Bridge Design (and Destruction!) Part 2

Summary

The video explores various bridge designs, starting with truss bridges, which distribute load across a frame. It explains how tension and compression impact different parts of the truss, showing how the outer segments tend to break first under load. The video then moves to suspension bridges, highlighting how steel cables transfer weight to the towers and anchors. Both bridge types are compared, demonstrating their strength and ability to span longer distances, but the arch bridge proves stronger. The finale humorously involves crushing a LEGO man.

Takeaways

• 🛠️ Truss bridges use a large frame called a truss, which distributes load across the structure, reducing the load on the deck.
• 🔗 The truss can be placed on top or below the bridge stack; in this case, it is on top.
• 📊 Different segments of the truss experience varying loads of either tension or compression.
• 🧮 The outer and top segments bear the largest loads, while middle segments experience no load in the specific test scenario.
• 🏗️ The top pieces of the truss, aligned with the wood grain, are stronger and less likely to break compared to the outer segments.
• 💪 The truss bridge held 32 pounds, making it 25% stronger than a beam bridge of the same length.
• 🌉 Suspension bridges use thick steel cables that transfer the load to towers and anchors at the ends of the bridge.
• 🪢 Supporting cables suspend the bridge deck from the main cables, both of which are always under tension.
• 📈 Suspension bridges are known for their ability to span longer distances than beam and arch bridges.
• 🎯 Truss and suspension bridges, though weaker than arch bridges in the test, are crucial for spanning large distances.

Q & A

What is a truss in a truss bridge?

A truss is a large frame that sits on top or below the bridge deck, used to distribute the load more efficiently across the structure.

How does a truss bridge distribute the load on the deck?

The truss frame distributes the load through its segments, allowing the deck to experience less direct load.

Why do the outer segments of the truss experience the largest loads?

The outer segments carry the largest loads because of the way the structure is designed to distribute the tension and compression forces.

Why didn't the top segments of the truss break despite carrying a large load?

The top segments didn't break because they were aligned along the grain of the wood, which makes the material stronger in that direction.

How much more weight did the truss bridge hold compared to the beam bridge?

The truss bridge held 32 pounds, which is 25% stronger than a beam bridge of the same length.

What are the key components of a suspension bridge?

A suspension bridge has thick steel cables that support the deck and transfer the load to towers and anchors, with supporting cables suspending the bridge deck.

Why are the cables in a suspension bridge always under tension?

The cables are always under tension because they transfer the load from the deck to the towers and anchors, maintaining the structure's integrity.

How did the suspension bridge in the model behave under load?

The cables transferred the load to the towers and anchor points, which helped maintain the integrity of the deck even under force.

Why are truss and suspension bridges better suited for longer spans compared to beam and arch bridges?

Truss and suspension bridges can span longer distances because their designs efficiently distribute loads and reduce the strain on the deck, unlike beam and arch bridges.

What was the final outcome of the bridge load tests in the video?

Both the truss and suspension bridges supported 32 pounds, similar to each other but weaker than the arch bridge, which was the strongest.

Outlines

00:00

🔨 Introduction to Truss Bridges

This paragraph introduces the truss bridge, a modern design in bridge engineering. It describes how a truss, a large frame that sits on top of the bridge, distributes the load across the structure. The design prevents the deck from bearing the full load by distributing it between tension and compression in different segments of the truss. The segment load distribution is shown as percentages of the total load, with the heaviest loads on the end and top segments.

💥 Breaking the Truss Bridge

In this section, the focus shifts to how truss bridges handle loads and what causes them to break. The paragraph discusses how, during a compression test, the outer segments of the truss break first due to carrying the heaviest loads. The top pieces, despite having similar loads, do not break because they are aligned with the wood grain, which strengthens them. The truss design is shown to increase load capacity by 25% compared to a beam bridge of the same length.

🌉 Suspension Bridges: The Basics

Here, the script introduces suspension bridges, describing their structure and function. Suspension bridges use thick steel cables to support the deck and transfer the load to the towers and anchors. The supporting cables hold the bridge deck from the main cables, which are always under tension. The towers bear the load through compression. The model bridge used in this demonstration simulates these concepts with wires, though some joining points and anchors are difficult to replicate on a small scale.

🏍 Suspension Bridge Test: Misfits vs Bike Gang

This part details a test of the suspension bridge, using fictional characters to illustrate load distribution. As force is applied to the bridge, the cables transfer the load to the towers and anchors, maintaining the deck's stability. Despite the tension and force applied, the bridge supports 32 pounds, the same as the truss bridge. The test shows how the suspension bridge maintains its integrity, preventing dramatic failures, even though the speaker humorously wishes to see a LEGO man launch.

📊 Comparing Bridge Strengths

This final paragraph compares the strengths of various bridge designs. The truss and suspension bridges are shown to support the same amount of weight (32 pounds) but are not as strong as the arch bridge. However, the real strength of truss and suspension bridges lies in their ability to span longer distances compared to beam and arch bridges, making them ideal for larger-scale constructions. The segment concludes with anticipation for a LEGO man being crushed, adding a playful tone.

Keywords

💡Truss bridge

A truss bridge uses a large frame called a truss to support the bridge. The truss distributes the load through its frame, reducing the load experienced by the deck. This concept is central to understanding the mechanics of truss bridges, as illustrated by the load calculations and segment analysis in the video.

💡Load distribution

Load distribution refers to how the weight and forces are spread across different parts of a structure. In the context of the video, it explains how truss and suspension bridges manage and balance the weight they carry, preventing excessive strain on any single part. Examples include the truss bridge's load distribution through its frame and the suspension bridge's transfer of load to towers and anchors.

