Member vs. Section capacity

[Effy555]

Hi,

Is there any rule when to use section capacity and when member capacity? Does member capacity include secondary effects already?

 

[JoshPlumSE]

What code are you talking about? Different codes use slightly different terminology and interaction equations and such.

 

[phamENG]

Agree with Josh that we need more info. But generally, section capacity and member capacity converge when member stability is satisfied through bracing, etc. If a member has to rely solely on its internal stiffness for stability (such as resistance to LTB), the member and section capacities will typically diverge with the overall capacity of the member being lower.

 

[Settingsun]

Your profile says Australia and terminology makes me think steel, so some extracts from AS4100 are in the image below for bending and compression. For bending, you can take Beta_m to be -1.0, Fy = 300MPa for rolled sections, Fy = 350MPa for hollow sections, to get simplified conservative limits. For compression, you can see that member capacity = section capacity for lambda_n < 15, and the reduction in capacity is still small for lambda_n = 25.

Member capacity includes allowance for some imperfections, but you need to do a second-order analysis (or moment magnification) in many cases.

 

[Tomfh]

Section capacity is when the section yields. Section capacity provides an upper limit to member capacity.

The rule is to use the lower of the two.

 

[liam1369]

Member capacity usally governs design as the member will typically fail via (LTB - Beam) or (Buckling - Column).

Section capacity is useful for very short elements (< 3.0m).

Always check the member capacity, a long beam will fail in member LTB way before developing its full section bending resistance.

 

[kostast88]

Member capacity is conservative. Use that.
Section capacity is satisfied de facto when member capacity is satisfied.

Member capacity accounts for Buckling/Axial Force effects etc.

Refer to the Blue Book (UK) from SCI for tables with member capacities in Europe. See how member flexural capacity reduces when buckling is considered (hint: decreases with length).
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