In construction concrete is almost never used without steel reinforcement
this is due to the cracking of concrete and its inability to transfer tension loads but what's the
purpose of the transverse reinforcements also known as stirrups in this video we go deeper
into the load carrying mechanics and the actual reason why stirrups are needed in concrete beams
The main purpose of a beam is to transfer loads to the supports to enable open spaces. Beams are some
of the most common structural members that appear everywhere. In this video we limit our focus only
to concrete beams under gravity loads. Though a lot of the concepts are generic and applicable to all
types of beams. Since beams are generally always subjected to bending, as explained in one of our
older videos, bending a member causes stretching of the material on one side and crushing on
the other side. The amount of stretching and crushing that the material feels is dependent
on the length size and material of the beam but of course the magnitude of the load as well
the length is the most significant parameter which makes longer beams more susceptible to bending
as the beam continues to be bent, cracks start to appear at the bottom side
which is being stretched. Once a crack develops the concrete loses its ability to carry tension loads
and therefore the tension has to be taken by the steel reinforcement
besides stretching and compressing the material also feels shearing forces. These forces attempt to
share of the beam as shown here. For simple beams sharing forces are the highest near the supports
due to the high forces exerted from the support while zero at the mid span of the beam
these forces are very destructive and usually cause sudden and brittle failure of the beam
The bending and shear load are the most important design considerations for a simple beam loaded
concentrically. These forces guide the design of the beam in terms of size amount of steel
and length. But how does a concrete beam resist these forces and why is the steel reinforcement
even necessary. The answer lies in the nature of concrete as a material. Concrete is strong
in resisting compression forces which makes it ideal for columns or walls, however concrete is
incredibly weak in resisting tension forces. In fact, in design engineers generally assume tension
strength to be zero. This is because in structural analysis in design concrete is assumed to be
cracked which prevents transferring of tensile forces. In reality even cracked concrete has
some small tension resistance however using this strength is risky and unreliable. For that reason
engineers are forced to reinforce the section of the member that is expected to experience
tension. For simple beams as we saw earlier this is always the bottom side but for some more complex
arrangements steel reinforcement could be required on both sides of the beam, if tension forces are to
be expected. One such example is a cantilever beam that has the top material in tension or stretching
and therefore has the top part of the beam reinforced. But if longitudinal rebar takes tension
on the bottom and concrete takes compression on the top then what is the purpose of the stirrups.
Many people in construction believe that the stirrups are only there to hold the longitudinal
reinforcement in place. Though that is true, it is not the main reason for their use similar to the
longitudinal reinforcement, the stirrups are also there to resist stresses which the concrete alone
is not great at resisting. These stresses originate from the shear forces mentioned earlier.
In fact, stirrups are also often referred to as shear reinforcement due to their purpose in
resisting shear forces. Stirrups get engaged when an inclined crack occurs in the beam these cracks
are often referred to as shear cracks however they are actually caused by tension forces as
we will see later. The cracks are microscopic and most of the time are invisible to the naked eye.
However, their presence is crucial in the design of concrete structures. Once the concrete is cracked,
no tensile forces are transmitted across the crack the only force that keeps the two sides
of the crack from sliding past one another is the interlocking of the concrete aggregates
the interlocking force can be very strong and can provide between 33 to 50 percent of the
shear resistance of a beam. However, as the crack widens the interlocking is significantly weakened
and the resistance drops which allows the two sides of the crack to slide past one another.
The sliding in granular materials causes motion perpendicular to the direction of the
applied force. This is the critical point where the stirrups get activated and prevent full separation
and sliding of the concrete. The stirrups prevent the sliding motion and begin to carry the shear
force that used to be carried by interlocking. Interlocking tends to lose its resistance
due to the widening of the crack and therefore it is unreliable and is ignored in the calculation of
the shear capacity of the beam. In fact, the only concrete shear resistance for regular
cast in place beams comes from the uncracked concrete on the compression side. The intrinsic
concrete shear stress is often insufficient for a safe design. Based on the applied shear forces and
the width of the inclined crack, engineers can calculate the amount of reinforcement needed
for higher shear loads near the supports the, stirrups are placed closely which ensures that
several of them will be engaged within an inclined crack. Near the middle of the beam where the shear
forces are low, designers tend to reduce the spacing or completely eliminate the stirrups.
Earlier we saw that the shearing forces act perpendicular to the beam and yet the crack
tends to follow a curved trajectory. Why is this the case? Why doesn't a beam crack vertically down
across its entire depth? To answer this question, we need to get slightly more technical and dive into
complex stress states caused by combined loading. When a beam is bent, its cross section is subjected
to normal and shear stresses. The distribution of each varies along the cross section as shown here
To be able to predict the propagation of the crack, we need to be able to determine in which direction
the beam is experiencing the highest tension loads since it is always the tension loads that actually
crack the concrete. The direction in which the highest tension loads act is called the principal
direction. This is the direction in which the material does not experience any shear stresses
but instead, all of the energy is stored in the normal stresses. This direction is different along
the depth of the beam and is shown here. We won't go deeper into how to calculate this direction,
if you're interested we suggest looking up topics such as "stress transformation" and "Mohr circle".
Since this is where the highest tension stresses act the crack tends to propagate in this direction
at the bottom of the beam there are no shear stresses and therefore the principal direction
also known as direction of highest normal stresses is already aligned along the length of the beam
which explains why the cracks start out vertical at the bottom of the beam. However, as the crack
propagates towards the inner part of the beam, the shear stresses experienced by the material
are non-zero which means that this is no longer the direction with the highest normal stresses
The principal direction is now slightly inclined and therefore the crack tends to also propagate
along the inclined path. It's because of these stress states that cracks follow
a curved trajectory occurring near the ends of a simple beam. Consequently, that's why the sure
reinforcement is placed in those regions. We hope this gave you a good introduction into the concept
and application of sheer reinforcing. Thanks for watching another video of the Engineering Hub,
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