D2.B.12 Physical Member Design
There are two methods available in STAAD.Pro for checking members against the requirements of AS 4100:
- Analytical member method - referered to as
MEMBER
Design - Physical member method - referred to as
PMEMBER
Design
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.12.1Modeling 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 User Interface - Using the tools in the Physical Member group on the Geometry ribbon tab, members can be manually or automatically formed.
D2.B.12.2Segment 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.
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:
- All user-defined restraints are checked to see if they are applied to the compression flange, with those that aren’t ignored;
- 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 adjacentL
restraints up to the nextF
,FR
,P
orPR
restraint are also ignored, regardless of whether they are placed on the critical or non-critical flange. Refer AS 4100 5.4.2.4.
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:
- 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;
- 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;
- 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.
Stiffness | Restraint Type |
---|---|
Most Stiff | FR |
↓ | F |
↓ | PR |
↓ | P |
↓ | L |
↓ | U |
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.
For design of cantilevers, the free tip should have
user-defined
U
restraints applied to both top and bottom flanges.
Uor
Lrestraints only, an error will be reported.
D2.B.12.3Automated PMEMBER Design Calculations
The AS 4100 PMEMBER Design automates many design calculations, including those required for segment / sub-segment design.
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 5.6.1.1. | 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
Fand FR, as well as Pand PRis 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.
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.
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.12.5Example
PARAMETER 1
CODE AUSTRALIAN
DMAX 0.4 PMEMBER 1 TO 42
DMIN 0.25 PMEMBER 1 TO 42
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
LHT 0.0 PMEMBER 1 TO 42
NSC 0.9 PMEMBER 1 TO 42
NSF 1.0 PMEMBER 1 TO 42
PBRACE BOTTOM 0.0 F 1.0 F PMEMBER 1 TO 42
PBRACE TOP 0.0 P 0.5 L 1.0 P PMEMBER 1 TO 42
SGR 0.0 PMEMBER 1 TO 42
TRACK 2.0 PMEMBER 1 TO 42
CHECK CODE PMEMBER 1 TO 42