Modified Strut and Tie Method and Truss Reinforcement for Shear Strengthening of Reinforced Concrete Deep Beams
Published at : 25 Jan 2021
Volume : IJtech
Vol 12, No 1 (2021)
DOI : https://doi.org/10.14716/ijtech.v12i1.3920
Harsha, G.S., Poluraju, P., 2021. Modified Strut & Tie Method and Truss Reinforcement for Shear Strengthening of Reinforced Concrete Deep Beams. International Journal of Technology. Volume 12(1), pp. 65-77
| Gangisetty Sri Harsha | Research Scholar, Department of Civil Engineering, Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP522502, India |
| Palleboina Poluraju | Sr. Assistant Professor, Department of Civil Engineering, VR Siddhartha Engineering College, Vijayawada, AP520007, India |
The
aim of this study was to evaluate and test the limitations of the Indian
Standard code 456-2000 related to deep beams, given that the code does not have
any provisions regarding the use of the strut and tie method (STM)of design.
This study validates the use of truss reinforcement and STM-shaped
reinforcements as alternatives to STM design. We conclude that horizontal web
reinforcement has a greater impact than vertical shear reinforcement. Deep
beams with truss reinforcement and STM-based reinforcement were shown to have
the highest shear strength capacity of all the deep beams. In the present
study, 21 deep beams were cast and used to analyze their shear and flexural
behavior. The specimens were divided into four groups based on length, width
and depth, percentage of tension reinforcement, and percentage of horizontal
and vertical shear reinforcement. The results revealed that truss-type
reinforcement configuration is stronger than vertical shear reinforcement, as
the former can resist 20% more load than the latter.
Deep beams; Shear reinforcement; Shear span depth ratio; Shear strength; Web reinforcement
Indian Standard (I.S) (2000)
code 456-2000 defines a deep beam as a beam whose length to depth ratio is less
than 2.0 for simply supported beams and less than 2.5 for continuous beams.
Deep beams are used in a variety of engineering applications, such as bridges
covering long spans, open rooms or halls of a building with no intermediate
column, a side wall in reinforced concrete water tanks, foundation pile caps,
transfer girders used to transfer loads safely if the soil bearing capacity is
inadequate, and in bunkers and silos used to store toxic materials. Deep beams
have two-dimensional action, unlike normal beams where the assumption plane
section continues to be plane after bending is not applicable here. The
deformation behavior of the normal beam is similar to that of the deep beam,
except that shear plays a key role in the deep beam (Niken
et al., 2017).
The factors that decrease catastrophic behavior and deflection are shear span to depth ratio, effective length to depth ratio, concrete compressive strength and vertical and horizontal reinforcement, effective beam depth, web reinforcement, type of loading, support conditions, and crack pattern. An increase in concrete strength leads to a significant increase in the fracture energy value (Siregar, 2017). When the deflection of the beam is less than 2.5 times the thickness, the experimental and analytical curves are about 1% using the average global stresses method (Benbouras et al., 2017). In 1909, (Talbot, 1909) declared that shear stress is a characteristic of the length, longitudinal reinforcement, and stiffness of a beam. He investigated on beams without web reinforcement and found that the shear strength of the concrete should also be considered in the design.
Many theories have been developed to describe shear
behavior and the shear capability of beams. Most experimental and field
research supports the theoretical studies predicts that a change in applied
moment along a beam’s length causes shear force. Studies have also found shear
span to depth ratio to have a significant influence on deep beams, with an
effective span to depth ratio having a qualitative influence on the failure
mode, and diagonal cracking strength was found to have a marginal influence on
deep beams(Tan et al., 1996). Aguilar et al. (2002)
examined four RC deep beams under monotonic loading to study their behavior and
strength. Specimens were designed in accordance with the American Concrete
Institute (ACI, 2014) 318-99 code clauses
10.7 and 11.8 and the strut and tie method (STM) of the ACI 318-02 code, as
shown in Appendix A. The load-carrying mechanism at failure in beams using STM
is intended to minimize the vertical and horizontal reinforcement, however, the
ACI 318-99 code’s reinforcement provision no longer replicates the behavior. The
percentage loss of flexural capacity of a beam without compressive pressure was
found to be higher than that of a beam with compressive pressure when different
bending moments were considered (Antonius et al.,
2019). Here, the test load obtained was twice that was calculated,
leading to the conclusion that the current ACI code must be improved. For a
shear span of 1.0 to 2.5, the shear failure of a deep beam beneath single-point
or two-point loading is due to the crushing of concrete in a compression zone (Zararis, 2003). Arabzadeh et al. (2011) investigated RC deep
beams using two independent resistance measurements according to ACI 318-05 and Canadian Standard Association (CSA). These
codes appear to be the most accurate and both have low variant and standard
deviation. The study found the angle of strut is inversely proportional to the
shear span to depth ratio and directly proportional to the horizontal web
reinforcement. An exact analysis of concrete deep beams is a complicated
problem, and a numerical method of analysis is required to predict the shear
strength(Enem et al., 2012). Khan and Ahmed (2013)
conducted an experimental evaluation to discover the ultimate shear strength of
deep beams using the STM in accordance with ACI 318-05. The study found that
Web reinforcement can be used to determine the shear strength of a deep beam.
