Steel: Required Strength vs. Nominal Strength vs. Allowable Strength vs. Design Strength

Steel is actually one of my favorite structural materials. For some reason, the design outlined by AISC just seems much more straightforward compared to other materials (e.g. concrete). Perhaps I just had a really good professor in college… (thanks Professor Uang!)

Anyways, if you are planning to take the PE or the SE but have very limited knowledge or experience with structural steel design, my goal is to be able to teach you 70% of what you need to know (for gravity design) with the least amount of effort.

I intend to cover these topics below in a number of posts (I'll start from the top down and if people find them useful, I'll continue. If not, perhaps I'll move on to talk about something else instead):

• Design strengths
• Load combinations
• Shapes and materials
• Flexural design
• Shear design
• Compression design
• Tension design
• Bolted connections
• Welded connections
• Composite beam design

…so hopefully by the end, you will know a thing or two about steel!

Let's get started with design strengths (this may be pretty basic to most of you out there, but we have to start somewhere).

Required Strength vs. Nominal Strength vs. Allowable Strength vs. Design Strength???

If you are new to “strength” design, all of these different terminologies may sound confusing to you. Let me just clear that up by using a simple diagram.

We'll start from the top. Basically, you have the demand, which is the required strength. This can be either ASD () or LRFD () depending on which load combination you are using (we'll talk about ASD & LRFD load combinations in another post).

In the manual, the subscript for required strength is “a”. Personally I find that slightly confusing so when I do my calculations, I like to write “req” as the subscript instead. Like this: .

Next, you have the nominal capacity (or nominal strength) which is the “unfactored” capacity. Meaning you basically calculate this out using the formulas in the manual for axial (), moment (), or shear () without applying the resistance factor or safety factor (more on this below).

ASD

From the nominal capacity, if you divide it by Ω, you end up with the allowable strength (e.g. . I like to call this ).

Ω is the safety factor which is always greater than or equal to one.

Note that for for seismic design, there is a thing called the overstrength factor which uses the symbol . This has nothing to do with the safety factor Ω that we use here, so make sure you don't get those two confused.

So now you got your demand and capacity, you can check to see if your design is “OK” or “No Good (NG)” (e.g. if , OK, otherwise NG).

LRFD

Similarly, you do the same thing for LRFD.

From the nominal capacity, you would multiply by to obtain the design strength (e.g. ).

 is the resistance factor and it's always less than or equal to one.

From there you check your demand vs capacity (e.g. if , OK, otherwise NG).

The End

From time to time, in the PE or the SE exam, there may be questions that specifically use the word “nominal” and then asks you to calculate some stuff out. By knowing the difference between these 4 different “strength” terminologies, you should have no problem understanding what you have to do.

Hopefully you find this helpful. Next up: load combinations.

Stay tuned.