Ductile detailing

Ductile Detailing of RC Frame Buildings
Training to LB technical staff on
Nepal National Building Code
Binay Shrestha

DETAILING OF RC MEMBERS
Detailing is often considered to be the preparation of
working drawings showing the size and location of the
reinforcement in a concrete structure.
Detailing involves the communication of the engineer’s
design to the contractors who build the structure. It
involves the translation of a good structural design
into the final structure.
Good detailing ensures that reinforcement and
concrete interact efficiently to provide satisfactory
behavior throughout the complete range of loading.

DETAILING OF RC MEMBERS
1. IS 456:2000 Indian Standard for Plane and
Reinforced Concrete - Code of Practice
(Fourth Revision)
2. IS 13920 Indian Standard for Ductile
Detailing of Reinforced Concrete
Structures Subjected to Seismic Forces -
Code of Practice

DUCTILE DETAILING
• Better behavior
• To improve ductility of building
Earthquake-resistant design is concerned
about ensuring that the damages in
buildings during earthquakes are of the
acceptable variety, and also that they
occur at the right places and in right
amounts.

MAKE BUILDING DUCTILE
The most common
construction material:
• Masonry.
• Concrete.
• Steel is used as
reinforcement.

DUCTILITY FOR GOOD SEISMIC PERFORMANCE
Brittle & Ductile BehaviourBrittle & Ductile Behaviour Chain Analogy for Ductile FailureChain Analogy for Ductile Failure

Our Practice for Detailing : Ignorance?

How do Beams resist forces
Long straight bars
(longitudinal bars)
placed along its length.
Closed loop of small diameter bars (stirrups)
placed vertical at regular interval along its
length.

Flexure (Bending) Failure:
ductile failure and hence is desirable.
a) Brittle failure
Relatively more steel is present on the tension face,
crushes in compression which is undesirable
b) Ductile failure
Relatively less steel is present on the tension face, steel
yield first and redistribution occurs in the beam until
eventually concrete crushes.
2 Basic Types of Failure in Beams

Shear Failure:
A beam may also fail due to shearing action.
Closed loop stirrups are provided to avoid such shearing
action. Shear damage occurs when the area of these
stirrups is insufficient.
Shear failure is brittle, and therefore, shear failure must
be avoided in the design of RC beams.
2 Basic Types of Failure in Beams

GENERAL REQUIREMENT IS13920-1993
• The factored axial stress on the member under earthquake
loading shall not exceed 0.1 fck.
• The member shall preferably have a width-to-depth ratio of
more than 0.3.
• The width of the member shall not be less than 200 mm.
• The depth D of the member shall preferably be not more
than 1/4 of the clear span.

BEAM
-Width, b ≥ 200mm
- Nominal Cover ≥ 25mm
-Width to effective depth ratio ≥ 0.3
-Steel percentage ≤ 2.5%
-minimum area of tension bar :
As/(bd) =0.85/(fy)
-minimum ratio = 0.24√(fck/fy)
-max. ratio = 0.025
-minimum shear bar
Asv/(b*Sv) ≥ 0.4/(0.87*Fy)
where ,Sv = stirrups spacing

Longitudinal Bars : are provided to resist flexure
Requires on both faces at the ends and on the bottom fa
ce at mid length
At least two bars go through the full length of the
beam at the top as well as at the bottom of the beam.
At the ends of the beams, the amount of steel
provided at the bottom is at least half of that at the
top.
Flexure

Stirrups help in three ways
Carry vertical shear force and resist diagonal crack.
Protect concrete from bulging outward due to flexure
Prevent buckling of compressed longitudinal bar due to
flexure
Shear

a)Diameter at least 6 mm but not less than 8 mm for
beam longer than 5 m.
b)135 hook at both end and 10 times diameter
extension.
c)Maximum spacing less than half the depth of beam.
d)Maximum spacing less than quarter the depth of
beam for twice the depth of beam from support.

At the location of the lap, the bars transfer large forces from
one to another
Laps of the longitudinal bars are:
a) Made away from the face of column.
b) Not made at locations where they are likely to stretch by large
amounts and yield (eg. Bottom bars at mid length of the beam)
At the location of laps, vertical stirrups should be provided at clos
er spacing

Calculation of Design Shear Force of
Beam : IS 13920

Column can sustain 2 types of damage:
a) Axial Flexural (Combined Compression
Bending) failure
b) Shear Failure (Brittle Damage) & must be
avoided by providing lateral ties
How do Column resist EQ

Design
-Width to Depth ratio ≥ 0.4
-0.8% ≤ Steel percent ≤ 4.0%
-Number of bars in rectangular section ≥ 4
-Longitudinal bar diameter (φ) ≥ 12mm
-Clear cover ≥ 40mm
-Minimum width of column = 300mm
Column

Closely spaced horizontal closed ties
help in three ways,
(i) they carry the horizontal shear
forces induced by earthquakes, and
thereby resist diagonal shear cracks,
(ii) they hold together the vertical
bars and prevent them from
excessively bending outwards, and
(iii) they contain the concrete in the
column within the closed loops. The
ends of the ties must be bent as 135°
hooks. Such hook ends prevent
opening of loops and consequently
buckling
Column