To identify the contribution of the steel to a beam’s failure, a softening
coefficient is introduced. Using this method, several conclusions have been
drawn, such as an increase in the vertical reinforcement increases the shear
strength, ultimate shear stress will increase with an increase in the
horizontal shear reinforcement, and the softening coefficient of the concrete
decreases with an increase in compressive strength.
Reineck and Todisco (2014)performed shear tests on
non-slender beams without stirrups under point load and concluded that STMs are
appropriate for RC deep beams at low slenderness ratios. With the increase of
shear span, Swami et al. (2015) found a
significant reduction in the preliminary cracking and failure, whereas the
crack width increased with the increase of load.
Faroque and Kumar (2015) investigated deep beams
using the Construction Industry Research and Information Association code, the
ACI code, and the IS code. The beams were designed in three lengths, from 4.5 m
to 5.5m. The results concluded that IS code was found to offer the maximum
reinforcement for all loading conditions and sizes.
Mihaylov et al. (2015)
validated the results of 129 published tests of continuous deep beams using the
three-degrees-of-freedom kinematic model
and local and global deformations with a number of settlements. Their
results were comparable to those using the nonlinear finite element model with
thousands of degrees of freedom. de DiosGaray-Moran and Lubell (2016)
tested the failure of eight large-scale specimens longitudinally reinforced
with deformed A1035 steel bars under stress ranging from 695 MPa to 988 MPa.
After the formation of diagonal cracks, the bars without web reinforcement
failed in a brittle manner. Bars containing effective shear span to depth
ratio, longitudinal reinforcement, and web reinforcement were determined to
have more influence on the failure mode. Failure in ductility was more likely
to occur with an increase in shear span to depth ratio and a decrease in
longitudinal reinforcement.
Yavuz (2016) took
a different approach to investigating STM by calculating the shear strength of
an RC deep beam with artificial neural networks (ANNs). Using different
parameters affecting shear taken from experimental statistics and the
literature database, they concluded that the ANN approach is better for
predicting shear strength when compared to STM. Ismail et al. (2017)
performed an experimental analysis of 24 beams to determine the parameters
affecting their shear capacity, such as shear span to depth ratio, web
reinforcement ratio, effective beam depth, and compressive strength of
concrete, using various international codes, such as ACI 318-14, American
Association of State Highway Transport Officials using Load and Resistant
Factor Design (AASHTO LRFD), Euro Code 2 (EC2), and Model Code 2010. They
concluded that the compressive strength and shear span to depth ratio have more
influence than other stress parameters on the shear strength of deep beams. The
major failure in deep beams is diagonal cracking failure, and the crack
increases with the increase in span to depth ratio. The remaining portions,
i.e., uncracked depths, resist the shear stress. The presence of shear
reinforcement in the middle region of the shear span will improve the strength
of a deep beam (Harsha and Poluraju,
2019). Beams created according to
the ACI code have been found to be satisfactory for normal concrete but not for
high-strength concrete, and those created according to AASHTO LRFD are less
efficient because of the less shear span to depth ratio and those created
following the EC2 and the Model Code 2010 are stable overall, but the stability
decreases as the concrete strength increases. Many studies have concluded that
STM is the best method, as the struts are placed in the path of the shear crack
propagation such that the shear effect will be reduced in that region. Since,
I.S. has no provisions related to the strut-and-tie design of deep beams, the
present study uses vertical web reinforcement area obtained in a normal deep
beam design arranged in the form of truss reinforcement to compare it with the
regular arrangement of reinforcement.
Horizontal
web reinforcement had more impact than vertical shear reinforcement in all the
specimens, irrespective of size. After the observation of all failures, it was concluded
that 0.5% horizontal web reinforcement is the optimum percentage in deep beams.
The deflection of the beams increased with an increase in shear reinforcement,
but load-carrying capacity also increased. The width of the cracks in the beam
decreased with an increase in the percentage of shear reinforcement. Deep beams
with truss reinforcement and STM-based reinforcement were shown to produce the
highest shear strength capacity in deep beams with normal web reinforcement.
Finally, truss-type reinforcement configuration was found to be more effective
than vertical shear reinforcement, as it was able to endure 20% more load.
I would like to acknowledge Dr. P. Poluraju, my
research supervisor, for helping in this investigation. I am also grateful to
the Koneru Lakshmaiah Education Foundation (KL Deemed to University) for their
continuous support and assistance.
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