Seismic Design and Detailing of RC Structures

Discontinuity in Construction Unfavorable for Seismic Design

Complicated Plans Asymmetrical Plan Discontinuous Elevations

Fig. 21.1: Unfavorable Discontinuity in Building Configurations

Code Prescribed Seismic Detailing of RC Structural Elements

1. Materials

Specification

Possible Explanation

ete n cr

f c 20 Mpa ( 3 ksi) for 3-storied Weak concretes have low shear and bong strengths and cannot take full o

or taller buildings advantage of subsequent design provisions C

f y 415 Mpa ( 60 ksi), preferably Lower strength steels have Steel

250 Mpa ( 36 ksi) (a) a long yield region, (b) greater ductility, (c) greater f ult /f y ratio

2. Flexural Members (members whose factored axial stress f c /10)

Specification

Possible Explanation To ensure lateral stability and improve torsional

b/d 0.3

resistance

To (a) decrease geometric error, Size

(b) facilitate rod placement Behavior and design of deeper members are

d L c /4

significantly different N s(top) and N s(bottom) 2 Construction requirement

To avoid brittle failure upon cracking t

c 0.1 f /f y (f c ,f y in ksi) at both top and bottom

To (a) cause steel yielding before concrete crushing en

and (b) avoid steel congestion m

0.025 at top or bottom

To ensure (a) adequate ductility and rce fo

A s(bottom)

0.5A s(top) at joint and

(b) minimum reinforcement for moment reversal ein

A s(bottom)/(top)

0.25A s(top) (max) at any section

Both top and bottom bars at an external joint must be R

To ensure (a) adequate bar anchorage, al

anchored L d +10d b from inner face of column with

(b) joint ductility

d in itu

90 bends

Lap splices are allowed for 50% of bars, only where Closely spaced stirrups are necessary within lap o n g

lengths because of the possibility of loss of concrete stirrups are provided @ d/4 or 4 c/c

L cover

Lap splice lengths L d and are not allowed within

Lap splices are not reliable under cyclic loading into distance of 2d from joints or near possible plastic the inelastic range

hinges

m Web reinforcements must consist of closed vertical To provide lateral support and ensure strength

stirrups with 135 hooks and 10d W fo en Design shear force is the maximum of (a) shear force

development of longitudinal bars eb rce t

t ( 3 ) extensions

It is desirable that the beams should yield in flexure ein

from analysis, (b) shear force due to vertical loads plus R

before failure in shear as required for flexural yielding of joints

To (a) provide resistance to shear, (b) confine Spacing of hoops within 2d (beginning at 2 ) at either concrete to improve ductility, (c) prevent

end of a beam must be d/4, 8d b ; elsewhere S t d/2

buckling of longitudinal compression bars

3. Axial Members (members whose factored axial stress f c /10)

Specification Possible Explanation To ensure lateral stability and improve torsional

b c /h c 0.4

resistance Size

To avoid (a) slender columns, b c 12 (b) column failure before beams

Lap splices are allowed only for 50% of bars, Closely spaced stirrups are necessary within lap lengths t

because of the possibility of loss of concrete cover al

only where stirrups are provided @ b c /4 or 4

en in m

Lap splices are not reliable under cyclic loading into the d u rce

Lap splice lengths L d and only allowed in the

center half of columns inelastic range it g n fo

To (a) ensure effectiveness and o

L ein (b) avoid congestion of longitudinal bars R

0.01 g 0.06

M c,ult 1.2 M b,ult To obtain ‘strong column weak beam condition’ to

avoid column failure before beams Transverse reinforcement must consist of closed spirals or rectangular/ circular hoops with 135

at joint

To provide lateral support and ensure strength development of longitudinal bars

hooks with 10d t ( 3 ) extensions Parallel legs of rectangular hoops must be spaced

To provide lateral support and ensure strength @ 12 c/c

development of longitudinal bars t

To (a) provide resistance to shear, (b) confine concrete

en to improve ductility, (c) prevent buckling of m

Spacing of hoops within L 0 ( d c ,h c /6, 18 ) at each

longitudinal compression bars rce

end of column must be b c /4, 4 ; else S t b c /2

Design shear force is the maximum of (a) shear fo

force from analysis, (b) shear force required for It is desirable that the columns should yield in flexure ein

before failure in shear R

flexural yielding of joints se

To provide resistance to the very high axial loads and er

Special confining reinforcement (i.e., S t b c /4, 4 )

should extend at least 12 into any footing flexural demands at the base sv

ran

Discontinued stiff members (e.g., shear walls, masonry T

Special confining reinforcement (i.e., S t b c /4, 4 )

should be provided over the entire height of walls, bracings, mezzanine floors) may develop columns supporting discontinued stiff members and significant forces and considerable inelastic response

extend L d into the member

For special confinement, area of To ensure load carrying capacity upto concrete spalling, taking into consideration the greater effectiveness of

circular spirals 0.11 S t d (f c /f y )(A g /A c 1),

circular spirals compared to rectangular hoops. It also

rectangular hoops 0.3 S t d (f c /f y )(A g /A c 1)

ensures toughness and ductility of columns

4. Joints of Frames

Specification

Possible Explanation

Special confining reinforcement t

To provide resistance to the shear force transmitted by framing members (i.e., S t b c /4, 4 ) should extend

se en and improve the bond between steel and concrete within the joint er m

through the joint sv rce ran fo

S t b c /2, 6 through joint with Some confinement is provided by the beams framing into the vertical T ein

beams of width b 0.75b c faces of the joint

@ 6.75 c/c

#3 stirrups

a b -Sp c

d @ 6.75 c/c e

2-legged #3 stirrups @ 3 (length 27 )

C-Sp

4-legged #3 stirrups @ 3 (through)

Lap-splices not allowed here Elsewhere, it is only allowed for 50% bars with special confinement

Anchorage at end joints L anch =L d + 10 d b L d for #7 bars = 0.04 A s f y /f c = 0.04 0.60 40/ (3/1000) 1.4 = 24.53 L anch = 24.53 + 10 7/8 = 33.29 ; i.e., 34

Extra #7 bar

All #6 bars

135 hooks with 4 extensions

Section a-a

Section b-b

Section c-c

Section d-d Section e-e

Beam Sections (with reinforcements)

Fig. 21.2: Typical Building Frame satisfying Provisions of Seismic Detailing

Seismic Design and Detailing of RC Structures