steel construction: columns & tension...
Transcript of steel construction: columns & tension...
F2013abn Steel Columns & Tension 1
Lecture 20
Architectural Structures
ARCH 331
lecture
twenty
steel construction:
columns & tension members
Cor-Ten Steel Sculpture By Richard Serra
Museum of Modern Art Fort Worth, TX
(AISC - Steel Structures of the Everyday)
ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 331
DR. ANNE NICHOLS
FALL 2013
F2013abn Steel Columns & Tension 2
Lecture 20
Architectural Structures
ARCH 331
Structural Steel
• standard rolled shapes
(W, C, L, T)
• tubing
• pipe
• built-up
F2013abn Steel Columns & Tension 3
Lecture 20
Architectural Structures
ARCH 331
Design Methods (revisited)
• know
– loads or lengths
• select
– section or load
– adequate for
strength and
no buckling
F2013abn Steel Columns & Tension 4
Lecture 20
Architectural Structures
ARCH 331
Allowable Stress Design (ASD)
• AICS 9th ed
• slenderness ratio
– for kl/r Cc = 126.1 with Fy = 36 ksi
= 107.0 with Fy = 50 ksi
22
23
12
..r
K l
E
SF
fF cr i t ica l
a
r
Kl
F2013abn Steel Columns & Tension 5
Lecture 20
Architectural Structures
ARCH 331
Cc and Euler’s Formula
• Kl/r < Cc
– short and stubby
– parabolic transition
• Kl/r > Cc
– Euler’s relationship
– < 200 preferred
y
cF
EC
22
F2013abn Steel Columns & Tension 6
Lecture 20
Architectural Structures
ARCH 331
Cc and Euler’s Formula
F2013abn Steel Columns & Tension 7
Lecture 20
Architectural Structures
ARCH 331
Short / Intermediate
• Le/r < Cc
– where
..2
12
2
SF
F
C
rKl
Fy
c
a
3
3
88
3
3
5..
cc C
rK l
C
rK l
SF
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Unified Design
• limit states for failure
1. yielding
2. buckling
Fe – elastic buckling stress (Euler)
Steel Columns & Tension 8
Lecture 20
Architectural Structures
ARCH 331
gc rnc AFP. 9 00
na
PP
ncu PP
ye
y
F.ForF
E.
r
KL440714
ye
y
F.ForF
E.
r
KL440714
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Unified Design
• Pn = FcrAg – for
– for
– where
Steel Columns & Tension 9
Lecture 20
Architectural Structures
ARCH 331
y
F
F
cr
y
F.FF
E.
r
KLe
y
6580714
ecr
y
F.FF
E.
r
KL8770714
22
rKL
EFe
F2013abn Steel Columns & Tension 10
Lecture 20
Architectural Structures
ARCH 331
Procedure for Analysis
1. calculate KL/r • biggest of KL/r with respect to x axes and y axis
2. find Fa or Fcr from appropriate equation • tables are available
3. compute Pallowable = FaA or Pn = FcrAg • or find factual = P/A
4. is P Pallowable (Pa Pn/)? or is Pu Pn? • yes: ok
• no: insufficient capacity and no good
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1. guess a size (pick a section)
2. calculate KL/r • biggest of KL/r with respect to x axes and y axis
3. find Fa or Fcr from appropriate equations • or find a chart
4. compute Pallowable = FaA (Pn/ = FcrAg)
or Pn = FcrAg • or find factual = P/A
Steel Columns & Tension 11
Lecture 20
Architectural Structures
ARCH 331
Procedure for Design
F2013abn Steel Columns & Tension 12
Lecture 20
Architectural Structures
ARCH 331
Procedure for Design (cont’d)
5. is P Pallowable (Pa Pn/)? or is Pu Pn? • yes: ok
• no: pick a bigger section and go back to step 2.
6. check design efficiency
• percentage of stress =
• if between 90-100%: good
• if < 90%: pick a smaller section and
go back to step 2.
%P
P
c
r 100
F2013abn Steel Columns & Tension 13
Lecture 20
Architectural Structures
ARCH 331
Column Charts, Fa (pg. 361-364)
F2013abn Steel Columns & Tension 16
Lecture 20
Architectural Structures
ARCH 331
Beam-Column Design
• moment magnification (P-)
Cm – modification factor for end conditions
= 0.6 – 0.4(M1/M2) or
0.85 restrained, 1.00 unrestrained
Pe1 – Euler buckling strength
)P/P(
CB
eu
m
1
11
f a c t o r e dm a xu MBM 1
22
1
rKl
EAPe
F2013abn Steel Columns & Tension 17
Lecture 20
Architectural Structures
ARCH 331
Beam-Column Design
• LRFD (Unified) Steel
– for
– for
Pr is required, Pc is capacity
c - resistance factor for compression = 0.9
b - resistance factor for bending = 0.9
019
820 .
M
M
M
M
P
P:.
P
P
nxb
ux
nxb
ux
nc
u
c
r
012
20 .M
M
M
M
P
P:.
P
P
nxb
ux
nxb
ux
nc
u
c
r
F2013abn Steel Columns & Tension 18
Lecture 20
Architectural Structures
ARCH 331
Design Steps Knowing Loads (revisited)
1. assume limiting stress
• buckling, axial stress,
combined stress
2. solve for r, A or S
3. pick trial section
4. analyze stresses
5. section ok?
6. stop when section is ok
F2013abn Steel Columns & Tension 19
Lecture 20
Architectural Structures
ARCH 331
Rigid Frame Design (revisited)
• columns in frames
– ends can be “flexible”
– stiffness affected by beams and column = EI/L
– for the joint • lc
is the column length of each column
• lb is the beam length of each beam
• measured center to center
b
c
lEI
lEI
G
F2013abn Steel Columns & Tension 20
Lecture 20
Architectural Structures
ARCH 331
Rigid Frame Design (revisited)
• column effective length, k
A
B
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• steel members can
have holes
• reduced area
• increased stress
Steel Columns & Tension 21
Lecture 20
Architectural Structures
ARCH 331
Tension Members
(AISC - Steel Structures of the Everyday)
g
stAAA holesallofgn
4
2
F2013abn Steel Columns & Tension 22
Lecture 20
Architectural Structures
ARCH 331
Effective Net Area
• likely path to “rip” across
• bolts divide transferred force too
• shear lag UAA ne