Consider the following statements in the light of IS : 456 – 2000:
1. There is an upper limit on the nominal shear stress in beams (even with shear reinforcement) due to the possibility of crushing of concrete in diagonal compression.
2. A rectangular concrete slab whose length is equal to its width may not be a two-way slab for certain definable support conditions.
Which of the above statements is/are correct?Concept:
Recent laboratory experiments confirmed that reinforced concrete in beams has shear strength even without any shear reinforcement. This shear strength (τ_{c}) depends on the grade of concrete and the percentage of tension steel in beams and this is known as the permissible shear stress in concrete.
On the other hand, the shear strength of reinforced concrete with the reinforcement is restricted to some maximum value τ_{cmax} depending on the grade of concrete.
Table 20 of IS 456 stipulates the maximum shear stress of reinforced concrete in beams τ_{cmax} as given below in Table. Under no circumstances, the nominal shear stress in beams τ_{v} shall exceed τ_{cmax }given in table for different grades of concrete.
Grade of concrete |
M20 |
M25 |
M30 |
M35 |
M40 and above |
τ_{cmax} (in MPa) |
2.8 |
3.1 |
3.5 |
3.7 |
4.0 |
When the shear stress in concrete exceeds these values it leads to brittle failure due to diagonal compression.
When, larger span /shorter span < 2, that is classified as two way slab
When, larger span /shorter span > 2, that is classified as one way slab. But if the support condition is like as in the figure shown below, then that is also a one way slab irrespective of the span ratio. when the slab is supported on two ends, it is a one way slab.
So, a rectangular concrete slab whose length is equal to its width may not be a two-way slab for certain definable support conditions.
Here, both the statements are correct.