disbursement in the concrete results in shallow, narrow, closely spaced micro-cracking in larger zones of the member. During this process, a UHPC member is able to deform excessively under increasing load, or in other words it behaves in a pronounced ductile manner. An example of this behaviour is depicted in the photograph on page 28.

Takes any desired shape with careful formwork

In UHPC members, the combination of narrow and shallow cracking and the material’s low porosity translates into excellent durability, and in turn less maintenance. Flowability and mouldability are two other characteristics of UHPC, which mean the material can take any desired shape with careful formwork. The properties of UHPC help the designers create a more sustainable structure; since the structure weight is minimized, less material is used; also less energy is required to produce the required construction materials.

We can compare the acceptance of UHPC with that of glass and carbon fibre reinforcement, technologies that have been in use for about 25 years and have standards available for their use. One of the reasons for the development of glass and fibre reinforcing bars was to enhance the durability of structures. However, their use is still not that common, perhaps because the bars cannot be used as a direct replacement for reinforcing steel and so require special design procedures. In comparison UHPC is a concrete and its design rules are similar to those for normal concrete, with some modifications.

While a few international standards and guidelines are available, there is a need for a North American standard design guideline. In March 2010, the UHPC North American Working Group was formed, and the efforts of its members over the past year resulted in the formation of American Concrete Institute Committee 239 “Ultra-High Performance Concrete.” In the near future, the committee expects to produce a report, guideline specifications, step-by-step design procedures, and material standards.

DIALOG has been involved in six UHPC projects including the two pedestrian projects described below. As the material is relatively new and due to the lack of North American UHPC design standards, the designs had to be validated through component and/or full-scale load testing. In all the tests, the predicted and observed behaviour in the elastic stress range (before cracking) matched closely. However, after cracking, the observed and measured properties such as ductility and strength always surpassed what was predicted.

Pedestrian Bridge over

Glenmore Trail at Legsby Road

The Glenmore Trail pedestrian bridge is a unique structure over a vital arterial road in southwest Calgary. The challenges of the site included constrained space for constructing piers, and a busy roadway that could not be shut down for extended periods. These issues led to the selection of a cantilever-type bridge that supports a UHPC precast drop-in girder.

The precast girder is designed as a simple span that forms part of a 52.9-m clear span. The precast girder is T-shaped and 33.6 m long. The remaining 9.65-m lengths at each end are post-tensioned, cast-in-place HPC cantilever girders. The bridge is located near a school, so as part of the design concept, educational art components that convey an understanding of the structural system components were incorporated into the finished structure.

Cross-sections at mid-span and at the ends of the drop-in girder are shown in the diagram above. The girder depth varies from 1.10 m at mid-span, to 1.40 m at the tips of the cantilever girders to accommodate bearings, dapped ends, and beam seats. The span (pier to pier) to depth at mid-span ratio of the bridge is 48.1, which is quite aggressive for similar bridge spans. The flange of the UHPC girder is only 80 mm thick near the edge, increasing to 200 mm at the face of the web. A 50-mm thick curb was added for drainage and to accommodate handrail support anchors.

One of the challenges in the design of the bridge T-section was its instability due to loading on one side of the section. This was overcome by relying on a combination of the UHPC strength and the addition of glass fibre reinforcement as passive reinforcement to arrest any cracking that may develop.

The UHPC girder was cast in a steel mould. The UHPC mix was prepared in a high-shear plant mixer, and about 40 cubic metres was required. It was mixed over a 16-hour batch cycle and was poured into four ready-mix trucks before pouring into the form. The 16-hour cycle was made possible using low heat/slow setting cement and admixtures. By temperature control, the working time and initial set was extended for more than 24 hours. Once the material was placed in the mould and covered, the temperature was slowly increased to initiate setting, and subsequently to accelerate strength gains.

The continuous pouring of the girder was the largest monolithic UHPC pour in the world at that time. The completed bridge has won awards from the Prestressed/Precast Concrete Institute and the American Concrete Institute Alberta Chapter.

Pedestrian Bridge over Country Hills

Boulevard at Sanderling Drive

This bridge connects two residential communities in northwest Calgary and forms part of the city’s trail system.

In order to avoi
d existing utilities under Country Hills Boulevard and to provide an unobstructed view across the busy roadway, the bridge was designed without a centre pier, and with a UHPC girder that is similar to the one used in the Glenmore Trail Bridge. The design was modified to a three-span continuous system, resulting in a more slender and attractive bridge.

The Sanderling bridge has a clear span of 64.4 metres and a drop-in UHPC girder that is the same length (33.6 m) as in the previous bridge. The remaining 15.4-m lengths at each end plus 22.75-m back spans were cast-in-place, post-tensioned HPC girders.

The three-span bridge was made continuous through cast-in-place joints and continuity post-tensioning. As bearings were not used at the ends of the UHPC girder, the dapped ends were eliminated and the girder depth was kept constant in a parabolic profile. The span-to-depth ratio at mid-span was an aggressive 61 due to the continuity.

Casting the UHPC for this girder followed a similar procedure to the Glenmore Trail project, except that batching was completed entirely in three ready-mix trucks which reduced the required production time.

Both projects demonstrate that bridges don’t just move people from one point to another — they provide a safe and pleasant environment for connecting communities and promoting recreation, as well as representing sustainable urban developments.cce

Gamal Ghoneim, P.Eng. is an associate with DIALOG in Calgary, and Sean Brown, P.Eng., is an associate with the company in Edmonton. Both are senior structural and bridge engineers.

Client/owner: Transportation Infrastructure , City of Calgary

Prime consultant and bridge engineers: DIALOG, Calgary (Jim Montgomery, P.Eng., Gerald Carson, P.Eng.)

Contractor: Graham Infrastructure

Testing: University of Calgary