RADIAL GATE DIS ISQUIETS DUE TO ZIN INC POURING DINAKAR R - PowerPoint PPT Presentation

Third National Dam Safety Conference 18-19 February 2017, Roorkee RADIAL GATE DIS ISQUIETS DUE TO ZIN INC POURING DINAKAR R MAHAJAN Mechanical organization, Water Resources Department, Pune, Maharashtra drzadap@gmail.com

ABSTRACT The joint of anchor girder and trunion bracket of radial gate is used to fill with zinc. The practice of Zinc pouring is prevalent in Maharashtra state since last 50 years. The art and technique of filling zinc, quality, subsequent assembly by tie studs, and use in practice decides the fate of joint and zinc pouring in due course of time. There are around 122 dams in Maharashtra having around 999 radial gates of different sizes and designs. Barring very few exceptions almost all of these gates are having zinc pouring at joints of anchor girder and trunion bracket. Since from beginning, and consequently due to ageing, some of the joints seem to be on the verge of collapse causing disquiets to radial gates of dams in state. This may be the case of dams in other states also following similar practice. The issue is cropping up slowly but definitely for individual gate and dams. It becomes important to tackle the issue immediately because the mass of moving parts of radial gate is too much to handle at ease. The erection of gate is done in a specific sequence and complete assembly is arrived at from pieces. Hence any repairs and restorations to this assembly becomes difficult due to this mass, added with space restrictions. Disassembly of gate is also not possible. As such the issue is discussed in this paper. So that timely preventive actions can be taken up by all concerned.

1 INTRODUCTION: PURPOSE OF ZINC POURING Pouring of zinc is predominantly done to accommodate the erection deviations of components. The usual sequence of erection of components followed for radial gate, Figure 1, is as under: Anchor girder and horizontal and vertical anchorages are erected with jig after arriving at specified level of construction. Once anchor girder is erected, trunion bracket is attached to it on both ends, by trunion tie studs, with intentionally keeping sufficient gap for pouring zinc. The gap is usually up to 8-12 mm according to on site available situation. The end arms on both sides are attached in overhung position. It is proceeded by fixing horizontal girders, and skin plates with Tee stiffeners. The gap has to ensure correct position of skin plate on sill beam, eliminating all manufacturing, and erection deviations and errors. Once the assembly is complete zinc is poured and finally tie studs are tightened with torque specified. This makes the joint and assembly final and cannot be reversed or altered in due course of time.

a) General Arrangement of Anchor Girder (b) Anchor Girder (d) Trunion Bracket

(c) Anchor Girder and Trunion Bracket Joint Details

1.1 Load, Properties of Zinc Pouring For most of the time zinc pouring is subjected to direct compression. In odd situations it would be subjected to shear, or other loads coming on anchor girder and trunion bracket. The joint is crucial for transmission of water load to concrete structure. It has to maintain the stability of entire assembly of gate. It is also the weakest and fragile joint, due to use of zinc, because of inherent weak compressive strength of zinc with respect to other structural steel and cast steel components used for anchor girder and trunion bracket respectively.

1.2 Initiation of Problems The problems for zinc pouring may get initiated from manufacturing stage itself. Figure 1 (e) and (f), Similarly, in usage, due to incorrectness in erection or other reasons, the joint and zinc pouring is subjected to different deficiencies. Since the zinc pouring is last in sequence of erection of radial gate, it inherently becomes very difficult to revert the earlier erection sequence for any repair and restoration to the joint. Hence it becomes vital to eliminate each problem as and when noticed. Theoretical failure modes of anchor girder and trunnion bracket joint would be compression, direct shear or tension failure, buckling failure, Failure due to twisting, Failure due to vibrations and torsion, etc. The joint should not fail in gate closed condition against full water storage.

(e) Joint Assembly before zinc pouring (f) Zinc pouring process Figure 1: Anchor Girder, Trunion Bracket and Zinc pouring details

2 DEFICIENCIES 2.1 Deficiencies of Zinc pouring For different dams and gates, deficiencies noticed for zinc pouring from time to time are, 1. Cavities and pours in initial filling. 2. Cracking of zinc pouring, removal of shards. 3. Not removal of weldments used at the time of zinc filling. Joint is made permanent by welding pieces, bars, stripes to anchor girder and trunion bracket. 4. Loosening of zinc pouring sheet, partial or complete and shaking. 5. Gap between anchor girder and trunion bracket due to various reasons. 6. No use of zinc pouring.

