Case Study

We have made investigation based on deviation informations received through mail dated 10/10/2021 in respect of tower TW-5 5 & gallery girder GR-5 & gr-6 for conveyor BC-11A & 11B. The nature of deviation and the corrective action proposed are furnished below.

A) Nature of deviation

• There is deviation(shifting) in supporting positions of end portal base of gallery girders and cap plate of tower legs
• Tower legs have been deviated from true vertical all along their height, resulting in inclination tower leg & shifting of cap plates from centre of base plates at founding level. No deviation between tower leg base plate and their foundation centres have been considered as per information of deviations received.
• The above deviations are in both directions along and across the conveyor, The deviations are substantial in some cases, much beyond the permissible limit as per IS 12843- 1989. 300 46 300 t tana

B) Study of deviation & corrective action proposed

• Shifting of gallery end portal base from tower leg cap plate were studied. We propose corrective action through modified detailing at tower leg cap plate position to bring the eccentric loading from gallery end portal base to centre of deviated tower leg cap plate, without causing any eccentric moment in weaker axis of tower leg. The eccentric moment in the stronger axis of tower shaft remain. The bending stress due to this eccentric moment in the tower shaft is well within permissible limit.
• Based on our study about the inclination of tower legs & shifting of their cap plates from their base plate positions at foundation level, it can be concluded that the increase in axial load in the deviated tower leg, from the vertical load of end portals of gallery girders are insignificant due to very gentle inclination of the tower legs. The horizontal component of the axial load are also very small and cause insignificant sway of tower structure due to smallness of the force and very high rigidity of the tower structure in both direction.
• Even through inclination of tower legs are very gentle, we propose to introduce plan bracings at different levels of tower for more equitable load distribution, better intigral action of tower as a whole & increased rigidity of tower structure.

C) General

• It is presumed that the structural system for the tower TW-5 & gallery girders GR-5 & GR-6 in respect of their arrangement, member section & sizes and connection details strictly conform to the latest drawings released by us- please ensure
• We strongly suggest to tie the tower leg through guy rope at top & intermediate positionsas per advise of erection expert to prevent any excessive sway due to any un foreseen deviation effect. This should be done prior to full load testing of conveyor.