General Structural Concerns

Transcription

1 1/28 General Structural Concerns Functionality / Stiffness deformations Stability equilibrium Strength material behaviour

2 2/28 Stability Loads act on structure tend to destabilise structure also tend to break elements Supports provide reactions must be such as to provide equilibrium

3 3/28 Loads Loads Static Dynamic Forces due to Settlements, Thermal effects,... Dead Loads (fixed) Live Loads (movable) Continuous Earthquakes Impact Self-Weight of Structure Fixed Building Elements Occupancy Environmental (snow,...) Wind

4 Loads Two main types dead loads - self-weight, fixed elements live loads - occupancy, contents, wind 4/28

5 5/28 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)

6 6/28 Dead Loads Permanent weight of structure non-moveable partitions built-ins, heavy equipment Roof Walls Floors Equipment Cowan, Gunaratnam and Wilson (1995). Structural Systems, Department of Architectural and Design Science

7 7/28 Dead Loads (cont.) How much does the stuff weigh? How much of each material is there? Dead loads

8 8/28 Dead Loads - Typical Values Bulk Material Weight/unit volume Sheet Material Weight/unit area Concrete, dense 23.5 kn/m 3 Gypsum plaster 13mm 0.22 kn/m 2 Hardwood 11.0 kn/m 3 Fibre cement 6mm 0.11 kn/m 2 Steel 76.9 kn/m 3 Brick 19.0 kn/m 3 Appendix A of SA loading code AS1170.1

9 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 9/28

10 Live Loads (cont1.) people move around may get heavy concentrations 10/28

11 11/28 Live Loads (cont2.) Could calculate - but tedious Codes specify loads for various types of occupancies AS specifies minimum floor live loads Uniformly Distributed (kpa) Concentrated (kn) - e.g. tall bookshelves

12 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 12/28

13 13/28 Wind Loads Both Pressure and Suction Always important for tall buildings But also important for low buildings - bracing

14 14/28 Wind loads on Buildings Pressure on the windward face Suction on other faces Suction on lowpitched roofs - < 30 0 Buildings need bracing and tying-down Wind can come from any direction wind

15 15/28 Wind Loads on Buildings (cont1.) may need to hold roof down

16 16/28 Wind Loads on Buildings (cont2.) Wind tends to overturn a tall building Acts as a vertical cantilever Overturning Moment Pressure Suction Reaction Resisting Moment Reaction

17 17/28 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

18 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 Sheltered by buildings 18/28

19 19/28 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

20 20/28 Seismic Loads Earthquakes cause damage by horizontal acceleration - may swing

21 Settlement, Temperature Loads Stresses caused by temperature changes Uneven settlement of foundations creates stresses - Gothic Cathedrals 21/28

22 22/28 Loads on Elements So far we have looked at the effect of loads on the building overall Now let s consider individual elements

23 23/28 Distributed Loads and Point Loads Floors, walls and roofs are generally distributed loads (kn per m or kpa) Other beams are point loads (kn) Distributed Load Point Loads Reactions

24 24/28 Effect of one Member on Another The forces at the supports are the reactions For equilibrium, the reactions just balance the loads Point Load on beam Point Loads from beam to beam Reaction from beam Point Load on column and reaction

25 Types of Reactions Simple Support Beam sitting on supports Provides vertical support only No horizontal reaction Allows rotation no moment developed V Rv Rv simple beam HH Rv Rv 25/28

26 26/28 Peter Smith & Mike Rosenman Types of Reactions Roller Support Provides vertical support only deliberately avoids horizontal restraint (allows expansion) Rv Rv a true roller support (only needed on very large structures)

27 27/28 Peter Smith & Mike Rosenman Types of Reactions Hinged (pin) Support Provides vertical and horizontal support, Allows rotation - no moment developed RH RH RV RV a definite hinged support (most simple supports just involve a beam sitting on something)

28 28/28 Types of Reactions Rigid Support RH RV M Provides V, H, and moment restraint, M Welded steel frame Cantilever beams or posts, and rigid frames Make sure you can physically achieve it!