Ar 351 Deprem Sunum 2 Li

8/3/2019 Ar 351 Deprem Sunum 2 Li

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IZMIR INSTITUTE OF TECHNOLOGY, DEPARTMENT OF ARCHITECTURE

Tuba NAN

STRUCTURAL IRREGULARITIESAND

SOLUTION SUGGESTIONS

Turkey, with more than 90% of its land being in

highly seismic regions, is an earthquake prone

country and has been struck by many

devastating earthquakes throughout history.

Therefore, it is too significant to design

earthquake resistant buildings in order to defend

the structures against significant earthquake

loads.

Earthquakes


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Turkey is located on Anatolian Peninsula on the Alp

Himalayan earthquake belt that is seismically active

region in the world. As a result of this, a great deal of

destructive earthquakes has happened in Turkey.

Seismic map of Turkey


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Earthquake directions

Earthquake Resistant Design

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Earthquake Resistant Building Design depends on:

1. Architectural Design

2. Structural Configuration

3. Seismic Zone

4. Soil Type

5. Proper Construction6. Proper Detailing

7. Building Inspection

ERD

Architects Civil Engineer

Building Constructor Municipalities


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Structural Irregularities according to TEC

Torsional Irregularity (A1)

Floor Discontinuity (A2)

Projections in Plan(A3)

Nonparallel Axis (A4)

Weak Storey(B1)

Soft Storey(B2)

Discontinuity of VerticalStructural Elements (B3)

1. Irregularities in Plan 2. Irregularities in Vertical

Direction

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Irregularities in Plan

Torsional irregularity is defined in the TEC-2007 as

the the case where torsional irregularity factorbiwhich is defined for any of the two orthogonal

earthquake directions as the ratio of the maximumstorey drift at any storey to the average storey drift

at the same storey in the same direction, is greater

than 1.2, as follows:

A1- Torsional Irregularity

bi = avgii max > 1.2


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A1- Torsional Irregularity

EARTHQUAKE

i(min)

i(max)

bi = avgii max > 1.2

Rigidity Center

ex

ey

EARTHQUAKE LOAD

BX

By

R

G Gravity Center

STRENGTH STRENGTH

EARTHQUAKE LOAD

G and RM

Gravity and Rigidity Centre


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the factors causing torsional irregularity can be

categorized as follows:

1. The plan geometry

2. Rigidity distribution

Effective factors on torsional irregularity

The plan geometry/ Form

Simple plan geometry Complex plan geometry


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Rigidity distribution

GR

ex

ey

DEPREM

GR

ex

ey

DEPREM

Rijit

ekirdek MRijit

ekirdek

Rigid core and torsion

Rigidity distribution

Different collapses

G

RM

M

GR


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Solutions for torsional irregularity

To separate the complex forms into simple and compact

forms by using seismic separation joints.


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Softening of acute angle reentrant corners:

b (a) b (b)

3 1.21 1.192 1.19 1.181 1.18 1.17

Ground 1.17 1.16


Strengthening of acute angle reentrant corners:

According to this solution the buildings, which have

acute angle corners such as the plan geometry ofL

and T type, are strengthened at weak points called

notch points by vertical structural members. Thismethod is widely used in America and Japon.


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Strengthening of acute angle reentrant corners:


Strengthening offlexible sides:

This solution suggests that rigid cores orstability walls can

be used for preventing deformation between the wings of the

building in case the rigidity center and the gravity center of the

building do not coincide due to the plan geometry of the

building.

For instance, open facades creates unbalanced perimeter in

a building. Moreover, this causes the formation of the rigid and

flexible sides in a building.


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Regular configuration of structural elements

The Vertical structural members should be orderedregularly in all directions.


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The vertical structural members should be connected with

beams to form a rectangular frame and provide the continuity

in rectangular frames.

It is desired that one should avoid from the beam-to-beam

connection (anchorage beam) without any vertical support.

Discontinuity of beams


One-way slabs cause large deformations and unexpected

shear stresses on the structural members. It should be

prevented. Slabs should work on both directions.

In Turkey, over-stretched one-way slabs are often used to

generate corridors in the apartment block projects. Both open and closed cantilever projections are widely

constructed in Turkey. If it is necessary to use, the continuity

between the beams is to be provided under the cantilever

slabs. Moreover, a side beam should be designed to prevent

critical displacement.


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Symmetric according to both

plan geometry and rigiditydistribution

Despite the symmetrical plan geometry and rigidity

distribution, structural elements type, their location in

plan and their sufficiency play the most effective role

on earthquake behaviour of structures.

If the structure has a central rigid core, it must be

supported with shear walls which are located on the

outer axis of the structure; otherwise it exposes to

high torsional irregularity.


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Irregular rigidity distribution

The models show favorable results against earthquake

loads coming from symmetry direction.

Asymmetry in the rigidity distribution cause excessive

torsional irregularity despite the symmetrical plan

geometry.

Shear walls should be positioned symmetrically toprovide similar rigidity distribution on both

earthquake directions.


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A2-Floor Discontinuity

According to the TEC-2007, type A2 irregularity which is

called floor discontinuities are described as follows:

In any floor;

I - The case where the total area of the openings including

those of stairs and elevator shafts exceeds 1/3 of the gross

floor area,

II The cases where local floor openings make it difficult

the safe transfer of seismic loads to vertical structuralelements,

III The cases of abrupt reductions in the in-plane stiffness

and strength of floors.


