Serviceability limit state and splicing types

Limit state of serviceability

It is the state beyond which the structure, although safe against collapse, is no longer fit for normal use. It is checked at service loads.

Requirements (IS 456:2000):

• Deflection: total deflection $\le$ span/250; after partitions and finishes $\le$ span/350 or 20 mm, whichever is less. This is ensured by limiting the span/effective depth ratio (20 for simply supported, 26 for continuous, 7 for cantilever).
• Cracking: crack width generally not exceeding 0.3 mm (0.2 mm where members are exposed to moisture or aggressive conditions); controlled by bar spacing and minimum steel.
• Durability: proper cover, cement content and water-cement ratio for the exposure.
• Vibration, fire resistance, fatigue where relevant.

Types of splicing of reinforcement

Splicing joins two bars when the available bar length is not enough. The types are:

1. Lap splice: the bars overlap. IS 456 cl. 26.2.5.1: lap length $\ge L_d$ or $30\phi$ (whichever is greater) in flexural tension, $2L_d$ or $30\phi$ in direct tension, and $L_d$ (compression) or $24\phi$ in compression; the straight length of the lap is at least $15\phi$ or 200 mm. Laps are staggered and kept away from points of maximum stress.
2. Welded splice: bars are joined by butt welding (flash butt or arc); used for bars of weldable grade.
3. Mechanical splice: threaded couplers, swaged sleeves or cold-forged couplers; suitable for large-diameter bars and seismic zones.
4. End bearing splice: the bar ends are cut square and held in contact by a sleeve (compression only).