PERMISSIBLE STRESSES (CLAUSE B-2, IS456:2000),Permissible Stresses in Concrete ( IS 456),Permissible Stress in Steel Reinforcement ( IS 456)
PERMISSIBLE STRESSES (CLAUSE B-2, IS456:2000)
The working stress method is based upon the concept of permissible stresses. Permissible stresses are obtained by dividing the ultimate strength of concrete or yield strength of steel (0.2% proof stress) by appropriate factors of safety. The factors of safety used in working stress method are:
(i) For concrete (a) in bending compression – 3.0
(b) in direct compression – 4.0
(ii) For steel – 1.78
There are greater chances of variation of strength of concrete due to improper compaction, inadequate curing and variation in the properties of concrete. The chances of variation in the properties of steel are less as it is fabricated in factories where good workmanship and better quality control is possible. So, lesser value of factor of safety is used for steel as compared to concrete.
The values of permissible stresses for various grades of concrete are given in Table 2.1 and for steel are given in Table 2.2.
TABLE 2.1.
Permissible Stresses in Concrete (Refer to Table 21, IS 456)
| Grade of Concrete | Permissible Stress in Compression | Permissible Stress in Bond (Average) | ||
| Bending σ cbc (N/mm2) | Direct σcc (N/mm2) | For Plain Bars in Tension τbd (N/mm2) | For HYSD Bars (N/mm2) | |
| – | – | – | – | – |
| M15 | 5.0 | 4.0 | 0.6 | 0.96 |
| M20 | 7.0 | 5.0 | 0.8 | 1.28 |
| M25 | 8.5 | 6.0 | 0.9 | 1.44 |
| M30 | 10.0 | 8.0 | 1.0 | 1.6 |
| M35 | 11.5 | 9.0 | 1.1 | 1.76 |
| M40 | 13.0 | 10.0 | 1.2 | 1.92 |
| M45 | 14.5 | 11.0 | 1.3 | 2.08 |
| M50 | 16.0 | 12.0 | 1.4 | 2.24 |
Notes:
(i) The bond stress given above for tension is increased by 25% for bars in compression.
(ii) The bond stress for plain bars is increased by 60% for deformed bars.
TABLE 2.2.
Permissible Stress in Steel Reinforcement (Refer Table 22, IS 456)
| S. No. | Type of Stress in Steel Reinforcement | Permissible Stresses in N/mm2 | High yield strength deformed bar (HYSD) conforming to IS 1786 (Grade Fe 415) | |
| Mild steel bars conforming to Grade I of IS 432 (Part I) | Medium tensile steel conforming to IS 432 (Part I) | |||
| 1. | Tension (σst or σsv )
(i) Upto and including 20mm (ii) Over 20mm |
140
130 |
Half the guaranted yield stress subject to maximum of 190
190 |
230
230 |
| 2. | Compression in column bars (σsc) | 130 | 130 | 190 |
| 3. | Compression in bars in beam or slab when compressive resistance of concrete is taken into account | The calculated compressive stresses in the surrounding concrete multiplied by 1.5 times the modular ratio or ssc whichever is lower=1.5mc or ssc | ||