in Erasmusrand, Pretoria was damaged in January 2015 and demolished by SANRAL during June 2016. SANRAL appointed the LEO-Superstructures Joint Venture for the design of a replacement bridge. The bridge spans across the 10-lane dual carriageway freeway providing access to the Waterkloof High School to the east from the suburbs to the west thereof.

The client required a replacement pedestrian bridge that spans across the freeway without a median support and fulfilling all vertical and horizontal clearance requirements. The bridge could be a concrete, steel or composite steel/concrete structure, similar in cost to the previous bridge. Aesthetics of the replacement bridge was an important consideration. The client required 5 conceptual designs which could be evaluated in terms of aesthetics, costs, maintenance requirements, constructability and traffic disruption and overall clearance.

Initially, the arch, being primarily a compression member, was envisaged to consist of precast concrete elements. The excessive weight associated with a concrete arch made transport and erection unfeasible and thereby a lighter steel solution was adopted.

The structure consists of a fixed steel arch supporting a composite steel/concrete deck with inclined square hollow steel struts between the arch and the deck. The deck merge with the arch at the crown thereby reducing the structural depth to a minimum. The arch consists of a stiffened closed steel plated box which transitions to an open trough filled with concrete over the crown.

Handling of the huge assembly was a major challenge during construction. Further challenges included manufacturing the curved members with inclined faces merging together which resulted in complicated geometry. Welding in confined areas in the box also proved challenging.

The structure was lift into position in two sections. The mere size of the assemblies complicated the erection. High precision erection was required to obtain a perfect fit with small tolerances. The main challenge was to place the second section. In order to get the base of the arch over the holding down bolts, the section had to be lifted inclined and then rotated in place afterwards. The abutment posed a geometric constraint which required that the end piece of the deck had to be installed and welded after the arch had been erected.

A full-strength site weld was required to join the arch at the median temporary support. The open trough section used in the arch minimised the welding and allowed that all welding could be performed from the top instead of all round.

The intention was to design a structure with an optimum structural form for the opening to be bridged. The aesthetic appeal lies in the slenderness of the structure which could be obtained through its structural functionality. The structural depth was reduced to a minimum over the crown by merging the deck into the arch. A balustrade with a high degree of transparency was chosen to accentuate the slenderness of the bridge. The sides of the arch and deck was sloped in opposite directions which introduced a separation line highlighting the outline of the arch and the deck.

The rise to span ratio of 0.07 is at the lower end of the feasible range for arches of this magnitude. With the low rise to span ratio the arch becomes susceptible to buckling and the effect of displacements becomes significant requiring third order large displacement stability calculations.  The design was balanced to achieve both an acceptable stability factor of safety and a high level of material utilization.

Due to the slenderness and optimum use of material, the potential exists for dynamic excitation to occur through rhythmic footfall loading. To avoid using Tuned Massed Dampers the stiffness and the mass of the structure was manipulated to reduce accelerations within acceptable limits.

Stability of the structure during erection was a critical aspect that required close collaboration between the design engineer and contractor. Each construction stage and each handling and transport process had to be analysed to ensure stability is maintained throughout. An innovative solution was developed to ensure stability as well as to limit displacements by temporarily stressing each segment with tie bars. The stressing was released after the arch had been closed thereby locking in the beneficial effect of the pre-loading forces on the permanent structure.

STRUCTURAL STEELWORK
Completion date of steelwork 15/4/2019
Completion date of full project 31/5/2019
Tons of structural steel used 105 Tons
Structural profiles used Combination of Platework and Tubular
SA content – if this is an export project 100% Local
PROJECT TEAM COMPANY
Nominator Cadcon (Pty) Ltd
Client/ Developer SANRAL
Structural Engineer Superstructures
Engineer LEO Consulting
Quantity Surveyor LEO Consulting
Project Manager LEO Consulting
Main Contractor Teichmann Ndungane
Steelwork Contractor CADCON
Steel Erector On Par
Corrosion Protection
Paintwork Contractor
DRAM