In the design of a two-way slab, what is the purpose of torsional reinforcement at the corners?
- ATo resist positive moments
- BTo resist negative moments
- CTo prevent corner liftingCorrect
- DTo increase shear capacity
Solution
Concept
Corner torsional reinforcement in two-way slabs
Formula
NONE
Step-by-step solution
1. In two-way slabs, corners that are prevented from lifting experience severe torsional moments due to restraint. 2. Torsional reinforcement is placed in top and bottom meshes at the exterior corners to resist high local stress concentrations. 3. This reinforcement prevents the corners of the slab from lifting and cracking under applied loads. Answer: C
Explanation
In two-way slabs, the corners tend to lift off their supports due to torsional moments developed near the boundaries. Torsional reinforcement placed at the corners resists these forces and prevents corner lifting and cracking. Thus, option C is correct.
Why the other options are wrong
A) Positive moments at mid-span are resisted by main bottom reinforcement rather than corner torsional steel. B) Negative moments over continuous supports are resisted by top steel across the support edges. D) Shear capacity in slabs is primarily provided by concrete depth and transverse shear reinforcement where needed.
Shortcut
Torsional reinforcement at corners prevents corner lifting in two-way slabs.
Common mistake
Assuming corner torsional reinforcement is intended to resist bending moments or increase shear capacity rather than prevent corner lifting.
Correct answer: C) To prevent corner lifting