The Mechanisms of Simple Truss Bridges:

I. Audience and Scope 

As we know today, bridges are one of the major roadworks used to transport people and goods, over a body of water or land. Much work goes into the construction of proper bridges, where civil engineers design durable structures to facilitate a safe and efficient flow. There are many different designs used in structural design. However, choosing the most effective design is dependent on the engineer, conditions, materials, etc.

Truss bridges are one of the oldest types of modern bridges. First being built with  wood, they now have transformed to using steel and other metal long-lasting materials. Relevant to our world today, the Quebec Bridge in Canada and built in 1917, is currently the longest truss bridge in the world. This bridge spans at a length of 549 meters, making it unique in its manufacturing. The middle of the bridge is a simple truss, as the outer constructions are more complex (JFE Engineering, 2020).

Figure 1: Quebec Bridge (Canada)

There are many different types of truss designs, however a simple truss bridge will be one of main focus. In this report, my target audience are college students preparing to take Statics. This is in hopes they have slight prior knowledge in structural engineering, as this class heavily focuses on truss bridges and calculating forces in members at each support. The terminology and concepts spoken about in this report, will enable students to have a better understanding of the: functions of these types of structures, how they are designed with structural analysis, and advantages of these bridges in comparison to others.

II. Function

A truss bridge is a selection of beams and other elements that create a rigid structure. This design consists of members organized into connected triangles that behave as a single object, coupled at joints known as fixed nodes (definition*). These grouping of triangles are manufactured from straight steel bars, that allow for the best truss designs to develop this secure arrangements of connection. The key component of its functionality is to provide long term durability and lasting effects to withhold extreme conditions over the years. There are many different parts that hold together this traditional design, which will be spoken about more specifically.

Figure 1.1: Truss Bridge Layout 

III. Several Mechanisms and Their Properties 

Here, it is key to see how properties of the truss bridge hold members and forces together, while new terminology is explained:

a) Members and Joints

Figure 1.2:

To begin, it is important to note the previous definition of a truss bridge (as highlighted above*). Members are load carrying components of a structure, interconnected at certain angles (see figure 1.1). Joints are fixed external supports, connected with two truss members. All external forces (loads and reactions) must only be applied at joints. The key is that members can carry certain loads in (1) tension, or (2) compression. A tension force simply means that the member can point away from the joint and pulls materials apart. A compression force is when the member point towards the joint, squeezing materials together. (See figure 1.2) One can know if a force is in tension or compression by calculating each members load, then balancing loads to function with one another (problems will later be done in Statics to help grow further knowledge*). Loads will be able to be calculated using method of sections (making a cut through a member where unknown forces are applied) or joints (calculating individual joints with no more than two unknown forces). When calculating forces, there are assumptions often made to analyze directions (compression ; remember: towards the joint or tension: away from the joint). It is key to establish equilibriums for each of the sections. Essentially, what this means is considering parameters in (x, y) directions (hence Fx=0, Fy=0 N). Additionally, all forces throughout the system must equal to 0 (N), to ensure that the bridge is fully balanced. This provides for equal distribution of weights. For example:

{Fx=0}

{Fy=0}

{M=0}

Examples shown below consider both methods in static problems, given loads at each joint:

i. Method of Sections:

Figure 1.3:  

* As seen in figure 1.3: method of sections makes a cut through the member to find the unknown forces applied. This is easier to do when forces in only a few members of trusses can be determined.

ii. Method of Joints: 

Figure 1.4:

*As seen in figure 1.4: method of joints looks at unknown joint (A), to find the (2) unknown forces that connect to that member. It is best to start calculations from left —> right of the truss bridge to calculate loads throughout effectively.

b. Chords, Bracing & Deck

Each truss is composed of chords that define the shape of the bridge (top, bottom,  vertical and diagonal). The bottom and outermost members are parallel to the deck. The top upper members in a truss are the top chords, typically in compression and parallel to the deck in the upper edge of the truss. The bottom members are the bottom outermost chords, typically in tension. The bracing essentially helps to stabilize the truss  bridge. The portal bracing connects the top of the end posts, while the sway bracing connects the the top chords to one another. The top lateral bracing is a crisscross structure connecting the top chords to one another, while the bottom lateral bracing under the deck connects the floor beams together (“Truss Bridges”, NA, ND). This enables to help picture what keeps the bridge intact, with its mechanisms in 3-D form.

Figure 1.5: Isometric View

c. Foundations

On another note, it is important to understand what solid foundation helps hold the weight of these structures. In this simplified version: the ends of a bridge are known to rest on abutments, which help and hold the soil that sits behind the support.  These abutments  are typically made out of concrete, as they allow for the greatest durability.  For additional support, piers may be used to support in the middle of the bridge. (“Truss Bridges”, NA, ND)

Figure 1.6: Types of Bridge Foundations

IV. Advantages and Longevity

What are the benefits of truss bridges in our world today? What advantages do they have from other bridges? These truss bridges have members organized to behave without bending or shearing. This reduces deflection (where an element can change its shape when a large load is applied). This is positive in that with load intensity, the shape and material of the members stay intact. In its longevity, the amount of materials needed are minimized, while the system is overall efficient and forces are distributed among number of members. These simple truss bridges have their ability to span long distances, to support heavy loads. In addition, unlike other bridge designs, roadways can be placed on top of the support structures. The load can be carried above the deck, making it versatile and economical to build. They are able to be constructed anywhere, not needing much effort to create a suitable environment to be properly built. Therefore, its nature and elements make it one of the most reliable bridges to last over decades, being state-of-the art in its technology.

