CROSS Safety Report
Computer analysis and slab design twisting moments
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Overview
A reporter has noticed on many occasions when checking reinforced concrete and post-tensioned slab designs that engineers often neglect to consider twisting moments when arriving at slab design moments.
Key Learning Outcomes
For civil and structural design engineers:
- If the torsional moments are required for equilibrium, they should be designed for
- One way to achieve this is to adopt the Wood-Armer method which is included in the post processing of most common structural software packages
- However, the user must first be aware of twisting moments and secondly switch on this option in the software package to arrive at the correct design moments
Full Report
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A reporter has noticed on many occasions, when checking reinforced concrete and post-tensioned slab designs, that engineers often neglect to consider twisting moments when arriving at slab design moments, derived from computer software analysis. For equilibrium, twisting moments Mxy must be distributed (i.e. summed) to the bending moments in the principal directions: Mx and My.
One way to achieve this is to adopt the Wood-Armer method which is included in the post processing of most common structural software packages. However, the user must first be aware of twisting moments and secondly switch on this option in the software package to arrive at the correct design moments.
Twisting moments often occur when the geometry of a structure is irregular. The magnitude of twisting moments can be significant. They occur in suspended slabs, ground bearing rafts and walls. The following references are given for further reading: ‘Advanced Structural Mechanics’ 2nd Edition by David Johnson, Thomas Telford, and ‘The Assessment of Reinforced Concrete Slabs’ by Denton and Burgoyne, The Structural Engineer, Vol 74, No.9
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Wood and Armer(1) proposed one of the most popular design methods that explicitly incorporate twisting moments in slab design. The method was developed by considering the normal moment yield criterion (Johansen’s yield criterion) aiming to prevent yielding in all directions. The method is incorporated in proprietary slab design packages and can be particularly relevant when skew slabs are being considered.
As the reporter says this option may have to be switched on depending upon the software. Simply adding the Mxy to the Mxx and Myy is a slightly conservative but simpler approach. Even simpler is to set the torsion stiffness very low such that torsion stresses in the chosen ‘equilibrium’ are very small.
Ignoring torsion is however a safety issue. If the torsional moments are required for equilibrium they should be designed for. Often ignoring them makes less than 10% difference but as suggested in the report, there can be cases where it is more significant. Ensuring the user understands the software and is able to validate the output remains one of our profession’s greatest challenges.
1. Wood, R.H., ‘The Reinforcement of Slabs in Accordance with a Pre-Determined Field of Moments,’ Concrete, V. 2, No. 2, 1968, pp. 69-76. (discussion by Armer)