Challenge 2
Q1) Structural components of a bridge are Substructure, superstructure, and foundation. Substructures are the vertical spans of a bridge which includes a Pier cap, Pier, and Pile cap while Superstructures are the horizontal span of a bridge which include the Deck, Girder, and Bearing. Foundation consists of piles which is one of the very crucial components which transfers all the load coming to the hard strata of the ground safely.
Q2)Any structure has various permanent non-structural functional components like railings which avoid overfalling of the people and vehicles, Electrical cables line, water supply lines, and fire lines which add extra dead weight on structures, therefore this kind of loads are termed as Superimposed Dead Load.
Q3)For a metro viaduct, various types of girders that can be used are I girders, Prestressed or Reinforced Girder, T Girder, Steel Box girders, Concrete box Girder, Steel plate girders, Pre-cast Pretensioned U girders.
Q4) Typically two types of bearings are used: one is Elastomeric Bearings and another one is Pot PTFE bearings. Elastomeric functions really well in the case of a span length of 25-27 m needed in the metro viaduct. At the same time, they are also cost-effective and can be replaced very easily. The pot PTFE bearing is more durable and costlier as well and is used only when elastomeric bearings can't be provided.
Q5)During the construction of metro viaducts across water passages, the topographical checklist that needs to be considered for designing the height of the viaducts are the width of the river, discharge of the river, Highest flood level(HFL) of the river and many other things. Among this Height of the viaduct is determined in accordance with the HFL(Highest Flood Level) of the river.
Q6)The different types of girders that can be used in a metro viaduct construction are Precast prestressed I Girder, Precast prestressed Box Gorder, Precast Pretensioned U girder, and Steel plate Girder. For designers
I girders are among the popular choice.
Precast prestressed Box Gorder is preferred when high torsion resistance is required as it behaves very well under torsion load as shear has a full path to cover along the periphery of the box.
Precast Pretensioned U girder is among the most preferred choice. Since it's an open section, it can be easily cast and can be launched very easily onto the piers, also it's lightweight because of the single section member. Metro rails are put at the bottom section as it's an open section the sides of the U girder act as parapets so no additional parapet is required reducing the SIDL. Overall the U girder has very less dead weight therefore the design of the foundation is optimized and also seismic load is also reduced on the U girder. Therefore practically U girder is very viable, economical, and easy & fast to construct.
Steel plate Girder is generally an option when a higher span length is required and where it is not feasible to go for shorter span lengths like 25 to 30 meters of the span length.
Q7) The choice of metro designers would be precast U girders in India because of the following advantages the U girder has to offer:
(i)It's easy and fast to cast due to its open section
(ii)Less SIDL because the side of the U girder acts as a parapet wall so no additional parapet is required
(iii)The metro movement takes place at the bottom of the deck therefore no additional sound barrier is required to be installed leading to an economy
(iv)Overall it has lesser dead weight which leads to a lesser seismic load and thus a more optimized economical foundation design.
Q8)Structures whose web are not connected by the top slab are classified as open web structures. Overall Open web section doesn't have a closed loop structure which is present in the box girder type of structures. It is basically open from the top. Therefore the advantages it carries are:
efficient, lightweight,
(i) Lightweight as less SIDL because the side of the U girder (open girder ) acts as a parapet wall so no additional parapet is required
(ii) Eefficient, as the metro movement takes place at the bottom of the deck therefore no additional sound barrier, is required to be installed leading to an economy
Q9) The load flow behavior in an open web girder is classified into two broad categories.
(i)Longitudinal direction and (ii) Transverse direction because the movement of the metro rail on the girder causes bending in both directions.
In the longitudinal direction, we need to design for the flexure and shear where prestressing takes all the loads while in the transverse design, we need to design only for the flexure where reinforcement takes all the loads as shear in this direction is negligible.
Q10) As per IRS CBC, Table 11(Clause 10.2.2) the stress limitations of SLS, the design is guided by the (a)allowable compressive strength for the triangular stress distribution for prestressed concrete girder bridge which is 0.4Fck as compared to 0.5 Fck for the RCC girder as only the extreme fiber experience such extreme stress. At the same time when it comes to uniform compressive stress, allowable compressive strength has been recommended as 0.3 Fck for prestressed girder as compared to 0.38 Fck for RCC girder. This is to avoid any chance of load reversal during its service life and to consider the possibility of the crushing of the concrete during the prestressing process.
(b)Allowable compression tension which is 0.75 fy is applicable only to the RCC girder because in prestressed concrete prestressed tendons also act as longitudinal reinforcement.
(c)And for the prestresses tendons, clause 16.8.1 dictates that the allowable stress limits should not be more than 75% of its ultimate tensile strength of it.
Q11) Based on the exposure condition of the concrete, different concrete widths are allowed. The exposure conditions have been broadly classified into three categories which are moderate, severe, and extreme and the crack width allowed is 0.2 mm,0.1 mm, and 0.1 mm respectively. The crack width of 0.2 mm at the moderate condition which is considered a normal exposure condition is based on the fact that the normal human eye can't perceive a crack width less than that.