Combined Shear and Torsion - Compression field method

[kww2008]

In earlier versions (pre 2005) of AASHTO LFRD Bridge Design Specifications, torsion was included in the determination of beta and theta in the "resistance" equations for epsilon_s. In the most recent Bridge Specs (2012), torsion was not included in the calculation of epsilon_s (Eqs 5.8.3.4.2-1, 5.8.3.4.2-2 and 5.8.3.4.2-3). However, in the current Canadian Standard, also using compression field method, torsion was included in similar equations.

Why are these two standards, both using the same method, treating combined shear and torsion differently ?

 

[VoyageofDiscovery]

The LRFD 2012 and CSA S6-06 (Oct 2011) approach looks consistent with epsilon referring to longitudinal strain. I can't speak for pre-2005 LRFD.

Both Canadian and US have sections on combined Shear and Torsion as well as you probably know.

HTH

VoD

 

[kww2008]

The commentary of LFRD 2012 seems to suggest that the shear contribution from torsion is not to be considered in the calculation of the longitudinal strain. In CSA S6-06, this shear contribution is included in calculating longitudinal strain. If my interpretation of the LFRD commentary is correct, then these two codes are not consistent in the calculation of the longitudinal strain.

 

[VoyageofDiscovery]

Have a look at this paper: ASSESSMENT OF THE NEW AASHTO DESIGN PROVISIONS FOR SHEAR AND COMBINED SHEAR/TORSION AND COMPARISON WITH THE EQUIVALENT ACI PROVISIONS

krex.k-state.edu/dspace/handle/2097/7011

CSA S6 Cl.8.9.3.19 formula appears on page 24.

VoD

 

[kww2008]

Thank you VoD. I have read this paper before your posting.

In Canadian CHDBC 2006, Clause 8.9.3.19 requires the shear contribution to be included in the calculation of the longitudinal strain. In AASHTO 2012 (6th Edn), the commentary on page 5-59 states "A stress limit for principal tension at the neutral axis in the web was added in 2004. This check requires shear demand, and not the resistance, to be modified for torsion". This seems to suggest that the shear contribution from torsion is not to be considered in the calculation of the longitudinal strain in AASHTO 2012. I have also come across an example from an American road authority for a bridge related structure which show shear contribution not included in the calculation of the longitudinal strain.

The key question is:
If using AASHTO 2012 for combined shear and torsion, should the shear contribution be included in the calculation of the longitudinal strain ?