General Structural Concerns

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General Structural Concerns

• Functionality / Stiffness
• deformations

• Stability
• equilibrium

• Strength
• material behaviour

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Stability

• Loads

• act on structure

• tend to destabilise structure

• also tend to break elements

• Supports

• provide reactions

• must be such as to
• provide equilibrium

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Loads
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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)

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

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

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Dead Loads

• Permanent weight of structure
• non-moveable partitions
• built-ins, heavy equipment

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

• How much does the stuff weigh?
• How much of each material is there?

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Dead Loads - Typical Values
Bulk Material
Weight/unit volume
Sheet Material
Weight/unit area
Concrete, dense Hardwood Steel Brick
23.5 kN/m3 11.0 kN/m3 76.9 kN/m3 19.0 kN/m3
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

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

• Uniformly Distributed (kPa)

• Concentrated (kN) - e.g. tall bookshelves

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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 gt 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

• Suction on other faces

• Suction on lowpitched roofs - lt 300

• Buildings need bracing and tying-down

• Wind can come from any direction

wind
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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

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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 sheltered
Wind
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Factors in Wind Loads (cont.)

• Shelter from anything permanent will reduce loads
• Shape of building affects loads
• Boxy vs streamlined

Pinchgut is exposed
Curved shapes would need special analysis
Shelteredby buildings
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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 lets 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 Loads
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Effect of one Member on Another

• The forces at the supports are the reactions
• For equilibrium, the reactions just balance the
  loads

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Types of Reactions Simple Support

• Beam sitting on supports

• Provides vertical support only

• No horizontal reaction

• Allows rotation
• no moment developed

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Types of Reactions Roller Support

• Provides vertical support only

• deliberately avoids
• horizontal restraint

(allows expansion)
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Types of ReactionsHinged (pin) Support

• Provides vertical and horizontal support,
• Allows rotation - no moment developed

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Types of Reactions Rigid Support

• Provides V, H, and moment restraint, M

• Cantilever beams or posts, and rigid frames

• Make sure you can physically achieve it!

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