Seismic Design of Reinforced Concrete...

1

Chapter 5

Earthquake Effect and

Seismic Design principles

Chapter 5 Earthquake Effect and

Seismic Design principles

5.1 Seismic conceptual design

5.2 Building classification and seismic fortification

5.3 Calculation Methodology of seismic action

5.4 Seismic Check of structural members and structural

lateral deformations

In the very early stage of building design, the configuration,

the basic material, structure, and framing of the building

have to be chosen.

The architects and the structural engineers should therefore

cooperate and thoroughly discuss the matter at this early

stage.

Seismic design in this stage is generally termed

seismic conceptual design


Bamboo

• Since earthquake is a stochastic process, the seismic action that

the building might be expected to be suffered in its life time

can not be quantified accurately.

• A building is not a homogenous block, but a complicated

assembly of parts. Considerable simplifications are always

needed in the structural analysis.

The building with good seismic performance can

hardly be obtained only by “accurate calculation’’.


• The desirable aspects of building configuration are

simplicity, regularity, and symmetry in both plan and

elevation.

• Irregularities, often unavoidable, contribute to the

complexity of structural behavior. Any irregularity in the

distribution of stiffness or mass is likely to lead to an

increased dynamic response.

5.1.1 Configuration Characteristics

• The first task of the designer will be to select a structural

system most conductive to satisfactory seismic performance

within the constraints dictated by architectural requirements. • Configuration is generally defined as building size and

shape, and the characteristic of proportion.

• The extended definition also includes the location of the

structural elements. And location of nonstructural

elements.

5.1.1 Configuration Characteristics

• Configuration largely determines the ways in which seismic

forces are distributed throughout the building, and also

influences the relative magnitude of those forces.

2

1. Building Height

• As a building grows taller, its period will tend to increase, and a

change in period means a change in the building response.

• It is easy to visualize the overturning forces associated with

height as a seismic problem.

• In Chinese Code for Seismic Design of Buildings, height limits

are imposed, relating to type of structure and earthquake

intensity. With the increasing of earthquake intensity, the

allowed maximum height is decreased.

Structure Noun Intensity

6、7

Intensity

8

Intensity

9

Frame 5 4 3 /

Plate-wall 6 5 4 /

Frame-wall, wall 7 6 5 4

Frame-tube 8 7 6 4

Tube in tube 8 8 7 5

Question： What is the purpose

for the requirements?

2. Aspect Ratio

• The term symmetry denotes a geometrical property of

building plan configuration.

• Structural symmetry means that the center of mass and

center of resistance (center of rigidity) are located at, or

close to the same point (unless live loads affect the actual

center of mass).

• Symmetrical forms are preferred.

3. Plan Configuration

T-shaped plan L-shaped plan

U-shaped plan Cruciform plan

Source from :1980 SEAOC Recommended Lateral Force Requirements and Commentary

Plan Arrangement — Irregularity

Split levels

Other complex shapes Setbacks

Multiple tower

Unusual high story Unusual low story

nonuniform mass distribution, or

converse Soft lower levels

3

Large openings in shear walls Interruption of columns

Interruption of beams Openings in diaphragms

Shear walls in some stories,

moment-resisting frames in

others

Interruption of vertical-

resisting elements

Abrupt changes in size of

members

Drastic changes in

mass/stiffness ratio

Cable-supported structures Shells

Staggered trusses Buildings on hillsides

Plan Arrangement — Regularity

Intensity L/B l /Bmax l / b

6，7 6.0 0.35 2.0

8，9 5.0 0.30 1.5

Length-Width ratio —— Control of torsion

Source from : Code of Seismic Design of Building （Code GB50011-2010)

Plan irregularity

Type of

irregularity

Definition

Torsional

irregularity

The maximum elastic floor displacement or inter-story

drift is more than 1.2 times of the corresponding average

of two ends of the floor.

Irregularity of

reentrant

corners

The projection beyond a reentrant corner is greater than 30

percent of the total plan dimension in the given direction.

Diaphragm

discontinuity

The dimensions and stiffness of diaphragm change

abruptly, including those having the effective width of

diaphragm less than 50 percent of the typical width, the

cutout or open area greater than 30 percent of the gross

enclosed floor area, or staggered floor.

