L2: Building Structures Flashcards
What is the main function of buildings?
To protect occupants from weather
What is the excess margin modern safety factors call for and what does this cover?
50-150% increase in structural capacity; covers minor material flaws, inaccuracies and other unavoidable errors
How does structure impact design for short/high rise buildings?
Short - design is common sense
High - structure highly affects design and appearance
What three items must be present in a building structure?
Stability
Strength and stiffness
Economy
What causes unstable structures to form?
Unbalanced forces
What do stable structures do under realistic loads?
Remain at rest
How stiff is wood compared to steel
Wood is less than 16 times steel’s stiffness
What are the types of building loads?
Dead (fixed), live (moving, weather)
What do forces do?
Exert motion or stress within a structural member
How can loads be distributed?
Uniform or concentrated
What are the types of forces?
Vertical
Horizontal
Moment (rotational)
How are vertical and horizontal forces balanced by a structure?
Vertical - with building foundation (accumulative)
Horizontal - bracing of structural frame (each floor separate)
What is stress and what stress occurs within materials?
Internal resistance to an applied force; compression, tension, shear, bending
What is compression and what strain does it cause?
Crushing/squashing causing shortening
What is tension and what strain does it cause?
Pulling/stretching causing lengthening
What is shear and what strain does it cause?
Force perpendicular to long axis; two forces moving against each other causing compression and tension (shortening/lengthening)
What is bending (moment) and what strain does it cause?
Compression above and tension below the theoretical mutual axis causing deflection (compression and tension)
What impacts the amount of bending?
Distance from point of rotation stiffness of materials
Where is bending stress greatest in a uniform load, point load, and cantilever (uniform or point)
Uniform - midspan
Point - at point
Cantilever - at support
What is the strain of materials?
The external reaction to an applied load
What determines a structures resistance to compression, tension and shear
Strength of material and cross sectional area
What stresses are steel good in?
All
What stress is concrete good in and what must be done to handle the other stresses?
Good in compression
Reinforced with rebar to handle tension
Reinforced with stirrups to handle shear
What determines a structures resistance to bending?
Moment of inertia; higher proportion of depth to width decreases deflection
Efficiency in shape - material bulk at centre is inefficient and adds dead load
What is a bearing wall?
A wall that supports any vertical load in addition to its own weight; system of compression materials most efficient if loaded uniformly
What are bearing walls limited by?
Height - would need thick base to support cumulative loads
Where are bearing walls used?
Common wood frame house - exterior and some interior walls support vertical loads
Tilt-wall construction (eg. shopping centres) - reinforced concrete formed and poured on ground and then lifted into position
Arches
Explain the forces in arches
Keystone sends compression loads to base of arch; these have vertical components which go into the ground and horizontal components which cause outward thrust
How is outward thrust handled?
Counterweighted - more mass alongside arch or arches placed in series (oppose)
Tied - tension cable/rod placed at base
What is a post and beam structure?
A system of horizontal and vertical members subject to bending, compression and shear; not laterally stable
What is the structural element hierarchy
Floor slab/deck Joists Beams Girders (Vertical) columns
What does a floor slab/deck do?
Initially receives live loads
What do joists do?
Directly support slab/deck
quite flimsy, closely spaced
What do beams do?
Receive loads from joists (or directly support floor if no joists)
What do girders do?
Receive loads from beams
What do columns do?
Receive loads from beam/girders and above columns to transfer to foundation
What are trusses?
Triangular spanning elements with axial tension and compression, commonly used to support roofing
