Understanding Beams: Behavior, Failure Modes, Stresses, and Types Used in Construction

The classification of external forces acting on a member can be done as body forces and surface forces. The division shear failure mode can also be divided into various sub-divisions which include; Shear compression stress where the development of diagonal crack into the compressive zone can cause the RC beam to fail, compressive failure or shear proper of struts which are realized in beams having small shear span to depth ratio and diagonal tension failure which entails the thrust line becoming very eccentric giving rise to flexural failure in the compressive zone.

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Introduction
A beam is a bar, processing length significantly greater than the depth and width. The
classification of the beam is done as moderate deep beams, shallow beams, and deep beams.
Beams are usually loaded in a normal direction to the longitudinal axis, while bars axially loaded
or twisted. The load-supporting action of plates resembles and to a certain extent that of the
beam. Deep beams are those type of beams whose depth are comparable to their spans and
sometimes a beam can also mean some structural elements that resist loads applied laterally to
their axis. There is a different variation that is shown by the behavior of deep beam as a result
of the existence of normal forces due to usual bending moments and shear forces. In
construction, the application of deep beams takes place in water tanks, nuclear reactors and
also underground bunkers. They normally transmit loads executed alongside their dimension to
their endpoints where the loads are transferred to walls, foundations, and columns. Beam
maybe;
 Both ends are supported to evade spin.
 Extending more than two supports.
 Supported at both ends to allow free rotation to take place.
In most cases in floor framing, beam is braced literally either by other beams framed into it or
by floor construction itself. A random cross section in a beam when exposed to a transverse
load will cause bending of the beam and there are certain effects arising as a result of bending
and these are buckling as well as twisting.

Conditions of equilibrium
The classification of external forces acting on a member can be done as body forces and surface
forces and the concentration of the surface force takes place when it is acting as a point and it
may be distributed uniformly or non-uniformly over a finite area. All forces acting on a member
are considered external forces but in the case of body forces, it acts slowly and steadily in static
loading.
The behavior of deep beams
When an effective ratio plan (L) to total depth (D) is below 2.0 and that of the continuous beam
is below than 2.5 is when a beam can be considered as a deep beam. An appropriate span is
simply the distance between the middle and the supports or 1.15 times the clear span
whichever is less (Nielsen, 2010, p. 275). The classification of beams can be done based on
Deep beam with or without openings, Continuous Deep Beam and Simply supported Deep
Beam (Hyun-Moo, 2008, p. 216).
The behavior of deep beam differs from the beams with normal sizes, needing special
considerations, designing as well as detailing of reinforcement. Their strength is controlled by
shear because of their proportion. The deep beam shear strength can be two to three times
superior to that predicted using equations. The transfer of load in the deep beam is done
directly from the application are to be supported by diagonal compression strut. The formation
of diagonal strut takes place in a direction parallel to a line from the load to support isolated

compression strut, which seems to be horizontal compression in the concrete and the tension
in the main reinforcement for load equilibration.
The designing and casting of beam take place for two points loading and two shear spans
200mm and 250mm. For design purpose, application of 50kN point load is applied on deep
beams. Shear rather than flexure assist in controlling the strength of deep beams so long as the
normal quantity of longitudinal reinforcement is used (Ng, 2015, p. 412). The compression in
diagonal direction and tension in a perpendicular direction is caused by the action of shear in
the beam web.
Failure Modes
The failure modes of the deep beam can be divided into two major categories;
Mode 1- Flexural failure mode
Mode 2-Shear failure modes
The shear failure mode can also be divided into various sub-divisions which include;
 Shear compression stress where the development of diagonal crack into the
compressive zone causes the RC beam to fail.
 Compressive failure or shear proper of struts which are realized in beams having small
shear span to depth ratio and this will also make the steeping line to be deep.
 Diagonal tension failure which entails the thrust line becoming very eccentric giving rise
to flexural failure in the compressive zone. This kind of failure arises from extension
tensile crack in the compressive region to flexural load (Nielsen, 2010, p. 74).

