ASD vs. LRFD
A Practical Perspective
In a recent discussion with a colleague, the topic of LRFD vs. ASD in steel design came up. He mentioned that he only designs steel using LRFD, whereas every firm I’ve worked at has had a company standard of using ASD.
This conversation made me think about something I was taught in undergrad:
I was told that I should only use LRFD for steel design for two main reasons:
It yields lighter sections.
It is more economical.
I accepted these points at face value as a student. After years of working in the industry and seeing real-world design and fabrication constraints, I do not believe them to be true.
Let’s methodically break down each claim and examine whether they hold up in practice.
Claim #1: LRFD Yields Lighter Sections
It is true that LRFD often produces nominally smaller sections due to the way load factors and resistance factors are applied. However, it is not a given that an LRFD-designed section will always be lighter than an ASD-designed section.
Worked Example: When Does LRFD Yield a Lighter Section?
Let’s compare a beam subjected to:
Dead Load (D): 99 k-ft
Live Load (L): 333 k-ft
LRFD Approach:
ASD Approach:
The results are nearly identical. This challenges the idea that LRFD always leads to lighter sections.
In fact, whether LRFD or ASD results in a smaller beam depends on the live-to-dead load ratio (L/D):
L/D > 3.36: ASD may yield a lighter section.
L/D < 3.36: LRFD may be more economical.
Many practical designs fall into the range where ASD and LRFD produce nearly identical results, meaning that the choice of method doesn’t automatically determine the most efficient design.
Claim #2: LRFD is More Economical
Even if LRFD does yield a lighter section in some cases, that doesn’t necessarily mean it is more economical. The economy of steel design isn’t determined by weight alone—it is determined by fabrication, labor, and constructability.
Steel Economy Isn’t Just About Weight
A lighter beam doesn’t always mean a more economical beam. Here’s a real-world example that proves the point:
A colleague of mine recently spoke to a fabricator who told him that a bottom flange cope on a small beam costs $200 per cope. If you have multiple beams framing into a girder, those costs add up fast.
So instead of using a smaller beam that requires expensive coping, why not just make the beam deeper (and potentially heavier) to eliminate that cost?
This principle applies broadly to steel construction:
Sometimes a heavier column avoids the need for doubler plates in moment frames, saving on fabrication costs.
A slightly deeper beam can reduce deflection and eliminate costly stiffeners.
A heavier beam might mean simpler connections that reduce labor costs.
The bottom line? Weight isn’t the only cost driver. Fabrication, labor, and detailing complexity all matter, often more than shaving off a few pounds of steel.
Labor is 60% of Steel Cost—Not Material
According to AISC’s Value Engineering article (AISC, 2000), material only accounts for about 30% of the total cost of a steel project, while labor makes up a staggering 60% of the cost.
This fundamentally changes how we evaluate "economy" in steel design. If labor costs dominate the total project cost, then:
A heavier beam that simplifies fabrication could be cheaper than a lighter beam with complex detailing.
Reducing the number of pieces to be handled, fabricated, and installed often yields greater cost savings than cutting down steel tonnage.
Minimizing welds, stiffeners, and intricate connections is often more effective than purely optimizing weight.
This means that choosing LRFD over ASD for the sake of weight savings alone is missing the bigger picture—labor costs drive the real economy of steel projects.
Deflection Usually Governs, Not Strength
Another critical point: most beams are governed by deflection, not strength. This means neither LRFD nor ASD has an inherent advantage.
For example, take a simple-span beam subject to uniform load. Deflection is given by:
where:
w is the distributed load,
L is the span length,
E is the modulus of elasticity,
I is the moment of inertia.
Deflection limits are usually serviceability criteria (e.g., L/360 for live load). Since both ASD and LRFD check service-level loads for deflection, neither method inherently leads to a better design in these cases.
Since deflection governs so many designs, and neither method has an inherent advantage here, why not use the simpler one?
ASD is the Superior Tool
We’ve now established that:
Neither LRFD nor ASD is more economical in a vacuum—it depends on L/D ratios.
A lighter section doesn’t always mean cost savings—fabrication costs matter.
Labor—not material—dominates steel project costs (AISC, 2000).
Deflection often governs design, meaning neither method has a structural advantage.
Given these facts, I prefer ASD for one simple reason: simplicity.
It’s easier to understand.
It’s faster to use.
It provides the same end results in most practical cases.
Since ASD gets me to the right answer faster, it is the better tool. Engineering is about efficiency—not just in material, but in the design process itself.
Citations:
AISC. (2000). Value Engineering in Structural Steel. Modern Steel Construction, April 2000. (Link)