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THE DESIGN OF SEISMIC ARRESTORS FOR PIERS WITH DISTINCT GIRDER TYPES

Updated: Dec 24, 2021

How i designed seismic arrestors for a very challenging metro project





INTRODUCTION

A seismic arrestor is characterised by its function to bear the impact of seismic forces on a bridge structure and the direction of forces it must bear. Depending upon the direction of seismic forces acting upon it, a seismic arrestor can be classified into a longitudinal and transverse seismic arrestor, resisting longitudinal and transverse forces respectively. Many factors decide the design of seismic arrestors and its subsequent location on the pier cap. It is imperative to know the type of superstructure it is resisting and the impact interface that it experiences.



GENERAL CONFIGURATOIN

In case of arrestors designed for box girders, they resist impact in both the longitudinal as well as transverse directions. The notch at the soffit of the box girder in the end segment adjusts itself to the arrestor dimensions with a minimum clearance of 50mm up to 150mm. The soffit being thick at this end span segment successfully dissipates the forces to the arrestor. In the case of an arrangement of I-section girder of either steel or PSC, two seismic arrestors, one for each direction, are placed upon the piercap. Each of the span might have a singular arrangement of transversely resistive arrestors and a common longitudinal arrestor.

In this case, the peculiarity is to be attributed to the change in the type of girders for both the spans.

Since the box girder on one side is not quite as deep compared to the I-section girders on the opposite side, the box girders have to be raised to compensate for the shortcoming in the depth to maintain the rail level. Hence, a pedestal wall is required to be introduced for the box girder span. This pedestal wall is so designed that it behaves as an integral part of the piercap itself and hosts the bearing pedestals as indicated in the sketch. The edge of bearing pedestal and the pedestal wall edge is maintained at minimum 100mm clearance. The longitudinal seismic arrestor is therefore raised to allow the impact of the box girder as well and is now impacted by both the types of girders with a different impact point. The longitudinal arrestor must also have the provision of a drain pipe that goes to through the pier and out. At the box girder section, we need to allow provision of launcher sleeves and check if it needs embedment into the piercap through the pedestal wall.

DESIGN

First, we need the preliminary dimensions using which as a stepping stone we can improve. Next, we need ultimate design loads. All the levels of different components must be known. The sa/g value is to be adjudged based upon seismic calculations and the appropriate zone factor is applied to the resulting calculus to get the final design loads.

The moment acting upon it is to be found through lever arm assumptions which vary as per the nature of the arrestor. We can either take the lever arm from the top of the impact interface downward or from the centre of the interface download.


To ascertain the forces in designing transverse arrestors for the I-section girders, the notch impacts a plane on the bearing pedestal and hence the lever arm is taken at the centre of the interface area downward.


The main aspect of design is to ascertain if the arrestor is to be designed as a bending member or as a corbel, which is found out by dividing the shear span by the effective depth. If less than 0.6-design it as a corbel. Corbels have extra reinforcement patterns that may cause congestion and placing them on site would be difficult; using a higher diameter of the bar might help here. The side face reinforcement used in corbels is usually to the tune of 0.5 % of the main. Nonetheless, the bearing pedestal, acting as a seismic arrestor, must include the bearing mesh reinforcement along with the design reinforcement at the top.

For the seismic arrestors for the box girders, there is an opening within the soffit in the end segment of the box girder. This Arrestor behaves both as a transverse as well as a longitudinal arrestor for the box girder but as a longitudinal arrestor for the I-section span. Having considered this, it must be expected that there will be two separate lever arms for this arrestor as illustrated. This leaves us with two different lever arms for moment consideration in design. In design, we will use the governing moment irrespective of the lever arm.

 

It is also worth noting that transversely, it is hit in both the directions as indicated in my sketch. Since now the box girder, having being raised by the pedestal wall, experiences a raise to match the rail level, it is imperative that the height of the arrestor be increased too; this results in a particularly taller arrestor with the interface of impact way up. This modifies the it mostly into a bending member designed like any other beam with some reinforcement scheme considerations. In the sketch, the dimensions of LxBxD given by 2000x700x1500mm is the approximate size that was used in this case.

The arrows indicate the direction of the longitudinal impact (comprising of seismic force, LWR forces, Braking, Wind, temperature, etc) and the transverse impact. When we design the member for transverse direction, the direction of this moment is taken as indicated with the shear span equalling our assumed point of impact to the pier top face. In the other direction, the member length remains 1500 but its depth is rendered almost 3 times. Hence, the design criteria shear span to depth ratio would be less than 0.6 and the corbel design considerations are initiated. Thus, this member behaves as a bending member in the longitudinal direction and as a corbel in the transverse direction.

Also note, that the interface is not left for concrete to clash with bare concrete but therein is placed a vertical bearing, the design and methodology of which I will elucidate in a separate article.

Further, the pedestal wall is an integral part of the piercap and is not supposed or designed to bear the seismic impact. The bending member is checked for crackwidth limitations.


VIDEO LINK

https://www.youtube.com/watch?v=bL7LrrXlL0E&t=102s

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