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(a)(b)

(c) (d)

If the ratio between the total areas of openings to the gross

floor area is greater than 1/3, the diaphragm should be

divided into simple and regular forms to provide the

continuity in the distribution of the earthquake forces on

slabs, and subsequently to the columns and shear walls.

The reinforcement around the corners and edges of the

openings may contribute to the continuity in floors.

The rigidity of the columns and beams around the openings

should be increased to balance the rigidity between floors.

Solutions for Floor discontinuity


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The projection ratio has significant role on earthquake

behaviour of structures. A3 irregularity which is called

projections in plan is the cases where projections

beyond the re-entrant corners in both of the two principal

directions in plan exceed the total plan dimensions of the

building in the respective directions by more than 20 %

(TEC, 2007).

A3- Projections in Plan

x>0.20Lx or y>0.20Ly


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Solutions for A3 irregularity

Seismic joints


The TEC-2007 describes the A4 type of irregularity which

is called Nonparallel Axes of Structural Elements as the

cases where the principal axes of vertical structural

elements in plan are not parallel to the consideredorthogonal earthquake directions.

A4- Nonparalel axis:


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A4- Nonparalel axis:

To separate the buiding to the regular and

simple parts by using sesimic separation joints.

Increasing the internal force values.

Solutions for A4 type of irregularity


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B1 type of irregularity is defined in the TEC-

2007 that in reinforced concrete buildings, the

case where in each of the orthogonal

earthquake directions, Strength Irregularity

Factorci, which is defined as the ratio of theeffective shear area of any storey to the effective

shear area of the storey immediately above, isless than 0.80.

Irregularities in Vertical directionB1- Weak storey

If the ratio is between 0.8 and 0.6, there exists

weak storey irregularity in structure.

If it is less than 0.6, the structure must beredesigned until appropriate range of values are

gained.

B1-Weak storey


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Ae = Aw + Ag + 0.15 Akci = (Ae)i / (Ae)i+1 < 0.80

Aw: Effective shear area

of column

Ag: Effective shear area ofshear walls

Ak: Effective shear area of

wall

B1-Weak storey / interstorey strength

irregularity

Earthquake loads are directly proportional with

the mass. Overturning moments will increase if

the gravity center moves from ground to the

upper levels.

B1- Weak storey

Inverted pyramid


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B1- Weak storeyVertical setbacks:

B1-Weak storey

Considerable overhangs


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Deficiency of shear walls in the ground floor

B1-Weak storey

To create partly setbacks as pyramidal

configuration (Figure 3.18)

To create seismic separation joints

To provide equal strength between stories To leave joint between column and wall

To make isolation

Solutions for B1- Weak storey


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Solutions for B1- Weak storey

B2 type of irregularity is defined in the TEC-2007 as the

case where in each of the two orthogonal earthquake

directions, stiffness irregularity factorki, which is definedas the ratio of the average storey drift at any storey to the

average storey drift at the storey immediately above or

below, is greater than 2.0

B2- Soft storey / Interstorey stiffness

irregularity


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[ki = (i/hi)ort / (i+1/hi+1)ort > 2.0 orki = (i /hi)ort / (i1/hi1)ort > 2.0]


irregularity


irregularity


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a) Add bracing elements which stiffen thecolumns up to a level

b) Add additional columns at ground storey toincrease the stiffness

c) Increase the cross-sections of the columns atfirst storey.

d) Add external buttresses

e) Create vaults on the ground floor

Solutions for B2- Soft storey


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Solutions for B2- Soft storey

a) Gusseted columns or the columns which rest oncantilever beams are prohibited

b) In the case where a column rest on a beam supportedwith columns at both ends, all internal forces consistingvertical loads and seismic loads from the earthquakedirection shall be increased by 50 % at all sections ofthe all beams and the columns which are adjacent tothe beam

B3-Discontinuity of Vertical Structural

Elements


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c) In no case the shear walls should be allowed to

rest under the columns

d) In no case the shear walls should be allowed to

rest on the beams


Elements


Elements


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Short column effect

Mezzanine floors

Mechanical floors

Hillside sides

Graded foundation

Adjacent columns to the openings

Stair landing


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Strong beam-weak column


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Seismic pounding effect

There are various parameters causing irregularity of

pounding in structures. They can be listed as follows:

a) Soft ground floors

b) Irregular plan geometry

c) Setbacks

d) Liquefaction

Pounding due to the torsion


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Pounding due to the liquefaction


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Minimum size of the seismic gaps should be 30

mm up to 6 m height. From thereon a minimum 10

mm shall be added for every 3 m height increment.

Seismic gaps


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Conclusions

Effective Factors for Structural Irregularities:

Architectural form (Plan Geometry)

Configuration of structural elements

(Rigidity Distribution)

The number of structural axis

The number of stories

Floor opening ratio

Positions of floor openings in plan

Positions of Shear walls according to the floor openingsProjection ratio & Projection direction or symmetry condition

Overhang directions & Symmetry in plan geometry & Building mass

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Structural configuration is much more important than plan geometry

Thank you for your interest

Tuba NAN

Ar 351 Deprem Sunum 2 Li

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