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D2.B.7 Member Resistances

The member resistance is calculated in STAAD according to the procedures outlined in AS 4100. Calculated design capacities are compared to corresponding axial, bending moment, and shear forces determined from the STAAD.Pro analysis. These are used to report the fail or pass status for the members designed.

Two types of design checks are typically performed per AS 4100:

  • Nominal section checks
  • Nominal member checks

The nominal section capacity refers to the capacity of a cross-section to resists applied loads, and accounts for cross-section yielding and local buckling effects. The nominal member capacity on the other hand refers to the capacity of a member to resist applied loads, and includes checks for global member buckling effects including Euler buckling, lateral-torsional buckling, etc.

D2.B.7.1 Axial Tension

The criteria governing the capacity of tension members are based on two limit states per AS 4100 Section 7. The limit state of yielding of the gross section is intended to prevent excessive elongation of the member.

The second limit state involves fracture at the section with the minimum effective net area φNt section axial tension capacities are calculated (Cl.7.2). Through the use of the NSF parameter (see D2.B.8 Design Parameters), you may specify the net section area. STAAD calculates the tension capacity of a member based on these two limit states per Cl.7.1 and Cl.7.2 respectively of AS 4100. Eccentric end connections can be taken into account using the KT correction factor, perCl.7.3. The fy yield stress is based on the minimum plate yield stress. Parameters FYLD, FU, and NSF are applicable for these calculations.

D2.B.7.2 Axial Compression

The compressive strength of members is based on limit states per AS 4100 Section 6. It is taken as the lesser of  nominal section capacity and nominal member capacity. Nominal section capacity, φNs , is a function of form factor (Cl.6.2.2), net area of the cross section, and yield stress of the material. Through the use of the NSC parameter (see D2.B.8 Design Parameters), you may specify the net section area. Note that this parameter is different from that corresponding to tension. The program automatically calculates the form factor. The kf form factors are calculated based on effective plate widths per Cl.6.2.4, and the fy yield stress is based on the minimum plate yield stress.

Nominal member capacity, φNc , is a function of nominal section capacity and member slenderness reduction factor (Cl.6.3.3). This value is calculated about both principal x and y axes. Here, you are required to supply the value of αb (Cl.6.3.3) through the ALB parameter (see D2.B.8 Design Parameters). The effective length for the calculation of compressive strength may be provided through the use of the parameters KY, KZ, LY, and LZ (see D2.B.8 Design Parameters).

D2.B.7.3 Bending

Bending capacities are calculated to AS 4100 Section 5. The allowable bending moment of members is determined as the lesser of nominal section capacity and nominal member capacity (ref. Cl.5.1). 

The nominal section moment capacity, φMs, is calculated about both principal x and y axes and is the capacity to resist cross-section yielding or local buckling and is expressed as the product of the yield stress of the material and the effective section modulus (ref. Cl.5.2). The effective section modulus is a function of section type (i.e., compact, noncompact, or slender) and minimum plate yield stress fy. The nominal member capacity depends on overall flexural-torsional buckling of the member (ref.Cl.5.3).

Note: For sections where the web and flange yield stresses (fy,web and fy.flange respectively) are different, the lower of the two yield stresses is applied to both the web and flange to determine the slenderness of these elements.

Member moment capacity, φMb , is calculated about the principal x axis only (ref. Cl.5.6). Critical flange effective cross-section restraints and corresponding design segment and sub-segments are used as the basis for calculating capacities.

D2.B.7.4 Interaction of Axial Force and Bending

Combined section bending and shear capacities are calculated using the shear and bending interaction method as per Cl.5.12.3.

Note: This check is only carried out where φVv section web shear capacities are calculated. Refer Table 1B.6-1 for details.

The member strength for sections subjected to axial compression and uniaxial or biaxial bending is obtained through the use of interaction equations. Here, the adequacy of a member is also examined against both section (ref. Cl.8.3.4) and member capacity (ref.Cl.8.4.5). These account for both in-plane and out-of-plane failures. If the summation of the left hand side of the equations, addressed by the above clauses, exceeds 1.0 or the allowable value provided using the RATIO parameter (see D2.B.8 Design Parameters), the member is considered to have FAILed under the loading condition.

D2.B.7.5 Shear

Section web shear capacity, φVv , is calculated per Cl.5.11, including both shear yield and shear buckling capacities. Once the capacity is obtained, the ratio of the shear force acting on the cross section to the shear capacity of the section is calculated. If any of the ratios (for both local Y & Z-axes) exceed 1.0 or the allowable value provided using the RATIO parameter (see D2.B.8 Design Parameters), the section is considered to have failed under shear.

Table 1B.6-1 below highlights which shear capacities are calculated for different profile types.

Table 1. Section Type Shear Checks
General Profile Type Australian Section Shear Checks


(i.e., parallel to minor principal y-axis)

WB, WC, UB, UC Calculated for web only
ANGLE EA, UA No checks performed
TUBE SHS, RHS Calculated parallel to both x & y principal axes
PIPE CHS Per AS 4100 5.11.4
Note: Only unstiffened web capacities are calculated. Stiffened webs are not considered. Bearing capacities are not considered.

D2.B.7.6 Torsion

STAAD.Pro does not design sections or members for torsion for AS 4100.

D2.B.7.7 Slenderness

The slenderness check is not used as a critical member ratio check. If a slenderness check is included and exceed unity (1.0), then it may be reported as the governing criteria. However, if it is less than unity, it will be reported but will not be used as the governing ratio.