In column where spacing between the corner bar
exceeds 300 mm:
Additional links with 180 hook ends for ties to be
effective in holding the concrete in its place and to
prevent the buckling of vertical bars
Column

Height of Confinement Zone:
larger of D, hc /6 or 450 mm
Spacing of ties in CZ:
< D/4
but >75 mm and < 100 mm
Spacing of ties in Lap Zone:
< D/2 and < 150 mm
Spacing of ties in rest :
< D/2

Lap splices shall be provided only in the central half of the member
length. It should be proportioned as a tension splice. Hoops shall be
provided over the entire splice length at spacing not exceeding 150
mm centre to centre. Not more than 50 percent of the bars shall be
spliced at one section.
Longitudanal Reinforcement
Transverse Reinforcement
The design shear force for columns shall be the maximum of:
a) calculated factored shear force as per analysis, and
b) a factored shear force given by….

CALCULATION OF DESIGN SHEAR FORCE
IN COLUMN IS 13920

COLUMN AND JOINT DETAILING
PROVISION OF
SPECIAL CONFINING
REINFORCEMENT IN
FOOTINGS

AREA OF CROSS SECTION OF SPECIAL
CONFINING REINFORCEMENT
• Area of cross section, Ash of the bar forming rectangular
hoop, to be used as special confining reinforcement shall not
be less than
Ash=0.18*S*hk*fck/fy*(Ag-Ak-1)
Where hk=longer dimension of the rectangular
confining hoop measured to its outer face
S= Pitch

Detailing for Reducing Short
Column Effect:
Provide Special Confining
Reinforcement in the form of
closely spaced ties
throughout the height and
into the column above.

Columns supporting
reactions from discontinued
stiff members, such as walls,
shall be provided with special
confining reinforcement over
their full height.
This reinforcement shall also
be placed above the
discontinuity for at least the
development length of the
largest longitudinal bar in
the column.
Where the column is
supported on a wall, this
reinforcement shall be
provided over the full height
of the column; it shall also be
provided below the
discontinuity for the same
development length.

Ash = area of the bar cross section,
S = pitch of spiral or spacing of hoops,
Dk = diameter of core measured to the outside of the spiral or hoop,
fck = characteristic compressive strength of concrete cube,
fy = yield stress of steel (of circular hoop or spiral),
Ag = gross area of the column cross section, and
Ak = area of the concrete core =
h = longer dimension of the rectangular confining hoop measured to
its outer face
The area of cross section, Ash, of the bar forming circular hoops
or spiral, to be used as special confining reinforcement, shall
not be less than

Deformability (ductility) of reinforced concrete members is a necessity. Note the obvious
differences of capability of concrete columns to take load after earthquake damage. The
reinforced column with more stirrups (ductile reinforcing) has an obvious capacity to
carry much more load than the column with less stirrups

Improper lapping of column bars
The pictures show damage concentration in the region of bar
lapping. Because of interaction between overlapped bars and
concrete for load transfer the overlapping section suffers higher
level of damage. This interaction is further coupled with lack of
stirrups which has led to buckling of bars, loss of concrete

BEAM COLUMN JOINT
In RC buildings, portions of columns that are common to
beams at their intersections are called beam-column
joints. Repairing damaged joints is difficult, and so
damage must be avoided.

Under earthquake moment,
the top & bottom beam bars
pull in opposite direction.
If size of joint is small or
concrete strength is low –
bar slips
Joint undergo geometric
distortion and if the size is
insufficient – diagonal crack
occur.
Can be controlled by –
providing large column size
– providing closed ties.

Column width – at least 20 times the diameter of largest
beam bar.
Closed loop steel ties in beam column joints confine the concrete in the
joint.
It also resist the shear force reducing the cracking & crushing of
concrete.

In Exterior Joint – beam bar
need to be anchored into
column to ensure proper
gripping.
In Interior Joint – beam
bars need to go through
the joint without any cut in
the joint region. Also, these
bars must be placed within
column bars and with no
bends.

Development length
Development length is given by
= diameter of bar
= stress in bar
=design bond stress given by 26.2.1.1

Concrete M20 Concrete M25 Concrete M30
 415 500 415 500 415 500
12 47 564 57 680 40 484 49 583 38 451 49 544
16 47 752 57 906 40 645 49 777 38 602 49 725
20 47 940 57 1133 40 806 49 971 38 752 49 906
25 47 1175 57 1416 40 1007 49 1214 38 940 49 1133
28 47 1316 57 1586 40 1128 49 1359 38 1053 49 1269
32 47 1504 57 1813 40 1289 49 1554 38 1204 49 1450

Since slabs are not significantly affected by lateral loads,
provisions of IS 13920 do not apply in this case . Following are
relevant provisions of IS 456:2000
-Reinforcement percentage ≥ 0.12%
-Nominal cover ≥ 15 mm
-Main steel spacing ≤ minimum of (3d, 300mm)
-Distributor steel spacing ≤ minimum of (4d, 450mm)
Slab

Ductile detailing

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Ductile detailing

Similar to Ductile detailing (20)

More from Binay Shrestha

More from Binay Shrestha (8)

Recently uploaded

Recently uploaded (20)

Ductile detailing