Structural Metal Framing Systems: Welded Steel vs. Aluminum Extrusion

A new project has you looking at structural metal framing systems. 

You know many use welded steel, while a smaller number use extruded modular aluminum. Others use a combination with a welded steel base and extruded aluminum guarding.

While steel framing has long been the assumed must-have for structural support, engineers today are discovering the hidden quality and value potential of aluminum frames. Both methods have strengths and weaknesses – and even systems in the same aluminum family perform very differently – so which is right for your project? 

(Related article: Modular Framing Design Considerations: How Aluminum Differs From Steel)

Pros & Cons of Each Metal Framing System

Steel framing systems use pieces welded together to form structurally sound joints. Aluminum framing systems snap together modularly with extruded, T-slotted components or – when exceptional quality is necessary – self-aligning, non-T-slot components. 

When choosing a system, consider your application’s needs for:

1. Strength
2. Weight
3. Service life (misalignment over time; corrosion resistance)
4. Assembly/modularity
5. Manufacturability & lead time
6. Cost (secondary steps, maintenance, etc.)

In cases where there are stark differences in performance between traditional extrusions and T-slot alternatives, we’ll mark them in red.

1. Strength

It’s true: Steel is a brute-force material, and aluminum is more of an engineered solution.

• Typical mild steel tensile strength: ~55,000 psi
• Standard aluminum tensile strength: ~40,000 psi

Welded Steel Frames – Strength

A welded steel profile offers higher tensile and yield strengths than aluminum extrusions of the same shape. For applications where high-load bearing capability is a must and weight doesn’t matter, welded steel is the way to go.

A variety of heat treatments are available to increase the material’s strength. However, heating and cooling around the weld joint can distort nearby components, putting added stress on the assembly.

Aluminum Extrusion Frames – Strength

Case closed, right? How much weight could an extrusion possibly hold with such a soft material?

Actually, aluminum extrusions’ strength supports many structural framing systems today without fear of failure. That’s why you can’t weigh (no pun intended) strength without also considering criteria #2 below.

But first, a few more strength considerations:

• For many aluminum structures, you can design workarounds for any material strength deficiencies. (Notably, many steel structures tend to be designed stronger than necessary.)
  
  
• T6 Temper is the default for aluminum extrusions, delivering about 40,000 psi yield strength.  Although weaker than steel on a psi basis, aluminum is more than 2x as strong pound-for-pound.
  
  
• Aluminum doesn’t handle fatigue as well as steel, though this can be designed around in most cases. For example, aluminum airplane wings hold up just fine under fatigue.
  
  
• The connection is the inherent weakness of a T-slotted aluminum system. Thanks to specialized connectors, self-aligning extruded systems let you bypass that issue. These brackets and other components form a mechanically locking bond, granting aluminum connections the stiffness and predictability of a weld.

Winner: Steel, with an asterisk

2. Weight

Why can’t you evaluate types of structural framing systems without factoring in weight? Because steel is about 3x as heavy as aluminum:

• Standard steel’s density: ~7.87 g/cm3
• Aluminum profile alloy density: ~2.71 g/cm3 

Welded Steel Frames – Weight

As the density values show, welded steel assemblies are much heavier. For projects that require lightweight structural design, think of criteria #1 and #2 as a ratio:

• Standard steel: ~47 strength/weight ratio

This ratio means steel is best for applications where strength and durability matter more than weight – such as bridge construction.

Aluminum Extrusion Frames – Weight

Extruded aluminum materials are lighter – so much so that they’re “stronger” pound-for-pound:

• Standard aluminum: 100 strength/weight ratio

This allows your structural framing design to be lighter in weight while achieving the same strength results as steel. Or, the same amount of weight with higher strength results!

This makes extrusions ideal for products where lightweight design is vital, such as in aerospace applications. Additionally, using lightweight material, you need fewer components to support the structure, often providing better operator access.

Winner: Aluminum

3. Service Life

There are two factors to consider when evaluating the useful lifetime of a system: structural integrity and environmental resistance.

Welded Steel Frames – Durability

“A steel resolve …” 

“A steel will …”

There’s a reason people use this material as a metaphor for endurance.

Welded steel’s higher yield strength means it withstands greater loads over time. A rigid material, welded steel notably survives better in applications where vibration is persistent, as it uses no fasteners that could come loose.

In engineering, just about every advantage comes with a “yeah, but …” In steel’s case, that’s exposure to saltwater or other corrosives, which can make sharp, flaky profiles out of a once-flawless frame. In some applications – especially outdoors – welded steel requires coating or galvanizing to pass the test of time.

Other considerations include:

• Welded joints have lower fatigue strength and more brittleness than their joined members. 
• High melting point and annealing temperatures (1300+°F, compared with 570° for aluminum) make steel better for applications subjected to high temperatures or fire. 
• Welded steel assemblies are 100% compatible with FEAs (finite element analyses), making it easier to predict their longevity.
  
  

Aluminum Extrusion Frames – Durability

“Are aluminum extrusions durable?” 

A loaded question. It’s time to break out the compliment sandwich.

First, the good: At no extra charge, aluminum provides a surface finish with superior corrosion resistance to even galvanized steel. This is because a protective oxide layer naturally forms on aluminum during exposure to the elements.

Now, onto the bad – and with traditional extrusion framing, there’s a lot of it.

On the raw material level, aluminum’s fatigue properties are poor compared to steel. On the assembly level, things really fall apart – literally.

A T-slot system is a simple and easy-to-manufacture technology that while versatile, has many engineering limitations. Put the frame on a vibrating or cycling application, and it loosens over time. Put stress on a small area, and the frame starts to yield.

