deep beams presentation2

7/21/2019 Deep Beams Presentation2

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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.



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 transerse section deiates appreciably

from straight line distribution assumed in theelementary beam theory.



The behaiour of a deep beam depends also on

how they are loaded ! special considerations should

be gien to this aspect in design.

"ere crac#ing 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 compressie stresses act

roughly parallel to the lines %oining the



load and the supports and the tensile stresses act

parallel to the bottom of the beam.

The fle&ural stresses at the bottom is constantoer much of the span.

The figure shows the crac# pattern and the

truss analogy of the same.



'ach of the three tension (), *+ and ' ties

hae crac#ed and at failure shaded region

would crush or the anchorage -ones at ' and

would fail. simplified truss model



( single span beam supporting a uniform loadacting on the top has the stress tra%ectories ,crac# pattern and simplified truss as shown.



( single span beam supporting a uniform loadacting on the lower face of the beam has thestress tra%ectories , crac# pattern and simplified

truss as shown.



The compression trajectories form an archwith the loads hanging from it. The crackpattern shows that the load is transferredupward by reinforcement until it acts onthe compression arch, which then transfersthe load down to the supports.

The force in the longitudinal tension tieswill be constant along the length of the

deep beam. This is the reason that thesteel must be anchored at the joints overthe reaction, failure of which is a majorcause of distress



Parameters inuencing deep beam

behavior are:

Width of support !

"verall depth of beam #

$%ective span &

Width ' Thickness of beam t

Type of loading, uniform w



The (in. thickness of deep beams should be based

on two considerations:

). *t should be thick enough to prevent buckling with

respect to its span + height i.e.

where t thickness of beam.

. 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



- ./& or ./#

i.e. - ./& when &'# 0 )

+ - ./# when &'# 1 )



2rom those values,

(u 3s.f s.- where f s .45 f y

The greater value of 3s is taken as tension steel.

( )( )

( )( ) L fy

Mu

L fy

Mu A

fyD

Mu

D fy

Mu

Z f

Mu A

s

s s

.

9.1

6.087.0

or

9.1

6.087.0.

==

===



#eep beams without holes



#esign procedure :

t ft av DVc

fck ft

Dav

Co

Dt ft D

avC Vc

VsVcVu

Dut

VuTv

.)35.0(72.0

becomesequationtheHence,

strength.tensiethe,5.0

!e"ths"an#hear

concrete$t.norma%or0.72toequae%%.&

$here..35.01 c)

b)

' '$here.

#hear omina!etermine a)

1

1

ma*

−=

=

=

−=

−=

+=

<=



When designing for shear, it is assumed that

concrete itself should carry at least /67 of

the ultimate shear.

This is ensured by choosing a suitable thk.

of beam by the following formula :

ft av D

Vut

)35.0(72.0

65.0

−

=



8hear capacities of tension steel + nominal web

steel should also be taken into account in

resisting the shear. Their shear capacity is

calculated as :

where,

! 6 9'mm for 2e)6

3s 3rea of tension steel provided.



+ 3ngle between the bar considered + the

critical diagonal crack.

y) #epth from the top of the beam to the point

where bar intersects the criticaldiagonal crack line.

n 9umber of bars including tension steel cut

by the assumed crack line

# Total depth of beam



;ertical steel 3v and <ori=ontal steel 3h

<ori=ontal steel bars acts as shear

reinforcement and also overcome the e%ects

of shrinkage + temperature.

The amounts speci>ed in *8:6/? are :

a@ ;ertical steel shall be .)7 for 2e)6, thebar diameter shall not be more than )mm

and spacing not more than AB thk. of beam or

6mm.



b@ <ori=ontal steel shall be .7 for 2e)6,

the bar diameter shall not be more than

)/mm + spacing not more than AB thk. of

beam or 6mm.

c@ 9ecessary side reinforcement should also be

provided.



#etailing of tension steel: *n deep beams, the tension steel is placed in

a =one of depth eCual to D.6#?.6&@

adjacent to the face of beam.

9o curtailment of the bars. *t should be bent

upwards at the ends to obtain adeCuate

anchorage + embedment .



). #etermine whether the given beam is deep or not.

. !heck its thickness.

A. #esign for eBure.

. #esign for minimum web steel + its distribution in

the beam.

6. #esign for shear.

/. !heck for bearing pressure at support + point

loading for local failures.

5. #etailing DEF*T*8< PF3!T*!$@

deep beams presentation2

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