How to Design a Basic Truss Bridge With Fusion 360!

When I first read the parameters for the Make It Bridge contest, I couldn't think of many problems that a pedestrian bridge would solve other than ya know... providing an easy path of travel across difficult terrain. But, the more I thought about it the more I realized that pedestrian bridges can provide much more not only for the individual person but also for the whole community. To showcase this, we will be designing a Pratt Truss bridge to go over a highway to highlight some of the advantages a town gains from adding such a structure. You may be wondering what is gained from building a pedestrian bridge over a highway. Sure it makes crossing safer but why not just drive across, or walk, and why does it need to be a truss bridge to begin with? These are all question I hope to address in this Instructable!

Contest Info: Homeschooled, 12th Grade

• Fusion 360 (Free for personal or educational use)

If you live in America or have ever visited for that matter, you will probably have noticed that there are a lot of roads and not a ton of sidewalks. Although it may not look the greatest, having a car dependent society is not the problem in and of itself. In fact, it has been a part of American culture for quite some time now. The problem is that there really is not anyway to get around if you do not have a car. Additionally, essential places like the grocery store, the hospital, or work are spread out due to urban sprawl instead of being clumped together as in many other countries that rely on development. Because of heavy reliance on the automobile, everything becomes more expensive without a car.

Let me explain. For example, if you want to go to the store and buy $5 worth of merchandise you can just hop in your car to go get it. However, if you don't own a car, and you live far from a store, the cost of these products increases significantly because you not only have to pay the $5, but also you have to spend significantly more money to pay someone to take you there, or you have to spend significant time walking to and from the store. These extra costs that are acquired during an exchange are called transaction costs.

If you would like to learn more about the problems with urban sprawl, the channel, Not Just Bikes, has a great playlist that discusses this topic in much greater detail.

I hope it is clear now that urban sprawl as a result of car dependency is probably not the way to go. So, how do we go about fixing this? While there is no quick fix, I do believe that the overall solution would be to encourage development of existing land over knocking down forests to build another strip mall. But, how does a pedestrian truss bridge encourage development? Obviously, a bridge on its own cannot accomplish this, but it can be a vital first step to get the ball rolling. As more and more people are encouraged to walk, businesses and cities will begin to accommodate for them vs the automobile. It will also improve the quality of life for those who do not have access to a car as it decreases those pesky transaction costs we talked about in the last step. Lastly, if designed correctly, a bridge can improve the overall look and feel of a town!

Now that we know why we need a pedestrian bridge in the first place, we can decide on a location. In my city, there is a massive highway (that everyone treats as a freeway) which runs straight through the middle of everything. People in the past built cities near rivers because they were the most efficient mode of transportation, and I would argue that highways have become a sort of modern river. They are the quickest and most convenient mode of transportation, so people build all along them further exacerbating the problem discussed in step one. However, people learned to build bridges over rivers but that logic has not progressed to highways. Keeping the problem in mind, I realized that there was a certain part of this highway in a heavily populated area that has a mall on the left and a shopping center on the right as seen above. These are obviously heavily traversed by pedestrians, but there was nothing linking them across the road other than a crosswalk which on a road of this size is not a considered an option by most people except those who most desperately need to cross. Those are the reasons I decided to locate my bridge in this spot, but the same train of thought could be applied by anyone following along.

So now that we have found a location, it would be good to understand why I have chosen to design a truss. First, truss bridges have a high strength to weight ratio meaning that for a given amount of materials, a truss can support more weight than many other bridge designs of the same weight. Second, the majority of all components in a truss bridge are supporting the structure in some way making the truss a very efficient and cost-effective design.

Why are truss bridges so strong to begin with? I'm glad you asked! Truss bridges and trusses in general are made up of many different triangles. That is the key! Triangles are some of the strongest shapes in nature. To understand this, think of a square with vertices made up of freely moving joints. Because the joints are free to move, pushing on any part of the square would cause the shape to deform. Now think of the same thing except with a triangle. A triangle would not deform (unless you push hard enough...) because for a triangle with sides of a fixed length there is only one set of corresponding angles. Therefore, instead of deforming, the forces acting on a triangle just get evenly redistributed to the joints.

I would also like to point out that this article by Ubani Obinna has helped me a ton in understanding the basics of bridge design. It has a much more in depth description than what I can give you, so if you would like to learn more, please head over to that link!