2.1.1 Cavities and Pours in Initial Filling Since, the zinc pouring is done on site at actual joint of anchor girder and trunion bracket on pier, the filling is done rather in a crude manner due to available site conditions. Ideal conditions cannot be created. Hence, the inherent deficiencies associated with casting process always crop up. Figure 2, Uniform filling of molten metal cannot be ascertained for available gap space and volume. This results in cavities and pours in the filled metal causing not uniform support for trunion bracket and uneven load transfer to anchor girder.

Khadakpurna Figure 2: Cavities and Lower Wardha pours in initial Nira Deoghar filling Nira Deoghar

2.1.2 Cracking of Zinc Pouring, Removal of Shards. For number of gates, the sheet of zinc pouring is found to be in cracked state. The shards become loose at places and can get removed from its location. Figure 3, Thus defeating the very purpose of zinc pouring. There are numerous reasons for this cracking. Nira Deoghar Radial Gate Size: 12 x 8 m = 5 no. Gate no 4 and 5, and Sina Kolegaon Gate no 18, zinc pouring is cracked to pieces as such shards are loose and can be removed by hand.

Nira Deoghar Figure 3: Cracking of zinc pouring, removal of shards Sina Kolegaon

2.1.3 Not Removal of Weldments Used at the Time of Zinc Filling To ensure correct position of trunion bracket against anchor girder, initially trunion bracket is temporarily welded to anchor girder with the help of small pieces of strips at different places. Also to arrest the outflow of molten zinc at the time of filling, the bottom and both vertical sides are sealed by placing and welding bars or strips at joints. Figure 4, This externally added support pieces, bars, strips, etc. should be removed finally once their temporarily intended purpose is over. However, it is forgotten in a convenient manner. For most of the times the joint remains permanent by these weldments to anchor girder and trunion bracket. This should not be desirable at all. This welding cancels the very purpose of zinc pouring and use of tie stud rods. This discrepancy is most common for almost 40% gate joints. This is a direct result of poor workmanship, and or lack of awareness.

Figure 4: Not removal of Khadakpurna weldments, i.e. pieces, bars and strips used for welding support at the time of zinc filling. Majalgaon

Figure 4: Not removal of weldments, i.e. pieces, bars and Susri strips used Lower Wardha for welding support at Sina Kolegaon the time of zinc filling. Totla doh

2.1.4 Loosening of Zinc Pouring Sheet, Partial or Complete and Shaking For some joints it is observed that the zinc pouring sheet has become loose and shaking by hand or in operation. Figure 5, This indicated that there is no proper cohesion of the joint and there is a relative movement of zinc sheet, trunion bracket and anchor girder. The loosening may be due to various reasons. The loose motion may be from earlier erection state due to initial imbalances of erection or from later operations of the gates.

Sina Kolegaon Figure 5: Loosening of zinc pouring sheet partial or complete and shaking

2.1.5 Gap Between Anchor Girder and Trunion Bracket at Zinc Pouring. For gate no 7 of Mula dam, Ahmednagar district, 4 mm gap is noticed at zinc pouring joint of anchor girder and trunion bracket. Figure 6, The gap increases while opening the gate, becomes maximum at top and reduces while closing the gate, becomes minimum at bottom. Gap indicates that there is relative movement of anchor girder and trunnion bracket separating each other, thus giving way for zinc pouring to incur gap. The movement must be due to load and corresponding strain or permanent set occurred in assembly in long term. This phenomenon is seen rare but can be thought of as a vital indication for gate failure. Gate no 7 entire zinc pouring sheet is loose from anchor girder side.

Figure 6: Gap between anchor girder and trunion bracket at Zinc pouring Mula Dam Radial Gate, Size: 40' x 25' (12.2 x 7.62 m) = 11 nos.

2.1.6 No Use of Zinc Pouring In some cases, Zinc pouring is not preferred for joints, hence not done. In this case, it can be visually seen that the mating of Anchor Girder and trunnion bracket surfaces is not proper. Gap and unevenness is seen for all joints. The trunion bracket rests on high spots of anchor girder plate surface. The geometry may change subsequently in use causing relative motions. Interestingly, IS 4623:2000 also do not mention zinc pouring practice anywhere.