Seismic Design of Reinforced Concrete Buildings

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Seismic Design of Reinforced Concrete Buildings

By Murat Saatcioglu

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Basic Principles of Design

𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 ≥ 𝐷𝐷𝐷𝐷𝐷𝐷𝐶𝐶𝐷𝐷𝐷𝐷

It is a good practice to reduce seismic demands to the extent possible. This can be done at the conceptual design stage by selecting a suitable structural system

 Select a suitable site with favorable soil conditions

 Avoid using unnecessary mass

 Use a simple structural layout with minimum torsional effects

 Avoid strength and stiffness taper along the height

 Avoid soft stories

 Provide sufficient lateral bracing and drift control by using concrete structural walls

 Isolate non-structural elements

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Seismic Amplification due to Soft Soil

Soil amplification

Liquefaction

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Unnecessary Use of Mass

Heavy Roofing Tiles Carried by URM Walls

Use of Soil for Plantation not Accounted for in design

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Unnecessary Use of Mass

Soil on Top of a Structural Slab

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Torsional Irregularity

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Torsional Irregularity

Increased shear demands on columns away from the shar-wall core

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Vertical Irregularity

Central core-wall was removed at the 3^rd floor level to accommodate a conference

hall and replaced by columns

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Vertical Irregularity (Soft Story)

El Centro County Services Building in California with a soft story. Also lack of column confinement

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Drift Control – Bracing by Shear Walls

Non-ductile concrete frames with URM infill

walls (left). Building stiffened by reinforced

concrete shear-walls (right) in the same

block

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Interference of Non-Structural Elements

Non-structural elements providing unintentional lateral support

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Seismic Design and Detailing Requirements of CSA A23.3-19 and ACI 318-19

Capacity design is employed…..

Selected elements are designed to yield while critical elements remain elastic

Design for

Strength and Deformability

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• 𝑓𝑓_𝑐𝑐^′ ≤ 80 𝑀𝑀𝑀𝑀𝐶𝐶 for normal-density concrete

• 𝑓𝑓_𝑐𝑐^′ ≤ 30 𝑀𝑀𝑀𝑀𝐶𝐶 for structural low-density concrete, unless demonstrated by tests that the strength and toughness is comparable to those elements built with normal-density concrete.

• If 𝑓𝑓_𝑦𝑦 > 400 𝑀𝑀𝑀𝑀𝐶𝐶 the increase in strain demands shall be considered in design and detailing. Higher grades of steel permitted in ACI-318.

• Reinforcement shall comply with CSA G30.18; weldable grade if R_d > 2.5.

GENERAL REQUIREMENTS Material Strength Limitations

ACI-318 Requirements

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Ductile Moment resisting frames (R

_d

= 4.0) Members Subjected to Predominant Flexure

• Longitudinal Reinforcement

M

_r

M

_r

M

_r⁺

> 1/2 M

_r

- -

M

_r⁺

> 1/2 M

_r

- -

Top and Botom 2 bars continuous M

r

- > 1/4 M

r

-

M

_r⁺

> 1/4 M

_r

-

Top and Bottom: 1.4b

_w

d / f

_y

≤ ρ ≤ 0.025

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Ductile Moment Resisting Frames (R

_d

= 4.0) Members Subjected to Predominant Flexure

• Transverse Reinforcement

c

1

h



n

s

1

s

₂

≤ d / 2

50 mm

2d

4 / d s

₁

≤

mm 300

s

₁

≤

bar . long b

1

8 ( d ) s ≤

hoop b

1

24 ( d ) s ≤

Hoops Sirrups with Hoops

seismic hooks

d

b

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Ductile Moment Resisting Frames (R

_d

= 4.0) Members Subjected to Predominant Flexure

W_f

M^-_pr M⁺_pr

(V_e)_left (V_e)_right

V_e = M^-_pr M⁺_pr l_n

l_n

W_f l_n 2 + +-

Plastic Hinge

• Shear Resistance Requirement

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Ductile Moment Resisting Frames (R

_d

= 4.0)

Members Subjected to Combined Flexure and Axial Load

• Longitudinal Reinforcement

Reduced maximum reinforcement ratio to

eliminate congestion of column cage and

concrete placement

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• Hoops, seismic crossties or spirals

• Spacing < 6d_b, 24d_t and ½ least dimension of member

Buckling Prevention Ties for Members Subjected to

Combined Flexure and Axial Load

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Concrete Confinement in Members Subjected to Combined Flexure and Axial Load

Well confined column

Poorly confined column

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CSA A23.3-2019 Confinement Steel Requirements

𝝆𝝆_𝒔𝒔

𝒌𝒌_𝒑𝒑 = 𝑷𝑷_𝒇𝒇 𝑨𝑨_𝒈𝒈𝜶𝜶_𝟏𝟏𝒇𝒇𝒇_𝒄𝒄

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𝑘𝑘_𝑛𝑛 = 𝐷𝐷_𝑙𝑙 (𝐷𝐷_𝑙𝑙 − 2)

Where, n_lis the number of laterally supported longitudinal bars

𝑘𝑘_𝑝𝑝 = 𝑀𝑀_𝑓𝑓 𝐴𝐴_𝑔𝑔𝛼𝛼₁𝑓𝑓𝒇_𝑐𝑐

CSA A23.3-2019 Confinement Steel Requirements

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CSA A23.3-2019 Confinement Steel Requirements

Spacing of transverse reinforcement for confinement of fully ductile columns:

Spacing of transverse reinforcement for confinement of moderately ductile columns:

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ACI 318-19 Confinement Steel Requirements

b_c : Core dimension measured to the outside of the perimeter hoop.