The problem with a lot of model bridges that I see is that they use oversimplified members. What I mean by that is that say for the main supports that span the width of the bridge they will use a simple rectangular cross section. In reality however, engineering is a bit more complicated. We must use something called an H-beam. H-beams are beneficial because they reduce cost by removing unnecessary material while retaining much of their strength. Furthermore, H-beams are more resistant to bending motions because they have most of the material focused near where the compression and tension forces are occurring. Essentially, all we need to know is that H-beams are very cost-effective which is why they are used so heavily in construction. Also note that for the purpose of this tutorial, I will not be going over the dimensions for the H-beams as we are simply focused on trying to understand the principle behind why they are used. If you would like a more detailed explanation than I can give you, please watch this wonderful video by Khan Academy.

As with anything, we will begin this bridge by designing the foundation. As you can see in the picture above, we need to create two large beams called bottom cords that will run the span of the bridge. These two beams will be connected by floor beams. Finally, we will add beams called stringers that sit on the floor boards and run parallel to the bottom cord to hold the actually pathway.

To begin this journey, we must sketch our main H-beams. Click on the Create Sketch button in the upper left-hand corner. Once you have finished the sketch for the bottom chords, click Finish Sketch in the upper right-hand corner. Next we must extrude our sketch to match our desired truss length. To do that, highlight the sketch you just made by clicking on it. Once it is highlighted, press the extrude hotkey (the letter E on your keyboard). Finally, type in your desired length. The process is the same for the floor beams and stringers, however it is important to note that when you extrude a sketch into another object it will either bond to that object or cut some of it away. This isn't really a problem unless you want to highlight or change a certain object later on. To get around this make sure the operation type in the extrude menu is set to New Body.

Next, we will add the vertical members that make up part of the actual truss. I placed them the same width apart as the floor beams in the previous step. Also note that because we are designing a truss bridge, we do not need to add vertical members to the very ends of the bridge since the diagonal members will connect to the ends of the bottom chord.

The height of these beams will control how tall your overall truss will be. Furthermore, the height you choose will actually change what type of truss bridge this is. If your vertical members are taller than the pedestrians traveling across, you will have created a through truss which is what I have done. However, if your vertical members are shorter than most of the pedestrians crossing, you will have created a semi-through truss. In both of these cases the live load is carried by the bottom chord. However, there is one last type of truss bridge which is the underslung truss. In an underslung truss, the truss is actually below the bridge itself providing support from underneath. This means that the live load is actually being carried by the top chord.

I feel that now is a good time to mention the different methods you could use to connect these H-beams. If you look at my design, you will see that the beams seem to just be resting on top of one another. This is because I assumed a welded connection which is just what it sounds like. The other popular form of connecting joints is using something called a gusset plate as shown above. Gusset plates can be used to connect members with bolts instead of permanently welding them together. Originally, I wanted to add gusset plates to the design, but I realized that this would create to many elements which could potentially cause problems with loading the design on some computers.

The next step is to sketch the H-beam that will go across the top of the vertical members. The easiest way I have found to accomplish this is to create this H-beam away from the rest of the design and moving it afterwards as opposed to sketching it on top to begin with. Be careful to sketch your H-beam so that when extruded it will be parallel to the bottom chord. This will make sense when we go to move it. Once you have created the top chord, we can make use of the move/copy function to move it into place. Open the move/copy menu by clicking on the icon in the toolbar. Now select the body you would like to move. Instead of using the arrows to move the object, we are going to make use of the Point to Point feature. This will allow you to select a reference point on the top chord that will move to a certain point on one of the vertical members. All you have to do is select the corner of the top chord as your origin point and then the corner of the vertical member on the end as the target point.

Now that we have our top and bottom cords installed, it is time to add the diagonal H-beams otherwise know as the web members. However, before we do that, now is a good time to make sure we understand how these members work. As discussed in a previous step, trusses work because they are made up of many triangles which by nature are very strong. This is an extremely important concept that we have to take into consideration when designing our truss. If the web members are not aligned correctly, it could be fatal to the design as our triangles would not exactly be... well ya know... triangles. In many real life cases, the members may not always be physically touching each other to form a triangle. However, this is not actually a problem. Imagine drawing a parallel line through the center of each member. As long as these lines intersect at the same point to form a triangle, our truss will hold. This works because although the members may not be physically touching, the forces are still acting on the same point meaning that we still have a triangle.

