Deep beams

• span/depth ratio of
simply supported beam is < 2,
continuous beam < 2.5,
it is classified as deep beam.
• Such structures are found in transfer girders and in shear wall
structures that resist lateral forces in buildings. It is also found in
some of the industrial buildings.

• RC deep beams, which fail with shear compression, are the
structural members having a shear span (orange line) to
effective depth (brack line) ratio, a/d, not exceeding 1.
• RC deep beams have many useful applications in building
structures such as transfer girders, wall footings, foundation
pile caps, floor diaphragms, and shear walls. Particularly, the
use of deep beams at the lower levels in tall buildings for both
residential and commercial purposes has increased rapidly
because of theirconvenience and economical efficiency.

• Deep beams are structural elements loaded as simple beams in
which a significant amount of the load is carried to the supports
by a compression force combining the load and the reaction. As
a result, the strain distribution is no longer considered linear,
and the shear deformations become significant when compared
to pure flexure.
• Floor slabs under horizontal load, short span beams carrying
heavy loads, and transfer girders are examples of deep beams.
Deep beam is a beam having large depth/thickness ratio and
shear span depth ratio less than 2.5for concentrated load and
less than 5.0 for distributed load. Because the geometry of deep
beams, their behavior is different with slender beam or
intermediate beam.

• Two-Dimensional Action, because of the dimension of deep beam
they behave as two-dimensional action rather than one-
dimensional action.
• Plane Section Do Not Remain Plane, the assumption of plane
section remain plane cannot be used in the deep beam design.
The strain distribution is not longer linear.
• Shear Deformation, the shear deformation cannot be neglected as
in the ordinary beam. The stress distribution is not linear even in
the elastic stage. At the ultimate limit state the shape of concrete
compressive stress block is not parabolic shape again

• Deep beams are structural elements loaded as beams but
having a large depth/thickness ratioand a shear span/depth
ratio not exceeding 2 for simple span and 2.5 for continuous
span, wherethe shear span is the clear span (ln) of the beam for
distributed load and the distance betweenthe point of
application of the load and the face of the support a for
concentrated load. Thisdefinition is some what arbitrary.
• better definition is deep beam is a beam in which asignificant
amount of the load is carried to the supports by a compression
thrust "oining the loadand the reactions. This occurs if the
above proportions are maintainedDeep beams are usually
found in transfer girders (girders support one or more
columnstransferring it laterally to other columns) used in
multi storey buildings to provide columnoffsets, in foundation
walls, pile caps, walls of rectangular tanks and bins, floor
diaphragms, andshear walls.

• The traditional principles of stress analysis are neither suitable nor
adequate to determine the strength of reinforced concrete deep
beams.
• In deep beams, the bending stress distribution across any transverse
section deviates appreciably from straight line distribution assumed
in the elementary beam theory.

• The behaviour of a deep beam depends also on how they are loaded
& special considerations should be given to this aspect in design.
• Here cracking will occur at one-third to one-half of the ultimate
load.
• In the single span beam supporting a concentrated load at mid span,
the compressive stresses act roughly parallel to the lines joining the

load and the supports and the tensile stresses act parallel to the
bottom of the beam.
The flexural stresses at the bottom is constant over much of the
span.
The figure shows the crack pattern and the truss analogy of the
same.

Each of the three tension AB, CD and EF ties have cracked and
at failure shaded region would crush or the anchorage zones at
E and F would fail.
simplified truss model

• A single span beam supporting a uniform load acting on the
top has the stress trajectories , crack pattern and simplified
truss as shown.

• A single span beam supporting a uniform load acting on the
lower face of the beam has the stress trajectories , crack
pattern and simplified truss as shown.

• The compression trajectories form an arch with the loads
hanging from it. The crack pattern shows that the load is
transferred upward by reinforcement until it acts on the
compression arch, which then transfers the load down to the
supports.
• The force in the longitudinal tension ties will be constant along
the length of the deep beam. This is the reason that the steel
must be anchored at the joints over the reaction, failure of
which is a major cause of distress

 Parameters influencing deep beam behavior are:
 Width of support = C
 Overall depth of beam = D
 Effective span = L
 Width / Thickness of beam = t
 Type of loading, uniform = w

The Min. thickness of deep beams should be based on two
considerations:
1. It should be thick enough to prevent buckling with respect to its
span & height i.e.
where t = thickness of beam.
2. The thickness should be such that the concrete itself should be able
to carry a good amount of the shear force that acts in the beam
without the assistance of any steel.
50&25 
t
L
t
D

• Z = 0.6L or 0.6D
i.e. Z = 0.6L when L/D < 1
& Z = 0.6D when L/D > 1

• From those values,
Mu = As.fs.Z where fs = 0.87 fy
• The greater value of As is taken as tension steel.
  
   Lfy
Mu
Lfy
Mu
A
fyD
Mu
Dfy
Mu
Zf
Mu
A
s
s
s
.
9.1
6.087.0
or
9.1
6.087.0.



• Design procedure :
tftavDVc
fckft
D
av
Co
Dtft
D
av
CVc
VsVcVu
Dut
Vu
Tv
.)35.0(72.0
becomesequationtheHence,
strength.tensilethe,5.0
Depth
spanShear
concretewt.normalfor0.72toequaleff.C
where..35.01c)
b)
TTvwhere
.
ShearNominalDeterminea)
1
1
max












• When designing for shear, it is assumed that concrete itself should
carry at least 65% of the ultimate shear.
• This is ensured by choosing a suitable thk. of beam by the following
formula :
ftavD
Vu
t
)35.0(72.0
65.0




n
D
y
AsCVs
1
2
2 Sin
1
. 

α = Angle between the bar considered & the critical diagonal crack.
y1= Depth from the top of the beam to the point where bar intersects the
critical diagonal crack line.
n = Number of bars including tension steel cut by the assumed crack line
D = Total depth of beam

• Vertical steel Av and Horizontal steel Ah
• Horizontal steel bars acts as shear reinforcement and also overcome
the effects of shrinkage & temperature.
• The amounts specified in IS:456-2000 are :
a) Vertical steel shall be 0.12% for Fe415, the bar diameter shall not be
more than 14mm and spacing not more than 3x thk. of beam or
450mm.

b) Horizontal steel shall be 0.20% for Fe415, the bar diameter shall not
be more than 16mm & spacing not more than 3x thk. of beam or
450mm.
c) Necessary side reinforcement should also be provided.

Detailing of tension steel:
In deep beams, the tension steel is placed in a zone of depth equal to
(0.25D-0.05L) adjacent to the face of beam.
No curtailment of the bars. It should be bent upwards at the ends to
obtain adequate anchorage & embedment .

1. Determine whether the given beam is deep or not.
2. Check its thickness.
3. Design for flexure.
4. Design for minimum web steel & its distribution in the beam.
5. Design for shear.
6. Check for bearing pressure at support & point loading for
local failures.
7. Detailing (BRITISH PRACTICE)

Thank you
Mr. VIKAS MEHTA
School of Mechanical and civil engineering
Shoolini University
Village Bajhol, Solan (H.P)
vikasmehta@shooliniuniversity.com
+91 9459268898

Deep beams

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Deep beams