💡Tension and compression

Tension and compression are forces that act on different parts of a bridge. Tension is a pulling force, while compression is a pushing force. The video discusses how different segments of the truss bridge experience either tension or compression, impacting their structural integrity.

💡Suspension bridge

A suspension bridge uses thick steel cables to support the deck and transfer the load to towers and anchors. The video highlights how these cables are always under tension and maintain the bridge's integrity by distributing the load. The suspension bridge in the video is shown to effectively support weight, similar to the truss bridge.

💡Deck

The deck is the flat surface of the bridge that carries the traffic load. The video emphasizes how the deck's load is managed differently by various bridge types. For instance, the truss bridge's deck benefits from the truss's load distribution, while the suspension bridge's deck is supported by cables.

💡Compression test

A compression test measures how much compression force a material or structure can withstand before failing. The video references a compression test as a means to demonstrate the breaking points of the truss segments, showing how the outer segments fail first due to handling the largest loads.

💡Arch bridge

An arch bridge uses a curved arch structure to support the load, which is transferred to the abutments at either end. The video contrasts the strength of the arch bridge with other types, noting its ability to support greater weight despite its different load management technique.

💡Steel cables

Steel cables are used in suspension bridges to support the deck and transfer loads. The video explains how these cables are always under tension and help distribute the weight to the towers and anchors, ensuring the stability of the bridge.

💡Towers and anchors

Towers and anchors are critical components of a suspension bridge. Towers support the main cables, experiencing compression, while anchors at the ends of the bridge hold the cables in place under tension. The video illustrates their role in maintaining the bridge's structure by distributing loads effectively.

💡Beam bridge

A beam bridge is a simple type of bridge supported by a horizontal beam. The video uses it as a baseline to compare the load-bearing capacities of more complex bridge types like truss and suspension bridges, highlighting how the latter can support more weight and span longer distances.

Highlights

Introduction to modern truss bridges, with a focus on their design and functionality.

Truss bridges use a large frame called a truss, which can be placed on top or below the bridge stack.

The design of the truss helps distribute the load across the frame, reducing stress on the deck.

Each segment of the truss experiences either tension or compression, based on the applied loads.

The outer and top segments of the truss experience the largest loads, leading to failure at these points.

Top segments of the truss are stronger due to alignment with the grain of the wood, making them more resistant to breaking.

The truss bridge supported 32 pounds, making it 25% stronger than a beam bridge of the same length.

Suspension bridges are introduced, utilizing steel cables to support the deck and transfer loads to the towers and anchors.

Main and supporting cables of a suspension bridge are always under tension, providing structural stability.

The towers experience compression while the cables transfer the load to the anchors.

Suspension bridges, like truss bridges, can support heavy loads and span longer distances.

Challenges in building a small-scale model of a suspension bridge include simulating the joining points and anchors.

The suspension bridge model supported 32 pounds, the same as the truss bridge.

Despite being strong, both truss and suspension bridges were weaker than the arch bridge in terms of weight support.

Truss and suspension bridges offer the advantage of spanning longer distances than beam and arch bridges.

Transcripts

In our last video, we looked at the simple designs

of beam and arch bridges.

Now let's move into the modern age with the truss bridge.

Truss bridges make use of a large frame

called a truss that sits on top or below the bridge stack.

In this case, it is on top.

While it may seem like we're only adding weight to the deck,

the design of the truss distributes the load

through the frame so that the deck not experience as much

of a load.

Each segment of the truss experiences different loads

of either tension or compression.

We apply two equal loads to the deck

and calculate the loads in each segment,

which are shown as percentages of the total load.

You can see that the largest loads

are on the end and top segments, while the middle segments have

none.

Remember that when we do the compression test-- spoiler

alert.

Let's see if this convict get shot out of his truss jail.

So how do you think the truss will break?

Discuss.

Thanks for coming Yoda.

I love your work.

As you can see, the outer segments of the truss

are the first to break, because they were handling the largest

part of the load.

The diagram showed that the outer and top segments

had the same loads.

Why didn't the top break?

That's because the top pieces are aligned

along the grain of the wood, and wood

is stronger in that direction.

Adding the truss allowed the same deck length to hold

32 pounds, which is 25% stronger than the beam

bridge of the same length.

The final type of bridge that we will discuss

is the iconic suspension bridge.

Although the only suspension bridge around

us is less than iconic, but the same principles apply.

Suspension bridges utilize thick steel cables

that support the deck and transfer the load to the towers

and to the anchors at the end of the bridge.

Supporting cables are used to suspend the bridge

deck from the main cables.

The main cables and supporting cables of the bridge

are always under tension.

The cables transfer the loads the towers,

which experience compression, and also

to the anchors at the end of the bridge.

In our model, we used wires for the main cables and supporting

cables.

Some of the construction is not ideal,

because it is difficult to simulate

some of the joining points and anchors on a small scale.

For this test, we need to full cast of characters--

the misfits versus the bike gang.

Oh, there's crazy guy again-- classic crazy guy.

As force is applied, the cables transfer the load out

to the towers and anchor points at the end.

This force distribution maintains the integrity

of the deck, so that even when it does break,

it doesn't really launch anyone.

Unfortunately.

I really wanted to see the crazy guy get launched.

This bridge supported 32 pounds, which is

the same as the truss bridge.

The truss and suspension bridges were

stronger than the long beam bridge,

but weaker than the arch bridge.

This may have been unexpected, but the real advantage

of truss and suspension bridges are

that they can span longer distances than beam and arch

bridges.

Now what we've all been waiting for-- crushing a LEGO man.