Civil Engineering March 2021 | Vol 29 No 2

18 March 2021 Civil Engineering The De Beers Interchange (Bridge B4407) was originally constructed in 1970 over the N2 to the west of Somerset West. The bridge was designed as a four-span 70 m long continuously post-tensioned deck, to carry the R44. The superstructure rests on elastomeric bearings at the piers and abut- ments. The piers are 900 mm diameter reinforced concrete columns founded on pad footings, while the abutments are spill through type walls with stone pitching. PRELIMINARY DESIGN As-built information was provided, which allowed the designers to determine the load capacity of the existing structure. As the structure was constructed in 1970, it was assumed that the original design had adopted the BS 153 (1954) loading system in its structural analysis, which called for a uniformly distributed linear traffic load of 2 200 pounds per foot to be applied, along with a single heavy axle line load of 2 700 pounds per foot. By comparison, current loading require- ments prescribed by TMH7 Part 1 and 2 have requirements of 20% to 60% higher than the BS 153 loading code. Despite this, the existing structure was assessed using SOFiSTiK finite element analysis software and deemed to comply with TMH7:1981. WIDENING OPTIONS The existing structure had sufficient ca- pacity to comply with TMH7:1981, however further widenings of the deck would impose additional dead loadings that the structure would not be able to accommodate. The initial proposal was a simple wid- ening strategy that involved adding a light- weight steel sidewalk at the farthest edge of the deck to accommodate pedestrians, and thus allowing for the reconfiguration of the road layout on the deck. However, this proposal added too much dead weight to the existing structure and was not pursued. The revised proposal involved building a new structural steel pedestrian bridge directly adjacent to the existing deck on both sides to accommodate pedestrians. This ensured that pedestrians could cross the bridge safely, while still allowing for a reconfiguration of the road over the deck. The structure would form an exoskeleton which incorporated a full height screen. This proposal was discarded due to po- tentially high maintenance costs because of the site’s proximity to the ocean. The proposed new pedestrian bridge option was then revised to reduce the maintenance requirement over the lifespan of the structure. The result was a pre-tensioned concrete structure, com- prising precast superstructure elements placed on an in situ substructure. The structure was also designed with a unique side profile to be visually appealing to the vehicle user below. COLUMN IMPACT LOADING Due to recent impact damage experienced by similar pedestrian structures, it was decided to increase the vehicle impact loadings that the piers would be required to accommodate. The resulting design is for triple the nominal forces required in TMH7. PRECAST BEAM DESIGN & SEQUENCING The superstructure is built up of three components: the precast beams, precast slabs and in situ stitching. The beams are designed to maximise their bending moment capacity at the mid span, while also maximising the shear capacity at the Upgrading the De Beers Interchange Chad Peacock Engineer HHO Consulting Engineers chad@hho.co.za Nadine De Mink Technologist HHO Consulting Engineers nadine@hho.co.za The De Beers Interchange in the Western Cape dates back to 1970. Although the structure was in good condition, it needed to be widened to allow for an additional traffic lane as part of an upgrade co-founded by the City of Cape Town and South African National Road Agency Limited (SANRAL). Aerial View of Completed pedestrian bridges (Credit: Bruce Sutherland)