1. GENERAL DESIGN DESCRIPTION
1.GENERAL DESIGN DESCRIPTION
There is increasing traffic congestion within the Midlands and the problem is imminently reported on a daily due the expansion of the city. This problem has raised an alarm and therefore, there is a proposal which requires that a flyover be constructed to ease the traffic flow at the junction of the major road which intersects all the major roads in the town. However, the major concerned is that this flyover should not interrupt the already existing primary roads within the area. Also, it is clear that the only traffic controlling point with the town is the major roundabout within the area. In early 50’s, there were trams along the major roads and they were used to serve the city however; these trams are currently not in use and serve as central reservations within the city. In fact, the trams now offer an opportunity of building a dual flyover carriage without necessarily adjusting the existing roads
Additionally, it is important to take into consideration the fact that the proposed flyover should be economical in terms of its elegant and its functionality as well. Obviously, the local environment and the apparent appearance of the town is also a major concerned which need to taken into consideration under the design aspect of this parameter. Some of the elements to be considered in the process include offices, premises, small businesses as well as local shops which are adjacent to the proposed carriage way. Therefore, the efficiency and minimum disruption during the construction is a key priority in the designing and the implementation of this project. This should even include stakeholders and the overall community as a whole
.OPTION 1: CONTINUOUS BEAM BRIDGE FLYOVER
In the construction of a flyover continuous beam bridge can be a type of the design to be used in the construction. This entails having two or even more beams which are jointed on the support edges. The continuous beams are of the two types based on the material science and these two categories include concrete continuous as well as the pre-stressed continuous beams. Moreover, for this flyover construction the continuous beams tends to have a number of advantages which ranges from few movements along the joints, smooth deck, good rigidity of the structure, span capability, reduced maintenance cost, resistance to wave and earthquake as well as strong applicability. Taking the span of the flyover to be 80.5m and the width of the flyover to be 20m, then this option will ensure that the deformations and rigidity structure are well organized in the round and have no adverse effects and are economical in the long run. Furthermore, the substructures will be made of the abutments and the piers. On the other hand the superstructure will mainly compose of the continuous beams along with the panels which prove safety parapet and walking surface. Moreover, the road span can be divided into four spans with each span measuring 20m. Moreover, the C50 concrete and motorway can be used in this option to build the continuous bridge
OPTION 1: CONTINUOUS BEAM BRIDGE FLYOVER
.OPTION 2: CABLE STAYED FLYOVER BRIDGE SYSTEM
Another construction method which can be used in this flyover bridge construction is the cable stayed technique. The method is also suitable based on the span of the proposed flyover that is 80m by a width of 20m. Some of the components of the cable stayed technique include cable, tower, pier, deck as well as the superstructure foundation. The choice of the technique will be based on the appearance and the behavior of the structure therefore enhancing the efficiency and increase discontinuity of the structural behavior. In the construction, the cable stays will be arranged in accordance with the pure fan system along with the radiating pair of stays. The stays are often referred to as pylon and the contractor have to use two of them at the top. In essence, this pylon will be used to support the vertical deck at the edges of the bridge deck. Additional cable planes can be used to allow for the makeable and invisible anchoring of the remaining plate grinders.
.KEY ISSUES
The maintenance cost for the cable bridges is economical, for instance one can easily repair the damaged cables at anytime. In addition, the cable bridges are durable even if the ground conditions are not good. In this aspect, the cable bridge design is the best choice for this but the engineer must recognized the both the deteriorating time as well as the service life time span. Das (1999, p.45), depicts on the two primary maintenance mechanism which can be employed in this design work. The first method is the preventative maintenance and this technique demarcates on the elements which must be considered as critical at earlier stage to avoid additional costs in the later days. The second bit is on essential maintenance and this focuses on keeping the bridge safer at all time. Therefore, if the two techniques are adopted and employed decisively then they will increase service life and safety of the flyover bridge. This design work will also require that carrying weight capacity and load limits to be subjected to the flyover be considered in the design work. However, the as the designer, it is important to consider the durability of the road at the expense of the state conditions in the meantime.
