Page 14 - June-2022

TECHNICAL PAPER

bars in the shear span regions as well as the longitudinal bars 5. DISCUSSION
at the mid-spans of test specimens. Shear load represents the
force demand in the shear span regions which is taken as one- The influence of the type of steel and the spacing of stirrups on
half of the total load. The yielding strain limits of the longitudinal the shear behavior of the test specimens has been discussed.
and transverse reinforcement bars are also shown in the figure The shear strength contribution of concrete from the test
along with the corresponding design and service load levels results was computed from the measured shear force just
for the specimens. The strain in stirrups in the shear zones was before the initiation of the diagonal shear crack using the shear
observed to increase in all specimens only after the formation load–transverse strain plots. The observed shear strengths
of inclined shear cracks. However, these stirrups reached their are compared with those predicted using the current code
yielding limits prior to the failure of specimens. The inclined provisions and other available shear design equations.
shear cracking in the specimen L500-ST500 was observed at a
shear force of 90 kN, whereas the yielding of stirrups was noted 5.1 Effect of steel grade
at a shear load of 450 kN. The specimen L500-ST550 showed
the first inclined shear crack at a shear force of 120 kN and Table 2 summarizes the observed shear strengths of specimens
the yielding of the stirrups in the shear span region at a shear with design strengths predicted using different building codes.
force of 520 kN. Similarly, the first shear crack in the specimen The effect of the type of steel reinforcement on the shear
L500-ST600 was observed at a shear force of 180 kN and the strength is compared with the observed values of test specimens
yielding of stirrups was noted at a shear force of 520 kN. The in Table 2. For the Set-I specimens having the same grade of
initiation of shear cracks in specimens L550-ST500 and L550- concrete, longitudinal steel reinforcement, and a/d ratio, the
ST550 was observed at shear loads of 170 kN and 180 kN, increase in the yield strength of transverse steel increased
respectively. The yielding of stirrups in these specimens was the shear strength of beams. The specimen L550-ST500 of
noted at corresponding shear forces of 520 kN and 460 kN. the Set-II category exhibited the higher shear strength for the
Similarly, the straining of stirrups in the specimen L550-ST600 same value of a/d ratio but with a higher concrete grade. The
was observed at a shear load of 115 kN and the stirrups reached measured shear strength values were normalized with respect
′
their yielding limit at a shear load of 300 kN. This showed that to the square root of the concrete compressive strength ( c f bd )
the increase in yield strength resulted in the delayed yielding of to eliminate the influence of grades of concrete as summarized
′
transverse stirrups of specimens of the same grade of concrete in Table 4. The normalized shear strengths (V exp c f bd ) of the
and a/d ratios. Set-I specimens were noted in the range 1.53-1.58. Test results
showed a small increase in the shear carrying capacity due to
As shown in Figure 9, the yielding of longitudinal bars in the the use of high-yield strength stirrups. The measured normalized
shear spans was observed in some test specimens. In the case shear strength of specimens was higher than the maximum
of specimens L500-ST500 and L500-ST500, longitudinal bars in shear strength limit of 0.83 f bd as per the current ACI code .
[3]
0.85
′
c
the shear span, where the shear failure was observed, reached This shows that the presence of discontinuous of regions in low-
yielding strain at nearly the same shear load levels. However, shear span beams increased their diagonal tension strengths.
the longitudinal bars in the other shear span reached their A reduction in the normalized shear strength was observed
yielding strain limits earlier than those in the failure shear span. in the Set-II specimens as the failure of these specimens was
The yielding of all longitudinal bars was observed in both shear flexure-dominated. The use of both high-strength concrete and
spans of the specimen L500-ST600. Similar behavior was also high-strength longitudinal steel reinforcement resulted in an
observed in specimen L550-ST550. However, the yielding of increased shear resistance of concrete beams.
longitudinal bars was not observed in the specimen L550-ST500.
The state of strain in the longitudinal bars at the mid-span of 5.2 Angle of diagonal shear crack
the test specimens at different shear load levels is also shown
in Figure 9. It is worth mentioning that the longitudinal steel Table 4 also summarizes the measured angles of inclination (θ)
in the shear span region showed much higher strain demand of diagonal shear cracks observed in the test specimens. The
as compared to those at the mid-spans before their complete values of θ in the Set-I specimens were varied in the range of
o
o
collapse. The shear load-strain response of the Set-II specimens 44.1 -54.0 . For the same grade of concrete and a/d ratio, the
was though nearly similar to that of the Set-I specimens, the angle of diagonal shear crack was reduced as the yield strength
magnitude of strain in the longitudinal steel at the mid-span was of transverse steel reinforcing was increased. For the Set-II
o
o
significantly higher than that of the stirrups in the shear span. It specimens, the value of θ was varied in the range of 34.0 -48.0 .
can be concluded that the yield strength limit for the transverse The specimen L550-ST600 having the higher a/d ratio exhibited a
o
steel reinforcement in the current codes may be increased smaller angle of the diagonal shear crack of 34.0 . For the higher
to 600 MPa in the shear design of beams. Concrete beams concrete compressive strength of concrete and high-strength
reinforced with high-strength longitudinal steel and different longitudinal steel, a smaller angle of diagonal shear crack was
concrete satisfied both strength and serviceability criteria. noted in the test specimens designed in accordance with the

24 THE INDIAN CONCRETE JOURNAL | JUNE 2022