In Chinese Code for Seismic Design of Buildings Reentrant corner

max3.0 BB

maxB maxBmax3.0 BB

maxB

max3.0 BB maxB

max3.0 BB

maxB

max3.0 BB

maxB

max3.0 BB max3.0 LL

maxL

4

Diaphragm Discontinuity

Bb 5.0B

B AA 3.00 LBA

• Generally the torsional response will inevitably occur even in

the symmetrical structure when attacked by the earthquake.

• In general case torsion arises from eccentricity in the building

layout. The accidental eccentricity are very likely caused by

construction which may change the actual center of resistance,

or the distribution of live loads in occupancy which affects the

actual center of mass.

Torsional irregularity

3. Plan Configuration

• The effective force exerted by lateral ground movement acts at

the center of mass of each floor creating a torsional moment

about the center of structural resistance.

• In addition, there exists torsional component in earthquake

ground motion.

Plan Configuration

Seismic force

C

Separated buildings The L-shaped building

• If the ground motion occurs with a north-south emphasis at the L-shaped

building, the wing oriented north-south will, tend to be stiffer than the wing

oriented east-west.

• The north-south wing, if it were a separate building, would tend to be deflect

less than the east-west wing, but the two wings are tied together and attempt to

move differentially at their notch, pulling and pushing each other.

4. Separation

Temperature Gap

When the length of a Frame (Cast-in-situ) is larger than 55m, or a wall

larger than 45m, it is better to set the Temperature Gap

Settlement gap

Between the main tower and the podiums and between the different kind

of foundations.

Seismic gap/joint

separating the irregular plan to regular plan，in case to reduce torsion.


The width of a gap: d

Frame System

when H15m，dframe＝70mm

in the area of intensity 6, 7,8, 9, every 5m、4m、3m、2m taller, +20mm

wider。

Frame-wall system, dframe-wall＝ 70%*dframe

Wall system, dwall ＝ 50%*dframe

Question: 1. Why the widths of gaps are different ? 2. What is the main reason?

3. How to reduce the unfavorable deformation?


5

• Regularity should prevail in elevation, in both the geometry and

the variation of story stiffness and strength, so as not to result

in soft story or weak story.

• The vertical configuration comprises uniformity and continuity,

avoiding drastic changes.

• The shape of rectangle, trapezoid, or triangle without abrupt

change is preferable.

• In Chinese Code for Seismic Design of Buildings, some

critical vertical irregularities are defined and quantified.

5. Vertical Configuration

Type of

irregularity

Definition

Lateral stiffness

irregularity

The lateral stiffness of the story is less than 70 percent of

that of the story above or less than 80 percent of the

average lateral stiffness of the three stories above; The

horizontal size of setback is larger than 25 percent of

total size of the adjacent lower story.

Discontinuity in

vertical lateral

force resisting

member

Loads applied on the vertical lateral force resisting

member (column, shear wall, and brace) are transferred

downward by horizontal transfer member (beam, truss

etc.).

Abrupt change

of story carrying

capacity

The shear strength of the story is less than 80 percent of

that of the story above.

Vertical irregularity

Discontinuity in vertical lateral force resisting system

1iK

iKi

i

iu

VK

10.7i iK K

iu

iV

inter-story drift of

the ith story

shear force of the

ith story

1 2 30.8( )3

i i ii

K K KK

3iK

2iK

1iK

iK

Lateral stiffness irregularity (soft story)

Abrupt changeof story carrying capacity (weak story)

1, iyQ

iyQ , , , 10.8y i y iQ Q

• Pure cases of soft-story or weak-story failures are rare and

generally the same floor is both soft and weak, therefore

justifying the use of the term soft/weak story instead.

• The soft/weak story problem is commonly magnified by

torsional response.

• Probably among all urban habitat structural problems, the

soft/weak story failures have been responsible for more

deaths and destruction than any other.

Vertical Configuration

6

• The soft/weak stories have consistently claimed a large

number of lives and have caused serious destruction during

many 20th century earthquakes.

• From Turkey to Taiwan and from northern California to

southern California , the mark of soft/weakstory failures

can be seen on the face of many collapsed structures.

