Challenge 2
1.What are the structural components of a bridge? Explain in detail about the foundation, sub structure and super structure components of the bridge
The various parts and components of a bridge are as follows:
<!-- [if !supportLists]-->1. <!--[endif]-->Deck
<!-- [if !supportLists]-->2. <!--[endif]-->Superstructure
<!-- [if !supportLists]-->3. <!--[endif]-->Substructure
<!-- [if !supportLists]-->4. <!--[endif]-->Foundation
<!-- [if !supportLists]-->5. <!--[endif]-->Girder or beam
<!-- [if !supportLists]-->6. <!--[endif]-->Bridge tower
<!-- [if !supportLists]-->7. <!--[endif]-->Pier
<!-- [if !supportLists]-->8. <!--[endif]-->Pier cap
<!-- [if !supportLists]-->9. <!--[endif]-->Bearings
<!-- [if !supportLists]-->10. <!--[endif]-->Piles
<!-- [if !supportLists]-->11. <!--[endif]-->Pile cap
<!-- [if !supportLists]-->12. <!--[endif]-->Bridge anchor
<!-- [if !supportLists]-->13. <!--[endif]-->Suspension cable
Superstructure
The portion supports the deck slab and girder and connects one sub-structure to the other. That means all the elements of the bridge attached to a supporting system can be categorized as superstructure.
Sub-structure
The parts of the bridge which support the superstructure and transmit all the structural loads of the bridge to the foundations. For example, piers, abutments, etc.
Foundation
Foundation is the portion that transmits loads to the bearing strata. Foundation is required to support the piers, bridge towers, and portal frames. Generally, piles and well foundations such as H-pile, bore piles, pipe piles, or precast concrete piles are adopted.
2.What is Super Imposed Dead Load means? What are loads that needs to be considered as SIDL for a metro viaduct bridge?
superimposed dead load is the dead load on the structure after slab and beams have been casted .. dead load is the load from slabs and columns and beams . in short superimposed dead load = floor finishes, tiles, water proofing , cable wires , wall loads , plumbing pipes, AC duct.
Following are the loads that need to be considered as SIDL for a metro viaduct :
<!-- [if !supportLists]-->i) <!--[endif]-->Deadload
<!-- [if !supportLists]-->ii) <!--[endif]-->Shrinkage creep
<!-- [if !supportLists]-->iii) <!--[endif]-->Live load
<!-- [if !supportLists]-->iv) <!--[endif]-->Coefficient of dynamic augment
<!-- [if !supportLists]-->v) <!--[endif]-->Footpath LL
<!-- [if !supportLists]-->vi) <!--[endif]-->Braking and traction
<!-- [if !supportLists]-->vii) <!--[endif]-->Centrifugal force
<!-- [if !supportLists]-->viii) <!--[endif]-->Seismic load
<!-- [if !supportLists]-->ix) <!--[endif]-->Wind load
<!-- [if !supportLists]-->x) <!--[endif]-->Buffer load
3.What are the different types of girders that can be used for a metro viaduct?
i) Precast Prestressed I-girder
ii) Precast Prestressed Box Girder
iii) Small segment box girder
iv) Precast Pretensioned U-girders
v) Steel plate girder
<!-- [if !supportLists]-->4. <!--[endif]-->What are bearings in a metro viaduct. Explain the types of bearings and analyses the types with respect to their advantages and disadvantages
Bridge bearings are structural equipment or devices installed between bridge substructure and superstructure to transfer the applied load including earthquake loads; wind loads; traffic loads; and superstructure self-weight. Bridge bearings also makes rooms for relative movements between superstructure and substructure, for instance, rotation movements and translational movements in horizontal and transverse direction. Bearing used in the construction of bridge structure is divided into two major categories namely expansion bearings and fixed bearings. The former permits both translational and rotational movements whereas the latter allow rotational and limited translational movements. There are several types of bridge bearings which have been employed in bridge construction which are discussed below.
Types of Bridge Bearings and their Details
Different types of bearings for bridges include:
<!-- [if !supportLists]-->· <!--[endif]-->Sliding bearings
<!-- [if !supportLists]-->· <!--[endif]-->Rocker and pin bearings
<!-- [if !supportLists]-->· <!--[endif]-->Roller bearings
<!-- [if !supportLists]-->· <!--[endif]-->Elastomeric bearings
<!-- [if !supportLists]-->· <!--[endif]-->Curved bearings
<!-- [if !supportLists]-->· <!--[endif]-->Pot bearings
<!-- [if !supportLists]-->· <!--[endif]-->Disk bearings
Sliding Bearings for Bridges
Sliding bearing consist of two metal plates, commonly stainless-steel plates, that slide relative to each other and hence makes room for translational movement and lubricating material between them.
