Challenge 7
1. Pier Caps are structural elements which support the superstructre (U-Girder/I-Girder etc) of metro viaducts. They transfer the loads coming from superstructre to the Pier.
Pier caps can be pre-cast or cast-in situ and RCC or PSC.
In case of Cast-in-situ pier caps the shuttering of Pier Caps is supported from ground or by brackets from pier and the concreting in done in situ.
In case of Pre-cast pier caps the casting of pier cap is done at casting yard. These pre-cast element are then transported to the site. At site they are erected at the desired location with the help of crane etc. After erecting in position, the stick concreting is done to integrate the Pier Cap with the pier. In case of Pre-stressed pier caps the post-tentioning in carried out after completion of stitch.
2. The two types of pier caps which can be used are as follows
i. RCC Pier caps - These pier caps can be pre-cast or cast-in-situ. These are made of Reinforcements and concrete only and are easy to construct. There is no requirement of ducts and strands and stressing etc which is required in PSC pier cap. These pier caps can be having larger depth and can be huge as compared to PSC pier caps. In case of cantilever more than 2m these types of pier cap are not recommended. These can be used in case of lighter load and in cases where the depth requirement does not affect the headroom clerance in the metro viaduct.
ii. PSC Pier Caps - These are pre-stressed pier caps. These are structurally more efficent and are widely used in Metro Viaducts. The post-tensioning involed various activities which are tedious. But due to good effieciency and good asthetics these pier caps are very commony used. These type of Pier caps are recommended for cantilever upto 3m. Beyond this a portal type of substructure is advisable.
3. The Table No.11 of IRS CBC Code provide the limitations for Serviceability Limit State for various materails.
The compressive stress limitations for RCC in case of Traingular stress due to bending are 0.5 fck and the same for PSC are 0.4 fck. For Uniform compressive stress in case of axial loading are 0.38 fck for RCC and 0.3fck for PSC.
The compressive stresses in RCC pier caps due to the external loading such as dead loads, live loads and hence these can reach their limits in case when the bridge is subjected to its maximum design loads. However in case of PSC pier caps the compressive stresses are generated due to pre-stressing force and are bound to arise as per applied pre-stress. These are reduced when dead load or live loads are applied. Hence considering these details and the chance of attainment of stress levels in RCC & PSC pier caps a higher allowable stress is specified for RCC and lower stresses for PSC in case of concrete.
The allowable stress in case of flexural stresses(0.5fck-RCC) are higher as compared to that for axial stresses(0.38fck-RCC) since in case of flexural stress the triangluar distrubution leads to average stress in section lower than the limiting value and this ensure some reserve capacity in the section. However in case of Axial Stress and rectangular stress distribution the average stress is same as limiting stress and section has no further reserve capacity.
The stress limit for reinforcement is given as 0.75fy for Reinforcement bar in RCC as this is derived based on bending moments in the section. In case of PSC the pre-stressing force to be applied in the strands defined the stress limitations and this is demeed satisfied by CL.16.8.1
4. The structural cracking of pier cap can occurs due to flexural or tensile loads on the section. The dimension i.e. the width of crack can be quantified by analysis of pier cap section. The width of crack needs to be limited as a serviceability requirements. These requirement are for apperance of cracked section and also for protection of reinfoirement in the cracked section.
The Table 10 of IRS CBC Code CL.10.2.1.a provides the crack widths for different exposure conditions i.e mild, moderate and severe.
The maximum allowable crack width in case of mild exposure is 0.2mm. This is the width of crack which may not be visible to the naked eyes of any person. The other lower values of crack widths (0.1mm) for moderate and severe condtion are based on the requiements of protection of reinforecement from corrison as water can enter throught these cracks and corrode the reinforecement.
5. Pier caps are designed as cantilever sections because majority of the loads coming on the pier cap are away from the pier section which support the pier cap. Due to this all these loads induce bending and shear on pier cap. As pier is the only central support, both the arms of pier cap are designed as cantilever sections.
6. The key elements considered for finalization of the size of the pier cap are bearing to bearing distance in transvere direction, size of bearing, size of bearing pedestals, C/L of bearing to C/L of Pier in longitudinal direction, size of the drainage pipes, size of seismic restrainer, distance from edge of bearing pedestal for accomodating the construction tolerance and also to support the structure in case of dislodgement.
7. The key elements for finalization of drainage system of a metro viaduct are the rainfall for given location which can be useful for finalization of the dia of discharge piep. The location of drainage hole in the superstructure. The location of the drainage hole in superstructre can be different for different types of superstructures and hence it is important to consider this aspect. The reinforcement detailing and location of prestressing cables and anchorages are also important to consider as these element shall not infringe with the drainage pipes in the pier cap. Locations of bearings, bearing pedestal and sesmic restrainer are also to be considered for planning of drainage system.
8. The key elements considered for finalization of the width of the pier cap are bearing to bearing distance in longitudinal direction, size of bearings, size of bearing pedestals, C/L of bearing to C/L of Pier in longitudinal direction, size of the drainage pipes, size of seismic restrainer, distance from edge of bearing pedestal for accomodating the construction tolerance and also to support the structure in case of dislodgement. The sizes and configuration of prestressing cables is also important for finalizing the width of the pier cap.
9. As the metro viaduct are always in the city areas, asthetics is a important aspect of the general arrangement of the structural element of the viaduct. The shape of the superstructre shall be such that it is aesthetically pleasing and shall also accomodate the clerance requirement with being economical. U-girder superstructrue is one of the system which cater all these requirements. The shapes of pier and pier caps shall also be finalized by considering the aesthetics and they shall also satify the structural requirements. The I-girder systems, Rectangular pier caps and portal etc shall be avoided as far as possbile as they may not be provide good appearance to the viaduct.