equilibrium and planar trusses - Texas A&M University
Transcript of equilibrium and planar trusses - Texas A&M University
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S2014abn Equilibrium 1
Lecture 3
Elements of Architectural Structures
ARCH 614
ELEMENTS OF ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 614
DR. ANNE NICHOLS
SPRING 2014
lecture
three
equilibrium
and planar trusses
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S2014abn Equilibrium 2
Lecture 3
Elements of Architectural Structures
ARCH 614
Equilibrium
• balanced
• steady
• resultant of forces on a particle is 0
X
X
=
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S2014abn Equilibrium 3
Lecture 3
Elements of Architectural Structures
ARCH 614
Equilibrium on a Point
• analytically
• Newton convinces us it will stay at rest
0 xx FR
0 yy FR
http://www.physics.umd.edu
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S2014abn Equilibrium 4
Lecture 3
Elements of Architectural Structures
ARCH 614
Equilibrium on a Point
• collinear force system
– ex: cables
0 xx FR 0 yy FR
0 lineinF
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S2014abn Equilibrium 5
Lecture 3
Elements of Architectural Structures
ARCH 614
Equilibrium on a Point
• concurrent force system
– ex: cables
0 xx FR
0 yy FR
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S2014abn Equilibrium 6
Lecture 3
Elements of Architectural Structures
ARCH 614
Free Body Diagram
• FBD (sketch)
• tool to see all forces on a body or a
point including
– external forces
– weights
– force reactions
– internal forces
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S2014abn Equilibrium 7
Lecture 3
Elements of Architectural Structures
ARCH 614
Free Body Diagram
• determine point
• FREE it from:
– ground
– supports & connections
• draw all external forces acting ON the body
– reactions (supporting forces)
– applied forces
– gravity
= m· g
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S2014abn Equilibrium 8
Lecture 3
Elements of Architectural Structures
ARCH 614
Free Body Diagram
• sketch FBD with relevant geometry
• resolve each force into components
– known & unknown angles – name them
– known & unknown forces – name them
• are any forces related to other forces?
• for the unknowns
• write only as many equilibrium equations as needed
• solve up to 2 equations
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S2014abn Equilibrium 9
Lecture 3
Elements of Architectural Structures
ARCH 614
Free Body Diagram
• solve equations
– most times 1 unknown easily solved
– plug into other equation(s)
• common to have unknowns of
– force magnitudes
– force angles
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S2014abn Equilibrium 10
Lecture 3
Elements of Architectural Structures
ARCH 614
Force Reactions
• result of applying force
• unknown size
• connection or support type
– known direction
– related to motion prevented
no vertical motion no translation
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S2014abn Equilibrium 11
Lecture 3
Elements of Architectural Structures
ARCH 614
Friction
• resistance to movement
• contact surfaces determine
• proportion of normal force ()
– opposite to slide direction
– static > kinetic
NμF
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S2014abn Equilibrium 12
Lecture 3
Elements of Architectural Structures
ARCH 614
Cable Reactions
• equilibrium:
– more reactions (4) than equations
– but, we have slope relationships
– x component the same everywhere
A
C
45 kN
4 m
2 m
B 6 m
45 kN
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S2014abn Equilibrium 13
Lecture 3
Elements of Architectural Structures
ARCH 614
Cable-Stayed Structures
• diagonal cables support horizontal
spans
• typically symmetrical
• Patcenter,
Rogers 1986
www.columbia.edu
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S2014abn Equilibrium 14
Lecture 3
Elements of Architectural Structures
ARCH 614
Patcenter, Rogers 1986
• column free space
• roof suspended
• solid steel ties
• steel frame supports masts
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S2014abn Equilibrium 15
Lecture 3
Elements of Architectural Structures
ARCH 614
Patcenter, Rogers 1986
• dashes – cables pulling
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S2014abn Equilibrium 16
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
• ancient (?) wood
– Romans 500 B.C.
• Renaissance revival
• 1800’s analysis
• efficient
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S2014abn Equilibrium 17
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
– analogous to cables and struts
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S2014abn Equilibrium 18
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
• comprised of straight members
• geometry with triangles is stable
• loads applied only at pin joints
http:// nisee.berkeley.edu/godden
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S2014abn Equilibrium 19
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
• 2 force members
– forces in line, equal and opposite
– compression
– tension
• 3 members connected
by 3 joints
• 2 more members need
1 more joint
32 nb
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S2014abn Equilibrium 20
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
• compression and tension
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S2014abn Equilibrium 21
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Structures
• statically determinate
• indeterminate
• unstable
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S2014abn Equilibrium 22
Lecture 3
Elements of Architectural Structures
ARCH 614
Trusses
• common designs
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S2014abn Equilibrium 23
Lecture 3
Elements of Architectural Structures
ARCH 614
Trusses
• common designs
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S2014abn Equilibrium 24
Lecture 3
Elements of Architectural Structures
ARCH 614
Trusses
• uses
– roofs & canopies
– long spans
– lateral bracing
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S2014abn Equilibrium 25
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Connections
• “pins”
http:// nisee.berkeley.edu/godden
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S2014abn Equilibrium 26
Lecture 3
Elements of Architectural Structures
ARCH 614
Sainsbury Center, Foster 1978
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S2014abn Equilibrium 27
Lecture 3
Elements of Architectural Structures
ARCH 614
Sainsbury Center, Foster 1978
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S2014abn Equilibrium 28
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Analysis
• visualize compression and tension from
deformed shape
http:// nisee.berkeley.edu/godden
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S2014abn Equilibrium 29
Lecture 3
Elements of Architectural Structures
ARCH 614
Truss Analysis
• Method of Joints
• Graphical Methods
• Method of Sections
• all rely on equilibrium
– of bodies
– internal equilibrium
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S2014abn Equilibrium 30
Lecture 3
Elements of Architectural Structures
ARCH 614
A B
C
P
F
E
D
P
Method of Joints
• isolate each joint
• enforce
equilibrium in
Fx and Fy
• can find all forces
• long
• easy to mess up
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S2014abn Equilibrium 31
Lecture 3
Elements of Architectural Structures
ARCH 614
Joint Cases
• two bodies connected
equal equal and 0
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S2014abn Equilibrium 32
Lecture 3
Elements of Architectural Structures
ARCH 614
Joint Cases
• three bodies with two in line
A
B
C A
B C
or or even
C B
A D
D D
0 x
0 0 x x
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S2014abn Equilibrium 33
Lecture 3
Elements of Architectural Structures
ARCH 614
Joint Cases
• crossed
A B
C
C
B
A
P
D
D
P
C
B
A
D
E
C B
A
D
E
equal
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S2014abn Equilibrium 34
Lecture 3
Elements of Architectural Structures
ARCH 614
Tools – Multiframe
• in computer lab
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S2014abn Equilibrium 35
Lecture 3
Elements of Architectural Structures
ARCH 614
• frame window
– define truss members
• or pre-defined truss
– select points, assign supports
– select members,
assign section &
assign pin ends
• load window
– select points,
add point load
Tools – Multiframe
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S2014abn Equilibrium 36
Lecture 3
Elements of Architectural Structures
ARCH 614
Tools – Multiframe
• to run analysis choose
– Analyze menu
• Linear
• plot
– choose options
• results
– choose
options