|INNOVATIVE DESIGN OF MANHOLE COVERS
In our 25 years experience of making precast concrete manhole covers and frames, we have realized that the current IS code has failed to meet some of the primary functions for which it was designed.
The IS code for Heavy Duty covers provides for 2mm thick mild steel sheet around the periphery of the cover and two mild steel lifting hooks placed about 2 to 3 inches away from the extreme edge of the cover. It also provides for a gap of 5mm around the cover between the cover and the frame besides a test load of 20 MT for failure.
All the above specifications except the test load have miserably failed to provide a cover which can be opened and closed as and when required, due to the following reasons explained. First, rusting and subsequent swelling of the mild steel rim compresses and hardens the dust collected in the 5mm gap between the cover and frame. This makes it practically impossible to remove the cover from the frame. Second, the hooks are so positioned that the workman who is trying to remove the cover by usually available tool, i.e pickaxe, gets no leverage. In this process, sometimes the workman pulls out the hook from the cover. Common sense would dictate that the fulcrum should be obtained from an object away from the object the workman is trying to lift. Third, the m.s. hooks corrode with time and ultimately fail.
Recognizing these issues, we have modified the design of the manhole cover in three areas:
- The metal strip around the cover has been deleted, since it was not to remain for a long time anyway (refer to Figure 1)
- Position of the lifting hooks has been shifted to the extreme edges of the cover so that when using a pickaxe to move the cover, the frame edge automatically becomes the fulcrum (refer to Figure 2)
- The mild steel hooks have been HOT DIP GALVANISED to prevent rusting of hooks for a long time to come (refer to Figure 3)
The ultimate result of these improvements is a product that is user friendly and low maintenance.
FIGURE 1: Corrosion of MS Strip makes it difficult to remove the cover off the frame
SOLUTION: ELIMINATE THE MS STRIP AROUND THE COVER
FIGURE 2: Positioning of lifting hooks does not facilitate easily lifting or moving the cover
SOLUTION: RELOCATE THE LIFTING HOOKS TO THE EDGE OF THE COVER
FIGURE 3: Eventual corrosion of lifting hooks again makes is difficult to open the cover
- MS Lifting Hooks are susceptible to corrosion with time
- Once corrosion is in an advanced stage the hooks fail and the cover cannot be opened in the traditional manner
- MS Lifting Hooks have been hot dipped galvanized to prevent corrosion
- These hooks will last a long time without failure
SOLUTION: HOT DIP GALVANIZE THE LIFTING HOOKS