Have you ever thought about truss structures for the main beams of a trailer? Trusses have some really nice advantages for stiffness and lightweight – mostly when the beams get long. They also come with a few disadvantages that are worth talking about.
We see trusses all the time in other industries. They are in buildings, bridges, cranes, towers, and so many other things. We do not see them as much with trailers, and that makes me wonder why. Even with the very nice advantages, somehow the distractions rule. So, I did some exploration.
Secondly, I have always wanted to design a long trailer using a truss for the main beams, so for a special customer project, we did. Here are some cool design things to consider about truss trailer beams.
What Is A Truss?
We will start with a super basic definition of a truss. From Wikipedia, “A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.” That is a rather unfulfilling definition, but it is hard to define a Truss without a full page and images. Because there are no limits to the arrangements possible, it is more of a concept than a definitive style. They come in 2D, like the rafters in a house; and in massive 3D, like the Eiffel Tower. Then, in a million contexts between.
This roof truss example is pretty simple. In a roof, like for your house, the trusses works together with many, in parallel, to hold the roof.
A much more complicated example is the Eiffel Tower. That is a very complex and massive truss.
The individual members of a truss are there to carry particular loads, usually in tension or compression only. While they usually take more space than a single solid beam, they can also be stronger, stiffer, and lighter weight. The combination of individual members can make a long span possible without a huge amount of material and weight.
So, let’s talk about the application of using a truss in a trailer for the main beams.
Why Truss Beams For A Trailer?
Looking at the needs of a long trailer – say something over 20 to 25 feet or so – we look at the stresses of the trailer frame and realize the beams we choose are more about stiffness than about strength. In other words, the dominant factor for the beam becomes more about how much it will deflect under load, than if it will fail.
We hit a limit for unacceptable deflection before we hit the limits for stress.
A long beam will bow a lot before it actually fails (breaks or permanently bends). So, we can make a long trailer with a “bouncy” beam, that will flex a lot, but not fail. However, the trailer would be spooky to pull with a load bouncing up and down or side to side. (And, bouncy can lead to other ways to fail, but that is not for this article.)
That gives us a good case to look at I-Beams – where the characteristics put more mass at the top and bottom than in the center. (Compare that to other beam shapes.)
When we think about even longer trailers – like those approaching 40 feet – then we must consider quite tall section beams. Those get real heavy, but the tallest portion is only really needed in the middle of the trailer.
So why not a truss? Truss trailer beams allow us to custom fit the trailer to the beam. We can tailor the beams to meet the needs of the trailer length and load. And, most important, the trailer will weigh less. In many cases, a lot less.
Example Truss Trailer Beams
If we pull in the loading and stresses image from a previous article on trailer engineering, we can easily see where a truss is useful. This image shows a more standard length trailer, but as the trailer gets long, the magnitude of the moments increase – even if the load stays the same. That means the beams must get taller and stronger to hold the same load on a longer trailer.
In the image, the Green line is the Moment, which is most definitely a function of beam length. Stress is the Red line, which a combination of the beam type and the moment. In design, for truss trailer beams, that gets a lot more complex.
The roof truss above is strongest at the center, because that is the tallest section. The center is also where the highest moments occur – which is why the design as a truss is so convenient.
There is not design space to make the center tall with truss trailer beams, but we can certainly take advantage. Here is an example of a truss trailer beam. The total length is quite long, 36 ft, and, the relative beam height is tall. Notice the amount of open space through the beams. Also, the taper at the ends.
The height of the truss trailer beams give them the stiffness. Materials give it strength. And, the open design reduces the weight. For a long trailer like this, these are some nice advantages.
Comparing Payload Capacity
As noted, one advantage of trusses is lighter weight beams. To design a trailer of this length with a traditional I-beam, the weight would be much higher. That means the available payload capacity is less for the same size trailer.
For instance, if we make a comparison for a trailer with a max capacity of 14,000 lbs.
- If a traditional full beam trailer weighs 5500 lbs, the payload capacity is 8500 lbs. (14,000 – 5500 = 8500).
- For the same with truss trailer beams, the trailer might weigh 4500 lbs, making the available payload capacity 9500 lbs. (14,000 – 4500 = 9500)
- The result is the trailer owner has 1000 lbs of extra payload capacity for a trailer of the same size and total capacity.
