Seismic-Design-Lec-2.pdf

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SEISMIC DESIGN REQUIREMENTS FOR

REINFORCED CONCRETE BUILDINGS

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MODEL BUILDING CODES

•

• A model building code is a document containing standardized

A model building code is a document containing standardized

building requirements applicable throughout the United States.

•

• The three model building codes in the United States were: the

The three model building codes in the United States were: the

Uniform Building Code (predominant in the west), the Standard

Building Code (predominant in the southeast), and the BOCA

National Building Code (predominant in the northeast), were

initiated between 1927 and 1950.

•

• The US Uniform Building Code was the most widely used seismic

The US Uniform Building Code was the most widely used seismic

code in the world, with its last edition published in 1997.

•

• Up to the year 2000, seismic design in the United States has been

Up to the year 2000, seismic design in the United States has been

based on one these three model building codes.

•

• Representatives

Representatives

from the three model codes formed the

International Code Council (ICC) in 1994, and in April 2000, the

ICC published the first edition of the International Building Code,

IBC-2000. In 2003, 2006, 2009 and 2012, the second, third fourth



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Initiation Of The Equivalent Static Lateral

Force Method

•

• The work done after the 1908 Reggio-Messina Earthquake in Sicily by a

The work done after the 1908 Reggio-Messina Earthquake in Sicily by a

committee appointed by the Italian government may be the origin of

the equivalent static lateral force method, in which a seismic coefficient

is applied to the mass of the structure, to produce the lateral force that

is approximately equivalent in effect to the dynamic loading of the

expected earthquake.

•

• The Japanese engineer Toshikata Sano independently developed in

The Japanese engineer Toshikata Sano independently developed in

1915

the idea of a lateral design force V proportional to the

building’s

weight W. This relationship can be written as

F = C W

, where

C

is a

lateral force coefficient, expressed as some percentage of gravity. The

first official implementation of

Sano’s

criterion was the specification C =

10 percent of gravity, issued as a part of the 1924 Japanese Urban

Building Law Enforcement Regulations in response to the destruction

caused by the great

1923 Kanto earthquake.

•

• In California, the Santa Barbara earthquake of 1925 motivated several

In California, the Santa Barbara earthquake of 1925 motivated several

communities to adopt codes with C as high as 20 percent of gravity.

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Development Of The Equivalent Static

Lateral Force Method

• The first edition of the U.S. Uniform Building Code (UBC) was published

in 1927 by the Pacific Coast building Officials (PCBO), contained an

optional seismic appendix.

• The seismic design provisions remained in an appendix to the UBC until

the publication of the 1961 UBC.

• In the 1997 edition of UBC the earthquake load (E) is a function of both

the horizontal and vertical components of the ground motion.

UBC/IBC Code s Lateral Force

UBC 1927- UBC 1946

F = C’W

UBC 1949- UBC 1958

F = C’W

UBC 1961- UBC 1973

V = ZKCW

UBC 1976- UBC 1979

V = ZIKCSW

UBC 1982- UBC 1985

V = ZIKCSW

UBC 1988- UBC 1994

V = ZICW/Rw

UBC 1997

V = CvIW/RT

IBC- 2000- IBC-2012

V = CsW

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Safety Concepts

• Structures designed in accordance with the

UBC

provisions

should generally be able to:

1. Resist

minor earthquakes without damage.

2. Resist

moderate earthquakes without structural damage, but

possibly some nonstructural damage.

3. Resist major earthquakes without collapse, but possibly some

structural and nonstructural damage.

• The UBC intended that structures be designed for “life-safety”

in the event of an earthquake with a 10-percent probability of

being exceeded in 50 years. The IBC intends design for “collapse

prevention” in a much larger earthquake, with a 2-percent

probability of being exceeded in 50.

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Seismic Codes Are Based On Earthquake

Historical Data

• The 1925 Santa Barbara earthquake led to the first introduction of

simple Newtonian concepts in the 1927 Uniform Building Code. As the

level of knowledge and data collected increases, these equations are

modified to better represent these forces.

• In response to the 1985 Mexico City earthquake, a fourth soil profile

type, , for very deep soft soils was added to the 1988 UBC, with the

factor equal to 2.0.

• The 1994 Northridge Earthquake resulted in addition of near-fault factor

to base shear equation, and prohibition on highly irregular structures in

near fault regions. Also, redundancy factor added to design forces.

• The 1997 UBC incorporated a number of important lessons learned from

the 1994 Northridge and the 1995 Kobe earthquake, where four site

coefficients use in the earlier 1994 UBC has been extended to six soil

profiles, which are determined by shear wave velocity, standard

penetration test, and undrained shear strength.

