2. Learning Objective 1
Explain the various loads exerted on a
building resulting from environmental
sources.
Building Construction
3–2
3. 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 it
Building Construction
3–3
4. Wind
• Basic effects
– Direct pressure
– Aerodynamic drag
– Negative pressure
• Secondary effects
– Rocking effect
– Vibration
– Clean-off effect
Building Construction
3–4
5. Calculating Direct
Pressure of Wind
p = Cv(2)
v = velocity
p = static pressure
C = numerical constant (0.00256) that
accounts for air mass and assumptions
of building behavior
Building Construction
3–5
6. 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-
dimensional (Continued)
Building Construction
3–6
7. 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
(Continued)
occupancies
Building Construction
3–7
8. Earthquakes
(Continued)
Building Construction
3–8
9. 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 Construction
3–9
10. Soil Pressure
• Exerts horizontal
pressure against
foundation
• Can only be
estimated
• Magnitude depends
on several factors
• Active or passive
Building Construction
3–10
11. Determining Soil Pressure
p = Cwh
p = pressure
h = depth of soil
w = density of soil
C = numerical constant that depends on
the physical properties of the soil
Building Construction
3–11
13. Learning Objective 2
Distinguish between the classifications
of loads based on origin and
movement.
Building Construction
3–13
14. 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
(Continued)
Building Construction
3–14
15. Live Loads and Dead Loads
• Distribution of loads
– Uniformly distributed loads
– Concentrated load
• Snow loads
• Water loads
Building Construction
3–15
16. 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 Construction
3–16
17. 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 Construction
3–17
18. Learning Objective 3
Recognize and discuss the internal
forces resulting from the loads and
forces applied to a structural member.
Building Construction
3–18
19. Interior Forces Created
by 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
(Continued)
Building Construction
3–19
21. Interior Forces Created
by 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 Construction
3–21
22. 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
(Continued)
Building Construction
3–22
23. 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
(Continued)
Building Construction
3–23
28. Columns
• Designed to support an
axial compressive load
• Thin columns fail by
buckling
• Short, squatty columns
fail by crushing
Building Construction
3–28
29. Arches
• Interior stresses primarily compressive
• Produce inclined forces at end supports
• Carry loads across a distance
Courtesy of Donny Howard
Building Construction
3–29
30. 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 Construction
3–30
31. 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 (Continued)
bending forces
Building Construction
3–31
32. 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
(Continued)
Building Construction
3–32
34. Space Frames
• Truss structures
developed in three
dimension
• Suited to support
uniformly
distributed loads
Courtesy of Ed Prendergast
Building Construction
3–34
35. 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 Construction
3–35
36. Learning Objective 5
Describe the basic structural systems.
Building Construction
3–36
37. 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 loads
Building Construction
3–37
38. Frame Structural Systems
• Walls enclose frame but
provide no structural support
• Steel stud wall framing
• Post and beam construction Courtesy of Ed Prendergast
• Rigid frames
• Truss frames
• Slab and column
frames
Building Construction
3–38
39. Shell and Membrane Systems
• Consist primarily of
enclosing surface with
stresses resulting from
applied loads occur
within the surface
Courtesy of Ed Prendergast
Building Construction
3–39
40. 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 Construction
3–40
41. 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 Construction
3–41
42. 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 Construction
3–42
43. 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)
Building Construction
3–43
44. Review Questions
4. How do dynamic loads differ
from static loads?
5. What is a membrane structure?
Building Construction
3–44
