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D2.B.12.6 Physical Member Restraints Specification

The PBRACE parameter is used to specify the restraint condition along the top and bottom flange of a PMEMBER.

General Format

PBRACE { TOP | BOTTOM } f1 r1 f2 r2f52 r52 (PMEMB pmember-list)
fn a fraction of the PMEMBER length where restraint condition is being specified. This value is any ratio between 0.0 and 1.0.
rn one of the possible restraint conditions as follows:
Table 1. Physical Member Restraint Types
Designation, rn Restraint Type Description
F Fully restrained  
P Partially restrained  
L Laterally restrained Cannot be specified at the ends of design members.
U Unrestrained Can only be applied at the ends of design members, and must be applied to both flanges to be effective.
CAUTION: Both top and bottom flanges can not be unrestrained at the same location (as this is unstable).
FR Fully and rotationally restrained  
PR Partially and rotationally restrained  
C Continuously restrained The flange is assumed to be continuously supported at that flange up to next restraint location. For continuously supported flange unbraced length is assumed to be zero.


PBRACE TOP 0.85 FR 0.33 PR 0.33 PR 0.25 F 0.75 L 0.5 PR 1.0 U 0.0 U
PBRACE BOTTOM 0.75 L 0.0 U 0.25 P 0.5 L -
1.0 U PMEMB 3 7


Refer to AS 4100 Section 5.5 for a full definition of the critical flange. Typically this will be the compression flange, except for segments with U restraint at one end, then it will be the tension flange (as is the case for cantilever portion at the end).

  • when gravity loads are dominant (i.e., negative local y-axis direction), the critical flange of a segment shall be the top flange (i.e., tension).
  • when upward wind loads are dominant (i.e., positive local y-axis direction), the critical flange shall be the bottom flange (i.e., tension).

Design physical members are divided into segments by "F", "P", "FR", "PR" or "U" effective section restraints. Segments are further broken down into sub-segments by "L" restraints, but only if the "L" restraints are deemed to be "effective". "L" restraints are only considered to be effective when positioned on the "critical" flange between "F", "P", "FR" or "FP" restraints. If an "L" restraint is positioned on the non-critical flange it shall be completely ignored. Further, if an "L" restraint is positioned between a "U" and an "F", "P", "FR" or "PR" restraint, it shall be ignored (regardless of whether it is on the critical or non-critical flange).

Design members must have either a F, P, FR, PR, or U restraint specified at both ends, for both flanges.

  • If UNL is not specified, segment length is used as UNL and used as L in effective length calculation as per 5.6.3.
  • If ALM i.e., α_m is not provided, automatic calculation of ALM is done based on moments within the segment.
  • If SKR i.e., Kr is not provided, it is automatically calculated based on table 5.6.3(3) considering restraint conditions are the end of the segment. If FR or PR is found at only one of the end, Kr is assumed to be 0.85; if FR or PR is found at both the ends, 0.70 is used as Kr.
  • If SKT i.e., Kt is not provided, it is automatically calculated based on Table 5.6.3(1) considering end restraints of the segment and section geometric information and segment length.
  • If SKL i.e., Kl is not provided, it is automatically calculated based on Table 5.6.3(2) considering end restraints of the segment, Load Height Position parameter, LHT and shear force variation within the segment.


  1. If PMEMBER list is not provided, all the PMEMBERS are restrained by same configuration.
  2. It is not necessary to provide the restraint locations in sequence as the program sorts them automatically.
  3. Unless specified, PMEMBER ends are assumed to be Fully Restrained (F).
  4. While designing any section of the member, effective restraints are searched on each side of the section along the critical flange.
  5. The types of restraints applied to the top and bottom flanges at each location determines the effective section restraints. These are outlined in the table below:

    Table 2. Restraint Meanings in Critical and Noncritical Flanges
    Case Flange Restraint on a Critical Flange Restraint on a Non-Critical Flange Effective Section Restraint
    I   U U U
    II 1 L Nothing L
    2 Nothing L None
    III 1 P or F Nothing or U F
    2 Nothing or U P or F P
    IV 1 PR or FR Nothing or U FR
    2 Nothing or U PR or FR PR
    V 1 L, P or F L, P, F, FR or PR F
    2 FR or PR L, P, F, FR or PR FR
    Note: The critical flange can change for each load case considered.
  6. If a C-type restraint is defined to a flange at a particular location of the PMEMBER, program will consider that continuous restraint to be effective up to the next bracing point of the point of contraflexure whichever is nearer to the continuous restraint.