A time-lapse video proves that Vesconite Superlube low-friction, high- load-carrying bearings perform as well as sliding bearings in large structures, including soccer stadiums. The video starts shortly after 7pm and shows structural movement throughout the evening, when the stadium concrete beams contract due to cooling, and continues past mid-day, when higher temperatures result in expansion. These subtle movements, when allowed by a bearing, ensure the safety and the structural stability of the stadium and the continued support of the superstructure by the support columns’ substructure. Since the stadium was designed to have a capacity of 55,000 specta- tors, expandable to 75,000, engineering performance was paramount to ensure the stadium withstands temperature- and moisture-related contractions and expansions, as well as earthquake and wind loads. History of the stadium The stadium was built more than ten years ago, and has been used for large sporting eventsThe original bearings were a bitumen-im- pregnated cloth. The columns and beams were made from traditional concrete. However, after eight years of use, excessive wear was found to the concrete of the continuous support beams and to the supporting columns. The bitumen-impregnated cloth had been torn or ripped away in some cases, and pieces of concrete in the beams and columns had disintegrated at the corners. Engineers believed that an unfortunate congruency of design and build- ing errors, combined with a choice of original bearing materials not suited to the design, necessitated an urgent review of bearing materials and expansion gap designs. Problem 1 - Small expansion gap All materials shrink and expand as temperature varies, so an expansion gap is required. Concrete has a known expansion rate so it is important that designs cater for this expansion. In this stadium in which Vesconite Superlube was eventually installed, the original bearings, made from bitumen-impregnated cloth, were employed at the expansion gap to prevent the columns from bonding with the continuous beam placed on top of them. The original contractors poured the column first and then used the bitumen- impregnated cloth to separate the column from the continuous beams. For a joint that doesn’t move much this would have been suitable. But, in the case of the stadium, the gap size and the material speci- fied proved inadequate and concrete was ripped out adjacent to the provided-for gaps. Structural bearings support stadium beam joints
Problem2 - Inadequate steel reinforcement and concrete cover Since concrete is good in compression but not in tension, many struc- tures, including bridges, have steel reinforcement to resist tensile forces. The steel tends to follow the contour of beams and columns with a certain amount of concrete cover. This prevents corrosion of the steel and the spalling of the concrete that results from exposure to the environment. However, at the stadium, concrete beam and column corners were inadequately strengthened with steel, and the corners cracked off leaving an inadequate bearing surface. Some steel was badly positioned, possibly because of a lack of over- sight during construction. The poured concrete moved, and the steel was not close enough to the surface to keep the corners strong. The Vesconite Superlube solution The existing bitumen-impregnated cloth was replaced with Vesconite Superlube, which is a low-friction, high-load-carrying bearing material. Since Vesconite Superlube was to be used as a sliding bearing, the bearing material was contained within two stainless-steel plates, which would slide relative to each other to allow for translational movements in a horizontal direction. Unlike various other stainless steel sliding bearing designs, the engi- neering contractor chose a top metal plate that arched down to join the bottom plate so as to reduce the ingress of dirt. The bottom plate was also longer than those traditionally used in sliding bearings and had a lip at each side; the length maintained the horizontal rigidity of the bearing during installation. Since there were significant engineering, design, and maintenance problems in place at the site, installing the Vesconite Superlube bearing solution involved several measures to address existing structural issues: 1. Hydraulic jacks were brought in to lift beams off the support - ing columns. 2. Where concrete columns or beams had had extensive pieces of concrete ripped off them, these were repaired by putting in form work and pouring concrete to ensure that the corners were intact. 3. Saws with large cutting wheels were used to cut out the ledges on which the Vesconite Superlube bearings were to be placed. Since Vesconite Superlube was to be used as a sliding bearing, the bearing material was contained within two stainless-steel plates, which would slide relative to each other to allow for translational movements in a horizontal direction.
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DECEMBER 2021 csengineermag.com
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