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D2.B.12 Physical Member Design

There are two methods available in STAAD.Pro for checking members against the requirements of AS 4100:

  1. Analytical member method
  2. Physical member method

Herein these are referred to as MEMBER Design and PMEMBER Design respectively.

Traditionally STAAD.Pro performed code checks based on single analytical members (i.e., single members between two nodes). This implementation remains in place as shown in the example in Section 1B.8. Physical Member (PMEMBER) Design on the other hand allows you to group single or multiple analytical members into a single physical design member for the purposes of design to AS 4100.

PMEMBER Design also has additional features, including:
  • automated steel grades based on section type;
  • automated tensile stress (fu) and yield stress (fy) values based on plate thicknesses;
  • automated segment / sub-segment design;
  • improved detailed design calculation output; and

Thus, it is strongly recommended that PMEMBER Design be used, even for the design of single analytical members.

D2.B.12.1 Modeling with Physical Members

Physical Members may be grouped by either of the following methods:

  • STAAD.Pro Editor - Directly specify physical members in the input file. Refer to TR.16.2 Physical Members for additional information.
  • Graphical Environment - Using the tools in the Physical Member group on the Geometry ribbon tab, members can be manually or automatically formed.
Note: When creating PMEMBERs for AS 4100, this must be performed in STAAD.Pro’s M. Creating Model Objects . Do not use the Steel Design workflow.

D2.B.12.2 Segment and Sub-Segment Layout

For calculation of member bending capacities about the principal x-axis, the PMEMBER Design uses the concept of segment / sub-segment design. By default PMEMBERs are automatically broken up into design segments and sub-segments based on calculated effective restraints. User-defined restraints assigned using the PBRACE design parameter are checked to see if they are effective (i.e., if they are placed on the critical flange as per AS 4100 5.5). Restraints not applied to the critical flange are ineffective and hence are completely ignored.

Refer to Section 1B.7 for further information on how user-defined restraints are applied using the PBRACE design parameter, including available restraint types, and restraint layout rules.

Note: Segment and sub-segment layouts for PMEMBERs may change for different load cases considered for design. Some restraints may be effective for one particular load case as they are found to apply to the critical flange, however for another load case may be found not to act on the critical flange, and found to be ineffective. In other words the critical flange can change for each load case considered.

Typically the critical flange will be the compression flange, except for segments with a "U" restraint at one end, in which case it will be the tension flange (as is the case for a cantilever).

The PMEMBER Design uses the following routine to determine effective cross-section restraints for each load case considered:

  1. first all user-defined restraints are checked to see if they are applied to the compression flange, with those that aren’t ignored;
  2. next a check is made to see if a "U" type restraint is found at either end of the PMEMBER. If this is the case then any adjacent "L" restraints up to the next "F", "FR", "P" or "PR" restraint are also ignored, regardless of whether they are placed on the critical or non-critical flange. Refer AS 4100

The compression flange in step 1 of the routine above is calculated based on the bending moments at the locations of the restraints being considered. If the bending moment is zero at the same location as a restraint then the following method is used to determine which flange is critical at the zero moment location:

  1. If the zero moment is at the end of the PMEMBER, then the compression flange is based on the bending moment at a small increment from then end;
  2. If the zero moment is along the PMEMBER and is a peak value, then the compression flange is based on the bending moment at a small increment from that location;
  3. If neither 1 or 2 above is valid, then the stiffer of the restraints at that location is taken. The stiffness of different restraint types from the most stiff to least stiff are taken as outlined in Table 1B.9-3.
Table 1. Assumed Order of Restraint Stiffness for Zero Moment Critical Flange
Stiffness Restraint Type
Most Stiff FR
Least Stiff None

Once the effective restraints have been determined, the PMEMBER is divided into segments bounded by "F", "P", "FR", "PR" or "U" effective restraints. These segments are then further divided into sub-segments by effective "L" restraints.

Note: Sub-segment lengths are not automatically checked to determine if they provide full lateral restraint as per AS 4100

For design of cantilevers, the free tip should have user-defined "U" restraints applied to both top and bottom flanges.

Note: If the effective restraints for any load case consist of "U" or "L" restraints only, an error will be reported.

D2.B.12.3 Automated PMEMBER Design Calculations

The AS 4100 PMEMBER Design automates many design calculations, including those required for segment / sub-segment design.

Table 2. Automated PMEMBER AS 4100 Design Parameters and Calculations
Automated Design Calculations PMEMBER Design Parameter Comments
αb compression member section constant per AS 4100 6.3.3. ALB  
αm moment modification factor per AS 4100 ALM Calculated based on moments distribution for individual segments and sub-segments.
fu tensile strength per AS 4100 2.1.2. FU Based on nominal steel grade specified using SGR design parameter and section type.
fy yield stress per AS 4100 2.1.1. FYLD Based on nominal steel grade specified using SGR design parameter and section type.
residual stress category for AS 4100 Table 5.2 and AS 4100 Table 6.2.4. IST Based on section type.
correction factor for distribution of forces in a tension member per AS 4100 7.3. KT Based on section type and eccentric end connection specified using EEC design parameter.
Load height position for automated calculation of the kl load height factor per AS 4100 Table 5.6.3(2). LHT

LHT is used for automating calculation of kl load height factors for segments and sub-segments, per AS 4100 Table 5.6.3(2).

See Load Height Position for details.

Segment and sub-segment layout. PBRACE Refer to the Segment and Sub-Segment Layout section above for details.
Nominal steel grade. SGR Based on section types.
kt twist restraint factor as per AS 4100 Table 5.6.3(1). SKT Based on effective end restraints for each segment / sub-segment.
kl load height factor as per AS 4100 Table 5.6.3(2). SKL Based on effective end restraints for each segment / sub-segment, and LHT design parameter (refer above).
kr lateral rotation restraint factor as per AS 4100 Table 5.6.3(3). SKR Based on effective end restraints for each segment / sub-segment. This is where the distinction between "F" and "FR", as well as "P" and "PR" is used.

D2.B.12.4 Load Height Position

When LHT is set to 1.0 to specify a top flange load height position, STAAD.Pro takes the top to be the positive local y-axis of the member.

Note: This may not literally be the top flange for say a column or beam with a beta angle. The local member axes can be viewed in the user interface by selecting Beam Orientation in the Diagrams Labels dialog (or by pressing <Ctrl+O>).

To automate kl using AS 4100 Table 5.6.3(2), the longitudinal position of the load also needs to be considered, i.e., as either "within segment" or "at segment end".

To determine which of these applies, the shear forces at the ends of each design segment / sub-segment is considered. If the shear force is found to have the same direction and magnitude at both ends, it is assumed that loads act at the segment end.

If on the other hand the shear force at each end is found to have different directions or magnitudes, loads are assumed to act within the segment.

Note: The above method includes an allowance for the self-weight of the member to be considered, as the self-weight always acts through the shear center.

The net sum of the end shears is also used to determine if the load is acting in the positive or negative local member y-axis direction. If LHT is set to 1.0 for top flange loading, the net sum is used to determine whether the top flange loading is acting to stabilise or destabilise the member for lateral torsional buckling. Negative local y-axis net loads act to destabilise the segments / sub-segments, whereas positive local y-axis net loads act to stabilise segments / sub-segments.

D2.B.12.5 Example

KX 0.75 PMEMBER 1 TO 42
KY 1.0 PMEMBER 1 TO 42
LX 4.5 PMEMBER 1 TO 42
LY 6.0 PMEMBER 1 TO 42
PBRACE TOP 0.0 P 0.5 L 1.0 P PMEMBER 1 TO 42