load bearing structure

8/10/2019 Load Bearing Structure

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Design of load-bearing structures19th lecture

Earthquake and buildings

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Contents

Introduction

The effect of earthquake on buildings

Conceptual design of buildings for earthquake

Design of slabs- the principles of design

- design for vertical forces

- design for horizontal forces

Summary


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Introduction 1.

The earthquakes do not kill men however the collapsed

building does.

Charles Richter

The earthquake does not read codes.

Thomas Paulay

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Introduction 2.

Design for earthquake: (Hungary)

It is a requirement of the building law.

Az OTK elrjaZaj- s rezgsvdelem

55. (1) Az ptmnyt s rszeit, szerkezeteit gy kell mretezni smegvalstani, hogy a krnyezetbl hat zaj- s rezgshatsoknak

(pl. szeizmikus s forgalmi rezgshatsoknak) az elrt mrtkben

ellenlljon, illetleg azt meghatrozott mrtkig csillaptsa.


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Introduction 3.There is earthquake in Hungary too !!!!!!

Date Location Magnitude,

M

EMS

intensity

Damage

2006 11. 23. Beregsurny 4,5 VI. Smalldamage

in buildings

2006 12. 31. Gymr 4,1 V. VI. 10 M Ft

building

damage

2007 03. 03. Rpcelak 2,9 No damage

The judgment of earthquakes by geophysicists are

changed.

2008 11.13 Murony 3,5 No damageIV.

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The earthquake 1.

a

b

Fault rupture

R

PP S

P

The origin of earthquake:

Ground surface

Wood-Anderson

seismograph

hypocenter

epicenter

Focaldepth


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Seismic waves:

Characterization by:

- acceleration

- velocity

- amplitude

The ground surface

moves horizontally

and vertically too.

The earthquake 2.

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The effect of movement

of soil on buildings:VibrationLateral forces, movement,

Internal forces,

Possibility of damage, and

collapse.

Horizontal vibration

Vertical vibration

Movement of the soil

Movement of the soil

Torsional vibration

Movement

of the soil

Movement of

the soil

Role of floors:Horizontal and tensional

vibration: distribution oflateral forces between bracing

elements.

Vertical vibration:balancing lateral forces and

transferring them to the

vertical load-bearing

elements.


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The earthquake 3.Measure of the effect of earthquake:

Magnitude:e.g.: Richter magnitude:

- M=logA; (A measured amplitude in m);

- maximal value: M = 9.

Intensity scale:e.g.: MCS-1917; MSK-1964, 1976, 1978;

newer EMS-1992, 1998

- 12 class; takes into account the effect of earthquakes on man, objectsnature and the extent of damage caused in buildings.

Soil acceleration:- Basic data for design; depends on direction and soil.

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The earthquake 4.Seismic map of Hungary:

The reference, horizontal peak ground acceleration,

agR, on rock.

EC8-NA


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The earthquake 5.

A fldrengs s a hatsa az pletekre.

The seismicity in Hungary:

Similar to Australia, but since the earthquake in Newcastle, 1989,

design for earthquake is part the design process.

4-5.

zone

design according to

EN 1998moderate

2-3.

zone

simplified design

method according to

EN 1998

low1. zone

Design methodSeismicityZone

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The earthquake 6.


Measure of the effect of earthquake :

EMS M ag/g 2.zone 3.zone 4-5.zone

I. 0,4

II. 1,5 < 0,001

III. 2,5 0,001 0,007

IV. 3,5 0,006 0,03

V. 4,4 0,015 0,06

VI. 5,2 0,03 0,15

VII. 6,0 0,07 0,36

VIII. 6,7 0,15 0,71

IX. 7,4 0,30 1,53

X. 8,0 0,51 3,06

XI. 8,5 1,53 3,56

XII. 8,9 > 2,04


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The earthquake 7.


European Macroseizmic Scale (EMS):

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The earthquake 8.


European Macroseizmic Scale (EMS):

VII. class: Damaging

- Most people are frightened and try to run outdoors. Many find it

difficult to stand, especially on the upper floors

- Furniture is shifted and top-heavy furniture may be overturned.

Objects fall from shelves in large numbers. Water splashes from

containers, tanks and pools.

- Masonry buildings: large, long cracks on most of the walls;

partitions, end walls, chimneys are collapsed.

- RC buildings: cracks on columns, beams, walls; cracks on partitions,

infill walls; covers and plaster are falling down.


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Conceptual design 1.Seismic response of the building, the expected

damage are strongly influenced by the architectural

design together with quality of materialand

execution.

Principles:

- simplicity, symmetry and regularity in plan;

- regularity in elevation;

- continuity in resistance and stiffness;

- redundancy and robust behavior;

- rigid floors;

- adequate foundation.

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Conceptual design 2.

Even the cleverest calculations and detailed design cannot

compensate for errors and defects in he conceptual design of

the structural and non-structural elements!

Close collaboration between architect and structural

engineer from the earliest planning stage!


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Conceptual design 3.Cost of the structure designed for earthquake

depends on:

design principles,

applied design method.

No significant extra costs thanks to themodern methods!

EN1998

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Avoid soft storey

ground floor!

Avoid soft storey

upper floors!


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Avoid asymmetrical

horizontal bracing!


