Ch. 3 Structural Design Features Flashcards
Any effect (or force) that a structure must resist
Load
gravity, wind, earthquakes or soil pressure
Total weight of building exerts a force on:
The soil beneath it
Most critical aspect of engineering design is determining the _________________ to which the structure will be subjected.
Type and magnitude of the forces
Forces wind exerts on a building: (3)
Direct pressure-IMPACT effect, may be reduced by STREAMLINING
Aerodynamic drag-winds fluid nature to FLOW around object, DRAG effect.
Negative pressure-SUCTION effect on downward side, resulting in OUTWARD pressure
Secondary effects produced from fundamental forces:
Rocking effects-back and forth from variations in velocity
Vibration-wind passing over surface (vibration) depending on harmonic characteristics
Clean-off effect-tendency to dislodge objects (canopies/parapets)
Major Earthquakes occur most frequently in parts of the world known as:
Fault zones OR
Zones of high probability
Zones of high probability:
- pacific coast of US/Canada
- Hawaii
- central Utah
- southern Illinois
- bldg codes provide seismic maps*
Vibrational motion produced by earthquakes can be ______.
Three dimensional
Shifting of earths surface can produce forces that are either:
- torsional OR
- resonant in nature (movements of large amplitude resulting from small force)
________ are movements of relatively large amplitude resulting from a small force applied at the natural frequency of a structure.
Resonant forces
Magnitude of forces in a bldg during earthquake depend on: (5)
- magnitude of vibratory motion
- type of foundation
- nature of soil under building
- stiffness of structure
- presence of damping mechanisms within bldg
- no mention of height*
One phenomenon associated with earthquakes is ____________. Occurs where the soil is of loose, sandy nature and saturated with water. Earthquake and particles try to move closer, but water prevents this. Reduction of strength in soil and shifting of foundations.
Soil liquefaction
Although movement of ground beneath a building can be three-dimensional, the _______ is the most significant force.
Horizontal motion
Tendency of a body to remain in motion or at rest until it is acted upon by force.
Inertia
keeps upper portion of building momentarily in initial position
Buildings with ________ are inherently _____ susceptible to damage from earthquakes than buildings with symmetrical design.
Geometric irregularities
More
Seismic joint between two sections of a building. The required _____ increases based on the _____ of the building
Width
Height
Provided to control vibration.
Operate in manner similar to door closer.
Typically installed at connection between columns/beams.
Absorb energy.
Damping mechanism.
2 design methods to protect bldgs against force of earthquakes
Use of shear walls or cross-bracing (long vibrational periods)
-symmetrically located (prevent torsional forces)
Use of continuous structures with high degree of redundancy in frames
-more members have to fail
continuity at joints absorb energy better than those free to rotate
2 types of base isolation
Elastomeric bearings -rubber or neoprene -change vibrational frequency Sliding systems (less common) -plates sliding on each other -isolate bldg from HORIZONTAL SHEAR force
Forces associated with ____ are difficult to determine accurately and can only be estimated
Soils
Soil Pressure-
Magnitude of pressure depends ________, _______, and _______.
Type of soil
Degree of cohesion
Moisture content
Soil against foundation
ACTIVE soil pressure
Foundation against soil
PASSIVE soil pressure
Other forces that may be exerted on building (3)
Temperature (members at periphery expand/contract)
Vibration
Shrinkage (wood)
Weight of any permanent part of the building
DEAD load-constant and immobile
Any load not fixed or permanent
LIVE load-usually contents, occupants and weight of SNOW or RAIN on roof. Wind or seismic.
________load is applied over a large area.
Uniformly distributed
_______ load is one that is applied at one point or over a small area. Highly localized loads.
Concentrated
Loads that are steady or applied gradually
Static loads-dead load of bldg, snow load, many live loads
Loads that involve motion
Dynamic load (aka SHOCK LOADING)-wind, FF's, vibration, falling objects *capable of delivering ENERGY to a structure IN ADDITION to the weight of the object*
_____ loads have the ability to cause failure after repeated cycles
Dynamic
When the support provided by a structural system is EQUAL to the applied loads, a condition known as _________ exists.
Equilibrium
Forces that resist the applied loads are known as ______
Reactions
Tension = those ______ forces that tend to pull things apart
Vertical OR Horizontal
Exterior loads create INTERIOR forces. The interior forces _______, _____, and ______.
Tension-pull material apart
Compression-squeeze the material
Shear-slide one plane of a material past an adjacent plane
Measurement of FORCE INTENSITY and is expressed as force units divided by area over which the force is applied.
Stress
Failure point of ordinary structural steel
36,000 psi
Exterior loads can be classified as: (3)
Axial
Eccentric
Torsional
_______ is a load that is perpendicular to the cross section but does NOT PASS through the center of the cross section.
- This load creates stresses that VARY across the section and may be both tensile AND compressive.
Eccentric
______ load is OFFSET from the center of the cross section of the structural member and at an ANGLE to or in the same plane as the cross section.
- produces a twisting effect that creates SHEAR stresses.
Torsional
__________ is a load applied to the CENTER of the cross-section of a structural member and PERPENDICULAR to that cross section.
- can be EITHER tensile or compressive, creating uniform stresses across the cross-section.