.SCHEME COMPARISON FOR OPTION-1 AND OPTION -2
The table below show the comparison between the two options picked
.
|
Parameter Being Compared |
Continuous Beam Bridge |
Cable Stayed Bridge |
|
The length of the span of the flyover |
15.25m + 30m + 15.25 m |
78m |
|
Main materials to be used in the process |
C50 concrete materials |
Steel, Prestressed concrete, cable, tower, pier and deck |
|
The support to be used in the construction |
Box Beams |
2 pier tower |
|
Aesthetic |
Not much appealing and simple |
Appealing, more attractive and beautiful |
|
Cost |
Economical and cheap |
Expensive and economical in the long run |
|
Construction method |
Scaffold as well as cast in-situ |
Balanced cantilever |
|
Construction period |
The time frame required for the project is Shorter |
Longer period is required in line with construction as far the method is concerned |
OPTION 2: CABLE STAYED FLYOVER BRIDGE SYSTEM
.RECOMMENDATIONS FOR PICKING OPTION 1
From the two options provided for the analysis, the flyover bridge can be build above the highway using the set pier and three continuous span beams. Although the second option of using cable-stayed bridge is appealing and beautiful, the span of the flyover which is 80m makes it economically expensive in the long run. Therefore, in the comparison the first option of using continuous becomes more efficient and economical thus, overrides the second option. In essence, the continuous beam bridge method offers an adoptable mechanism which can be ascertain with a short period of time and the one which have low difficulty in terms of the overall work. Therefore, in this design construction and with all specifications the best option to be used in this process is the continuous beam bridge construction mechanism (Harris, 2018).
.CONSTRUCTION METHOD
A flyover bridge deck is defined as a bridge which goes over the section of another roadway and this is basically done to ease traffic. Over the past decades, the only technique which has been used in the construction of the bridges is the cable-stayed method. The cable-stayed method is a technique which is based on either steel or concrete however; these elements can be combined with other materials (Zhu et al., 2015 p.2000). The bridge deck flyover can be made from three elements and such elements include truss deck, steel box as well as the concrete prestressed box deck. In this design work, the consultant engineer picks on the prestressed concrete box and this is supported by the cables at the support edges of the flyover. The advantage of this method is that the technique does not rely on any bond as well as the result self-weight deflections in the structure will just cancel out (Ai, 2017). Although, there are other technical methods which the engineer could had opted for however; the choice on the method was based on the below parameters
The scale of the entire bridge
The overall problems which one can encounter during the design
The span lengths regularity
Both the vertical and the horizontal bridge decks profiles
Material costs to be used in the construction
Soil mechanics and its division nature
The weather of the site
The allocated and scheduled time and work frame for the project
In addition, different companies tend to sue different construction methods when erecting bridges and some of the key constructions which they employ include
One, Staging also known as false- work; is used in developing temporary scaffolding and framing which are used to support the various structural elements while carrying out the construction. Two, Span-by-span method; is a method in which the fabrication is done to support the structural elements through a creation of the sequential process span in situ (Kim et al., 2017 p.70).
.CONSTRUCTION PROCEDURE
KEY ISSUES
First, overall alignments are carried out and in the process the flyover laid out is located with the respect to the abutment and piers. In addition, excavation is carried out to measured level which ensures that the abutment and piers are correctly piled at the appropriate depth. Second, the concrete are placed at the appropriate positions at the bottom section to form a firm foundation. This process is done within the confinements of the ground and formworks. Thirdly, the constructors through the help and supervision of the engineers in the field embark on placing the abutment concrete and the pier columns. Fourthly, the process is successfully completed and meets the engineering and the BS criteria then, the workers embark on building the pier caps. At the same time the more abutments are also built. Thereafter, the precast grinders, steel and box grinders are then placed. Also, the works can build the deck using either the concrete or the slabs. Finally, the workers complete the bridge flyover construction by building the roadways, the guardrails as well as painting the pavement markings (Tollazzi et al., 2016 p.50).
.ENVIRONMENTAL SUSTAINABLITY
The following are some of the safety and construction constraint mechanism which will put in place during the flyover construction (Farooq and Akram, 2018). It is important fence the construction site so as to protect human and animals from access the site to reduce accident instances occurring in the site. Sprinkling of water to dusts generated and using of the dust and building nets to prevent falling rocks from causing accidents and to control the resulting air pollution. Developing of the diversions and using the flag persons to direct traffic during the construction process. Authorization of only permitted persons to access the site and such persons include permitted workers, constructor, engineers and consultant engineers (Domaneschi et al., 2015, February p.160).