A friction force is generated in sliding bearing and it is imposed on substructure, superstructure and sliding bearing itself. So, it may be required to provide lubricant such as polytetrafluoroethylene (PTFE) to decline generated friction. It is specified by ASSHTO that sliding bearing cannot be used unless the bridge span is smaller than 15m. This is because sliding bearing cannot be purely used if the bridge experiences rotation movement. However, the span restriction for sliding bearing utilization can be disregarded when it is used in combination with other bearing types.
Rocker and Pin Bearings for Bridge Structures
Rocker is an expansion bearing composed of curved surface at the bottom, which accommodate translational movement and a pin at the top makes room for rotation movement
Pin bearing is a fixed bearing that make room for rotation movement through the application of steel pin. It has similar structure and component like rocker bearing apart from the bottom of pin bearing which is flat and fixed to the concrete pier, as can be observed
Both rocker and pin bearings are mainly employed in steel bridge structure. Rocker and pin bearing should be considered when the bridge movement is adequately known and described, since such bearings can make rooms for both translational and rotational movements in one direction only. These bearings are likely to suffer deterioration and corrosion, so it is necessary to conduct regular inspection and maintenance.
Roller Bearings for Bridges
Roller bearing can be used in the construction of reinforced concrete and steel bridge structure. There are two main configurations including single roller bearing which is composed of one roller placed between two plates and multiple roller bearing that consist of several rollers installed between two plates. The former as shown in Figure-5 can accommodate both rotation and translation movement in longitudinal direction and it is cheap to manufacture but its vertical load capacity is limited. In contrary, the latter as shown in Figure-6 can make room for translation movement only and rotation movement can be accommodated if rollers are combined with pin bearing. Multiple roller bearings are expensive and support considerably large vertical loads. Regular inspection and rehabilitation should be conducted since roller bearing are susceptible to corrosion and damages.
Elastomeric Bearings for Bridge Structures
It consists of elastomer manufactured from synthetic or natural rubber and can take both translation and rotation movements through elastomer deformation. The ability of elastomer to carry large vertical loads is because of reinforcement provision that prevents lateral bulging of elastomer. There are number of elastomeric bearing pads classified based on types of reinforcements used. For example, steel reinforced, plain, fiberglass reinforced and cotton duck reinforced elastomeric bearing pads. Strength and response of each type is different, steel reinforced elastomeric bearing is the strongest one and plain elastomeric pad is the weakest. Elastomeric bearing is neither expensive nor requires considerable maintenance, that is why it the most desired bearing type.
Curved Bearings for Bridges
It consists of two curved plate that match each other as shown in Figure 8. If curved bearing is cylindrical, then it only accommodates rotation movements. However, both rotation and translational movements can be dealt with if curved bearing is spherical. Due to the fact that both gravity loads and curved geometry generate lateral resistance against and consequently lateral movement would be limited, that is why polytetrafluoroethylene (PTFE) slider is adhered to the bearings in order to make rooms for lateral movements. Details of curve bearings can be seen
Pot Bearings for Bridge Structures
As shown in Figure-9, pot bearing consists of elastomeric disk confined in a pot, steel piston that is properly tailored into the pot wall and flat sealing rings which keeps elastomeric inside the pot. Pot bearing can support considerable vertical loads and it is commonly transferred through steel piston to the elastomeric disc which is almost incompressible. As far as lateral load is concerned, it is transferred as the steel pistol moves toward pot wall. Translational movement is limited in pure pot bearing that is why PTFE are introduced to the sliding surface to make rooms for translation movement.
Disk Bearings for Bridges
Rotation movement is accommodated through the deformation of elastomer whereas the translation movement is considered through the application of PTFE slider.
The utilized elastomer should be adequately hard to support vertical loads without suffering large deformations and sufficiently flexible to allow rotational movement. Both vertical loads and lateral loads are supported by elastomeric disk and metal ring in the center of the bearing respectively.
