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DESIGN OF CANTILEVER PIER CAPS FOR METRO BRIDGES

Updated: Apr 12, 2021

Cantilever Pier caps are structural components that have a complex arrangement, nonetheless modelling them is quite simple on MIDAS civil.

General Configuration

Before we begin designing the pier cap, a crisp picture of the arrangement must be systematized. In this context, the configuration shown in the sketch would readily help. What makes this piercap special is the incidence of two types of girders-concrete I-Sections and a Single Celled Box girders. The metro line meets a depot line that advances to the metro depot. The main line of the metro is designed using the box girders a depot line is designed using I-section girders. Both these girder systems have 2 Metro rail tracks each.

The length of the pier cap, about 14m, hosts 10 bearing pedestals 5 on each side; 33 seismic arrestors and respective offsets left for the minimum clearance and cover provisions. Both sides of the pier have a span of about 25m and 28 m on either sides. The depth of the cap at pier face, 1.5m in total, was adjudged based upon previous experience and sufficiency of the section to resist shear and cantilever moment. The zone of pier is the part which







LOADS AND MOMENTS

The pier cap experiences some critical combination of loads apart from the self weight and dead load of girders. Live load combinations must be visualised, derailment load cases must be added for both left and right sides. One critical case is the passage of launching girder over the pier location. The pier cap has to have the provision to fix the launcher screws in the pier cap, this must be in accordance with the screw offset in the type of launcher we are dealing with. The longitudinal moment in the piercap is by far nullified by the occurrence of the same type and number of girders on both the sides. The distance from the centre point of the bearing to the face of the pier is the lever arm that is considered in the resulting calculus. In this case, both the spans on either sides of the pier cap have the same number of girders and spans that almost nullify the longitudinal moment i.e 25m and 28m respectively. This leaves us with a considerably less balance of longitudinal moment residual in the piercap. This residual longitudinal moment acts as torsion in the section of the piercap. Hence an additional check for torsional moment and a respective torsional reinforcement has to be provided.


The transverse moment is the main design moment for the piercap. Since the piercap can be likened to two oppositely protruding cantilever beams ebbing from a common pier, we need to ascertain what side of the cantilever is the side generating the maximum transverse moment. In our case, as indicated in the third sketch, there are 4 bearings(1,2,3,4) on the transverse LHS and the RHS hosts 2 bearings(5,6). Each of the bearings have a different lever arm and hence an ascending magnitude of moment from the face of the pier on both sides. It is to be understood that the bearing that lies within the pier zone does not contribute to the pool of transverse moments that sum up to the acting moment. Hence bearing number 4 does not contribute to the transverse moment and the vertical load is directly transferred to the pier. Hence the governing transverse moment is the sum of the moments contributed by bearings 1,2 and 3 for which the design is to be initiated.

The CG of each of th girder systems and the piercap section itself must be known. Any transverse moment that emanates from the additional DL, SIDL, LL loads is applied at the respective


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