university of sydney – desa 1102 structures loads & supports peter smith & mike rosenman...

University of Sydney – DESA 1102 Structures LOADS & SUPPORTS

Peter Smith & Mike Rosenman

General Structural Concerns

Functionality / Stiffness

deformations

Stability

equilibrium

Strength

material behaviour

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Stability Loads

Supports

● act on structure

● tend to destabilise structure

● must be such as to

provide equilibrium

● provide reactions

● also tend to break elements

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LoadsLoads

DynamicStatic

Dead Loads(fixed)

Forces due to

Settlements, Thermal effects,

...

Live Loads(movable)

Continuous Impact

Self-Weightof

Structure

FixedBuildingElements

Earthquakes

Wind

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Occupancy Environmental(snow, ...)



Loads

Two main types

dead loads - self-weight,

fixed elements

live loads - occupancy, contents, wind

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Loads (cont.)

The building materials impose dead loads (fixed, vertical)

Wind and earthquake impose live loads (variable, mostly horizontal)

The occupants and contents impose live loads (variable, mostly vertical)

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Dead Loads Permanent weight of structure

● non-moveable partitions● built-ins, heavy equipment

Roof

WallsFloors

Equipment

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Cowan, Gunaratnam and Wilson (1995). Structural Systems, Department of Architectural and Design Science



Dead Loads (cont.) How much does the stuff weigh?

How much of each material is there?

Dead loads

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Dead Loads - Typical Values

Bulk Material Weight/unitvolume

Concrete, dense

Hardwood

Steel

Brick

23.5 kN/m3

11.0 kN/m3

76.9 kN/m3

19.0 kN/m3

Sheet Material Weight/unitarea

Gypsum plaster 13mm

Fibre cement 6mm

0.22 kN/m2

0.11 kN/m2

Appendix A of SA loading code AS1170.1

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Live Loads Furniture, Equipment, People, Snow Moveable Partitions May or may not be acting all the time

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Cowan, Gunaratnam and Wilson (1995). Structural Systems, Department of Architectural and Design Science



Live Loads (cont1.)

people move around

may get heavy concentrations

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Live Loads (cont2.) Could calculate - but tedious

Codes specify loads for various types of occupancies

AS 1170.1 specifies minimum floor live loads

Concentrated (kN) - e.g. tall bookshelves

Uniformly Distributed (kPa)

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Live Loads (cont3.) Building Codes give minimum values Domestic live loads range from 1.5 kPa Corridors and balconies are generally 4kPa, to

allow for crowding Most stores and workshops are >= 5 kPa

Live loads

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Wind Loads Both Pressure and Suction Always important for tall buildings But also important for low buildings -

bracing

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Wind loads on Buildings Pressure on the windward face

wind

Wind can come from any direction

Suction on other faces

Suction on lowpitched roofs - < 300

Buildings need bracing and tying-down

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Wind Loads on Buildings (cont1.)

may need to hold roof down

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Wind Loads on Buildings (cont2.)

Wind tends to overturn a tall building Acts as a vertical cantilever

Resisting Moment

PressureSuction

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OverturningMoment

Reaction

Reaction



Factors in Wind Speeds General wind speed in the region

● (pressure varies with square of the speed)

Local topography affects wind patterns Wind speed increases with altitude Wind speed decreases with terrain

roughness Very exposed

More shelteredWind

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Factors in Wind Loads (cont.)

Shelter from anything permanent will reduce loads

Shape of building affects loads● Boxy vs streamlined

Shelteredby buildings

Pinchgut is exposed

Curved shapes would need special analysis

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Wind Loads on Elements Timber Framing Code has a procedure for finding

maximum wind speeds Timber Framing Code also has simplified rules for

bracing single-storey houses

In non-cyclone areas, wind loads in the 1kPa range

Multiply by the area exposed to wind

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Seismic Loads

Earthquakes cause damage by horizontal acceleration - may swing

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Settlement, Temperature Loads

Stresses caused by temperature changes

Uneven settlement of

foundations creates stresses

- Gothic Cathedrals

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Loads on Elements

So far we have looked at the effect of loads on the building overall

Now let’s consider individual elements

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Distributed Loads and Point Loads

Floors, walls and roofs are generally distributed loads (kN per m or kPa)

Other beams are point loads (kN)

Point LoadsDistributed Load

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Reactions



Point Load on beam

Effect of one Member on Another

The forces at the supports are the reactions

For equilibrium, the reactions just balance the loads

Point Loadon columnand reaction

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Point Loadsfrom beam to beam

Reaction from beam



Types of Reactions Simple Support

Beam sitting on supports

Hsimple beam

V

RvRv

H

Provides vertical support only

No horizontal reaction

Allows rotation no moment developed

V

RvRv25/28

RvRv



Types of Reactions Roller Support

Provides vertical support only

a true roller support(only needed on

very large structures)

deliberately avoids

horizontal restraint (allows expansion)

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RvRv



Types of ReactionsHinged (pin) Support

Provides vertical and horizontal support,

Allows rotation - no moment developed

a definite ‘hinged’ support(most simple supports just involve

a beam sitting on something)

RV

RH

RV

RH

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Types of Reactions — Rigid Support

Provides V, H, and moment restraint, M

Welded steel frame

RV

RH

M

Make sure you can physically achieve it!

Cantilever beams or posts, and rigid frames

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university of sydney – desa 1102 structures loads & supports peter smith & mike rosenman...

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