Internally, when loads exerted on the beam do not induce axially then compressive, tensile and
shear stresses experience normal results from the loads exerted on them. The original length of
the beam reduced under the gravity loads to encircle a minor radius arc at the beam topmost
results into compression while to enclose a larger radius arc, stretching of the beam is slightly
done at the beam bottommost with the similar length of the beam, and so under tension. In
beam, there is sagging which is a mode of deformation whereby the beam upper surface is in
compression as under a vertical load. In the middle, the same original length is similar to the
radial arc of bending. There is a certain reinforced solid beam whereby the tensile forces and
concrete are fully compressed by a tendon made of steel thus they are referred to as pre-
stressed concrete. And under the state of loading, they produce a compression than the tension
targeted when fabricated. (Ng, 2015, p. 1134).
Pure bending in a beam occurs over a span of a set portion and the bending moment is
continuous over that portion. When the bending moment is constant over the portion is when
the beam is said to be in a state of pure bending. On the other hand, when the shear force over
the portion is zero is when a portion of the beam is said to be in a situation of pure bending.
Pure bending is sometimes being known as a loading which produces no shear force within the
beam. The impact the shear stress has on the normal stress distribution is negligible though,
there is the minimal impact the shear stress has on bending of the beam. The shear stress must
vanish at the beam uppermost and bottommost since there are no forces applied in the
horizontal direction (Muttoni, 2012, p. 531).

Stresses in Beams
The stresses caused by the bending moment are known as bending stress, and flexure formula
shows the relationship between the bending moment and stresses. The beam has an axial
plane of symmetry which is taken to be the XY plane and the vertical shear force caused by the
shear stress will alter the section at the plane. The deformation caused by the vertical shear
force a negligible in the beams which are slenderly matched to the deformation instigated by
bending. The neutral surface becomes curved upon deformation.
Procedure for determining bending stress
Stresses at a given point
 Decide the neutral axis is situated.
 Use the method of sections in determining the bending moment M at the cross section
containing the given point.
 Computation of inertia moment of the cross-sectional area.
 Determining the y-coordinate of the given point.
 Computing the bending stress from My/1.
 Drawing the bending moment diagram by one of the methods and identify the
maximum bending moment that has the largest magnitude.
 Computation of the inertia moment of the cross-sectional area about the neutral axis.
The stresses in the beam include;
Bending stress

Here, when a load is experienced on a beam then it experiences a normal compressive stress
thus making it an example of normal stress (Hsu, 2010, p. 87).
Normal stresses
A beam subjected to a positive moment creates a concave-upward curvature. This means that
the material near the top of the beam is positioned in compression along the x-direction with
the lower region in tension, the stress becomes zero at the transition between the compressive
and tensile regions. And this is the beam neutral axis (Ramachandran, 2016, p. 532). The crack
and growth initiation from the lower tensile will be experienced when materials such as glass or
chalk tend to fail in tension. The type of stress mentioned above takes place when a member is
loaded by an axial force and it takes place when a member is placed in compression or tension
(Rots, 2008, p. 76).
Shear stresses
The beams are bent by transverse loads as a result of the introduction of compressive normal
strain and axial tensile beams x-direction. They cause shear effects that seem to slide
perpendicular planes peripherally to each other (Hsu, 2010, p. 98). The shearing stresses
associated with the shearing effect leads to vertical shear force, and the shear stress on vertical
planes must be escorted by an equivalent stress on the straight plane (Ramachandran, 2016, p.
631). This is because accompaniment of the shear stress on vertical planes must be done by an
equal stress on horizontal planes and these particular horizontal stress must become zero. A
perpendicular application of load in normal stress and shear stress is normally being noticed
when the application of a load to an area parallel.

There are two common type of failure and they include;
The compression failure
In ductile flexural failure, after yielding of the reinforcement, the deformation of the beam
becomes important and deflection of the structure takes place in a ductile way if there is no
possible force relocation (Rodrigues, 2017, p. 89). The softening of beam compressive flange
takes place and the section center of rotation decreases thus decreasing the internal level arm.
When the ultimate capacity of the concrete compressive zone is attained then ductile flexural
failure occurs.
Shear failure
In shear flexure failure or web of the beam, the propagation of the inclined flexural shear
cracks, as well as the minimization of the various capabilities of the shear transfer mechanism,
results from high local tensile stress in the web. It is difficult to find a static equilibrium when
there is a minimal shear transfer capacity acting between adjacent portions. The failure of
failure mechanism is regarded as sliding shear taking place alongside a crack in a beam in
absence of shear underpinning and yielding of stirrups in a beam with shear reinforcement
(Crist, 2011, p. 55).
Brittle flexural failure
Failure in a beam with large reinforcement failure takes place by crushing the concrete in the
compressive zone before yielding of the flexural reinforcement.
Shear compression failure

