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In this beautiful flyover, there are a few high-tension steel tendons protected in conduits. Unfortunately, they are corroding. Can you guess the impact of this on the flyover after 3–4 months? Why is this happening?
You may not believe this, but modern flyovers use these kinds of empty box girders for construction. Interestingly, the smart engineering of these girders allows male–female interlocking of many such segments. Now, let’s insert a steel tendon via the holes in these box girders. Suppose you tightened the cable. How the cables in tension will affect the load carrying capacity of the flyover? To answer this question we need to first understand how a flyover transfers weight.
The weight of the entire superstructure, including the vehicle, is first transferred to the bridge bearing, then to the pier, from there to the pile cap, and finally to the piles.
The very first stage of flyover construction the engineers are searching for a hard strata layer here. If the entire weight of the vehicles and the flyover is borne by the hard strata, such a structure will be super stable. Once they find the hard strata, the workers begin to insert the reinforcement bars, then pour concrete. After a 14-day curing period, the pile foundation is ready. You can transmit a tremendous amount of force to these piles, and the strong, hard strata will make sure that the pile structure is stable. You can easily guess what will happen to the flyover if the piles do not reach the hard strata.
This is why the piles are super important in a flyover. All these piles are connected to a single concrete structure called a pile cap. Now, the construction of the piers begins. Piers end with the construction of these beautifully curved pier canopies. Obviously, to pump the concrete at this height, you need powerful concrete pumps. It’s an impressive sight to see all these well-aligned and beautiful piers standing, waiting to invite their next component: the hero of the project—the box girders.
The box girders are prefabricated. Welcome to the box girder fabrication site. This bottom formwork and the inside form are the key components in the girder fabrication. Look at how they are able to move with the help of hydraulic power. The steel structure is placed inside the form first. The concrete is poured and allowed to cure. First, the inside formwork is removed. Now, it’s time to remove the bottom formwork. You can see a lot of protruding shapes and holes in the box girder. Why do we need them? We will explore soon.
The segmental launching gantry — the biggest machine used in this project—is almost a robot, a robot with four legs. Yes, this machine can move to the next segment once it is done with the assembly of one segment. This is how it does it.
Initially, the launching gantry is supported on these support brackets. The LG now starts the lifting and assembly work of the box girders. The girder assembly between one pair of piers is now complete. To do the girder assembly of the next section, the LG should reach this position. To achieve this, the launching gantry has to make use of all its four legs. First, the rear leg is supported on the deck. Now, even if you remove the rear middle leg, there is no issue. The LG is still stable. Next, the rear middle leg is supported on the road deck. Now, the launching gantry can move forward. The front leg gets supported on the next pier cap. Now, the rear middle leg moves forward and gets supported on the front of the deck. It’s time to move the support bracket forward and support it on the next pier. Next, the rear leg is retracted, and the entire launching gantry is moved forward again. If you observe the position of the LG now, it is the same as the position of the LG just before the assembly of the previous segment: the main legs are in the center of the gantry and the gantry is properly centered. From this position, the gantry can start the new launching process and this cycle is repeated.
Now, let’s see some details of box girder assembly. Workers apply epoxy resin on the faces of the girders. You can see how the male and female protrusions perfectly fit together. After the assembly of one segment is over, there will be a small gap left between them. This is deliberate. If the engineers had planned to complete the segment perfectly with the girders, there would be no way to fit the last girder. The small gap left is closed with reinforcement bars and concrete.
Engineers are now doing the most crucial operation: insertion of super-strong steel tendon wires via the holes of the box girders. They are even tightening the wires. At this stage, even if the gantry crane removes the support, the segment will be strong and stable. The segment can even carry the weight of at least 20 cars.
Here is the big question: Are the steel tendons that the workers inserted and tightened just for bonding the box girders together? Even if the workers had not inserted the tendon wires, the girders would be stable. However, after a few months of operation, cracks will initiate from the bottom of the girders and finally lead to a tragedy. Welcome to the brilliant engineering of concrete post-stressing.
