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University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
General Structural Concerns
Functionality / Stiffnessdeformations
Stabilityequilibrium
Strengthmaterial behaviour
1/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
StabilityLoads
Supports
● act on structure● tend to destabilise structure
●must be such as to provide equilibrium
● provide reactions
● also tend to break elements
2/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
LoadsLoads
DynamicStatic
Dead Loads(fixed)
Forces due to
Settlements, Thermal effects,
...
Live Loads(movable)
Continuous Impact
Self-Weightof
Structure
FixedBuildingElements
Earthquakes
Wind
3/28
Occupancy Environmental(snow, ...)
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Loads
Two main types
dead loads - self-weight, fixed elements
live loads - occupancy, contents, wind
4/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
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)
5/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Dead Loads
Permanent weight of structure● non-moveable partitions● built-ins, heavy equipment
Roof
WallsFloors
Equipment
6/28
Cowan, Gunaratnam and Wilson (1995). Structural Systems, Department of Architectural and Design Science
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Dead Loads (cont.)How much does the stuff weigh?
How much of each material is there?
Dead loads
7/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
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 plaster13mm
Fibre cement 6mm
0.22 kN/m2
0.11 kN/m2
Appendix A of SA loading code AS1170.1
8/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Live LoadsFurniture, Equipment, People, SnowMoveable PartitionsMay or may not be acting all the time
9/28
Cowan, Gunaratnam and Wilson (1995). Structural Systems, Department of Architectural and Design Science
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Live Loads (cont1.)
people move around
may get heavy concentrations
10/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
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)
11/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Live Loads (cont3.)Building Codes give minimum valuesDomestic live loads range from 1.5 kPaCorridors and balconies are generally 4kPa, to allow for crowdingMost stores and workshops are >= 5 kPa
Live loads
12/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Wind LoadsBoth Pressure and SuctionAlways important for tall buildingsBut also important for low buildings -bracing
13/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Wind loads on BuildingsPressure 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
14/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Wind Loads on Buildings (cont1.)
may need to hold roof down
15/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Wind Loads on Buildings (cont2.)Wind tends to overturn a tall buildingActs as a vertical cantilever
Resisting Moment
Pressure Suction
16/28
OverturningMoment
Reaction
Reaction
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Factors in Wind SpeedsGeneral wind speed in the region● (pressure varies with square of the
speed)Local topography affects wind patternsWind speed increases with altitudeWind speed decreases with terrain roughness Very exposed
More shelteredWind
17/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Factors in Wind Loads (cont.)Shelter from anything permanent will reduce loadsShape of building affects loads●Boxy vs streamlined
Shelteredby buildings
Pinchgut is exposed
Curved shapes would need special analysis
18/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Wind Loads on Elements
Timber Framing Code has a procedure for finding maximum wind speedsTimber 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
19/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Seismic Loads
Earthquakes cause damage by horizontal acceleration - may swing
20/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Settlement, Temperature Loads
Stresses caused by temperature changes
Uneven settlement of foundations creates stresses - Gothic Cathedrals
21/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Loads on Elements
So far we have looked at the effect of loads on the building overall
Now let’s consider individual elements
22/28
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Distributed Loads and Point LoadsFloors, walls and roofs are generally distributed loads (kN per m or kPa)Other beams are point loads (kN)
Point LoadsDistributed Load
23/28
Reactions
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Point Load on beam
Effect of one Member on Another
The forces at the supports are the reactionsFor equilibrium, the reactions just balance the loads
Point Loadon columnand reaction
24/28
Point Loadsfrom beam to beam
Reaction from beam
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
Types of Reactions Simple Support
Beam sitting on supports
Hsimple beam
V
RvRv
H
Provides vertical support only
No horizontal reaction
Allows rotationno moment developed
V
RvRv25/28
RvRv
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
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)
26/28
RvRv
University of Sydney – DESA 1102 Structures LOADS & SUPPORTS
Peter Smith & Mike Rosenman
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
27/28