This type of failure as a result as a result of high principal compressive stresses in the region
between induced shear cracks and this failure mode is associated with high amounts of shear
reinforcement.
Types of Beams in Construction
The beams used in construction are many in number and their classification is done based on
the following conditions;
 According to the state of support.
 According to materials for construction.
 According to geometry
 According to the equilibrium condition.
 According to the shape of the cross-section.
 According to the method of construction.
Based on the support condition
In this kind of condition, one end of the beam is pinned while the other end is supported by the
roller thus rotation is allowed to take place freely without interference. Both, as well as bending
stress, is sustained here. (Rodrigues, 2017, p. 439).
Fixed Beam
In this kind of beam, there is support rendered on both ends of the beam to curb spinning and
it is also known as a built-in beam. Fixing moments are produced by the fixed ends other than
the reactions.

Cantilever Beam
The fixing and freeing of the beam at one end is known as a cantilever beam. The distribution of
load is done back to the support by the beam where the forcing of the load against a moment
and shear stress takes place (Hyun-Moo, 2008, p. 67). Cantilever beams permit the creation of
some bridges, as well as balconies.
Continuous Beam
There is a distribution of support along the whole length of this type of beam.
Reinforced concrete beams
The construction of this type of beam is done using concrete and reinforcement.
T-section
A reinforced concrete slab is normally used in the construction of the above-mentioned beam
is. In order to increase the concrete strength of compression, there is a need for constructing T-
beam which is isolated while in the case inverted T-beam is constructed based on the
requirements of lading imposed.
Precast concrete beam
The manufacture of this kind of beam is done in factories and can be controlled as compared
with the construction on site. There is a greater quality of concrete beam and there are
numerous cross-sectional shapes can be manufactured for example Double beam, inverted T-
beam, Double T-beam and many more (Rots, 2008, p. 56).

Prestressed concrete beam
Application of loads on a beam after stressing strands helps in constructing this kind beam. The
variations of pre-stressed concrete include post-tensioned concrete beam as well as a pre-
tensioned concrete beam.
Beam in a rectangular shape
The beam mentioned above is mostly applied when constructing a reinforced concrete building
and other structures.
Timber Beams
The mentioned beam which was used in the past and it is constructed from timber but its
application is significantly declined now (Schiff, 2013, p. 235)
Conclusion
The classification of external forces acting on a member can be done as body forces and surface
forces. The concentration of the surface force takes place when it is acting as a point and it may
be distributed uniformly or non-uniformly over a finite area. All forces acting on a member are
considered external forces but in the case of body forces, it acts slowly and steadily in static
loading.
The division shear failure mode can also be divided into various sub-divisions which include;
Shear compression stress where the development of diagonal crack into the compressive zone
causes the RC beam to fail. And in case of compressive failure or shear proper of struts which
are realized in beams having small shear span to depth ratio and this will also make the

steeping line to be deep. Diagonal tension failure which entails the thrust line becoming very
eccentric giving rise to flexural failure in the compressive zone. This kind of failure arises from
extension tensile crack in the compressive region to flexural load.
References
Crist, R. (2011). Static and dynamic shear behavior in the beam (Vol. 2). Grupo Planeta.
Hsu, T. (2010). A unified theory of concrete structures (Vols. 245-276). China Publishing Company.

Hyun-Moo. (2008). Bridge maintenance, safety maintenance and health monitoring (Vol. 5). Reed
Elsevier.
Muttoni, A. (2012). Design of concrete structures with stress fields (Vol. 5). OLMA Media Group.
Ng, K. M. (2015). Strengthening of reinforced concrete deep beams (Vol. 6). Thomson Reuters.
Nielsen, N. (2010). Limit analysis and concrete plasticity (Vol. 3). Media participation.
Ramachandran, V. (2016). Concrete admixture handbook (Vol. 4). Adventure Works Press.
Rodrigues, H. (2017). Facing the challenges in structural engineering (Vol. 3). Carlton Books.
Rots, H. (2008). Finite elements in civil engineering application (Vol. 1). McGraw-Hill Education.
Schiff, D. (2013). Dynamic analysis and failure modes of simple structures (Vol. 3). Wolters Kluwer.