Challenge 2

NOTE : For related figures word file is also attached as this work space is only meant for writing text and do not support images. On including images hear with answers , work space start hanging. The matter was discussed with RM and technical team , they suggested to attach separate word file for images so please take into the consideration.

Q-1 : What are the structural components of Bridge ? Explain in detail about the foundation, substructure and super structure components of bridge

Aim & Introduction: Components of bridge to be listed and foundation , substructures and super structure are to be explained in detail.

Ans / Explanation : Following are the components of bridge-

· Super structure : It consist of Parapet, Deck, girder and bearing

· Substructure : Pier cap , pier and pile cap forms sub structure

· Foundation : Pile is foundation part of the bridge

Super structure : Parapet is non structural part but it has functional requirement. It prevents the over falling of the vehicles and also support the people who are using the bridge as a walk way. Deck is supported on girder which itself support the rail track. Deck is generally of RCC construction. Then we have girder which may be of RCC , pre stressed or steel. The girder may be I girder , U girder or box girder. Girder is placed on bearing. Bearing may be elastomeric type or Pot PTF type depending upon load to be transferred.

Substructure : Below bearing we have pear cap which may be of RCC or PSC. Then there will be pear which may be circular or rectangular. Generally circular pear are adopted as it fits with the land scape of the city. Below pier, pile cap is constructed which connects the pier with the pile and facilitate the load transfer from the super structure to the ground.

Foundation : Pile is the foundation part of the bridge and they need to behave like a single structure for which pile cap is casted integrally with the piles. Piles may be caste in situ or pre cast

Result : Thus bridge components including their detal is explained above

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Q-2 : What is Super Imposed Dead Load means? What are loads that needs to be considered as SIDL for a metro viaduct bridge?

Aim & Introduction : Super Imposed Dead Load is required to be explained.

Ans/Explanation : Super Impose Dead Load is the load that are added to the structure in addition to the self weight of the structure . SIDL include all the weights of the material on the structure that are not structural elements but are permanent and may vary with time . It includes weight of the track from the plinth , rails , fasteners , cables ,para hand-rail , OHE mast , cable trough , ,signaling equipment etc. It includes weight of the track from the plinth , , parapetExample of SIDL or metro viaduct bridge are railing , Wearing surface , Electrical cables , Rail

Ans/Explanation : Railing /Parapet and electrical cable tray constitutes the super imposed dead load

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Q-3: What are the different types of girders that can be used for a metro viaduct?

Aim & Introduction: Different types of girder that can be used in metro viaduct are to be listed.

Ans /Explanation: Following are the different types of the girder that can be used in metro viaduct depending upon the span–

· Prestress precast I girder

· Precast prestressed Box-girder

· Prestress precast U girder

· Segmental box girder

· Steel plate girder- a) Steel I- girder b) Steel box girder

· Steel truss girder

Result: Thus different types of girders have been explained above.

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Q-4: What are bearings in a metro viaduct. Explain the types of bearings and analyses the types with respect to their advantages and disadvantages

Aim & Introduction: Bearing in metro is to be defined. Types of bearing also to be explained and to analyse on the basis of their advantages and disadvantages.

Ans/Explanation: There are following two types of bearing used in metro-

1) Elastomeric bearing

2) POT PTFE bearing

Elastomeric bearing: The purpose of the elastomeric bearings is to support a bridge or other heavy structure in a way that permits the load to shift slightly, in a horizontal direction, relative to the ground or foundation and are normally adopted when span length of metro viaduct ranges typically from 25 m to 28 m . They are cost effective and can be easily replaces in case wear and tear. So these bearing are provided when span length and loads are less. Without such bearings, the bridge support might crack or fracture when it moves due to ground movements or thermal expansion and contraction. Without such bearings, the bridge support might crack or fracture when it moves due to ground movements or thermal expansion and contraction. ( Fig. is attached in word file)

POT PTFE bearing is a bearing carries vertical load by compression on an elastomeric disc confined in a steel cylinder which accommodates rotations by deformations of the disc are more durable and costlier so provided when span length is too high and load on the bearing are too high. The elastomer under a triaxial pressure offers low resistance to deformation but high vertical stiffness. ( Fig. is attached in word file)

Result: Bearing types and their usages has been explained as above.