Vertical Configuration Soft/weak story collapse of a residential complex during

the 1906 San Francisco Earthquake

Soft/weak story failure of a residential reinforced concrete

complex during the 1999 Turkey earthquake

Soft/weak story failure of a reinforced concrete

building during the 1999 Taiwan earthquake

Soft-story failure of an open-front building during

the 1989 Loma Prieta earthquake

1.The structure with plan irregularity

• The three-dimensional analytical model must be applied in the

structural analysis.

5.1.2 Additional requirements for the analysis of

irregular configurations

7

2.The structure with vertical irregularity

• The story shear force of the soft/weak story must be increased

by 15%.

• The elasto-plastic analysis must be conducted for the tall

building exceeding the specified height.

• The shear strength of the weak story should not be less than 65

percent of that of the adjacent upper story for the structure

with abrupt change of story carrying capacity.

3. The structure with both plan irregularity

and vertical irregularity

• The above two terms must be followed simultaneously.

• In addition, for the structure with seriously irregularity, time

history analysis must be carried out to check the analytic

results obtained by modal analysis.

• For the building with seriously irregularly configuration,

seismic joints can be provided to separate the building into

simple and regular individual units. There must be enough

clearance at the seismic joints so that the adjoining portions do

not pound each other.

• The design characteristic of redundancy plays an important role in

seismic performance.

• Placing resisting members on the perimeter whenever possible is

always desirable.

• The detailing of connections is a key factor in seismic performance,

since the more integrated and interconnected a structure is the

more load distribution possibilities there are.

• The lower of the density center of a building, the better resistant

performance.

5.1.3 Configuration Influences on Seismic Performance

Better

5.1.4 Essentials of Structural System

1. Selection of materials and types of construction

• Structural materials have their own performance characteristics

and should be selected according to the location and condition

of the building to be planned in order to accomplish safe,

economical, and superior architecture.

• High strength-to-weight ratio

• High deformability

• Low degradation

• High uniformity

• Reasonable cost

• Steel structures

• Steel and reinforced-concrete composite structures

• Wooden structures

• Cast-in-situ reinforced concrete structures

• Precast concrete structures

• Prestressed concrete structures

• Masonry structures

• Mixed structures

2. Major Characteristics of Building structures

Requirements for seismic structural system

1. It shall have a clear analytical model and reasonable path for

seismic action transfer.

2. It should have several lines of defense against earthquakes. It

should avoid loss of either earthquake resistance capacity or

gravity load capacity of the whole system due to damage to

part of the structure or members.

3. It shall possess the necessary strength, adequate

deformability, and better energy dissipation ability.

Summary

8

4. It should possess a rational distribution of stiffness and

strength, avoid weakening of some parts of the structure due

to local weakening or abrupt changes; avoid appearance of

extremely large concentration of stress and plastic

deformation; when weak parts do appear, measures should be

taken to enhance their earthquake resistance capacity.

5. It should have similar dynamic characteristics in the direction

of individual primary axis.

6. Designing the connections and details of a structure to be

earthquake resistant is almost as important as checking the

structure’s overall dynamic behavior.

Simplicity

Regular

Symmetry

Integral Construction

Redundancy

Seismic

Conceptual Design

Three-level

1. Seismic fortification objectives

5.2 Building classification and seismic fortification

1) The first level

When buildings designed based on the code are subjected to the

influence of frequently occurred earthquakes with an intensity of

less than the fortification intensity of the region, they will not be, or

will be only slightly damaged and will continue to be serviceable

without repair.

------------- No damage under minor earthquake.

2) The second level

When buildings are subjected to the influence of earthquakes equal to the

fortification intensity of the region, they may be damaged but will still be

serviceable after ordinary repair or without repair.

------ Repairable under moderate earthquake

3) The third level

When buildings are subjected to the influence of expected rare

earthquakes with an intensity higher than the fortification intensity of

the region, they will not collapse nor suffer damage that would

endanger human lives.

------------- No collapse under major earthquake.

1) The first level

By the elastic analysis, the carrying capacity of the structure is checked

under the fundamental combination of effects of seismic action of minor

earthquake and other loads, and the elastic seismic deformation is checked

under the action of minor earthquake.

Calculation is necessary under the first level.

2. Seismic design method

2) The second level

The objective of this level is realized mainly by seismic conceptual

design and constructional measures or detailing.

3) The third level

The elasto-plastic deformation is checked under the action under rare

earthquake.