Here are the concrete flyovers we made. Let’s attach these end caps and also let’s insert these rebars inside into the flyovers. In one of the girders I am tightening the rebars and keeping them in extreme tension. The other girder has rebars, but not under any stress. Now, let’s test the weight carrying capacity of these two flyovers with the help of a car. This is the girder without post stressing. As the car transferred some amount of weight, it underwent a sudden failure. The girder with the tightened rebars is able to survive more weight. This one also failed, but this time the failure was more gradual with increased load capacity. That’s the advantage of post-stressing concrete.
When you keep the steel tendons in tension and when they are not allowed to go back to their original length, the concrete gets compressed. What’s the advantage of it? When the concrete segment is supported from its two ends, the segment bends as shown. Due to this the bottom portion of the segment will be in tension, and the top portion will be in compression. Concrete is good in compression and bad in tension. This means the bottom section, which experiences tensile stress, will easily develop cracks. The remedy for this is to keep the entire concrete block in good compression, well before the bridge construction is over. In this case, tensile stress may get produced on the bottom region due to the live load, but much lower than the previous case. This way, the engineers make sure that the bridge will last for decades without developing cracks.
These powerful hydraulic jacks do the difficult job of tensioning the steel tendons. The high pressure fluid moves the piston and the cables get tightened. You can see that after tensioning the cable, the workers release the pressure, remove the machine and cut off the extra wire. Then why don’t these cables, which are in high tension, contract and go back to their original length?
You may not believe this, but modern flyovers use these kinds of empty box girders for construction. Interestingly, the smart engineering of these girders allows male–female interlocking of many such segments. Now, let’s insert a steel tendon via the holes in these box girders. Suppose you tightened the cable. How the cables in tension will affect the load carrying capacity of the flyover? To answer this question we need to first understand how a flyover transfers weight.
The weight of the entire superstructure, including the vehicle, is first transferred to the bridge bearing, then to the pier, from there to the pile cap, and finally to the piles.
The very first stage of flyover construction the engineers are searching for a hard strata layer here. If the entire weight of the vehicles and the flyover is borne by the hard strata, such a structure will be super stable. Once they find the hard strata, the workers begin to insert the reinforcement bars, then pour concrete. After a 14-day curing period, the pile foundation is ready. You can transmit a tremendous amount of force to these piles, and the strong, hard strata will make sure that the pile structure is stable. You can easily guess what will happen to the flyover if the piles do not reach the hard strata.
This is why the piles are super important in a flyover. All these piles are connected to a single concrete structure called a pile cap. Now, the construction of the piers begins. Piers end with the construction of these beautifully curved pier canopies. Obviously, to pump the concrete at this height, you need powerful concrete pumps. It’s an impressive sight to see all these well-aligned and beautiful piers standing, waiting to invite their next component: the hero of the project—the box girders.
The box girders are prefabricated. Welcome to the box girder fabrication site. This bottom formwork and the inside form are the key components in the girder fabrication. Look at how they are able to move with the help of hydraulic power. The steel structure is placed inside the form first. The concrete is poured and allowed to cure. First, the inside formwork is removed. Now, it’s time to remove the bottom formwork. You can see a lot of protruding shapes and holes in the box girder. Why do we need them? We will explore soon.
The segmental launching gantry — the biggest machine used in this project—is almost a robot, a robot with four legs. Yes, this machine can move to the next segment once it is done with the assembly of one segment. This is how it does it.
Initially, the launching gantry is supported on these support brackets. The LG now starts the lifting and assembly work of the box girders. The girder assembly between one pair of piers is now complete. To do the girder assembly of the next section, the LG should reach this position. To achieve this, the launching gantry has to make use of all its four legs. First, the rear leg is supported on the deck. Now, even if you remove the rear middle leg, there is no issue. The LG is still stable. Next, the rear middle leg is supported on the road deck. Now, the launching gantry can move forward. The front leg gets supported on the next pier cap. Now, the rear middle leg moves forward and gets supported on the front of the deck. It’s time to move the support bracket forward and support it on the next pier. Next, the rear leg is retracted, and the entire launching gantry is moved forward again. If you observe the position of the LG now, it is the same as the position of the LG just before the assembly of the previous segment: the main legs are in the center of the gantry and the gantry is properly centered. From this position, the gantry can start the new launching process and this cycle is repeated.
