Which of the following are the design steps of restrained slabs? 1. Slabs are considered as divided in each direction into middle strips and edge strips 2. The middle strips are one-fourth of the width and edge strips are three-quarters of the width 3. Tension reinforcement provided at mid-span in the middle strip shall extend in lower part of the slab to within 0·25l of continuous edge or 0·15l of a discontinuous edge, where l is the length of the span Select the correct answer.
- A1 and 3 onlyCorrect
- B1 and 2 only
- C2 and 3 only
- D1, 2 and 3
Solution
Concept
Strips and detailing of restrained rectangular slabs
Formula
Middle strip = ¾ of the width; each edge strip = ⅛ of the width
Step-by-step solution
1. The slab is divided into middle and edge strips in each direction (statement 1 correct). 2. The middle strip is three-quarters of the width and each edge strip is one-eighth (so statement 2, which reverses the proportions, is wrong). 3. Bottom steel at mid-span in the middle strip extends to within 0.25 l of a continuous edge or 0.15 l of a discontinuous edge (statement 3 correct, IS 456 D-1.7).
Explanation
IS 456 Annex D treats the edge strips as having less reinforcement because the moments are smaller there. The middle strip takes the large central moments and forms the major part of the width.
Why the other options are wrong
Statement 2 swaps the width of the middle and edge strips, so options containing it are incorrect.
Shortcut
Middle strip ¾, edge strips ⅛ + ⅛.
Common mistake
Taking the middle strip as one-fourth.
Correct answer: A) 1 and 3 only