Martin Ogle is Principal Design Consultant in the Engineering and Materials Group. He joined TWI in 1978 after a career in consulting engineering, where he was involved in the design, contract supervision, checking and repair of civil engineering structures. These included a wide range of steel and composite bridges, most notably the cable-stayed Batman Bridge in Tasmania where he was Deputy Resident Engineer for four years. He was responsible for much of the drafting on BS 5400 Part 10 Fatigue of bridges, including the creation of a new fatigue loading specification.
At TWI he was Head of the Structures Section until 1985. He has been involved in welding design problems on bridges, safety barriers, buildings, masts, power plant, offshore platforms, cranes, and machines. One of his roles is to ensure that TWI's many assets - including laboratory testing theoretical analysis, and on-site inspection and monitoring - are used to full advantage in solving Industrial Members' problems. He has been active in drafting design standards for welded steel and aluminium structures for national, European and international application. He has also drafted practical workmanship and quality standards for contract use in the building bridge and transportation industries.
A carefully specified procedure made refurbishment a feasible alternative to repair for two heavy girders. Martin Ogle explains.
Industrial Member Lloyds' British recently approached TWI with a difficult problem. They had undertaken refurbishment of two 9t overhead grabbing cranes which were in bad shape. The 15m span cranes had been working for 17 years in Powderhall Refuse Disposal Works in Edinburgh ( Fig.1). Their job was to move refuse from a large storage pit to various hoppers for processing. This involved continuous lifting and traversing of the grab both along and across the pit.
This work had taken its toll. Fatigue cracks had been found in welds joining the webs to the top flange of the main box girders ( Fig.2). This had been exacerbated by the fact that the longitudinal weld was a single fillet located directly below the crab rail.
A further problem was that the cranes had been severely damaged by a fire in the refuse pit. Apart from destroying much of the electrical gear the heat had also distorted the girder webs.
The cranes were required to work for a further 20 years. They would also be expected to work with a loading capacity of 60 t/hr instead of the current 30 t/hr. Thus in terms of its workload the future fatigue life would have to be more than double the previous life.
Lloyd's British had to make an urgent decision as to whether it was practicable to restore the girders. If no satisfactory solution could be found the girders would have to be scrapped and new ones built at considerable cost.
Various methods of restoring the cracked web-to-flange weld were considered. Concern was expressed that if the existing weld was cut out the thermal stresses locked up in the plates at the time of the fire might be released and cause unacceptable distortion. Strengthening of the girder by welding on additional material could also cause other fatigue and distortion problems. Lloyd's British decided to seek specialist advice from TWI.
Recommendations
The problem and proposed restoration plans were carefully studied by the Engineering and Arc Weld Departments at Abington. A report to Lloyd's British recommended that repair of the existing fillet weld was a practicable proposition, provided that a very careful procedure was followed to control distortion. This advice was accepted by Lloyd's British, who asked TWI for a detailed repair procedure so that work could proceed straight away.
Repair procedure
The repair involved grinding out the existing 6mm fillet weld in 150mm lengths and replacing it with a partial penetration butt weld reinforced with a fillet. The throat of the weld would be increased from 4 to 6mm (
Fig.3). Calculation showed, on the basis of previous performance, that the fatigue design life of 20 years could be achieved. As fatigue life is inversely proportional to stress range cubed, the 50% increase in throat dimension would give a life improvement of more than three times. In addition, the new crab was lighter than the old, thus bringing down the applied stresses induced by crab traverses.
The reasons for adopting a partial penetration rather than a full penetration butt weld were threefold. First, the amount of grinding out of the joint was less. Second, the amount of new weld metal to be deposited was less and hence the effect on distortion reduced. Third, the quality of the weld root would be better through not having a feather edge preparation.
Distortion
Control of distortion was achieved as follows:
- Strongbacks were specified to control angular distortion of the top flange ( Fig.4 and 5). These were purpose made and wedged into position at the location being welded.
- Strongbacks were specified to control bowing of the web plate ( Fig.4 and 5). These were tack welded to the exterior vertical stiffeners and prevented outward bowing of the webs during angular rotation of the asymmetric weld. The two girders were also held apart with struts.
- A carefully planned sequence of grinding and rewelding was specified ( Fig.6 and 7). This was a fairly detailed procedure. It was designed to ensure that:
- The length of detached web was not long enough to buckle under applied load plus residual compression;
- The length of each weld pass laid down in any one location was kept to a practical minimum to reduce the angular distortion on web and flange, the existing weld at either end of the gap helping to provide angular restraint;
- The pass sequence was staggered to prevent heat build up in any one location by using a skip and back-step sequence.
Results
Samples from weld procedure trials were taken and sent to Abington for approval. The site work was supervised by Lloyd's British following TWI's laid down procedures. Regular liaison was maintained during the repair programme so that advice could be obtained on any unexpected problems.
The final result was reported to be very satisfactory. Distortions of the top flange were less than 1mm. Overall bow in plan view reached 2mm and ended up as 1mm, which was within the tolerance for the crab rails. This was well below the upper bound estimate of 10mm for continuous weld passes without interpass cooling. The web distortions were also acceptable.
Careful attention to design detail and welding procedure made possible a satisfactory repair to the crane girders. This enabled significant savings to be made on the refurbishment contract.