<!-- [if !supportLists]-->5. <!--[endif]-->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?
i) The planning for designing the height of the viaducts Highest Flood Level need to be checked and from that parameter the viaduct need to be designed.
ii) The height of the bridge is governed by other consents like Overhead electric cables which have to be placed on some certain cleareance that the moving metro can pass through it easily.
iii) The width of the river bed needs to be taken under consideration.
iv) The depth of the river needs to be checked for placing the piles in a proper place.
<!-- [if !supportLists]-->6. <!--[endif]-->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.
i) Precast Prestressed I-girder- This is the most important part applied in superstructure which is I-girder. A designer has to see the span length and depending upon the span length the girders are placed.
ii) Precast Prestressed Box Girder- They are the Closed Web Structures which behaves very well in torsion loading in which shear due to torsion covers the larger area as is it closed web.
iii) Small segment box girder- These are the box girders which are divided into small parts because at some place we cannot lift the large box girders as it can disturb the traffic and surrounding so these small segments which is 3m in length can be used upto to 40-45 m span length.
iv) Precast Pretensioned U-girders- The most favourable girders which are used is U-girders they are better in quality, they are open section. Placing this girders can save our time and it can be placed in one go. The span length of this girders are 25-28m . The sides of this girders can be used as parapet/railing.
v) Steel plate girder- This girders are used when the span length of corridor is more than 30m. They are constructed with steel plate and is placed on concrete columns.
<!-- [if !supportLists]-->7. <!--[endif]-->Explain why the choice of metro designers would be precast U girders in India.
<!-- [if !supportLists]-->U-<!--[endif]-->girders will involve casting the entire span of a viaduct between two piers in single piece whereas box-girders will have many segments between two piers. One U-girder between two pillars can be erected overnight, against 5-6 days required in case of box girders. “U-girders are cast in yard like box-girders
Advantages to this type of construction compared to conventional construction include: Lower fabrication times. Faster construction. Ability to span longer distances.
U-Trough Girder construction method is more economical in terms of time and cost. For instance, U-Trough Girder's volume of concrete can be reduced due to a narrower cross section. With the U-Trough technology, at least one span can be installed in a day against three days using the segmental box girder method.
8.What is an open web structure? Brief the advantages of the system.
Open web girders are used for track bridges over large rivers and valleys. Standard length of open web girders in railways are 30.5, 45.7, 61.0 and 76.2 meters.
Mostly open web type girders are used for track bridges over valleys and large rivers on account of economy in cost. This is achieved due to low depth of bottom of girder from the rail level (for through girder bridges). Presently steel used is conforming to IS: 2062 Grade 'B0' for rolled sections.
The primary members of the girder are Bottom chord, Top chord, End Rakers, Diagonals, Vertical members and Floor system comprising of Cross girders and Rail bearers or stringers.
9.Explain the load flow behavior in a open web girder.
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 analyse this section as 3D element which can bend in longitudinal and transverse direction.
<!-- [if !supportLists]-->10. <!--[endif]-->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.
10.2.2. Stress Limitations- To prevent unacceptable deformations from occurring, compressive stresses in concrete and stresses in steel should be calculated by linear elastic analysis for the load combinations given under 11.2 in any of the following applications : a) for all prestressed concrete construction; b) for all composite construction; c) where the effects of differential settlement, temperature difference, the creep and shrinkage of concrete are not considered at the ultimate state : 10.2.2.1 For reinforced concrete and prestressed concrete, the compressive and tensile stress limitations are as specified in Table 11. TABLE 11. STRESS LIMTATIONS FOR THE SERVICEABILITY LIMIT STATE. (Clause 10.2.2) Type of construction Material Type of stress under design loading RCC PSC Triangular or near triangular compressive stress distribution (e.g. due to bending) 0.50 fck Concrete 0.40fck Uniform or near uniform compressive stress (e.g. due to axial loading) 0.38 fck 0.30fck
Reinforcement Compression Tension 0.75 fy Not applicable Prestressing tendons Tension Not applicable Deemed to be satisfied by 16.8.1 NOTE 1– The above stress limitations have been derived from 12.2 after making allowance according to Table 13 (see 12.4.2). NOTE 2 - See 17.3.3 for limiting flexural stresses in joints for post tensioned segmental construction.
<!-- [if !supportLists]-->11. <!--[endif]-->Explain your understanding of crack widths.Brief the three different design crack width with respect to IRS CBC
Design crack widths, as calculated in accordance with 15.9.8.2, shall not exceed the values given in Table 10 under the loading given below
