The Pacaltsdorp footbridge is a new and valuable link crossing the N2 national highway for the commuters from the new Pacaltsdorp township accessing work opportunities in George and the surrounding industrial areas. For SMEC South Africa, the challenge set by SANRAL was to create a design that complemented both the surrounding built and natural environments. Of course, it also had to attract pedestrians towards it and away from the highway.
First, concepts with simply supported precast concrete beams and cast-in-situ continuous concrete decks were considered. However, after some review, the idea of giving the bridge some individual character gained support. More natural forms were investigated and the concept of a multispan arch structure supporting a stress ribbon deck was chosen. The shallow arches flow across the highway and the stress ribbon across the arch.
The project was viewed as a valuable opportunity to showcase the engineers’ ability to positively shape the built environment with their creative skills. For this reason, the design team took on the challenge of designing and building the world’s first multispan arch structure supporting a stress ribbon deck.
The footbridge is a 65m long, continuous, four-span, self-anchored, arch-supported stress ribbon bridge with a maximum span of 21.2 m. The bridge has semi-integral abutments and a 220 mm thick concrete stress ribbon deck that spans up to 12.6 m between the crests of the arch sections. The chosen form is a case of opposites attract. The stress ribbon is a tension structure following a shallow catenary. In this instance, it is restrained by the compressive strength of the arch that is able to resist the large horizontal forces needed to maintain the stress ribbon’s profile.
This creates a self-anchored structural system and only vertical uplift forces need to be resisted at the abutments. The structural system also offers some great practical advantages in that it is semi-integral with no expansion joints.
The stress ribbon, itself, consists of pre-stressing strands cast into the reinforced concrete deck section. The strand is positioned parallel to the longitudinal axis of the deck slab. As a result, the concrete deck section gains compressive stress from the radial force generated by the prestressing cable as it follows the path of the catenary. The applied anchorage force is resisted by the thrust of the arch section.
An attractive, unseen feature of the form is that it uses the material of choice, concrete, to its best advantage. Under permanent loads, both the arch and the stress ribbon sections remain in compression throughout the structures design life.
Being conventional in some forms, a further positive was that the proposed design could stand with and apart from its counterparts. The only hesitancy during the design development process was that an undulating pathway would be created. However, the gradual slopes and shallow sag makes this form accessible to all. The final structural form is considered to be attractive both visually and technically. The fact that the bridge is semi-integral and that the concrete sections, remain predominantly in compression is appealing to a bridge designer’s goals of creating a durable structure. The fact that the bridge can be set out with simple circular curves and constructed with conventional formworks forms was also pleasing.
Source: CESA 2016 Consulting Engineers South Africa
Date: Aug 2016