The above numbers can make a big difference. While this is a round number example, it is right in the ballpark of what we see in design.
So what about the disadvantages of truss trailer beams?
Disadvantages of Trusses in Trailer Design
Probably the 2 biggest disadvantages are in design and in construction. What?
First, Design
A single beam, long or short is much easier to design and analyze than a truss made up of many pieces. Then, since it is a bunch of pieces, not one big beefy beam, give care that the trailer cannot twist or collapse in some other way. Much harder to analyze properly with trusses to capture the various load cases.
Next, beam height. Trusses work best when there is space to setup the proper angles for the members. For truss trailer beams, that means height, vertical height, which is often at a premium, because generally we like low trailer decks. This is a significant trade-off to consider in the design phase, and it affects both the weight and strength of the truss trailer beams.
For the trailer in the image above, the Deck-Over design helps because the available space under the relatively high (over the tires) deck height. Compare that to other lower deck designs like Drive-Over fender trailers or the more traditional car haulers and utility style trailers where it is much more difficult to achieve the vertical space for truss trailer beams.
Also for design, there is more variability in how the trailer may be made. More tolerance, which the design and analysis must consider.
Second, Truss Trailer Beams Construction
Yes, a long beam with a lot of small pieces can be a pain to set-up accurately. It is also hard to weld together without warping or twisting. Weld distortion is a big deal in this type of construction. The setup, layout, and welding to make it all correct and straight is an advanced fabrication skill. (Not recommended for beginners.)
Next, it is very time consuming to cut all the bars (at various angles), measure, layout, and weld so many joints. Even the painting is more time consuming, (and uses more paint because there is more overspray). For production, labor costs for all the additional work will eat up a lot.
True, there is some savings in material cost because the truss trailer is lighter, but that is easily offset with the extra work to build it.
Other Disadvantages
Some minor disadvantage is localized weakness if you have high point loads. This is where support for point loads is between the truss diagonal bars. That means the top / or bottom / member of the truss trailer beam must be bigger, stronger material to adequately support between the diagonal supports.
Next, damage. Local damage affects the entire truss. If you damage one section of the truss trailer beam, you have weakened the whole beam, and it is hard to repair. Of course, we can design around some of this by anticipating areas of strength to avoid damage. Even so, this is important to realize with a truss trailer.
Finally, cleaning takes more time after the trailer tows in wet or muddy conditions. There are a lot of small surfaces to clean with a truss.
What Do We Offer?
This article would be incomplete if we did not discuss how to get a truss trailer.
First, we do have one truss trailer beams design in the Deck-Over Trailer Plans area of our Plans Store. Specifically we have 36 foot, 10K Truss Frame Trailer Plans available for purchase. More will eventually come.
Second, if that size or configuration does not satisfy your needs, please contact us. We do custom trailer design. Custom trailer plans with all the engineering are expensive, but if you need them, we welcome the opportunity to quote on the plans you want. Give us a shout.
For lower deck options, we also offer relatively long, high capacity car hauler trailers. Then, for full width, we have trailer plans for Drive Over Fender designs. Though, these have an I-Beam construction rather than truss trailer beams.
As you build a truss trailer, you will need to know about joining beams. Most smaller material like the top and bottom rails of a truss are not available in full 40′ lengths, so you will have to join sections. The trailer plans include the specifics you need, but here is another article with a lot of good information about the essence of extending beams.
Wrapping It Up
The advantages of a truss are inviting and quite attractive. Start with strong, stiff, and lighter weight. Yeah, those are some great things for a trailer – especially for a really long trailer. These are the offer in using trusses for trailer beams.
The biggest disadvantages for truss trailer beams are in design and especially in construction. For production, the additional labor is not insignificant. From a DIY standpoint, the design is not so easy to be sure it is good enough. But also from a DIY standpoint, the extra time to build is not nearly as important – as long as the skills are sufficient for the advanced fabrication techniques.
So, to some people the disadvantages make the truss trailer beams unattractive. To other people, a truss is exactly the right ticket to achieve the goals of a long trailer.
It is certainly something to think about if you are in the market to build a long trailer.
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