Basis of structural design 1

8/10/2019 Basis of structural design 1

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Some structures can fail

12.02.2009. Mall under construction in Oradea




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19.12.2008 failure of a silo near Vinga

Design criteria

Suitability for its function: a building should be designed

and realised in a manner that will offer to its users acertain function

Safety and serviceability:

Structures should resist loads and other external actions withoutcollapse, protecting its inhabitants

Structures should not develop excessive deformations andcracks, nor vibrate alarmingly

Aesthetics: buildings should be aesthetically pleasant,both individually and as a group

Economy: generally, the above three criteria need to befulfilled with a limited budget

Cost to design and build a structure

Maintenance cost during the planned life


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Structural materials

A building consists of the structure and othercomponents used in order to protect and provide forbuilding function and aesthetics (cladding, partitions,floors, etc.)

Structural material is the one which is used in those partsof the structure which carry loads and give it strengthand stiffness

Properties ofstructural materials:

strength

stiffness

ductility

.

.

.

deformation

Structural materials: properties

Strength (ultimate stress): the

stress (load per unit area of thecross-section) at which thefailure takes place

tension

compression

Stiffness: the resistance of anelastic body to deformation

Ductility: capacity of the material

to deform into the inelastic rangewithout significant loss of itsload-bearing capacity

ductility

stiffness

strength

force

deformation


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Structural materials: ductility

ductile

force

deformation

brittle

force

deformation

Ductile materials: able to deform significantly into theinelastic range

Brittle materials:

fail suddenly by cracking or splintering

much weaker in tension than in compression


"Traditional" materials: used by builders and engineers

since the ancient times

Stone and timber: occur naturally

Bricks: man-made

sun-dried clay/mud bricks - from 4500 B.C.

fired bricks - from 3000 B.C.

calcium silicate bricks

Ancient concrete:

lime mixed with stone and sand: early civ. of the Middle East

"hydraulic cement" - lime, stone, sand and silicates: Romans

Stone, bricks, ancient concrete:

weak

weaker in tension than in compression

Stone and bricks masonry: units interconnected by evenweaker mortar


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Timber: substantial tensile strength along the grain

weak in compression and across the grain (difficult to realiseconnections in tension)

"Modern" materials: Portland cement concrete, steel,aluminium , etc.

Portland cement concrete:

mixture of Portland cement, water, aggregates

weaker in tension

brittle

Steel (iron with low carbon content) andAluminium (duraluminium alloy):

strong in tension and compression

ductile

Structural materials: strength

-2000

Very high-strengthprestressingwires

202Normal usePortlandcementconcrete

Modern

606High strength

355355Mild steel

Iron andsteel

700700High strengthsteel

450450Aluminium alloy (dural)

-3.5Across grain

30120Along grainTimber(spruce)

CompressionTensile

606Brick

405Limestone

20040GraniteStone

Traditional

Ultimate strength u(N/mm2)Material


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Specific strength

For structures subjected to tension/compression, as thesize of an object increases, its strength increases withthe square of the ruling dimensions, while the weightincreases with its cube

For each type of structure there is a maximum possiblesize beyond which it cannot carry even its own weight

Consequences:

it is impossible to construct structures of enormous size

there is a limit to natural structures (trees, animals, etc.)

larger a structure becomes, stockier and more bulky it gets

large bridges are heavier in proportions than smaller ones

bones of elephants are stockier and thicker than the ones of mice

proportions of aquatic animals are almost unaffected by their size(weight is almost entirely supported by buoyancy)

Specific strength

-26700-2000


90090202Normal usePortlandcementconcrete

Modern

2700270606High strength

45004500355355Mild steel

Iron andsteel

80008000600600High strengthsteel

1700017000450450Aluminium alloy (dural)

-700-3.5Across grain

60002400030120Along grainTimber(spruce)

CompressionTensileCompressionTensile

3200320606Brick

1800225405Limestone

7000140020040GraniteStone

Traditional

Specific strength S(m)Ultimate strength u(N/mm2)Material


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Specific strength

Stone, brick and concrete: used in compression Steel: used in tension

Timber: excellent performance in terms of specificstrength, especially in tension

Aluminium: high specific strength

Aircrafts must carry loads and must be capable of being

raised into the air under their own power materials withhigh specific strength

wood was extensively used in early planes modern material: aluminium

Structural materials: stress-strain curves

Stress-strain curves

provide "at a glance"information on:

strength

stiffness

ductility

Elastic region Inelastic region

Steel: elastic regionis almost linear

Stone, brick,

concrete, aluminium:elastic region isnot linear


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Structural materials: stress-strain curves

Steel and aluminium: excellent ductility Concrete, brick: brittle

Modulus of elasticity: E= /

Unloading after loading in the elastic range NOpermanent deformations

Unloading after loading in the inelastic rangepermanent deformations present

Permanent deformations need to be avoided in structures

under service loads stresses should be kept in theelastic region under service loads

factor of safety = ultimate strength / design stress

Structural materials: stiffness

Excessive flexibility is undesirable in structures

people dislike noticeable vibration and deflections in buildingsand bridges

large vibrations and deflections can damage (brittle) non-structural components (partitions, glazing, floors, etc.)

Materials with large stiffness are generally desirable(steel is more advantageous than aluminium from thispoint of view)

Elastic efficiency of materials:

average stress in the bar:

= ALg/ (2A) = Lg/ 2

extension of the bar under its own weight

= L / E= L2g/ (2E) = L2 / (2M)

specific modulus of the material - a measure of material stiffness

M= E/ (g)the higher the value of M, the less it will extend under its ownweight


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Structural materials: stiffness

The extension of a bar under its own weight isproportional to the square of the scale (a bar which is 10times longer than a reference one will extend 102 = 100times more than the reference one)

Structural materials: stiffness and ductility

Lowductility

2.80210 000


Brittle1.1225 000Normal usePortland

cementconcrete

Modern

1.8040 000High strength

Largeductility

2.80210 000Mild steel

Iron and

steel

Moderateductility

2.80210 000High strengthsteel

Ductile2.8070 000Aluminium alloy (dural)

--Across grainNA

3.0015 000Along grainTimber(spruce)

DuctilitySpecific modulus

M(m105)

Modulus of elasticityE(N/mm2)

1.6030 000Brick

1.3530 000Limestone Brittle

1.5745 000GraniteStone

Traditional

Material


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Structural materials: ductility

Ductility is important for the "ultimate" behaviour ofstructures

Most structures are designed to respond in the elasticrange under service loads, but, given the uncertainties inreal strength of material, behaviour of the structure,magnitude of loading, and accidental actions, a structurecan be subjected to inelastic deformations

A ductile material will sustain large deformations beforecollapsing, "warning" the people inside

A ductile material allows for redistribution of stresses instatically indeterminate structures, which are able tosupport larger loads than in the case of a structurerealised of brittle material

Documents

Basis of structural design 1