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

Based on R1.1.1.9.1 of ACI 318-08, for UBC 1991 through 1997, Seismic

Zones 0 and 1 are classified as classified as zones of low seismic risk. Thus,

provisions of chapters 1 through 19 and chapter 22 are considered sufficient

for structures located in these zones.



Seismic Zone 2 is classified as a zone of moderate seismic risk, and zones

3 and 4 are classified as zones of high seismic risk. Structures located in

these zones are to be detailed as per chapter 21 of ACI 318-08 Code.



For Seismic Design Categories A and B of IBC 2000 through 2012,

detailing is done according to provisions of chapters 1 through 19 and

chapter 22 of ACI 318-08. Seismic Design Categories C, D, E and F are

detailed as per the provisions of chapter 21.

Detailing Requirements of ACI 318-08

Code/Standard

Level of Seismic Risk

Low

Moderate

High

IBC 2000-2012

SDC A, B

SDC C

SDC D, E, F

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Major Changes from UBC 1994

(1) Soil Profile Types:

The four Site Coefficients S

₁

to S

₄

of the UBC 1994, which are independent of

the level of ground shaking, were expanded to six soil profile types, which are

dependent on the seismic zone factors, in the 1997 UBC (S

_A

to S

_F

) based on

previous earthquake records. The new soil profile types were based on soil

characteristics for the top 30 m of the soil. The shear wave velocity, standard

penetration test and undrained shear strength are used to identify the soil

profile types.

(2) Structural Framing Systems:

In addition to the four basic framing systems (bearing wall, building frame,

moment-resisting frame, and dual), two new structural system classifications

were introduced: cantilevered column systems and shear wall-frame

interaction systems.

(3) Load Combinations:

The 1997 UBC seismic design provisions are based on strength-level design

rather than service-level design.

(4) Earthquake Loads:

In the 1997 UBC, the earthquake load (E ) is a function of both the horizontal

Seismic

Design According To 1997 UBC

The Static Lateral Force Procedure

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Applicability

The static lateral force procedure may be used for the following structures:



All structures, regular or irregular (Table A1), in Seismic Zone no. 1 (Table

A-2) and in Occupancy Categories 4 and 5 (Table A-3) in Seismic Zone 2.



Regular structures under 73 m in height with lateral force resistance

provided by systems given in Table (A-4) except for structures located in soil

profile type SF, that have a period greater than 0.70 sec. (see Table A-5 for

soil profiles).



Irregular structures not more than five stories or 20 m in height.



Structures having a flexible upper portion supported on a rigid lower

portion where both portions of the structure considered separately can be

classified as being regular, the average story stiffness of the lower portion is

at least ten times the average stiffness of the upper portion and the period of

the entire structure is not greater than 1.10 times the period of the upper

Seismic

Design According To 1997 UBC

The Static Lateral Force Procedure

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Design Base Shear, V

The total design base shear in a given direction is to be

determined from the following formula.

The total design base shear need

not exceed

the following:

The total design base shear shall

not be less

than the

following:

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Where

V = total design lateral force or shear at the base.

W = total seismic dead load

In storage and warehouse occupancies, a minimum of 25 % of floor live load is to be

considered.

Total weight of permanent equipment is to be included.

Where a partition load is used in floor design, a load of not less than 50 kg/m

2

_{is to be}

included.

I = Building importance factor given in Table (A-3).

Z = Seismic Zone factor, shown in Table (A-2).

R = response modification factor for lateral force resisting system, shown in Table

(A-4).

C

a

= acceleration-dependent seismic coefficient, shown in Table (A-6).

C

v

= velocity-dependent seismic coefficient, shown in Table (A-7).

T = elastic fundamental period of vibration, in seconds, of the structure in the direction

under consideration evaluated from the following equations:

For reinforced concrete moment-resisting frames,

For other buildings,

Alternatively, for shear walls,

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Where

h

n

= total height of building in meters

A

c

= combined effective area, in m

2

_{, of the shear walls in the first story of}

the structure, given by

D

e

=the length, in meters, of each shear wall in the first story in the direction

parallel to the applied forces.

A

i

= cross-sectional area of individual shear walls in the direction of loads in

m

2

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Table (A-2): Seismic zone factor Z

Note: The zone shall be determined from the seismic zone map

(Graphs A-1 and A-2).

Table (A-3):Occupancy Importance Factors

Tables And Graphs

Zone

1 2A

2B

3

4

Z

_0.075

_0.15

_0.20

_0.30

_0.40

Occupancy Category

Seismic Importance Factor, I

1-Essential

facilities

1.25 2-Hazardous facilities

1.25 3-Special occupancy structures

1.00 4-Standard occupancy

structures

1.00 5-Miscellaneous

structures

1.00 

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Table (A-4): Structural Systems

Tables And Graphs (Contd.)

