D5.C.5.4 Members Subject to Torsion

Note: This feature requires STAAD.Pro V8i (SELECTseries 2) build 2007.07 or later.

General

Eurocode 3 (EN 1993-1-1:2005) gives very limited guidance for the analysis and design of torsion members. While both elastic and plastic analyses are permitted generally, the design analysis methods for torsion discussed within EC3 are primarily based on elastic methods. Also, only the first yield design resistance is specifically discussed for torsion members. Furthermore, there is no guidance on section classification nor on how to allow for the effects of local buckling on the design resistance for combined torsional effects. EC3 also does not specifically deal with members subject to combined bending and torsion and loosely states that the yield criteria (Eqn 6.1 in the code) can be used for elastic verification.

The method used by STAAD.Pro is therefore based on the SCI publication P057: Design of members subject to combined bending and torsion. Though this publication is based on the British standard BS 5950-1, the principles from this document are applied in the context of Eurocode 3.

Note: At the time this feature has been implemented in STAAD.Pro, SCI are in the process of updating document P057 to be in accordance with Eurocode 3. Hence this method might be subject to modifications subject to the publication of a newer version of P057. The NCCI document SN007b-EN-EU: Torsion will also be referenced where appropriate.

Code Basis

Torsion design in EC3 is given in Cl. 6.2.7 of EN 1993-1-1:2005. Therefore, this clause is used primarily for this implementation.

EN 1993-1-1:2005 does not deal with members subject to the combined effects of torsion and lateral torsional buckling. However, EN 1993-1-6 considers such a condition in Appendix A. Therefore, STAAD.pro uses Appendix A of EN 1993-1-6 to check for members subject to combined torsion and LTB.

The following clauses from EC3 are then considered:

Cl. 6.2.7(1)
Cl. 6.2.7(9)
Cl. 6.2.7(5)
EC-3 -6 App A

Note: STAAD.Pro does, however, use this clause (6.2.7) to report the output for all torsion checks. Also any distortional deformations and any amplification in the torsional or shear stresses due to distortions will be neglected by the program.

Clause 6.2.7(1)

States that for members subject to torsion, the design torsional moment T_Ed at each cross section should satisfy:

T_Ed / R_Rd ≤ 1.0

Where:
- T_Rd is the design torsional resistance of the cross section.
This is the primary condition that will need to be satisfied for members subject to torsion. The method for working out the torsional resistance T_Rd, for the various cases is dealt in the following sections.
Cl. 6.2.7(9)

States that:

For combined shear force and torsional moment, the plastic shear resistance accounting for torsional effects should be reduced from V_pl,Rd to V_pl,T,Rd and the design shear force should satisfy:

V_Ed / V_pl,T,Rd ≤ 1.0

The code also gives means to evaluate V_pl,T,Rd in equations 6.26 to 6.28. These equations, however, only deal with I/H sections, Channel sections, and structural hollow sections (RHS, SHS, CHS). Therefore, the application of Cl. 6.2.7(9) is only performed for these section profiles.
Cl 6.2.7(5)

States that the yield criteria given in Cl. 6.2.1(5) of EN 1993-1-1:2005 may be used for elastic verification. STAAD.Pro evaluates the stresses due to the various actions on the cross section and applies this yield criterion.

The program allows for two types of checks for members subject to torsion for EC3 design:

Basic Stress Check: This method is intended to be a simplified stress check for torsional effects. This method will produce the output corresponding to Cl. 6.2.7(5) of EN 1993-1-1.
Detailed Checks: This method will perform a full torsional analysis of the member. All four of the clause checks mentioned earlier will be performed.

The details of these checks are as described below.

You have the option to choose the method to be used for a specific member or group of members. This will be facilitated by setting the value of the TORSION. The TORSION parameter set to zero by default, which results in torsion checks only being performed if the member is subject to torsional moments (i.e., for this default setting, the program will ignore torsion checks if there is no torsional moment in the member). Setting the value of the TORSION parameter to three (3) will cause the program to ignore all torsional moments. The detailed output (i.e., TRACK 2) will indicate that torsion has been ignored for that particular member. The details of setting the values to one (1) or two (2) and the corresponding checks performed are as described below. Refer to D5.C.6 Design Parameters for additional details.

Note: If the TORSION parameter is set to 1 or 2, the program will perform the appropriate checks even if the member is not subject to torsional moments. In such cases, the program will perform the checks with a value of zero for the torsional moment.