Now, let’s see some details of box girder assembly. Workers apply epoxy resin on the faces of the girders. You can see how the male and female protrusions perfectly fit together. After the assembly of one segment is over, there will be a small gap left between them. This is deliberate. If the engineers had planned to complete the segment perfectly with the girders, there would be no way to fit the last girder. The small gap left is closed with reinforcement bars and concrete.
Engineers are now doing the most crucial operation: insertion of super-strong steel tendon wires via the holes of the box girders. They are even tightening the wires. At this stage, even if the gantry crane removes the support, the segment will be strong and stable. The segment can even carry the weight of at least 20 cars.
Here is the big question: Are the steel tendons that the workers inserted and tightened just for bonding the box girders together? Even if the workers had not inserted the tendon wires, the girders would be stable. However, after a few months of operation, cracks will initiate from the bottom of the girders and finally lead to a tragedy. Welcome to the brilliant engineering of concrete post-stressing.
Here are the concrete flyovers we made. Let’s attach these end caps and also let’s insert these rebars inside into the flyovers. In one of the girders I am tightening the rebars and keeping them in extreme tension. The other girder has rebars, but not under any stress. Now, let’s test the weight carrying capacity of these two flyovers with the help of a car. This is the girder without post stressing. As the car transferred some amount of weight, it underwent a sudden failure. The girder with the tightened rebars is able to survive more weight. This one also failed, but this time the failure was more gradual with increased load capacity. That’s the advantage of post-stressing concrete.
When you keep the steel tendons in tension and when they are not allowed to go back to their original length, the concrete gets compressed. What’s the advantage of it? When the concrete segment is supported from its two ends, the segment bends as shown. Due to this the bottom portion of the segment will be in tension, and the top portion will be in compression. Concrete is good in compression and bad in tension. This means the bottom section, which experiences tensile stress, will easily develop cracks. The remedy for this is to keep the entire concrete block in good compression, well before the bridge construction is over. In this case, tensile stress may get produced on the bottom region due to the live load, but much lower than the previous case. This way, the engineers make sure that the bridge will last for decades without developing cracks.
These powerful hydraulic jacks do the difficult job of tensioning the steel tendons. The high pressure fluid moves the piston and the cables get tightened. You can see that after tensioning the cable, the workers release the pressure, remove the machine and cut off the extra wire. Then why don’t these cables, which are in high tension, contract and go back to their original length?
To understand this logically, we 3D printed a simplified model of the hydraulic jack and the entire post-stressing mechanism. You won’t believe this. These simple wedges do the tricky job of keeping the tensioned cables in place even after the hydraulic jack is removed. The clever engineering of the wedges makes sure that the cable can be stretched outward, but once stretched outward, it will not go inward, even if you remove all other accessories. Here is how it works.
One thing we should keep in mind is that the wedges are not strongly connected with the tendon wires. What will happen if the block surrounding the wedges moves like this? The block will close the gaps in the wedges, and the wedges will form a strong connection with the tendon wire. Obviously, the cable will be stretched toward the right. Now, what about this situation? Here, somebody is pulling the cable from the right. Here, the wedges won’t form any strong connection with the tendon, and the tendon wire will stretch toward the right freely. Keep these two tricky concepts in mind; we are going to explore the clever engineering of post-tensioning.
When the jack moves from left to right, the outer wedges are tightened, and the cable moves from left to right. It is undergoing a tension operation. The inner wedges cannot block this cable movement, since these wedges are not getting tight with the cable. After achieving sufficient tension in the cable the engineers release the pressure on the piston. Here, obviously, the cable wants to retract inward. However, as soon as it moves a little, the inner wedges will get tight with the cable, and further motion of the cable will be arrested. In short, these wedges will ensure that the cables remain in tension forever. What a clever engineering feat to keep the cables always in tension! Simplicity at its best. Now you may remove the accessories and cut the cable from here. The cable inside the flyover will remain in tension.
In reality a hydraulic jack is able to stretch more than a dozen of tendons in one go. We had planned for 3 cable stretching initially. Since our plastic components were not able to carry this load, we settled with a single cable stretching. It’s remarkable how just a few high-tension steel cables can dramatically increase the strength of a concrete flyover.