Lateral-

force

resisting system

description

R

_{Height limit}

Zones 3&4.

(meters)

Bearing Wall

Concrete

shear

walls

4.5

48 Building Frame

Concrete

shear

walls

5.5

73 Moment-Resisting

Frame

SMRF

IMRF

OMRF

8.5

5.5

3.5 N.L

----Dual

Shear

wall

+

SMRF

Shear

wall

+

IMRF

8.5

6.5 N.L

48 Cantilevered Column

Building

Cantilevered

column elements

2.2

10 Shear-wall

Frame

Interaction

5.5

48

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Table (A-6): Seismic coefficient Ca

Footnote: Site-specific geotechnical investigation and dynamic response

analysis shall be performed to determine seismic coefficients for soil

Profile Type .

Tables And Graphs (Contd.)

Soil

Profile

Type

Seismic Zone Factor, Z

Z =0.075

Z = 0.15

Z = 0.2

Z = 0.3

SA

0.06

0.12

0.16

0.24 SB

0.08

0.15

0.20

0.30 SC

0.09

0.18

0.24

0.33 SD

0.12

0.22

0.28

0.36 SE

0.19

0.30

0.34

0.36 SF

See Footnote

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Graph (A-1): Palestine’s seismic zone factors (Source: International

Handbook of Earthquake Engineering , Mario Paz)

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Graph (A-2): Palestine’s seismic zone factors (Source: Annajah

National University)

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Vertical Distribution of Force:

The base shear which is evaluated from the following equation, is distributed over

the height of the building.

Where:

The shear force at each story is given

The overturning moment is given by

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Horizontal Distribution of Force:

The design story shear in any direction, is distributed to the various elements

of the lateral force-resisting system in proportion to their rigidities.

Horizontal Torsional Moment:

The torsional design moment at a given story is given by moment resulting

from eccentricities between applied design lateral forces applied through each

story’s center of mass at levels above the story and the center of stiffness of

the vertical elements of the story, in addition to the accidental torsion

(calculated by displacing the calculated center of mass in each direction a

distance equal to 5 % of the building dimension at that level perpendicular to

the direction of the force under consideration).

Interactions of Shear Walls with Each Other:

In the following figure the slabs act as horizontal diaphragms extending

between cantilever walls and they are expected to ensure that the positions of

the walls, relative to each other, don't change during lateral displacement of

the floors. The flexural resistance of rectangular walls with respect to their

weak axes may be neglected in lateral load analysis.

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The distribution of the total seismic load Fx , or Fy among all cantilever walls

may be approximated by the following expressions:

Fix = Fix’ + Fix’’

and Fiy = Fiy’ + Fiy’’

Where

Fix’ =

load induced in wall by inter-story translation only, in x-direction

Fiy’ =

load induced in wall by inter-story translation only, in y-direction

Fix’’ =

load induced in wall by inter-story torsion only, in x-direction

Fiy’’ =

load induced in wall by inter-story torsion only, in y-direction

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The force resisted by wall i due to inter-story translation, in x-direction, is given by

The force resisted by wall i due to inter-story translation , in y-direction, is given by

The force resisted by wall i due to inter-story torsion, in x-direction, is given by

The force resisted by wall i due to inter-story torsion, in y-direction, is given by Where:

x

i = x-coordinate of a wall w.r.t the C.R of the lateral load resisting system

y

i = y-coordinate of a wall w.r.t the C.R of the lateral load resisting system

e

x = eccentricity resulting from non-coincidence of center of gravity C.G and center of rigidity C.R, in x-direction

e

y = eccentricity resulting from non-coincidence of center of gravity C.G and center of rigidity C.R, in y-direction

F

x = total external load to be resisted by all walls, in x-direction

F

y = total external load to be resisted by all walls, in y-direction

I

x i = second moment of area of a wall about x-axis

I

i y = second moment of area of a wall about y-axis

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 According to Chapters 2 and 21 of ACI 318-02, structural walls are defined as being walls proportioned to resist combinations of shears, moments, and axial forces induced by earthquake motions. A shear wall is a structural wall. Reinforced concrete structural walls are categorized as follows:

 Ordinary reinforced concrete structural walls, which are walls complying with the requirements of Chapters 1 through 18.

 Special reinforced concrete structural walls, which are cast-in-place walls complying with the requirements of 21.2 and 21.7 in addition to the requirements for ordinary reinforced concrete structural walls.

Special Provisions For Earthquake Resistance

According to Clause 1.1.8.3 of ACI 318-02, the seismic risk level of a region is regulated by the legally adopted general building code of which ACI 318-02 forms a part, or determined by local authority.

According to Clauses 1.1.8.1 and 21.2.1.2 of ACI 318-02 in regions of low seismic risk, provisions of Chapter 21 are to be applied (chapters 1 through 18 are applicable).