Result: Bearing types and their usages has been explained as above.

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Q-5 : During the construction of metro viaducts across water passages, explain the topographical check list that needs to be considered for the designing the height of the viaducts?

Aim & Introduction: Topographical check list that need to be considered during construction of metro viaduct across the water passage is to be explained.

Ans/Explanation: Maximum flood level and overhead electric cable are the topographical check that need to be consider for designing the height of metro viaduct across the water passage. Height of the bridge should be such that water may remain at level well below the bearing on the situation of high flood level as electrical cable may be running along the rail track over the girder. Similarly bridge height should be such that metro can safely pass below the overhead electric line.

Result: Therefore, criteria of designing height of the bridge with reference to topographical check has been explained above.

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Q-6 : What are the different types of girders that can be used in a metro viaduct construction? Explain the differences of the girders and elucidate how a designer has to choose his girder type whilst designing.

Ans/Explanation: Following are the different types of the girder that can be used in metro viaduct depending upon the span–

· Precast pretension U-girder

· Steel plate girder- a) Steel I- girder b) Steel box girder

· Precast prestressed Box-girder

· Steel truss girder when

Advantages of each type of girder:

U-girder: Upton the span length of 30 m where tortional shear force is not prominent. it behaves reasonably well. It is an open section so it can be casted easily in one go. It is lighter section so can easily transported and placed in position on the girder. Its web act as noise barrier as metro run on the lower deck and most of the noise is produced in the area confined by the webs of the girder so extra noise barrier are required to be installed , moreover its web also act as parapet so there cost saving of separate parapet

Steel plate girder : When span length is more than 45 m and up to 70 m or 80 m , we go for steel plate girder / steel box girder. Steel box girder are lighter than concrete box girder because in steel box girder very light steel plates are required because its higher strength.

I-girder : It has wider space at top and bottom of the section due to its shape. These wider space is used to place reinforcement and PT cables. It is vey thin section so it has got very less dead weight therefore design of foundation also gets optimised .

Box girder : It has close section so it behaves very well under tortional load as shear gets full path along the periphery to travel. i.e. it has more area for the tortional shear flow. So it is consider to be better section for tortional loading. Box girder is suitable for span length of 30 to 45 m in order to take care of tortional shear force.

Steel truss girder : When span is very large say 80 m or more then truss girders are adopted.

Result: Thus different types of girders have been explained above with their advantages and choice of usage.

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Q-7 : Explain why the choice of metro designers would be precast U girders in India.

Aim & Introduction: To be explained why metro designer would prefer U-girder in India .

Ans/Explanation: Metro designer generally prefer to use U- girder due to the following reasons-

Ø It is an open section in which webs are not connected with top deck

Ø U-girder is a light section so economical

Ø Its web are used as parapet so extra cost is involved in the construction of seprate parapet

Ø Noise pollution can be prevented through the web of U-girder

Ø Being light weight , it can be easily transported to the site and can be placed on girder.

Ø Being precast quality control is better.’

Result: Thus, preference of U-girder by metro designer has been explained above

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Q-8 : What is an open web structure ? Brief the advantages of the system.

Aim & Introduction: Open web structure is to be defined and its advantages are to be stated.

Ans/Explanation: The structure which are open from the top, i.e. web is not connected with top slab is called open web structure.

Advantages of open web structure:

Ø These are light section so economical

Ø These have open web acting as parapet so no extra cost is involved in the construction of separate parapet

Ø Since metro run at lower deck so the web of the U girder act as noise barrier, therefore no separate noise barriers are required to be installed

Ø Being light weight, it can be easily transported to the site and can be placed on girder.

Ø Being precast quality control is better.’

Ø They have open section so can be easily casted in one go.

Result: Thus open web structure is defend and its advantages are explained

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Q-9: Explain the load flow behavior in an open web girder

Aim & Introduction: Load flow behaviour in open web girder need to be explained.