Traditional T-slots require regular realigning to maintain a semblance of precision. This downtime to fix the system only does so much – it’ll still need replacement long before a welded structure.

Back to the positive! Self-aligning extrusion systems borrow engineering concepts from aerospace and trucking to eliminate wiggling and weakening. These advanced T-slot alternatives offer …

• Angled bolts
• High-strength brackets
• Solid rivets
• Threaded inserts

… to create bonded connections that stay stiff as a weld, even during long periods of vibration or cyclic loading.

And because of these connections, you can assume the assembly will act like a welded structure. That opens the possibility of an FEA, which is impossible to do effectively with traditional T-slots because of all the variables.

Winner: Steel for structural integrity, aluminum for corrosion resistance, self-aligning extrusions for both

4. Modularity & Assembly

Modular Steel Framing Systems

You have little room for error when welding, drilling, and cutting. Simply put, it’s tough to rework a completed assembly made from welded steel.

Repurposing and repairing steel-based framing systems often require outside help, adding weeks to your downtime.

Say you want to add a horizontal cross-member 6 months after purchase so you can mount some equipment. There’s no snap-on solution, so you have to:

• Cut again
• Sand again
• Weld again
• Stress relieve again
• Clean again
• Paint again

When something damages your steel framing, adjustments are just as tedious.

Even repurposing is a hassle. Steel is less portable due to its weight, so moving it to another space or facility takes a lot of effort.

Modular Aluminum Framing Systems

Modular aluminum profile systems allow you to attach or remove components anywhere, anytime. There’s less pressure to design the ultimate assembly that’ll last for eternity.

The modular customizability makes structural aluminum extrusions valuable for products that might need changes after installation. Want to mount a control panel to your year-old structure? With T-slot extrusions, it’s simple:

• Order precut or fully assembled parts
• Insert your new T-nut and fastener
• Tighten the connection

All of this is possible for a single, average-skilled worker to complete with hand tools. If you need to move the build, it’ll take less effort to tear down and move the lightweight extruded framing.

Advanced, self-aligning framing extrusions are fundamentally similar to T-slots – no need to drill or tap holes. However, the auto-aligning features take value a step further. 

The parts of a self-aligning system come on and off even faster and easier than traditional T-slots. More importantly, you can do this over and over and maintain perfect precision.

Winner: Aluminum

5. Manufacturability

Welded Steel Frames – Manufacturability

Because it’s hard and brittle, steel is harder to manipulate.

• Removing or bending material on steel workpieces is a chore
• The steel frame will, of course, require welding
• You’ll need to drill and tap holes

All of this takes more effort when the material has low machinability and formability ratings.

Aluminum Extrusion Frames – Manufacturability

Aluminum is soft and easy to form without compromising its sturdiness. You can create intricate shapes with wall thicknesses as low as 0.06”.

The lightweight nature of aluminum also makes extrusions easier to work with, since they require less effort to move through the production process.

Of course, you may not need to form anything – the library of premade T-slot parts may suffice for your design. There are several thousand bars and other shapes, with many profiles available precut.

Note that self-aligning aluminum extrusion framing systems come in far fewer shapes because the technology hasn’t been around as long. There’s a chance your project will need a custom-made component.

Winner: Aluminum

6. Cost

At face value, aluminum framing components cost more than welded steel profiles due to the complexity of the extrusion process and the higher raw material cost.

However, that surface-deep advantage doesn’t tell the whole truth.

Welded Steel Framing – Cost

To judge the true cost of steel structural framing, consider how the other criteria factor in:

1. Strength – A strong and tough carbon steel is still reasonably priced vs. aluminum …
2. Weight – … However, steel’s weight makes it more difficult to work with, often requiring specialized equipment when manufacturing, transporting, and installing.
3. Durability – Steel can absorb far more impact and stress, but it’ll corrode quickly in certain applications.
4. Assembly – It takes at least 2x longer to assemble steel frames, and since they’re less modular, future changes add labor, travel, and downtime costs.
5. Manufacturability – Steel’s stubbornness makes it cost-inefficient to fabricate, and you’ll have to hire a welder if you lack a skilled one in-house.
   
   

Aluminum Extrusion Framing – Cost

These same performance criteria impact the cost of aluminum framing too:

1. Strength – Special, high-strength alloys cost extra
2. Weight – You can buy fewer pounds of aluminum and still meet performance specs competitive with steel.
3. Durability – Any aluminum extrusion helps you avoid corrosion-maintenance costs, but only self-aligning systems eliminate lost revenue from downtime (which is why the brackets cost more).
4. Assembly – Connecting the frame is relatively quick and doesn't require skilled labor. The ability to modify assemblies extends the value of your initial investment.
5. Manufacturability – Aluminum profiles save you the expenses of welding and other operations, especially when buying precut parts.

Winner: Aluminum T-slots up-front; self-aligning extrusions for long-term

Steel vs. Aluminum: Your Winning Metal Framing System Is …

There’s much more to the “steel vs. aluminum” debate than raw strength and material cost. 

Structural metal framing systems made with welded steel, standard extrusions, and advanced extrusions each have a few selling points:

• Strength: Steel, IF power is the overwhelming priority
• Weight: Aluminum, WHEN you can use enough material to make up for strength concerns
• Durability: Self-aligning extrusions, IF you need both structural stability and corrosion resistance
• Modularity: Aluminum
• Manufacturability: Aluminum
• Cost: Self-aligning extrusions, IF lowest initial cost isn’t your only criteria

The moral is this: Don’t limit aluminum to where your old assumptions tell you it’s OK to use. Premium solutions sometimes come at a premium price, so weigh your up-front budget vs. your need for long-term structural and production stability.

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