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Avoid short

columns!

nagy meredeksg

nyomatkbra

nyrsi

tnkremenetel!

Avoid partially

infilled frames!

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Gap is needed!


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Use the slabs to tie in

the elements and

distribute the forces!

Rigid floor.

disadvantegous advantageous

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Supporting load:

- as a plate: - loads due to gravity;- lateral load due to vertical

vibration;

support: vertical structural elements- as a diaphragm: - wind load;

- lateral load due to horizontal

vibration;

- lateral load due to torsional

vibration;

support: vertical bracing elements

Floors 1.


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Floors 2.Floor as a diaphragm:

RIGID FLOOR:Relative idea: depends on the

rigidity (deformation) of the

floor and bracing elements.

Displacement: rigid body

translation, rotation.

FLEXIBLE

FLOOR

SEMI RIGID

FLOOR

Simple calculation methods. 3D building model.

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Floors 3.Making rigid floor:

Building material, building construction

method:

Rigid floors: - cast in situ RC floors;

- prefabricated RC composite floor;

- corrugated metal sheet composite

floor.


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By seismic joints:

NOYES

NO YES

Rigid unitsVertical

displacement

Horizontal

displacem

ent

Changing ground motion

x < 4y

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By additional bracing elements:

Flexible floor Rigid floor


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Avoiding stress concentration:

Locations for stress concentration

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NO

NO

NO

NO

NO

NO NO

NO

NOYES

YES

YES YES

YES

YES

YES

YES

YES YES


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NO NO

NO

NO

YESIGEN

YES

YES

YES

NO

YES YES YESYES YESNO YES

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Analysis 1.

Design of floors for earthquake

according to Eurocode 8

EN 1998 Design of structures for earthquakeresistance: supplement to the other Eurocodestandards and should be used together them.


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Analysis 2.Ultimate Limit State: Resistance

Ed RdEd effect of action for seismic action,

Combination according to: EN 1990: 6.4.3.4

Ed=E{Gk,j; P; Aed; 2,iQk,i}, Gk,j+ Aed+ 2,iQk,i

Design value of seismic action: Aed= IAek,

I importance factor

Rd design resistance of structural elements.

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Analysis 3.

Guidance:

Mass of the building used for dynamic analysis:

Gk,j+ E,iQk,i , E,i= 2,i

Residential building (A):E=0,15; 2=0,3; =0,5 (independently occupied stories)


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Analysis 4.Analysis of the stiffening system:

Static analysis: lateral force method of analysis

Dynamic analysis: modal response spectrum analysis

Both uses response spectra, which considers:

- the effect of soil: soil factor

- type of building: importance class

- the behavior of the stiffening system: behavior factor

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Analysis 5.

Design response spectra:

Period (s)

Designresponsespectra(Sd

/ag

)


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Analysis 6.Analysis of floors for vertical acceleration:

- The vertical mass forces are defined by, avg,

vertical response spectra.

- avg is at most 45 90 % of the horizontal

acceleration.

- avg, max= 0,15gx 0.9 = 0,13g, that is 13% of the

total mass.

- Not necessary to take into account.

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Analysis 7.Analysis of floors for horizontal acceleration :- Analysis of stiffening system results the forces acting

on the shear walls.

1. Find the forces acting on the floor. Calculation of the bending

moment and the shear force in the plane of the floor. (Membranemodel, strut and tie model.)

2. Checking of shear resistance of the floor in its plane.

3. Analysis of thejoint between the floor and the stiffening

elements.

4. Calculation of edge reinforcement, reinforcement in ring beams.

5. Calculation the deflection of the floor in its plane. Is the floor

rigid?


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Analysis 8.Analysis of floors for horizontal acceleration:

Edge reinforcement, reinforcement in ring beams:

Reinforcement of the ring beam is lapped as usual.

Edge beams and cross beams are used.

Crossbeams

Cross

beams

Edge beam

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Analysis 9.Analysis of floors for horizontal acceleration:

Example: four storey building, stiffened by RC walls, (Dulcska-Kollr)

1. zone: ag = 0,08g

320 kN

641 kN

1112 kN

961 kN

Continuous ring beam


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Analysis 10.Example: four storey building, stiffened by RC walls, (Dulcska-Kollr)

V

M

111,2

444,8

222,4

3336

27,8

1112

416

Compressed column

283

Top floor:

Continuous ring beam

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Composite floor 1.

szls tmaszkzbens tmasz

a fels vasals 33%-avasals a tmasz felett

vasals a tmasz felett

a fvasals 33%-a

kapcsol vasals

a kregpanel feltmaszkodik a falra

a kregpanel nem tmaszkodik fel a falra


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Composite floor 2.

Joint between prefabricated element and cast in situ concrete:

Direct connection is needed for the dynamic load.

Friction is diminishes, plates may buckle.

kapcsol elemekacl trrcs

egyttdolgozst biztost elemek elrendezse

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Composite floor 3.

Free edge:

Edge beam is important.

No Yes Yes

fggnyfal fggnyfal fggnyfal

A szabadszegly kialaktsa


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Literature:Chopra, A.K.:Dynamics of structures. Theory and

applications to earthquake engineering.

Prentice Hall, New Jersey 1995.

Charleson, A.: Seismic Design for Architects.

Elsevier Ltd., Architectural Press 2008.

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