Now that we have our top and bottom cords installed, it is time to add the diagonal H-beams otherwise know as the web members. Since we are designing a Pratt truss, our web members need to be angled away from the vertical members. If that is confusing, think of making an arrow that points downward out of the middle vertical beam and then simply copy-paste the remaining web members till you get to the end of the truss. As for the web members on the ends of the truss, we will need to rotate these 180 degrees so that the perimeter of our truss resembles a trapezoid. This can be easily accomplished in Fusion 360 by utilizing the move/copy feature in the top tool bar. Simply, hover of the middle of the web member where your cursor should snap to the middle. If it does not snap to the middle as is sometimes the case, you can get around this by sketching a construction line that is half of the length of the beam on the surface to create a reference point. After you have the middle of the beam selected with the move/copy tool, use the menu that should appear on the righthand side of the screen to rotate the member 180 degrees.

One of the last things to do is to add the deck. The deck simply refers to the actual surface that bears the live load. The material could be anywhere from concrete to wood. I choose to design mine out of wooden planks as wood can be very strong and light. I also thought it looked the best. Ultimately, we do not want this bridge to be an eye-sore as we are trying to encourage people to use it.

To design the deck, I began by sketching a rectangle where the dimensions had a ratio of about 10:1 meaning that it was 10x as long as it was wide. After you finish that, extrude the sketch to your preferred height. Now just repeat 200 times for every plank of wood on the deck...

Alright I'm just messing with ya. Thankfully, there is an easier way to do it. After you have created the first plank, click on the dropdown menu in the top tool bar that says create, and look for the pattern function. When you click on the pattern function it will give you several pattern types to work with, but we are looking for the Rectangular Pattern option. This function requires two parameters: an object, the wooden plank in our case, and an axis of travel. Simply, click on the new object you just created so Fusion 360 knows what to make the pattern out of then click on the axis with the arrow pointing in the direction that you want the pattern to appear. You will then be given the option to decide how long you want it to be and how many of each object you would like to add. It's that easy!

Note that because I have chosen to put this bridge over a highway, I had to add a second identical truss to the design so that it could span the entire distance. Your scenario may be different and one truss might be enough. Heck! you may even need 3 or 4 trusses. There is nothing wrong with doing this as long as you add a support in between each bridge. This works because as long as the support is stable and can support the weight you have essentially shortened the gap the needs to be crossed.

Remember, one of the purposes of this bridge is to give those who cannot drive a safe method of travel. With this in mind, I choose to design some elevators to access the bridge since many times those who cannot drive are physically disabled. Therefore, it does not make sense to just use a staircase. I won't go over how to design the elevators themselves as that kind of distracts from the point of this tutorial. Furthermore, your location may not even require it. For example, your location may work best with a staircase or ramp. Regardless, your scenario is most likely not going to be the same as mine, so do what works best for you!

We have now finished the physical truss. All that is left to do is to add some practical and aesthetic elements to our design. If you were to walk across our bridge as is, you would probably see that it is not too safe as there is nothing stopping people from falling through the trusses to the side. To fix this, I simply added a couple of bars that go along the side of the bridge that also double as a handrail. These are super simply to add just create a sketch at one end of the bridge that resembles the cross section of a handrail and extrude to the other side using the To Object extent type in the extrude menu on the right side of your screen.

Lastly, if you have not been adding color and texture as we go along, your bridge is probably a solid greyish color as this is the default in Fusion 360. We can remedy this by using the appearance function. You can access this function by pressing the letter A on your keyboard. Upon doing so, you will be presented with a host of paints, textures, and colors that you can use to make your bridge come to life! All you have to do is click on the color you want, wait for it to download, and drag it onto the object you want to color. It is also worth noting that you can change the appearance of multiple faces at one time by preselecting them. To preselect multiple objects, just click on all the faces you want to color while holding down the ctrl key then just drop the color onto one face, and all of the preselected faces should change to the same color.

Congratulations! You have just designed your own pedestrian truss bridge. My goal with this tutorial was to showcase the general steps one might go through to design their own truss bridge in Fusion 360. Your bridge probably won't look the same as mine, but that's ok. Add your own unique creativity to your design so that people are encouraged to use it. Remember, this bridge is not the end itself rather the means to an end. Ultimately, the goal is to encourage future development in safer and cheaper transportation methods, and I believe we have accomplished that!

Then Jesus told his disciples, "If anyone would come after me, let him deny himself and take up his cross and follow me. For whoever would save his life will lose it, but whoever loses his life for my sake will find it. For what will it profit a man if he gains the whole world and forfeits his soul? Or what shall a man give in return for his soul? For the Son of Man is going to come with his angels in the glory of his Father, and then he will repay each person according to what he has done. - Matthew 16:24-27

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