Civil Engineering March 2021 | Vol 29 No 2

16 March 2021 Civil Engineering After reviewing related literature, consulting with AECOM’s UK special- ists, and considering the construction methodology, it was hypothesised that the method of strand installation (strand by strand push method after concreting) had resulted in the entanglement of the individual strands (van Wijk, 2019). When pushing the first few strands it was found that the strands tend to spiral around the inside perimeter of the 120 mm diameter duct. It was also recorded that the final few strands were pushed with much more difficulty and were therefore pulled instead during the original installation. It was therefore deemed possible that there could be a difference in elongated length between the strands pushed first and those pulled last which could also result in non-uniform stress distributions in the tendon strands when tensioned. The faulty fully-stressed tendons were destressed, discarded, and replaced with new strands. Strands that were installed in the remaining ducts were removed and re-installed using only the pull through method. Extensions measured after successful re-installation of strands were within 6% of the theoretical elongations and were accepted. Cast anchor fatigue and service life A critical component of the tied-arch structural form are the anchor plates that form the connection between the hangers and the concrete elements. The structural form dictates that the anchors are subject to the following: Q Q A free tensile load of which (in a failure situation) a significant propor- tion of the load would still be applied up to rupture. Q Q Limited redundancy with potential susceptibility to “unzipping” failure. Q Q The form of the anchor connection results in a partial creation of a hidden critical element that means subsequent complete inspection will be difficult and replacement will be practically impossible. This critical structural component is subject to high fatigue over the life of the bridge structure. The design originally re- quired that anchor plates be manufactured from structural grade cast steel, and that the first cast assembly of each anchor plate type be destructively tested and analysed by a specialist metallurgy third party. The destructive testing of full-size cast steel anchor plates, together with smaller samples (coupons), was conducted as part of the specified quality control measures in accordance with the European speci- fication for cast steel products. Cast steel strength parameters, as well as durability and fatigue properties, were verified by testing and reported by the specialist metallurgist. It was found that, although the strength requirements were easily achieved, the ductility and fatigue-resis- tance parameters were not adequate over the section depth in the full-scale anchor plates, even though the manufacturer expected the results to be acceptable. All test parameters were achieved in the smaller coupons, but not over the section depth of the full-scale elements. Several remedial measures were performed with revised trial heat treatment processes, but the required result was still not achieved. Unfortunately, the completed cast anchor plates had to be rejected and new anchor plates manufactured. A redesigned compound steel plate option was then also considered since the steel type and thickness that could be successfully welded and ma- chined was readily available in South Africa. Manufacturing duration also became an important consideration at this stage. Jack pressure (MPa) 60 50 40 30 20 10 0 Tendon 10 0.0025 600 400 200 0 Tendon extension (mm) 0.0035 0.0045 0.0055 0.0065 Tendon 3 Tendon 4 Tendon 9 0.0065 0.0025 0.0025 Duct 3, 4, 9 and 10 – Wobble “k“ sensitivity analysis (μ = 30) Figure 10 Sensitivity analysis of Wobble factor k Figure 11 Test stressing and lift-off testing Figure 12 Video footage of inside of duct and debris extracted from duct Project team Client Western Cape Government, Department Transport & Public Works Consulting engineer AECOM SA Contractor Haw & Inglis Civil Engineering Temporary works designer Maffeis Engineering Specialist subcontractors Amsteele Systems, Allweld Marine & Industrial, Form-Scaff, Nyeleti Consulting & Zutari Concrete supplier Afrimat Environmental consultant SLR Consulting