The structure that connects the ground and the flyover deck is called an abutment. The angle of the abutment is crucial for the smooth entry of vehicles onto the flyover. The abutments are filled with soil. Now, the vehicles can enjoy a smooth ride over the flyovers.
Ever wondered how engineers come up with these clever and complicated interchange designs? Let’s now skim through the basics of interchange designs.
The three-way interchange design is commonly used when one road terminates at another road, facilitating movements in and out of a through route. This design minimizes the use of land and construction costs and, most importantly, minimizes the need for lane weaving. It is commonly found at the ends of expressways or as a transition.
The cloverleaf interchange might be the most beautiful innovation of civil engineering. This design, which resembles a clover leaf, is used when two highways intersect. The design allows free-flowing movements without the need for traffic signals. In a cloverleaf design, anyone can move to any road without any hassles. Just observe these line animations to understand how this design achieves this. However, the cloverleaf design requires a significant amount of space. This design also causes weaving issues as vehicles enter and exit the loops.
The diamond interchange is a popular solution when a highway intersects a secondary road. Its design consists of four ramps forming a diamond shape, hence the name. This type of interchange is efficient in areas of moderate traffic volume, offering easy and direct access between the roads with minimal land use. It supports high traffic volumes and speeds but can be limited by traffic signals that may cause delays during peak times.
A modification of the full cloverleaf, the partial cloverleaf interchange eliminates some of the loops to reduce space requirements. The most important advantage is that it minimizes weaving problems found in traditional cloverleaf designs. This interchange type combines the features of a cloverleaf and a diamond interchange. Parclo interchanges are particularly effective in suburban traffic.
Engineers either grout the post-tensioning cables or they just keep them inside a conduit. In the first case, the post-tensioning cables are bonded to the grout. In the second case, they are not bonded. In the case of unbonded post-tensioning, grease should be added to the conduit to avoid corrosion. Bonded post-tensioning produces greater resistance to corrosion. However, bonded post-tensioning needs more cables in order to achieve the same compression level as the unbonded method.
You might have noticed these kinds of gaps and strange arrangements in flyovers. If you delve into the details, you'll be amazed at the intricacies of engineering. These are expansion joints. In civil engineering, it’s common practice to leave a gap in lengthy concrete structures. Materials expand or contract during temperature changes and these gaps allow for this motion. If you constrain the structure too much, it will crack during temperature changes. However, we cannot simply leave a gap in the road. Expansion joints allow for the movement of the road deck while simultaneously preventing gaps. This green member is the main support of the expansion joint. Interestingly this member is not fixed to the road, it just floats on two bearings. This means this support member won’t block the movement of the road decks in any way. The support boxes of the expansion joint are free to slide along the green member. The portion you see on the flyover is a collection of lamellas with rubber strips between them. Now, you may observe this animation and understand the entire working of the expansion joints. Let’s watch the working of the expansion joint one more time - this time from the perspective of a pedestrian.
This also means that the road deck must be resting on bearings. Thanks to these bearings and expansion joints, flyovers can accommodate changes in environmental temperature.
The flyover we discussed so far is known as a concrete-based flyover, which is the most popular choice. However, if you check the flyover in San Antonio, Texas, you will be able to see a lot of steel girders underneath. These are steel structure flyovers. The details of steel structure flyovers are illustrated in this visual. They are mainly used when the situation demands a shorter construction period. They are lighter and stronger compared to concrete flyovers. The main shortcoming of steel flyovers is that they are more costly compared to concrete flyovers.
One thing we should keep in mind is that the wedges are not strongly connected with the tendon wires. What will happen if the block surrounding the wedges moves like this? The block will close the gaps in the wedges, and the wedges will form a strong connection with the tendon wire. Obviously, the cable will be stretched toward the right. Now, what about this situation? Here, somebody is pulling the cable from the right. Here, the wedges won’t form any strong connection with the tendon, and the tendon wire will stretch toward the right freely. Keep these two tricky concepts in mind; we are going to explore the clever engineering of post-tensioning.