According to Clause 1.1.8.2 of ACI 318-02, in regions of moderate or high seismic risk, provisions of Chapter 21 are to be satisfied. In regions of moderate seismic risk, ordinary or special shear walls are to be used for resisting forces induced by earthquake motions as specified in Clause 21.2.1.3 of the code.

According to Clause 21.2.1.4 of ACI 318-02, in regions oh high seismic risk, special structural walls complying with 21.2 through 21.10 are to be used for resisting forces induced by earthquake motions.

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Building Frame System:

Based on section 1627 of UBC-1997, it is essentially a complete space frame

that provides support for gravity loads.

Moment Frames:

Based on ACI 2.1, 21.1 and 21.2, are defined as frames in which members

and joints resist forces through flexure, shear, and axial force. Moment

frames are categorized as follows:

Ordinary Moment Frames:

Concrete frames complying with the requirements of Chapters 1 through 18

of the ACI Code. They are used in regions of low-seismic risk.

Intermediate Moment Frames:

Concrete frames complying with the requirements of 21.2.2.3 and 21.12 in

addition to the requirements for ordinary moment frames. They are used in

regions of moderate-seismic risk.

Special Moment Frames: Concrete frames complying with the requirements

of 21.2 through 21.5, in addition to the requirements for ordinary moment

frames. They are used in regions of moderate and high-seismic risks.

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Earthquake Loads

Based on UBC 1630.1.1, horizontal earthquake loads to be used in the above-stated load combinations are determined as follows:

Where:

E = earthquake load resulting from the combination of the horizontal component , and the vertical component,

E _h = the earthquake load due to the base shear, V

E _v = the load effects resulting from the vertical component of the earthquake ground motion and is equal to the addition of to the dead load effects D

Ρ = redundancy factor, to increase the effects of earthquake loads on structures with few lateral force resisting elements (taken as 1.0 where z =0, 1 or 2)

Load Combinations

Loads ACI 818-02 UBC-1997

Dead (D) and Live (L) 1.2 D + 1.6 L 1.32 D + 1.1 L Dead (D), Live (L)

and Earthquake (E)

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The shear wall is designed as a cantilever beam fixed at the base, to transfer load to the foundation. Shear force, bending moment, and axial load are maximum at the base of the wall.

Types of Reinforcement

 To control cracking, shear reinforcement is required in the horizontal and vertical directions, to resist in plane shear forces.

 The vertical reinforcement in the wall serves as flexural reinforcement. If large moment capacity is required, additional reinforcement can be placed at the ends of the wall within the section itself, or within enlargements at the ends. The heavily reinforced or enlarged sections are called boundary elements.

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Shear Design

 According to ACI 11.1.1, nominal shear strength

V

n is given as

Where

V

c is nominal shear strength provided by concrete and

V

s is nominal shear strength provided by the reinforcement.

 Based on ACI 11.10.3,

V

n is limited by the following equation.

 The shear strength provided by concrete

V

c is given by any of the following equations, as applicable.

h

= thickness of wall

d =

effective depth in the direction of bending, may be taken as 0.8

l

w , as stated in ACI 11.10.4

A

g = gross area of wall thickness

N

= factored axial load

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Shear Reinforcement

 When the factored shear force exceeds Ф

V

c/2,

-Horizontal reinforcement ration

ρ

his not to be less than 0.0025. Spacing of this reinforcement

S

2 is not to exceed the smallest of lw/5 , 3h and 45 cm.

- Vertical reinforcement ratio

ρ

n is not to be taken less than

Nor 0.0025.

 According to ACI 11.10.9.1, when the factored shear force

V

u exceeds Ф

V

c , horizontal shear reinforcement must be provided according to the following equation. Spacing of this reinforcement

S

1 is not to exceed the smallest of lw/3, 3h and 45 cm.

Where:

A

v = Area of horizontal shear reinforcement within a distance S2.

Ρ

h

=

ratio of horizontal shear reinforcement area to gross concrete area of vertical section.

Ρ

n = ratio of vertical shear reinforcement area to gross concrete area of horizontal section.

Design of Ordinary Shear Walls

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Flexural Design

 The wall must be designed to resist the bending moment at the base and the axial force produced by the wall weight or the vertical loads it carries. Thus, it is considered as a beam-column.

 For rectangular shear walls containing uniformly distributed vertical reinforcement and subjected to an axial load smaller than that producing balanced failure, the following equation, developed by Cardenas and Magura in ACI SP-36 in 1973, can be used to determine the approximate moment capacity of the wall.

Where:

C = distance from the extreme compression fiber to the neutral axis

l

w = horizontal length of wall

P

u = factored axial compressive load

f

y = yield strength of reinforcement

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Reinforcement