Ans/Explanation:

In a open web girder it is saucer type girder which can be bend in longitudinal direction and as in the transverse direction which means perpendicular to the movement of the trains. We have to design this open web girder in saucer like bending which means it has to be designed for longitudinal bending and transverse bending. In longitudinal bending we have to design it for flexure and for shear And in transverse direction we have to design it for flexure because the shear effects in shorter span are not too high. We have to analyze this section as 3D element which can bend in longitudinal and transverse direction.

Result: Load flow behavior in open web girder has been explained as above

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Q-10 : As per IRS CBC, Table 11(Clause 10.2.2) the stress limitations of SLS, Explain your understanding of the table and how it correlated while designing a viaduct for metro rail construction.

Aim & Introduction: As per IRS CBC, Table 11(Clause 10.2.2) the stress limitations of SLS, explain your understanding of the table and how it correlated while designing a viaduct for metro rail construction.

Ans/Explanation: As per IRS CBC, Table 11 , clause 10.2.2 the stress limitation of SLS are as given below-

Material	Type of stress under design loading	Type of construction
		RCC	PSC
Concrete	Triangular or near triangular compressive stress distribution (i.e. due to bending	0.5fck	0.4fck
	Uniform or near uniform compressive stress ( i.e. due to axial loading )	0.38fck	0.3fck
Reinforcement	Compression Tension	0.75fy	Not applicable
Pre-stressing tendons	Tension	Not applicable	Deemed to be satisfied by 16.8.1

In case of triangular distribution of stresses or nearly triangular stresses (In case of pure bending) in RCC, permissible stress is 0.5f_ck . At extreme fibre stress would be 0.5f_ck which will reduce linearly to 0 at neutral axis. Thus average stress at any fiber would be less than 0.5f_ck.

For uniform stress or nearly uniform stresses (In case of axial forces) the permissible value of stress in RCC is 0.38f_ck.. Here stress through the section is 00.5f_ck , therefore average stress would also be 0.5f_ck. This is why permissible stresses are 0.38 f_ck.

in case of PSC construction, the permissible value of stresses for triangular and uniform stress distribution are 0.4fck.and 0.3fck. respectively. Lesser value of stresses i.e.0.4fck.and 0.3fck. are generally observed due to transfer of prestresses. In prestress section it has got maximum compression stress when external loading is minimum and the prestress has just been transferred to the section. So, in that case we do not want to crush the section and we know that when load actually comes on the section which is the external load, the dead load, the SIDL, and the live load this compressive stress is accordingly balance by the tensile stress at that particular section and we do not want the section to be crushed, therefore we allow slightly lesser stress. Second reason is that stress of 0.4f_ck will be finally neutralise and thus there will be reversal of stress which we do not want because cyclic reversal of stresses will case the fatigue.

For reinforcement, the permissible value of compressive or tensile stresses is 0.75f_y . For PSC section this is not applicable because in PSC section all the load is taken by prestressing cable.

Result: The stress limitations of SLS as per IRS CBC, Table 11(Clause 10.2.2) for designing a viaduct for metro rail construction. has been explained as above.

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Q-11 : Explain your understanding of crack widths. Brief the three different design crack width with respect to IRS CBC.

Aim & Introduction: Crack width is to be explained as per my understanding. Three types of crack width as per IRS CBC is required to be explained.

Ans/Explanation: Crack width is permissible limit of crack in concrete to be allowed without affecting the serviceability criteria of the structure and reinforcement may not be corroded due to moderate, severe and extreme condition of exposure.

Corresponding to the different exposure condition of the structure different crack width are defined as clause 10.2.1a, table-10 of IRS, CBC as mentioned below-

Exposure condition Design Crack width in mm

Moderate 0.2

Severe 0.1

Extreme 0.1

Moderate conditions are the normal condition of exposure of structure in which per permissible crack width is fixed as 0.2 mm which human eyes cannot perceive. Thus, Crack width is a serviceability criterion. Structures perceive to be safe by the people who are actually using the structures.

For structures under sever and extreme conditions where there is very high variation in temperature or structure is situated near sea lower value of crack width i.e. 0.1 mm is permitted so that reinforcement may not be corroded.

Result: Thus, crack width under different exposure conditions has been explained.

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