Bldg Construction Chapter 03
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Transcript of Bldg Construction Chapter 03
Building Construction Related to the Fire
Service
Chapter 3 — The Way Buildings are Built: Structural Design
Features
Learning Objective 1
Explain the various loads exerted on a building resulting from environmental sources.
Building Construction3–2
Load and Gravity
• Load – Any effect or force that a structure must resist
• Gravity– Force on building through weight of
building components and contents– Snow, ice, or water on building adds
weight– Total weight of building exerts force on
soil beneath itBuilding Construction
3–3
Wind
• Basic effects– Direct pressure– Aerodynamic drag– Negative pressure
• Secondary effects– Rocking effect– Vibration– Clean-off effect
Building Construction3–4
Calculating Direct Pressure of Wind
p = Cv(2)v = velocityp = static pressureC = numerical constant (0.00256) that accounts for air mass and assumptions of building behavior
Building Construction3–5
Earthquakes
• Slippage between tectonic plates of earth’s surface
• Subjects buildings to seismic forces• Occur most frequently in fault zones• Seismic loads far more complex
than those for wind• Vibrational motion can be 3-
dimensionalBuilding Construction
3–6
(Continued)
Earthquakes
• Torsional, resonant forces• Force magnitude depends on factors• Most significant force is horizontal
motion• Buildings with geometric irregularities
more susceptible to earthquake damage• Building codes require stronger seismic
bracing for buildings with larger occupancies
Building Construction3–7
(Continued)
Earthquakes
Building Construction3–8
(Continued)
Earthquakes
• Design methods to protect against forces– Increase stiffness using symmetrical
shear walls and cross bracing– Continuous structures with high
degree of redundancy in structural frames
– Damping mechanisms– Base isolation
Building Construction3–9
Soil Pressure
• Exerts horizontal pressure against foundation
• Can only be estimated
• Magnitude depends on several factors
• Active or passive
Building Construction3–10
Determining Soil Pressure
p = Cwhp = pressureh = depth of soilw = density of soilC = numerical constant that depends on the physical properties of the soil
Building Construction3–11
Other Forces
• Temperature• Vibration• Shrinkage
Building Construction3–12
Learning Objective 2
Distinguish between the classifications of loads based on origin and movement.
Building Construction3–13
Live Loads and Dead Loads
• Classification of types of forces resulting from gravity
• Dead – Weight of any permanent part of a building
• Live – Any load that is not fixed or permanent
Building Construction3–14
(Continued)
Live Loads and Dead Loads
• Distribution of loads– Uniformly distributed loads– Concentrated load
• Snow loads• Water loads
Building Construction3–15
Static and Dynamic Loads
• Static – Steady or applied gradually– Dead loads– Many live loads
• Dynamic – Involves motion– Wind, earthquakes, vibration, falling
objects– Capable of delivering energy in
addition to weight
Building Construction3–16
Structural Equilibrium and Reactions
• Equilibrium – When support provided by structural system is equal to the applied loads
• Reactions – Forces that resist the applied loads
Building Construction3–17
Learning Objective 3
Recognize and discuss the internal forces resulting from the loads and forces applied to a structural member.
Building Construction3–18
Interior Forces Createdby Exterior Loads
• Tension – Pull material apart• Compression – Squeeze material• Shear – Slide one plane past an
adjacent plane• Strength of materials varies with
direction of interior forces
Building Construction3–19
(Continued)
Interior Forces Createdby Exterior Loads
Building Construction3–20
(Continued)
Interior Forces Createdby Exterior Loads
• Stress – Quantity used to evaluate magnitude of interior forces
• Factors of safety• Stresses typically occur in
combination within a member• Keeping stresses within allowable
values determines shape and size of structural members
Building Construction3–21
Exterior Loads Classified by Manner Applied
• Axial – Applied to center of cross section of structural member and perpendicular to that cross section
• Eccentric – Perpendicular to the cross section of the structural member but does not pass through the center of the cross section
Building Construction3–22
(Continued)
Exterior Loads Classified by Manner Applied
• Torsional – Offset from center of cross section of the structural member and at an angle to or in the same plane as the cross section
• Loads change under fire conditions due to thermal energy released
Building Construction3–23
(Continued)
Exterior Loads Classified by Manner Applied
Building Construction3–24
Learning Objective 4
Describe the basic structural components.
Building Construction3–25
Beams
• Simply supported• Cantilever• Continuous• Restrained• Primary design consideration –
Ability to resist bending from applied loads
Building Construction3–26
(Continued)
Courtesy of Ed Prendergast
Beams
Building Construction3–27
Columns
• Designed to support an axial compressive load
• Thin columns fail by buckling
• Short, squatty columns fail by crushing
Building Construction3–28
Arches
• Interior stresses primarily compressive
• Produce inclined forces at end supports
• Carry loads across a distance
Building Construction3–29
Courtesy of Donny Howard
Cables
• Flexible members used to support roofs and brace tents and restrain pneumatic structures
• When used to support loads over distance, will assume shape of parabola
• Stresses are tension
Building Construction3–30
Trusses
• Framed units made up of group of triangles in one plane
• If loads applied at only point of intersection of truss members, only compressive or tensile stresses
• If has curved top chord, subjected to bending forces
Building Construction3–31
(Continued)
Trusses
• Provide inherently rigid frame• Potential for early failure under
adverse conditions; failure of any portion of top or bottom chords results in failure of truss
Building Construction3–32
(Continued)
Trusses
Building Construction3–33
Space Frames
• Truss structures developed in three dimension
• Suited to support uniformly distributed loads
Building Construction3–34
Courtesy of Ed Prendergast
Truss Connectors
• Wood truss – Pins, bolts, gusset plates, adhesives, brackets, metal straps
• Steel trusses – Steel gusset plates, rivets, welds
• If connector fails, truss will fail
Building Construction3–35
Learning Objective 5
Describe the basic structural systems.
Building Construction3–36
Structural Bearing Walls
• Span elements such as beams, trusses, and precast concrete slabs
• Usually exterior walls with interior support system of columns and beams
• Provide lateral support along direction of the wall
• Subjected to compressive loadsBuilding Construction
3–37
Frame Structural Systems
• Walls enclose frame but provide no structural support
• Steel stud wall framing• Post and beam construction• Rigid frames• Truss frames• Slab and column
frames
Building Construction3–38
Courtesy of Ed Prendergast
Shell and Membrane Systems
• Consist primarily of enclosing surface with stresses resulting from applied loads occur within the surface
Building Construction3–39
Courtesy of Ed Prendergast
Membrane Structure
• Thin stretched flexible material• Addressed in building codes if life
of 180 days or more• Addressed in fire codes if life of
180 days or less• Cannot resist compressive forces• Advantages
Building Construction3–40
Shell Structure
• Rigid three-dimensional structures having a thickness that is small compared to other structural materials
• Regular geometric shapes• Most commonly constructed of
concrete
Building Construction3–41
Summary
• Forces exerted on buildings arise from many sources and determine how a building’s structural system is designed.
• A variety of structural components are available to support the applied loads.
• Under fire conditions, loads and stresses are subject to change in magnitude and direction.
Building Construction3–42
Building Construction
Review Questions
1.What is a load?2.What factors determine the magnitude of the forces developed within a building during an earthquake?3.What is a dead load?
(Continued)
3–43
Review Questions
4.How do dynamic loads differ from static loads?5.What is a membrane structure?
Building Construction3–44