.COST ESTIMATE
The total budget for the bridge is mainly dived into two sub-classes that is the substructure and the superstructure. On the portion of the superstructure, the mainly structure budget is for the deck pavement, parapet and the beam. On the other hand, the substructure consists of the piers and the main budget is on amount to be used to purchase the pier material pieces. C50 concrete is the main material which is used in the construction of the main bridge and it is estimated at 109?/m3. Moreover, the bridge thickness cost is estimated at 43.55?/m3 per 150mm footings. Other costs include 57.99?/m3 for the beam exceeding 1m2; the cost for the parapet 0.5m high with the dimension of 125mm by 125mm is estimated 19.2?/m. Finally, another cost to be considered in this process is the cost of columns along with the piers which exceeds 1m2 and the amount is estimated at 57.99?/m (Hameed et al., 2016).
.RECOMMONDATIONS
Backfill and permanent loads may cause settlements along the bridge decks of the flyover. There the variations for settlements is an important element which need to be taken into consideration when design the flyover. There is a likelihood of a monotonically variable settlements formed as time goes. Therefore, an imminent measure must be put in place to tackle the mess if need be in the long run. In fact, the monotonically variable settlements may be in two forms either they may operate in the same direction or may make the structures to become statically indeterminate as time goes. Apparently, settlements tend to interact with the concrete slabs and elements. This results into the creep development where there is a livelihood of a differential settlement affecting the structure (Chandra et al., 2016). The impact of the differential concrete elements and the settlements interactions may affect either a part or the structure as a whole. Therefore, proper mechanisms need to be taken into account to handle this paramount effect (Pino et al., 2017).
Both the controlled deformation and forces are the main causes of the permanent actions on the structures. Prestressing is thus classified as pertinent and permanent action based on this basis as the two actions makes it to impose an effect on the structure. Additionally, it is important for the structural engineer to distinguish the actions imposed on the structure by the prestressing effects through classifying them into various sub-classes in the meantime. The classification of the prestressing can be based on the prestress by tendons and the prestress by forced deformation at carry (Salatoom and Taneerananon, 2015 p.7).
Another essential element to consider under traffic loads is the carriageway width denoted as w. keen consideration is viable when it comes to measuring of the height kerbs and the parameter need to measured as height not more than 100m. Also, the inner vehicle restraint limits systems should be considered in the process (Bao et al., 2015). Some of the aspects which must not be included in the process include the vehicle fixed distance, central reservation kerbs and the restraints systems as well as the vehicle restraints system width. It should not include the distance between fixed vehicle restraint systems or kerbs of a central reservation nor the widths of these vehicle restraint systems. The above analysis for the carriageway can be represented as shown in the diagrams below
SCHEME COMPARISON FOR OPTION-1 AND OPTION -2
2.CALCULATIONS
.LOAD PATH ABD OVERALL STABILITY
In this bridge design context, it is important to consider the bearings and relative movements which are likely to occur at the support and superstructure. This is important to avoid shrinkage, creep, thermal effects, wind loading as well as the foundation settlement. In the design all the bearings are arranged at the respective edges of the piers bridge. The bearings are mainly used in the process as they offer frictions with low polytetrafluoroethylene properties
LOAD ANALYSIS
Considerations are taken for the roadway and the box beam when computing the self-load. In analyzing the box beam material concrete C50 is used.
In the analysis it is clear that box beam area is given as
But for the same box beam to be used in the analysis
Therefore, the load box beam is computed as
Taking the thickness of the to be design to be 0.008m and the width of the flyover to be (3.65m +3.65m) and using C30 concrete
Then, the area of the flyover is given as = 0.0584
Thus, the load of the flyover = 0.0584x = 14.6
The load of the road: max height of the flyover = 7m
Hence, maximum load for the design can be given as 7m x 4x 1000×100 = 2800kN
Structural Analysis
The structural analysis for the flyover in terms of the bending and shear forces can be depicted as shown in the diagrams below.