Axial
Larger structural systems can be constructed from several basic components, including some of the following: (BCACTSC)
Beams Columns Arches Cables Trusses Space frames Connectors
______ is the structural member that can carry loads perpendicular to its longitudinal dimension.
Beam
The primary design consideration of beams is their:
Ability to resist bending from the APPLIED loads.
____________ introduces stresses across the cross-section of a beam that are not uniform.
The bending moment = the force multiplied by the distance at which a bending moment is applied.
At the middle of the beam, a point known as ________, the tension and compression stresses are actually ZERO.
Neutral axis
________ are structural members designed to support an axial compressive load.
Columns
stresses=primarily compressive, not designed to withstand stresses due to bending
Columns
In cases of failure, tall, thin columns fail by _______ and short, squats columns fail by _______.
Buckling
Crushing
If supports at the ends of the arches shift because of ______ or ______, bending stresses may develop in the arch.
Settling soil
Thermal expansion
Used to carry loads across a distance and have application as support for roofs and entrances in masonry buildings.
Arches
Flexible structural members
Used to support loads over a distance will assume the shape of a ____________. Stresses are _______ stresses.
Cables
Parabola
Tension
________ are framed structural units made up of a group of triangles in ONE plane.
Trusses
_______ are truss structures that are developed into three dimension.
Space frames
(Trusses) If loads are applied only at the POINT OF INTERSECTION of the truss members, only _____ or _____ stresses will affect the members of the truss.
If loads are applied to the truss BETWEEN THE INTERSECTION POINTS of the members, which will also create _________.
Compressive or Tensile
Bending forces
Curved members of a bowstring truss are unavoidably subjected to __________.
bending forces
Joints of trusses may be formed by ____, _____, ____, _____, or _____.
- pin connections
- welding
- gusset plates
- strap connectors
- structural adhesives
Lightweight wood OR metal trusses, known as _______, have become very common in FLOOR construction, taking the place of solid joists.
Truss joists
Lightweight wood trusses are also commonly used in _____ construction.
Roof
Lightweight steel trusses, known as _________, are also used for floor and roof construction in fire-resistive and non-combustible construction.
Bar joists
______ trusses even lighter than steel bar joists are available, made of ______ or ______, and assembled with ______.
Roof
Galvanized steel studs or channels
Self-drilling screws
Design of space frames is more complicated than with two-dimensional trusses because:
The forces must be ANALYZED in three dimensions
Wood truss connectors
- gusset plates
- metal straps
- adhesives
- pins and bolts
- brackets
Members of steel trusses joined using:
- steel gusset plates
- rivets
- welds
_______ can affect the behavior of trusses under fire conditions.
Quality control in the manufacture of trusses
Uses the walls of the bldg to support spanning elements such as beams, trusses and precast concrete slab.
Bearing wall structures
Materials used for bearing walls include:
- concrete masonry units (cmu’s or concrete blocks)
- bricks
- stone
- concrete panels
Types of structural frame construction (besides wood):
- steel stud framing
- post and beam construction
- rigid frames
- truss frames
- slab and column frames
Relatively closely spaced vertical studs connected by top and bottom horizontal members with diagonal bracing for stability refers to:
Steel Stud Wall Framing
rigid wall panel
Uses a series of vertical elements to support horizontal elements that are subject to TRANSVERSE loads
Post and Beam
Besides wood, masonry for posts, steel and precast for posts and beams
Vertical posts spaced 24 in. apart
Minimum dimensions larger than studs in stud wall
Posts 6 x 8 in. when supporting roofs
Diagonal braces to withstand lateral loads
Post and Beam Construction
When the joints between columns and a beam are reinforced so bending stresses can be transmitted through the joints, the structural system is known as ________.
Rigid Frames
The peak of the roof has a hinged connection to allow for slight movement. This type of rigid frame can be constructed of _____, _____, or _____.
Steel, laminated wood, or reinforced concrete
Rigid Frames
The ________ usually must be reinforced and will be the LAST portion of the assembly to fail under fire conditions.
Joints
Slab and column frames are most frequently encountered in _____________.
Concrete structures
Floor slabs supported by columns. Intersection between slab and column is a region of high stress, usually reinforced by a _______ or _______.
Capital or drop panel
Membrane structure distinguished from a simple tent by:
It’s permanence
- tent = short periods
- membrane structure = permanent
Bldgs that consist primarily of an enclosing surface and in which the STRESSES resulting from the applied loads occur within the SURFACE.
Surface system
Structure with an enclosing surface of a thin stretched flexible material
Membrane structure
_________ structures are rigid three-dimensional structures having a thickness that is small compared to other structural material dimensions. They lend themselves to regular geometric shapes such as:
- cones
- domes
- barrel vaults
- folded plates
Shell
Most common concrete
Also plywood or fiberglass
Building codes typically address membrane structures with a life of _________ while fire codes address those to be used for __________.
BC 180 days or more
FC Less than 180 days
Fabric roofs weigh only about __________.
2 lbs per square foot
Increasing the stiffness of a building reduces its susceptibility to ground motions that have a ________________ period.
When areas where ___________ periods are expected, the use of a flexible
Long (meaning slow) vibrational.
Short vibrational.
US and Canada, frames are usually _____ and sometimes _____.
Steel
Aluminum
Fabrics cannot resist ____ forces; therefore cables and masts or tubular or solid frameworks must support the fabrics. Frameworks can be made of ______, _______ or _______.
Compressive
Wood, concrete or steel