Two-stage

In Summary:

Level 1 Level 2 Level 3

minor earthquake

level

moderate

earthquake level

major

earthquake level

No damage Repairable No collapse

1) Elastic force

2) Elastic deformation

Structural detailing 1) Elastoplastic

deformation

2. Seismic design method

9

49

Probability Density Function (PDF) of Earthquake

Intensity is look like the 3rd Extreme Value Distribution

(地震烈度的概率密度函数符合极限Ⅲ型分布）

( )( )

( )

The maximum value of Earthquake Intencity = ;

Earthquake Intencity

Shape Parameter;

Mean Intensity.

Ik 1 k

k

k If I e

12

I

k

，

；

3. minor / moderate / major earthquake

50

the 3rd Extreme Value Distribution

Exceedance Probability (EP)

PDF

Eq. Intensity

3. minor / major / major earthquake

51

Distribution Function:

When ( ) . %

EP ( ) . %

1I F I e 36 8

1 F I 63 2

：

：

kI

eIF

)(

EP in Design Reference

Period（设计基准期内的

超越概率）is 10%

Basic Intensity

Moderate Earthquake

Mean Intensity：Minor Earthquake


EP in Design Reference

Period is 2~3%

Rare occurred Intensity

Major Earthquake

52

Fortification Intensity（设防烈度）：

In 50 years, if EP is 10%，

the return years is 475。

Frequently occurred Intensity

(小震，多遇地震)：

In 50 years, if EP is 63.5%，

the return years is 50；

It is about 1.55 grade lower than Fortification Intensity。

Rare Occurred Intensity（大震，罕遇地震）：

In 50 years, if EP is 3%~2%，the return years is 1461~2475。

It is about 1 grade higher than Fortification Intensity。


53

1. 中国国家标准建筑抗震设防分类标准（GB50223)，根据

建筑物重要性、在地震中及地震后破坏对社会和经济造

成的影响及在抗震防灾中的作用，将建筑分为四类：

• 甲类建筑(特殊设防类)

• 乙类建筑(重点设防类）

• 丙类建筑(标准设防类)

• 丁类建筑(适度设防类)

5.2 建筑的分类与建筑抗震设防

Q: 为什么要分类？

如何分类？

如何设防？

一、建筑抗震设防分类

54

•甲类建筑(特殊设防类)——地震破坏后会对社会有严重

影响，对国民经济有巨大损失或有特殊要求的建筑物。专

门研究确定。如生命线工程。

•乙类建筑(重点设防类)——地震中使用功能不能中断或

需迅速恢复的建筑物及破坏后会对社会有重大影响，对国

民经济有重大损失。如医院、中小学校校舍、大型商场或

人员集中的公共建筑等。


10

55

•丙类建筑(标准设防类)——地震破坏后对社会有一般影

响，对国民经济有一般损失的建筑物。如一般工业与民用

建筑、公共建筑、住宅、旅馆、厂房等。

•丁类建筑(适度设防类)——地震破坏后对社会影响轻微，

对国民经济影响轻微，且不致影响其它甲、乙、丙类建筑

的建筑物。如临时建筑或附属建筑物等。


56

1. 甲类建筑，地震作用应高于本地区抗震设防烈度的要

求，其值应按批准的地震安全性评价结果确定。6〜8

度设防时，提高1度进行抗震设防，9度时应比9度设防

更高的要求。

2. 乙类建筑：地震作用应符合本地区抗震设防烈度的要

求，一般情况6〜8度时，提高1度采取抗震措施，9度

时应比9度设防更高的要求。

二、抗震设防标准抗震作用计算、构造措施


57

3. 丙类建筑：地震作用和抗震措施均应符合本地区抗震

设防烈度要求。

4. 丁类建筑：一般情况下，地震作用应符合本地区抗震

设防烈度要求，抗震措施可适当降低，但6度抗震时不

降低。

5. 抗震设防烈度为6度时，除特殊要求外，一般情况下对

乙类、丙类和丁类建筑可不进行地震作用计算。

（高层建筑需进行地震作用下和计算）

5.2 建筑的分类与建筑抗震设防 Summary

Building classification and seismic fortification

Review and read this chapter, please!

Do Group work, please!

Thanks!

Seismic Design of Reinforced Concrete...

Documents

Transcript of Seismic Design of Reinforced Concrete...