Braced Structures

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Earthquake Resistance

Philosophy of Structures

Date April 5, 2002.

Sanaullah Khan Kakar

Contents



Shear Walls



Moment Resisting Frames



Braced Structures



Framed Tube Structures



Hybrid Structures



Conclusion

Shear Walls

First

used

in

1940
,

may

be

described

as

vertical,

cantilevered

beams,

which

resist

lateral

wind

and

seismic

loads

acting

on

a

building

transmitted

to

them

by

the

floor

diaphragms
.



Effects in Earthquake

Building frame system
with Shear Walls


Shear Walls

Typical arrangement of shear walls


Shear Walls

A

simple

building

with

shear

walls

at

its

ends
.

Ground

motion

enters

the

building

and

creates

inertial

forces

which

move

the

floor

diaphragms
.

This

movement

is

resisted

by

the

shear

walls,

and

the

forces

are

transmitted

back

down

to

the

foundation
.


Shear wall: vertical analogy
as cantilever beams


Moment
-
Resisting Frames

Moment
-
resisting

frames

are

structures

having

the

traditional

beam
-
column

framing
.

They

carry

the

gravity

loads

that

are

imposed

on

the

floor

system
.

The

floors

also

function

as

horizontal

diaphragm

elements

that

transfer

lateral

forces

to

the

girders

and

columns
.

In

addition,

the

girders

resist

high

moments

and

shears

at

the

ends

of

their

lengths,

which

are,

in

turn,

transferred

to

the

column

system
.

As

a

result,

columns

and

beams

can

become

quite

large
.

Moment Resisting
Frame System


Effects of Earthquake


Moment
-
Resisting Frames

Moment
-
Resisting Frames



Moment
-
resisting

frames

can

be

constructed

of

steel,

concrete,

or

masonry
.




Consist

of

beams

and

columns

in

which

bending

of

these

members

provides

the

resistance

to

lateral

forces
.



There

are

two

primary

types

of

moment

frames,

ordinary

and

special
.



Special

moment
-
resisting

frames

are

detailed

to

ensure

ductile

behavior

of

the

beam
-
to
-
column

joints

and

are

normally

used

in

zones

of

higher

seismicity
.




Steel

moment
-
resisting

frames

have

been

under

intensive

study

and

testing
.


Braced Structures

Braced

frames

develop

their

resistance

to

lateral

forces

by

the

bracing

action

of

diagonal

members
.

The

braces

induce

forces

in

the

associated

beams

and

columns

so

that

all

work

together

like

a

truss

with

all

members

subjected

to

stresses

that

are

primarily

axial
.


Braced Structure


Braced Structures



Braced

frames

act

in

the

same

manner

as

shear

walls,

though

they

may

be

of

lower

resistance

depending

on

their

detailed

design
.



Bracing

generally

takes

the

form

of

steel

rolled

sections,

circular

bar

sections,

or

tubes
;

vibrating

forces

may

cause

it

to

elongate

or

compress,

in

this

case

it

loses

its

effectiveness

and

permits

large

deformations

or

collapse

of

the

vertical

structure
.


Braced Structure


Braced Structures

Comparison of Moment
-
Resisting Frames and Braced Structures


Bracing

can

be

used

to

stop

buildings

swaying

over
.

It

helps

buildings

stand

up

to

the

sideways

forces

that

can

occur

during

earthquakes

or

high

winds
.

Bracing

members

can

work

in

tension

or

in

compression
.

Effects of Earthquake


Braced Structures

Framed Tube Structures

A framed tube structure is a new addition to
the structural systems. Khan is generally
credited with its invention in the 1960s.


"a

three

dimensional

space

structure

composed

of

three,

four,

or

possibly

more

frames,

braced

frames,

or

shear

walls,

joined

at

or

near

their

edges

to

form

a

vertical

tube
-
like

structural

system

capable

of

resisting

lateral

forces

in

any

direction

by

cantilevering

from

the

foundation
.
"


Dr.Fazlur Rehman Khan

Framed Tube Structures

The

framed
-
tube

structure

has

its

columns

closely

spaced

around

the

perimeter

of

the

building,

rather

than

scattered

throughout

the

footprint,

while

stiff

spandrel

beams

connect

these

columns

at

every

floor

level
.

This

structural

system

was

first

implemented

in

1964

in

the

construction

of

the

DeWitt
-
Chestnut

Apartments

in

Chicago,

a

43
-
story

reinforced

concrete

tower

designed

by

Dr
.
Fazlur

Rehman

Khan

and

his

colleagues

at

Skidmore,

Owings

&

Merrill

(SOM)
.

Because

of

its

great

relative

strength

and

stiffness,

the

tubular

form

immediately

became

a

standard

in

high
-
rise

design
.


The revolutionary framed
-
tube
structural system was first seen
in the construction of the
DeWitt
-
Chestnut Apartments in
Chicago.


Looking

up

at

the

Sears

Tower,

the

worlds

tallest

building

for

over

twenty

years
.

The

design

for

this

1454
-
foot

tall

tower

introduced

the

bundled

tube

structural

system,

as

well

as

a

new

vocabulary

in

architectural

form
.



Framed Tube Structures

In

1998

the

city

of

Chicago

named

the

intersection

of

Jackson

and

Franklin

Streets

(located

at

the

foot

of

the

Sears

Tower)

“Fazlur

R
.

Khan

Way
.


Framed Tube Structures

Hybrid Structures

Structures

are

often

built

in

which

the

lateral

resistance

is

provided

by

a

mixture

of

structures
.

The

most

common

are

moment

resisting

frames

combined

either

with

structural

walls

or

diagonal

bracing
.

Both

externally

and

internally

the

new

Toyota

building

is

visually

exciting



Hybrid Structures



£
25

million

Toyota

headquarters

on

the

North

Downs

near

Epsom

racecourse

in

Surrey
.



The

hybrid

structural

frame

which

successfully

combines

precast

and

insitu

concrete

with

structural

steelwork
.



The

resulting

hybrid

structure

was

erected

at

a

speed

to

rival

that

of

steel

construction

and

has

the

bonus

of

a

high

quality

exposed

finish

that

absorbs

temperature

fluctuations
.

To

paraphrase

the

Toyota

advertisement
:

“the

structure

in

front

is

a

hybrid”
.

There

is

a

refined

juxtaposition

of

the

structural

elements

Conclusion

Structural systems that are suitable for
different ranges of number of stories