The steel analysis demarcates on the durability and stability of the structure as time goes by with reference the overall design. In this design there is assumption that the main members of the bridge can either be concrete of appropriate grades within a characteristic strength range from 35 N/mm2 up to 50 N/mm2 or steel of S355 grade (Arya, 2015). However, before embarking on analysis using this assumption, it is important to check on the impacts of the characteristics strengths of the steel bar with reference to the BS 5950 as this will help in ascertaining the level of buckling as far as the slenderness ratio is concerned. First, by using the BS 5950 chart for the S355 analysis with a range of 35N/mm2 to 50N/mm2, we obtain the follow values from the data
|
Steel grade and design strength py (N/mm2 ) |
|
|
Slenderness ratio λ |
Characteristic Strength of Steel pc (N/mm2 ) |
|
35 |
313 |
|
40 |
301 |
|
42 |
296 |
|
44 |
291 |
|
46 |
286 |
|
48 |
280 |
|
50 |
275 |
RECOMMENDATIONS FOR PICKING OPTION 1
The above analysis can be used to compute a design graph which shows the trend of the impacts of the Characteristic Strength of Steel pc (N/mm2) against the Slenderness ratio λ (Marfani et al., 2018 p.56).
Therefore, from the graph above, it is clear that the Slenderness ratio λ is inversely proportional to the Characteristic Strength of Steel pc (N/mm2) of the steel material used. Therefore, in order to increase the durability of the structure it will important to use materials which have high Characteristic Strength in line with Steel pc but with lower slenderness ratio (Jose et al., 2018 p.4076).
ULTIMATE LIMIT STATE IN CABLE
The research conducted of the past decades indicates that there is a clear understanding of the ultimate load behaviors as far as the reinforced concrete beam analysis is concerned. The British Standards or BS has various theories which try to examine and evaluate the assumptions and the impropriety of the aspect and these theories as follows
The first theory poses on the concrete reinforcement and strains and it states that both the strains and the concrete strain are directly proportional the overall distance as long as the reference is taken with the neutral axis (Alavi et al., 2017 p.19). Second theory demarcates that the ultimate limit state is only achievable at the point where the concrete strain will reach the extreme compression fiber and the specified value at that point is feu. The third theory emphases on the failure and the stress to the strain curve and it stipulates that at that point the compressive concrete stresses are idealized. The fourth assumption states that provided the ultimate stress limit is used then the overall tensile strength of the overall concrete beam is ignored as a whole.
Computation of the Design Strength
The computation of the design strength is given by characteristic strength divided by the partial safety factor with reference to the load and depicted as follows
Computation of the Design Load
Also, the computation of the design load is given by characteristic strength multiply by the partial safety factor with reference to the load and depicted as follows
Design load = characteristic load × γf
CONSTRUCTION METHOD
(1600x 2) N/mm2 = 3200 N/mm2
.SERVICEABILITY LIMIT STATE
Serviceability limits aims at defining the working loads of the structure and its behaviors. However, there are other elements which are often affected by the limit states in terms of their performance. Some of them include fire resistance and durability and thus, they must be considered in any design work. In fact, designer has the obligation of not judging which elements to be considered in a design work but rather have to compute and compare their adverse effects before making any assumption and pre-judgment (Mututantri et al., 2015).
On the contrary, the designer will only have to pick one of the elements after evaluating it and denote it as the critical limit state upon which other parameters will be computed. Thus, this will hasten the process as the other elements will be compared and checked on this basis of analysis in the long run. For instance, in the sizing of the beam, the reinforced concrete, shear as well as the ultimate limit state are often used in coming up with the best alternative design (Agarwal et al., 2018).
From that analysis, the design can then be checked against the remaining cracking and deflection elements. Moreover, serviceability is quite critical and important when tackling the deflection for the concrete slabs as it gives critical design effects (Alam et al., 2015 p.318). Furthermore, it is based on this parameter that the designer must ensure that the design meets also the legal and BS gauge such as shear, cracking and bending. However, the designer will have to assume certain values when handling the strength and the loading impacts of the composing materials in the structure. Such elements will require that the design have elaborated as well as advance understanding on the characteristics values and limit state impacts (Zhou et al., 2015).
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