D2.B.7 Member Resistances
The member resistance is calculated in STAAD.Pro 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 - this refers to the capacity of a cross-section to resists applied loads, and accounts for cross-section yielding and local buckling effects
- Nominal member checks - this 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.Pro 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 classification (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).
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.
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.
General Profile Type | Australian Section | Shear Checks |
---|---|---|
I-SECTION (i.e., parallel to minor principal y-axis) |
WB, WC, UB, UC | Calculated for web only |
T-SECTION | BT, CT | |
CHANNEL | PFC | |
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 |
D2.B.7.7 Shear and Bending Interaction
The program will report if M* > ϕMs (i.e., shear bending interaction has failed the check as per Cl. 5.12.3). If M* > ϕMs, then Vvm = 0.6 × Vv.
D2.B.7.8 Torsion
STAAD.Pro does not design sections or members for torsion for AS 4100.
D2.B.7.9 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.