Page 16 - June-2022

TECHNICAL PAPER

strengths of test specimens. The predicted shear strengths were shear load-transverse strain plots. The magnitude of strain
computed considering the actual measured material properties in the transverse steel reinforcement was zero in the early
and without using the partial safety factors (or strength- stages of loading in all test specimens. This showed that the
reduction factors). shear resistance of beams was primarily due to the concrete

The measured shear strengths of specimens were found to be contribution only. The shear load corresponding to the initiation
2.6-3.0 times the shear strengths predicted by ACI 318-19 and of strain in the transverse reinforcement was considered as
[3]
IS: 456 (2000) except the specimen L550-ST600. The CSA code the peak shear resistance of concrete. Table 6 summarizes the
[2]
measured values of peak shear forces (V' c ) observed in the test
predicted values were a little more conservative as the V exp / V u
ratio was noted in the range of 3.2-3.5. The V exp / V u ratio was also specimens prior to the initiation of strain in the transverse steel.
computed in the range of 2.5-2.8 using the design equations The measured concrete shear stresses (τ c,exp ) were also compared
proposed by Sahoo [23] . This shows that these design provisions with the nominal concrete shear stresses (τ cn ) computed using
have adequate safety margin against the shear failure of beams the design provisions of the ACI 318-19 , the CSA A23.3-04 [24] ,
[3]
having the a/d ratios of 1.05 and reinforced with high-strength and Sahoo [23] considering the measured material properties.
steel. It is worth mentioning that the lower a/d ratio results in the The concrete shear resistance for Set-I specimens was noted
shear span dominated by the discontinuous regions (D-regions). in the range of 1.67-3.04 MPa with the maximum value of τ c,exp
The presence of D-regions increased the diagonal tension
strengths of RC beams due to contribution of arch actions to observed in the specimen L500-ST600. The value of τ c,exp for
some extent. In the case of Set-II specimens, a relatively smaller the Set-II specimens was noted in the range of 2.05-2.91 MPa.
safety margin was observed in the predicted shear strengths The specimen L550-ST600 exhibited the smallest τ c,exp value of
using the current design codes even though the a/d ratios for 2.05 MPa among all the specimens.
two specimens were nearly same as that of the Set-I specimens.
In these specimens, no diagonal tension failure was observed. On comparing the nominal shear resistance of the Set-I
Since the failure of specimen L550-ST600 was primarily flexure, specimens, the ACI code predicted the maximum value of τ cn
the shear strength computed may not represent the true shear as 1.02 MPa; whereas, the CSA code provides the minimum
capacity of the beam. Though this beam was designed to fail value of τ cn equal to 0.4 MPa. The equations proposed by Sahoo
in shear like other specimens, very conservative value of design [23] resulted in the concrete shear strength equal to 80 % of the
shear strengths recommended by IS: 456 (2000) for high- ACI code predictions. The measured concrete shear stresses
[2]
strength concrete combined with the high-yield strength steel were 1.63-2.97 times the value predicted using the ACI code
(600 MPa) of steel stirrups resulted in the shear strength higher provisions. The maximum τ c,exp / τ cn value was computed as high
than the flexural strength of this specimen. Nevertheless, the as 8.9 using the CSA code provisions. For the Set-II specimens,
observed shear strength of this specimen was in the range of the τ c,exp / τ cn ratios for the ACI code prediction were in the range
1.7-1.8 times the predicted shear strengths (Table 4). In general, 1.63-2.38 and the corresponding range for the CSA code
the current code provisions for the shear design of RC beams of prediction was 3.17-5.64. Thus, the shear design provisions have
low a/d ratios were found to be very conservative.
sufficient safety margin in the design concrete shear stress to
avoid the brittle shear failures of the RC beams of low a/d ratios.
5.4 Contribution of concrete to shear strength
However, the level of safety margin was found to be higher in
The shear contribution of concrete in test specimens was the shear strength contribution of transverse steel reinforcement
conservatively measured using the results shown in the than that of the concrete.

Table 6: Comparison of measured concrete shear resistance with the code predictions

SET SPECIMEN V ' c kN τ c,exp , MPa ACI 318 [3] Sahoo [23] CSA A23.3 [24]
τ cn , MPa τ c,exp τ cn , MPa τ c,exp τ cn , MPa τ c,exp
τ cn τ cn τ cn
L500-ST500 97.5 1.67 1.02 1.63 0.83 1.95 0.40 4.89
Set-I L500-ST550 150.0 2.57 1.02 2.51 0.83 3.01 0.40 7.52

L500-ST600 177.5 3.04 1.02 2.97 0.83 3.56 0.40 8.90
L550-ST500 170.0 2.91 1.22 2.38 0.94 2.53 0.45 5.64
Set-II L550-ST550 170.0 2.91 1.22 2.38 0.94 2.53 0.46 5.55

L550-ST600 120.0 2.05 1.26 1.63 0.96 1.69 0.53 3.17
Notes: V ' c = Shear force at strain initiation in stirrups; τ cn = Nominal concrete shear stress; τ c,exp = Measured concrete shear stress.

26 THE INDIAN CONCRETE JOURNAL | JUNE 2022