When the jack moves from left to right, the outer wedges are tightened, and the cable moves from left to right. It is undergoing a tension operation. The inner wedges cannot block this cable movement, since these wedges are not getting tight with the cable. After achieving sufficient tension in the cable the engineers release the pressure on the piston. Here, obviously, the cable wants to retract inward. However, as soon as it moves a little, the inner wedges will get tight with the cable, and further motion of the cable will be arrested. In short, these wedges will ensure that the cables remain in tension forever. What a clever engineering feat to keep the cables always in tension! Simplicity at its best. Now you may remove the accessories and cut the cable from here. The cable inside the flyover will remain in tension.
In reality a hydraulic jack is able to stretch more than a dozen of tendons in one go. We had planned for 3 cable stretching initially. Since our plastic components were not able to carry this load, we settled with a single cable stretching. It’s remarkable how just a few high-tension steel cables can dramatically increase the strength of a concrete flyover.
The structure that connects the ground and the flyover deck is called an abutment. The angle of the abutment is crucial for the smooth entry of vehicles onto the flyover. The abutments are filled with soil. Now, the vehicles can enjoy a smooth ride over the flyovers.
Ever wondered how engineers come up with these clever and complicated interchange designs? Let’s now skim through the basics of interchange designs.
The three-way interchange design is commonly used when one road terminates at another road, facilitating movements in and out of a through route. This design minimizes the use of land and construction costs and, most importantly, minimizes the need for lane weaving. It is commonly found at the ends of expressways or as a transition.
The cloverleaf interchange might be the most beautiful innovation of civil engineering. This design, which resembles a clover leaf, is used when two highways intersect. The design allows free-flowing movements without the need for traffic signals. In a cloverleaf design, anyone can move to any road without any hassles. Just observe these line animations to understand how this design achieves this. However, the cloverleaf design requires a significant amount of space. This design also causes weaving issues as vehicles enter and exit the loops.
The diamond interchange is a popular solution when a highway intersects a secondary road. Its design consists of four ramps forming a diamond shape, hence the name. This type of interchange is efficient in areas of moderate traffic volume, offering easy and direct access between the roads with minimal land use. It supports high traffic volumes and speeds but can be limited by traffic signals that may cause delays during peak times.
A modification of the full cloverleaf, the partial cloverleaf interchange eliminates some of the loops to reduce space requirements. The most important advantage is that it minimizes weaving problems found in traditional cloverleaf designs. This interchange type combines the features of a cloverleaf and a diamond interchange. Parclo interchanges are particularly effective in suburban traffic.
Engineers either grout the post-tensioning cables or they just keep them inside a conduit. In the first case, the post-tensioning cables are bonded to the grout. In the second case, they are not bonded. In the case of unbonded post-tensioning, grease should be added to the conduit to avoid corrosion. Bonded post-tensioning produces greater resistance to corrosion. However, bonded post-tensioning needs more cables in order to achieve the same compression level as the unbonded method.
You might have noticed these kinds of gaps and strange arrangements in flyovers. If you delve into the details, you'll be amazed at the intricacies of engineering. These are expansion joints. In civil engineering, it’s common practice to leave a gap in lengthy concrete structures. Materials expand or contract during temperature changes and these gaps allow for this motion. If you constrain the structure too much, it will crack during temperature changes. However, we cannot simply leave a gap in the road. Expansion joints allow for the movement of the road deck while simultaneously preventing gaps. This green member is the main support of the expansion joint. Interestingly this member is not fixed to the road, it just floats on two bearings. This means this support member won’t block the movement of the road decks in any way. The support boxes of the expansion joint are free to slide along the green member. The portion you see on the flyover is a collection of lamellas with rubber strips between them. Now, you may observe this animation and understand the entire working of the expansion joints. Let’s watch the working of the expansion joint one more time - this time from the perspective of a pedestrian.
This also means that the road deck must be resting on bearings. Thanks to these bearings and expansion joints, flyovers can accommodate changes in environmental temperature.
The flyover we discussed so far is known as a concrete-based flyover, which is the most popular choice. However, if you check the flyover in San Antonio, Texas, you will be able to see a lot of steel girders underneath. These are steel structure flyovers. The details of steel structure flyovers are illustrated in this visual. They are mainly used when the situation demands a shorter construction period. They are lighter and stronger compared to concrete flyovers. The main shortcoming of steel flyovers is that they are more costly compared to concrete flyovers.