TR.37.5 Direct Analysis
General Format
PERFORM DIRECT ANALYSIS ( {LRFD | ASD} TAUTOL f1 DISPTOL f2 ITERDIRECT i3 (REDUCEDEI i4) (PDITER i5) (TBITER) PRINT print-options )
print-options = { LOAD DATA | STATICS CHECK | STATICS LOAD | BOTH | ALL }
This command directs the program to perform the analysis that includes:
- Reduce Axial & Flexure stiffness to 80% for members selected in the Define Direct input. The 80% applies only to analysis.
- Solving the static case which has notional loads included.
- Perform iterations of the iterative PDelta with SmallDelta analysis procedure (default 15 iterations).
- Solving simultaneous equations for displacements;
- Compute Tau-b of AISC 05 Direct Analysis Appendix 7 based on required strength versus yield strength.
- Reforming the global joint stiffness matrix.
- Solving simultaneous equations for displacements;
- Repeat steps c) through g) until converged or ITERDIRECT iterations are reached.
A Direct Analysis will correctly reflect the secondary effects of a combination of load cases only if they are defined using the REPEAT LOAD specification (TR.32.11 Repeat Load Specification) and/or REFERENCE LOAD specification (TR.33 Reference Load Cases - Application). Direct analysis will not be performed for LOAD COMBINATIONS cases.
Notional loads must be defined using a DEFINE NOTIONAL table.
A list of members which will have their initial Tau-b value set and/or have their Axial stiffness reduced and/or their flexural stiffness reduced must be entered using a DEFINE DIRECT table.
For information on NOTIONAL LOADS, see sections TR.31.7 Definition of Direct Analysis Members and TR.32.13 Notional Loads.
PDELTA iterative load adjustments are computed for frame members only. They are not calculated for plate or solid elements. The results are based on P-large&small Delta effects (refer to Option 1 in TR.37.2 P-Delta Analysis Options).
Convergence occurs when 2 consecutive iterations have all member tau-b values the same within a tolerance, TAUTOL, and displacements & rotations the same within a tolerance, DISPTOL.
LRFD is the default (all generated loads are factored by 1.0). If ASD entered then loads are factored by 1.6 for the Pdelta and Tau-b calculations. ASD final results are based on the final displacements divided by 1.6
If resulting displacements are diverging, then the P-Delta iterations will be terminated and the current iteration results will be used as the final results for that load case.
| Parameter Name | Default Value | Description |
|---|---|---|
| TAUTOL f1 | 0.01 | Tau-b tolerance f1 is normally 0.001 to 1.0. |
| DISPTOL f2 |
0.01 inch (displacement) 0.01 radians (rotation) |
Displacement tolerance f2 should not be too tight. The value is in current length units. |
| ITERDIRECT i3 | 1 | Limits the number of iterations. A value for i3 between 1 to 10 is typically sufficient. |
| REDUCEDEI i4 | 1 |
Integer, i4, specifies whether to use the reduced EI (Tau-b × 0.8 × EI) for member section moment and section displacement.
|
| PDiter i5 | 15 | The number of iterations, i5, used in the iterative PDelta with SmallDelta analysis procedure within Direct Analysis; 5 to 25 iterations is the normal range. The default is recommended. |
| TBITER | - | If this command is present, then the analysis procedure will iterate Tau-b. |