Comparison: Steel vs Aluminum – Part 2
In the debate about Steel vs Aluminum in beams like trailer frames and gantry cranes, there is one big piece of the equation you don’t often hear so much about. Elasticity . . . . What? It has to do with the stiffness of a material.
The engineering term is “Modulus of Elasticity” and it has a lot to do with how much a beam will deflect under a given load. It is a property of the material — Aluminum, Steel, Titanium, Plastic — they all have a different “Elastic” property.
In discussions of Strength and Stiffness, another related engineering principle is often referred to as the “I”, or “Area Moment of Inertia“. This has to do with the size of the beam. Oversimplified, it’s the amount of material, it’s distribution in the beam cross section, and the distance from the top stressed portion to the bottom stressed portion. Follow the above links for more complete descriptions.
In essence, these 2 principles combine in a beam, any beam, to define how “Stiff” it is. This is not “Strength” — though they are often related.
Let’s look at an example using a simple Gantry Crane top beam, and let’s look at it through the lens of Finite Element Analysis (FEA). We’ll use this fun engineering tool to make our comparison of Steel vs Aluminum because it gives some great visual perspectives.
Insight From Engineering Tools
To start, we build two cranes — one with a Steel top beam, and one with an Aluminum top beam. All the legs are the same, in steel, so we can easily see the differences of just the top beam. We’ll then load both gantry beams with 2500 lbs hanging from the center of each beam. This image shows how the loading is setup.
The legs are each constrained at the bottom. For simplicity we assume they are fixed, knowing in reality that’s not exactly true, but for the simple comparative analysis it will do. We also bond (weld) the legs to the top beam, which again, is not exactly true, but good enough for the insight we’re seeking.
For this first run, both beams are 5″ I-Beams. The only difference is one beam is Steel, and one is Aluminum. When loaded, (and exaggerated) they look something like this.
This image shows a deformation scale of 30X. That means what you see is 30 times more deflection than in reality. We use this kind of exaggeration to get a better visual picture of what is actually happening. It’s easier to see, and the exaggeration is the same for both beams, so we can get a feel for the difference of Steel vs Aluminum.
The actual deflection of the Steel beam is just a little less than 0.2″. (Take these numbers with a grain of salt because of the setup assumptions). Deflection of the Aluminum beam is almost 0.5″. Note that stress is very similar except for the hot spots around the leg connection. Those spots really come from the greater deflection, which the legs try to resist. Of course you can’t really weld steel legs to an aluminum top beam. Yet, this is an interesting symptom of the connection when a deflecting beam transfers some deflection into the connecting members. Keep this in mind for later.
From a practical standpoint, if the top beam of a gantry crane deflects 0.5″, it could be hard to move the load along the beam because you’d actually be pulling it up a comparatively steep slope. 2500 pounds is a significant weight. Yes, the deflection moves with the load, and that is a compensation, but it is still less than desireable.
6″ Aluminum I-Beam
In an attempt to reduce deflection and strengthen the joints, let’s replace the 5″ aluminum beam with a 6″ beam. This is the result.
In this image we can see that deflection is definitely less — about 0.3″ — but still more than the Steel I-Beam. Stress is also down, which is good, but it is OK above too. We also see the hot spots drastically reduced because the beam deflection is less. All moves in the right direction.
The change we see in beam deflection is due to the greater “I”, or “Area Moment of Inertia”. Aluminum material is still the same, only the size has changed. Giving it a taller section from 5″ to 6″ is part of it, but also, the 6″ I-Beam has a larger cross sectional area.
At this point we can decide if the 0.3″ deflection is acceptable. In some cases it may be, but for this analysis, since the deflection of the aluminum beam is still more than the steel, let’s change the top aluminum beam again. Going one standard size up puts it at 8″. We’ll do this to illustrate this elasticity difference in comparing the materials — Steel vs Aluminum beam.
8″ Aluminum I-Beam
In the image, we can easily see the 8″ Aluminum I-Beam further reduces stress as well as deflection. The hot spots are gone, and this time deflection is below our 0.2″ steel beam target. We have success.
Deflection Comparison: Steel vs Aluminum Beam
So what have we learned? This is a simple example of where deflection is the driving factor in the design rather than stress. Even the first example had acceptable strength as indicated by the stress (if we strengthened the corners with a gusset). Really, for longer beams, deflection is usually more the concern. Can you imagine your tiny house if the trailer deflected half an inch in the front and back? (And even more over bumps?) You’d have some big cracks to deal with.
This is not to say Aluminum is a bad material for crane beams or trailer frames. Not at all. I actually love aluminum. The above analysis just highlights one of the caveats comparing a Steel vs Aluminum beam.
There are certainly many advantages to using aluminum as pointed out in our Aluminum vs Steel Part 1 post. There are also some disadvantages including the extra deflection in highlight above. This simple study with a gantry crane beam is easy to see and understand. It is also easy to superimpose the concepts onto other applications like trailer design. If we think about the main beam of a trailer frame for instance, if deflection is to be addressed, then the main beams will need to be bigger (taller) for the aluminum trailer. That may make the deck height go up some — which may be fine. Also, keep in mind that the longer the beams (longer trailer), the greater the height difference.
We have not changed a thing in the debate over Steel vs Aluminum. More importantly, that’s not what we set out to do — because there is little point to debating. Both are great materials. Both have good advantages for most applications. Our purpose is simply to broaden perspective and give one more tool for making decisions. The choice of material should be the best one for the job. Enjoy.