A_ch : Core area A_g : Gross area

𝐾𝐾_𝑓𝑓 = 𝑓𝑓_𝑐𝑐^′

172 + 0.6 ≥ 1.0 𝐾𝐾_𝑛𝑛 = 𝐷𝐷_ℓ

𝐷𝐷_ℓ − 2

𝐷𝐷_ℓ: Number of laterally supported longitudinal bars

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ACI 318-19 Confinement Steel Requirements

Maximum spacing requirements for transverse confinement reinforcement:

a) One-fourth of the minimum column dimension

b) For Grade 60 (400 MPa), 6d_b of the smallest longitudinal c) For Grade 80 (550 MPa), 5dbar _b of the smallest longitudinal d) Sbar_o as calculated below:

h_x is the largest x_i (see the figure). It should not

exceed 150 mm and it need not be less than 100 mm

75 mm

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Ductile Moment Resisting Frames (R

_d

= 4.0)

Members Subjected to Combined Flexure and Axial Load

V_col = M^acol M^bcol

l_u + M^-_pr

M⁺_pr

lu

M⁺_pr M^-_pr

M_col^a

M^b_col

V_col Vcol

• Shear Resistance

𝑉𝑉_{𝑐𝑐𝑐𝑐𝑙𝑙} = 𝑀𝑀_{𝑐𝑐𝑐𝑐𝑙𝑙}^𝑎𝑎 + 𝑀𝑀_{𝑐𝑐𝑐𝑐𝑙𝑙}^𝑏𝑏 ℓ_𝑢𝑢

• The factored shear need not exceed that obtained from structural analysis under factored load combinations with R_dR_o = 1.3

• The values of θ ≥ 45^o and β ≤ 0.10 shall be used in shear design

• The transverse reinforcement shall be hoops or spirals.

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Ductile Moment Resisting Frames (R

_d

= 4.0)

Strong Column - Weak beam Concept

M

^anc

M

^bnc

M

^lpb

M

^rpb

∑ M

^nc

≥ ∑ M

^pb

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• Joint Shear Resistance (Clause 21.3.3.1.2)

A

s

A'

s

C

1

= T

1

C

2

= T

2

T

1

= 1. 25 A'

s

f

y

T

2

= 1. 25 A

s

f

y

x x

V

e

V

e

V

x-x

= V

e

- T

2

- C

1

Joints in Ductile Moment Resisting Frames (R

_d

= 4.0)

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• Shear Resistance (Clause 21.3.3.4.1)

j c

c

j

2 . 2 f ' A

V = λ φ

j c

c

j

1 . 6 f ' A

V = λ φ

j c

c

j

1 . 3 f ' A

V = λ φ

Continue half the column transverse reinforcement into the joint

Continue the column transverse reinforcement into the joint

Continue the column transverse reinforcement into the joint

Joint Shear in Ductile Moment Resisting Frames (R

_d

= 4.0)

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Joint Shear Failure

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The same as those for ductile moment resisting frame elements, except:

• For beams, the clear span of the member shall be not less than three times the beam effective depth.

• For columns, the shortest cross-sectional dimension shall be not less than 250 mm.

Moderately Ductile Moment Resisting Frames (R

_d

= 2.5)

M

_r

M

_r

M

r+

> 1/3 M

r

- -

M

r+

> 1/3 M

r

- M

r

- > 1/5 M

r

- -

M

r+

> 1/5 M

r

-

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c1

h

n

s1 s₂ ≤ d/ 2 50 mm

2d

4 / d s₁ ≤

mm 300

s₁ ≤

bar . long b 1 8(d ) s ≤

hoop b

1 24(d ) s ≤

Hoops Sirrups with Hoops seismic hooks

db

Other requirements that are different than those for ductile moment resisting frame elements:

• Probable moment resistance is replaced by nominal moment resistance (for beam, column and joint shear calculations, as well as for checking the strong column-weak beam

requirement).

• Confinement reinforcement spacing is relaxed to h/2.

• Joint shear limits are reduced.

Moderately Ductile Moment Resisting Frames (R

_d

= 2.5)

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Flexural Shear walls

t

As

ℓ

h p

h w

ℓ_w

Concentrated vertical

reinforcement & ties requirements

Design V & M

Minimum wall thickness requirements Concrete and reinforcement requirements

Distributed vertical and horizontal reinforcement requirements

Minimum height of plastic hinge region

h_w / ℓ_w > 2.0

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A_s ≥ 2-15M

≥ 4 bars in 2 layers (plastic hinge)

ρ_v= 0.0015A_g ρ_h = 0.002A_g

R_d = 1.5 R_d = 2.0 R_d = 3.5

A_s ≥ 0.0005b_w ℓ_w

≥ 0.00075b_w ℓ_w (plastic hinge) ρ_v= 0.0025A_g ρ_h = 0.0025A_g

A_s ≥ 0.001b_w ℓ_w

≥ 0.0015b_w ℓ_w (plastic hinge) ρ_v= 0.0025A_g ρ_h = 0.0025A_g

hp

hw

ℓw

t t t

As As As

ℓw ℓw

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Questions and Comments…