V. Conclusion

With all this being said, it is important to hold this information relevant in accordance to preforming Statics problems. Bridge designs are known to be unique and hold major distinctions between one another. Simple truss bridges are the most durable in their entirety. It is important to keep in mind that using the method of sections or joints, is up to the student. However, it is dependent on what is being solved for. The end result should create an equilibrium situation, to balance all features and loads. Simple truss bridge triangular designs, enable to help the bridge support other parts. This produces a stable and effective design, with a bottom support of abutments or sometimes even piers. Therefore, its dynamic weight-bearing system makes it so successful in its longevity.

VI. Citations

1. https://i.cbc.ca/1.2854734.1417281738!/http:/https://i.cbc.ca/1.2854734.1417281738!/cpImage/httpImage/image.jpg_gen/derivatives/16x9_780/quebec-weather-20140108-topix.jpgcpImage/httpImage/image.jpg_gen/derivatives/16x9_780/quebec-weather-20140108-topix.jpg (figure 1: Quebec bridge)
2. Truss bridges｜JFE Engineering Corporation. (2020). Retrieved February 21, 2020, from http://www.jfe-eng.co.jp/en/products/infrastructure/bridge/br04.html
3. 3 Methods for Truss Analysis. (n.d.). Retrieved February 3, 2020, from http://www.engineersdaily.com/2011/01/3-methods-for-truss-analysis.html  (figures 1.2, 1.3)
4. https://i.ytimg.com/vi/EXNZ6_dVhPw/maxresdefault.jpg (figure 1.4)
5. Truss Bridges. (n.d.). Retrieved February 4, 2020, from http://www.darrellcausey.com/mrcausey/wp-content/uploads/2018/03/Bridge-Type-Handout.pdf (figure 1.5, 1.6)

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The learning process:

Being that I took Statics and no other engineering classes prior, this was the only expertise I held in civil engineering. Having retained a lot of information from Statics, this class put a lot of attention towards simple truss bridges. In my experience, I have not written a technical description in my college career. Therefore, one of my biggest obstacles was finding ways to format this paper. I began by researching the best ways to format my thoughts, looking at several examples and suggested tips. In writing the description: I used the elements of the truss in its structure, methods to calculating loads, and its foundations. 

When going through the feedback process, the experience was very interesting. The first person to read my description was my mom, having no prior knowledge in structural engineering. Initially, she found it troubling to understand the topics at hand. When reading the paper for the second time, she found it very intriguing and informative. She believed that having diagrams with captions, successfully highlighted key concepts. From this, my mom was able to fully follow along, understanding how weights and loads are applied in trusses. She stated that my format and order of words was prominent in discussing functions and each part of the truss. 

Later in the classroom, peer reviewing was done to present constructive criticisms in editing. This method allows for the exchange of thoughts and ideas in writing techniques. Essentially, it was seen that similar points of discussion from my mom, were met with student Jay Garcia (undeclared major). Jay Garcia additionally added that it was very relevant to connect advantages of these bridges to others, which evidently portrayed the need for these bridge designs around the world. He stated that the format of my paper was very organized, visually appealing, and easy to follow. I continued to explain that truss bridges are one of the oldest traditional bridge models, where members are able to stay intact from its minimal materials/shape. In their constructions, this helps to produce valuable longevity in their ability to span long distances. 

Over time when sitting down with Professor McDonald, her feedback had been most effective. Her idea of introducing examples of knowledgeable truss bridges (like the Quebec Truss Bridge), creates the reader to visualize real life examples. In addition, she allowed me to summarize all my information into my conclusion, keeping key elements. This is necessary in allowing the reader to fully accomplish what had been projected in the description. 

Overall, this process enabled me to recognize how to write technical descriptions. This pushed me to explore my writing skills and habits, to present my work in the most concise manner. Being that no format was given, this allowed me to figure out what would work best for me in my topic. Essential components included audience, scope, functions, advantages, and conclusion. For further reference, it is key to remember to fully explain concepts thoroughly for functional and successful comprehension in material. Quality over quantity provides the most accuracy, while getting to the point and avoiding rambling thoughts. 

In this writing, I especially made sure to avoid specific terminology and only display relevant equations that will later be talked about in Statics. This placed importance on these equations and general concepts. Future curiosities would be to see how my core audience (Statics students) would react when reading this report. This would be helpful to know how they approach problem sets. Additionally, my hope to any potential readers, is an acquired motivation to learn more about structural engineering. Structures exist everywhere in our world today. Some of a few examples are: buildings, bridges, tunnels, that require complex and greater thoughts in their building mechanisms. It is best to keep informed and continue to grow knowledge!