Causes of Lifting Edges in Two-Way Slabs

What causes the lifting of the edges in a two-way slab?

A. Resultant shear force at the ends
B. Torsional moments on the slab
C. Unbalanced moments on the slab
D. Heavy shear load at the mid span

Answer:

The lifting of the edges of a two-way slab occurs due to unbalanced moments on the slab (C).

The behavior of a two-way slab is influenced by the way loads are distributed and transferred through the structure. When a load is applied to a slab, it induces bending moments and shear forces. In the case of a two-way slab, these moments and forces interact in both the longitudinal and transverse directions.

Unbalanced moments arise when the applied loads are not symmetrically distributed across the slab. This can occur due to variations in the magnitude of loads, support conditions, or irregularities in the structure. As the slab undergoes deflection, these unbalanced moments lead to the development of upward forces along the edges of the slab, causing the edges to lift.

In contrast, options A and B have less direct influence on the lifting of the slab edges. Option A, the resultant shear force at the ends, primarily affects the shear capacity of the slab and its distribution. Option B, torsional moments on the slab, refers to twisting effects that usually occur in non-rectangular or irregularly loaded slabs, but it is not the primary cause of edge lifting. Option D is not a relevant factor in causing edge lifting in a two-way slab.

In conclusion, unbalanced moments resulting from load distribution asymmetry play a significant role in causing the edges of a two-way slab to lift. This phenomenon underscores the importance of properly designing and analyzing the structural elements to ensure the safe and efficient performance of the slab under different loading conditions.