# What really is the difference between allowable stress design and ultimate limit state design?

I'm trying to understand this difference. From what I've read, ASD basically deals with making sure the stress in a structure does not exceed a certain limit, the yield stress of the material. It ensures that structures remain in their elastic region while subject to designed structural loads.

Limit state design on the other hand defines a set of ultimate limit states against which the structure has to be checked. These states can be static equilibrium of the structure (EQU), the strength of a structure (STR) and fatigue damage of the structure (FAT).

But don't these two design philosophies in the end boil down to the same thing, making sure the structure (steel structure for example) does not reach a stress too high? For example, we might check a steel beam against loads by calculating the expected characteristic loads such as wind and dead load, combine these according to different defined ultimate states and then check that the stress resulting from these does not exceed the yield stress of the beam. But isn't this in the end what allowable stress design does? What is the practical difference between the two?

EDIT: Making further research, I noticed that limit state codes do sometimes allow yield to occur. For example, in Eurocode 3, part 1-1, 6.2.5, it is stated that the resistance of a cross section against bending is determined using the plastic section modulus.

In this Youtube video, the differences are given:

Here the difference is given to be that in ASD, we use service loads which are the expected loads without any safety factors applied. Aren't these what are known as characteristic loads in limit state design? In contrast, it is stated that limit state design uses factored loads. So why is it that in ASD we use loads without any factors? Aren't safety factors there to take uncertainties in things like material properties and modeling theories into account? Why don't they exist in ASD?

• I'm having a bit of trouble understanding your question. You emphasized your question regarding the "practical difference" between the methods, but also explained how to apply each method (service loads and low allowable stress in ASD vs. factored loads and higher allowable stress in USD). That's basically the definition of the "practical difference" between them. You also ask if they boil down to the same thing and, well, yes. They are different methods to make sure the structure is safe, so fundamentally they try to satisfy the same constraints.
– Wasabi
Jan 25, 2021 at 12:58
• They just take different paths to get to the same final conclusion. Is your question basically "why do we have these two methods" and "is there anything one does better than the other?"
– Wasabi
Jan 25, 2021 at 12:59
• I also have some hard time formulating my question. For one, why is not plastic behavior not allowed in ASD but is allowed in USD? I mean, I am starting to understand the difference between the two as the fact that in ASD we only apply one safety factor: we calculate nominal loads and resistance and then apply a safety factor. In USD, we apply safety factors to both resistance and the loads. But what is it about the nature of USD that allows us to consider plastic behavior? Differences between the two are often listed as one allowing only elasticity, while other allowing also plasticity. Jan 25, 2021 at 21:26
• Then another difference often listed is that one uses only one safety factor, while other uses factors for both resistance and loads. But what ties these differences together? Why is it that when we consider safety factors for both resistance and loads (UTS) we can also allow for plasticity? Jan 25, 2021 at 21:27
• I'm not familiar with ASD since I'm only being taught USD at university. What comes to mind is that maybe ASD does not separate design into ultimate and serviceability limits? That would explain why plastic behavior is not allowed in ASD as permanent deformations that are inherent in plastic behavior probably break serviceability demands? Then, as design is split into ultimate limit and serviceability design, when can allow plasticity in ultimate states as they don't consider serviceability demands. After all, plasticity of a member does not necessarily mean collapse of the structure.. Jan 25, 2021 at 21:50