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DESIGN OF MULTI-LEVEL STATION PIERS FOR ELEVATED METRO CORRIDORS

Updated: Aug 9, 2022

There are many types of piers in a typical Metro Mass Transit System anywhere. Station piers are the most critical piers that are designed for an elevated metro system in any city. A series of these piers constitute an entire metro station.


Generally, these are designed as huge expanded double portals the design of which I will expound in some other article. A unique type of station pier is a singular pier with freely hanging cantilevers branching from either sides, supporting the entire station upon its long protruding arms, like a tree, this article is about the design of such types of piers. While we are aware of these piers in our cities, it is interesting to know that this remarkable edifice is a blossom of structural engineering.


COMPONENTS


The station pier can be broken down into 3 main parts-

1. Main Pier

2. Pier Above The Concourse Arm

3. Pier Above The Pier Cap

Their geometric dimensions reduce along their height upwards proportionally as is indicated in the sketch in red hatch. Then topmost end is socketed into the platform arm above and the bottom most pier is socketed into the pile cap below (usually > 1.5m thick).

It is of essence to know the number of levels that are necessitated. Usually, two levels are a common practice unless an additional level is needed for a junction or a subsidiary pedestrian footbridge is branching out of the station.

The following video will throw more light on the components of Metro Staion Piers:



The main pier, usually rectangular with chamfered edges, transfers all the loads to the foundation. At levels appropriate, arms branch out of the main pier stem, these arms are double cantilever arms which are essentially post tensioned. The arm at the concourse level is called the concourse arm, the one at the platform level is the platform arm and if necessitated there is a property development arm below the concourse arm. These arms are aligned with bearing pedestals that host the girders at that level. With Prestressing anchors at the ends, the cables traverse throughout theses arms. These arms are cast on site with scaffolding arrangement to be specifically designed.

The pier above the concourse arm is a dimensionally reduced rectangular pier perched atop the concourse arm, nonetheless connected to the main pier.

The pier above the pier cap is a much shrunken version of the main pier. This is essentially pegged into the platform arm and is responsible for transferring the loads form the platform arm to the piers below. The peculiarity in


The concourse arm is the level at which the pedestrian level is hosted upon and counters for tickets and other stores are established. Its structural geometry is but an inverted T-beam or L-beam depending upon their instance of position-the end piers have and L shaped concourse arm mostly, marking the end of the station. In order to complete the slab at this level, a number of girders are placed upon this arm upon the bearings as indicated. These girders, again, can be either steel or PSC based on choice and configuration schema. Not to mention, there are seismic arrestors to curb the lateral movement of these girders upon the arm, refer figure. Their design is carried in two parts – first as a cantilever rectangular beam and second as a two-way mirrored corbel. The design of a concourse arm I will cover in some other article some other time.

The platform arm is structural entity that is the structural component holding the platform from which the trains are accessed .Structurally, it is to be understood in two parts, the two branches of the cantilever on either sides and the central portion (Protruding Longitudinally). Both of these differ from the nature of the forces they resist and the profile of the cables within apart from their geometry.

The pier cap at the end of the station pier is for the main girder with two seismic arrestors on either sides for the girders respectively. This seismic arrestor is designed the same way as has been described in my previous article.

We must establish a distinction between the types of girders that are to be found in station pier-

1. Concourse arm girders (on both sides)

2. Platform arm girders (on both sides)

3. Main bridge girder that is placed on the bearings at the top of the piercap next to yh the platform arm.

These types are all indicated in the sketches above.

Both CC (Concourse) arm girders & PF (Platform) arm girders are fundamentally I-section girders of either steel or PSC and upon it is the deck slab for pedestrian movement, etc.


DESIGN

While designing the pier 4 types of forces dominate the essence of design namely:

· Axial

· Shear

· Moment (transvers + longitudinal)

· Torsion


TRANSVERSE MOMENT


At first glance at the Force diagram (Transvers section), one may liken this pier to a tree with branches spreading against each other. The bearing points or rather positions of the girders are those shear generating points that are primarily responsible for moments that develops in the arms and then radiates to the main pier. Naturally, each of these bearing points have a different lever arm which must subsequently develop an increasing magnitude of moment outward proportional to individual offsets from the pier face. Also note, that the individual bearing reactions might not be identical, unless the floor above is uniformly loaded in all aspects. These reactions are influenced by the positioning of SIDL on the floor above apart from the DL and LL. Hence each of these reactions of varying magnitude along with separate lever arms is to contribute to a collective moment that is to be experienced by the pier at that very level. This same moment will also be contributed from opposite branches of the CC arm. Hence the net moment thereon will be the moment acting at that node.

The same approach is valid for the platform arm and the two reactions from the piercap, as sketched. Also note, that this net moment can be controlled and nullified while designing the arms and profiling them, the mechanism of which I will discuss in a separate article.


LONGITUDINAL MOMENT


The longitudinal moment results in torsion in transverse section of the arms. Hence, extra torsional reinforcement provision is mandatory for arms. While torsional effect is analysed, only the rectangular portion of the arm is considered for reinforcement.

The design of a CC arm is primarily influenced by the longitudinal moment while designing for a corbel and the transverse moment is dominant while designing it as a cantilever beam, both f these aspects of design I will try to cover in a separated article on CC arms.


TORSION


It is interesting to track the nature of torsion that is encountered in the structure. Since this force is mainly dealt with in the design of the arms, not much impact is considered for the design of the pier itself. Nonetheless, a section of the CC arm is sketched indicating the torsional effect upon it.


POST TENSIONED CABLE PROFILE


An overview of the cable profiling is illustrated in my sketch. The profiles encountered in such a pier are customised for each of the arm; Pay close attention to the profile of the platform arm and the special section attached to itself at the central portion. The profiling is fundamentally set to mimic the onset of moments in the arms for which the pier serves as a converging focal. Notice that the eccentricity of the cable profiles is kept at maximum when it is traversing the pier zone.


AXIAL FORCE


As any typical column constructed in buildings, station piers experience the axial force as a summation of axial values as per the number of levels until the foundations. As illustrated in the sketch, you notice the axial force diagram augmenting in magnitude at the levels with the net moment summing up upwards-down until the face of the pile cap. When lateral force is considered in design, the height (H1, H2, H3) of the pier section assumes a vital role as a vertical lever arm, again contributing to the ultimate moment(C). The CC arm and the PF arm and the respective loads and masses can be lumped into a mass and rendering the entire edifice into a two nodal system for seismic analysis.

While we have disintegrated the pier into parts, each of the columns are separately designed. When designing the pier above the concourse arm, the finished level of the concourse arm will be considered as the base level and net moment at CC arm top level will be the design ultimate moment for this portion of the column, in this case Moment-B. Respectively, the pier above the pier cap will be designed for Moment-A.



REINFORCEMENT


It is to be ensured that the main reinforcement is designed in both the transverse and the longitudinal directions for the respective moments. Secondly, the reinforcements (RF) from the pier below must adequately socket into the pier above as also into the arms on either sides as indicated in my sketch. The RF from the arms are separately designed and schemed taking into account the pier RF. To avoid the congestion of RF at the junctions, Larger diameters upon number of bars be used.


This article was also published on special request in metro rail magazine in 2020

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