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Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 1
Braced frameA Column
B Beam
D Brace
E Gusset plate
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 2
Eight-story steel braced frameAssumeHigh strength steel (yield strength Fy = 50 ksi ) Fy = 50 ksiAverage wind pressure P = 30 psfGravity load = 50 psf Column (psf) Beam (psf) Brace (psf)DL 70 70 70
Column load (per foot on beam)w = 90 psf x 30’/1000 w = 2.7 klfBeam loadw = 100 psf x 30’/1000 w = 3 klfBase shearV = 30 psf x 45 x 90’/1000 V = 122 kOverturn moment MM = 122 k x51’ M = 6222 k’
Column P lateral = M/30’ P gravity = n w A tributary Σ Pa + d P = 0 P = 8x2.7x15 = 324 k 324 k
h =
96-
6 =
90’
51’
Brace force (see vectors) P = 78 kNote: brace alters from tension to compression
design for compression, due to buckling
30’45
’
V=122 k
LL 50x0.4 = 20 50x0.6 = 30 50Total 90 100 120
b + c P=6222 k’/30’= 207 k P = 8x2.7x30 = 648 k 855 k
V=122 k
P=78 kP=78 k
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 3
Column P KL length Use Pall vs. Pa + d 324 k 12’ W14x61 410 > 324
Brace 78 k 20’ W14x43 117 > 78b + c 855 k 12’ W14x120 919 > 855
Column and brace design (use AISC column table)
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 4
Bending momentM = wL2/8 = 2.7 klf x 302/8 M = 304 k’Section Modulus S = M/Fb = 304 k’x12”/ 30 ksi S = 122 in3
Use W18x71 127 > 122Deflection Δ = (5/384) W L3/ (E I)Δ = (5/384) (2.7x30) (30x12)3/ (30,000x1170) Δ = 1.4”Δallowable = L/240 = (30x12)/240 = 1.5 1.5 > 1.4, okNote: while the center bay beams are supported by braces, UBC requires such beams to be designed without brace supportB
ea
m d
esig
n
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 5
Design 4th story columnsAssumeHigh strength steel (yield strength Fy = 50 ksi ) Fy = 50 ksiAverage wind pressure P = 30 psfGravity load = 50 psf Column (psf) Beam (psf) Brace (psf)DL 70 70 70
Column load (per foot on beam)w = 90 psf x 30’/1000 w = 2.7 klfBeam loadw = 100 psf x 30’/1000 w = 3 klf4th floor shear VV = 30 psf x 45 x 3.5 x 12’/1000 V = 56.7 kOverturn moment MLever arm = 3.5x12’/2+6’ = 27’M = 56.7 k x 27’ M = 1531 k’
Column P lateral = M/30’ P gravity = n w A tributary Σ P
27’
Brace force (use vectors) P = 36 k
LL 50x0.4 = 20 50x0.6 = 30 50Total 90 100 120
b + c P=1531 k’/30’= 51 k P = 4x2.7x30 = 324 k 375 kV=56.7 k
P=36 kP=36 k
a + d P = 0 P = 4x2.7x15 = 162 k 162 k
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 6
Column P KL length Use Pall vs. P
b + c 375 k 12’ W14x61 410 > 375
Column and brace design (use AISC column table)
Brace 36 k 20’ W8x31 97 > 36
a + d 162 k 12’ W8x31 189 > 162
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 7
Str
uctu
ral
bra
cin
g o
pti
on
sre
gard
ing
num
ber o
f joi
nts
and
brac
e le
ngth
Arc
hit
ectu
ral
bra
cin
g o
pti
on
sre
gard
ing
loca
tions
of o
peni
ngs
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 8
Bra
cin
g c
on
fig
ura
tio
ns
First interstate Bank Los AngelesArchitect: I M PeiEngineer: CBM
Combines framed tube exterior with braced core
Moment frame Braced frame Combineddrift drift drift
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 9
1 IBM building PittsburghArchitect: Curtis and DavisEngineer: Worthington & Skilling
2 Alcoa building San FranciscoArchitect/Engineer: SOM
3 Federal Reserve Banc BostonArchitect: Hugh StubbinsEngineer: Le Messurier
1 Onterie Center ChicagoArchitect/Engineer: SOM
2 Proposed 142 story tower ChicagoArchitect: Kay Vierk JanisE
xpre
sse
d b
racin
g
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 10
John Hancock tower ChicagoArchitect/Engineer: SOM
Braced tube
AT&T tower, SeattleArchitect: Basetti et allEngineer: Skilling et all
Braced superstructure with4 columns of 12 ksi high strength concrete in 10’ φ steel tubes
Bank of China Hong KongArchitect: I M PeiEngineer: Ove Arup
Braced superstructure defines architectural form
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 11
Exp
resse
d b
racin
gAlcoa Building San FranciscoArchitect/Engineer: SOM
John Hancock Tower ChicagoArchitect/Engineer: SOM
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 12
Be
lt t
russ/o
utr
igg
er
joins
exter
ior co
lumns
to tr
ansfe
r she
ar to
redu
ce dr
ift
1 Georgia-Pacific tower, AtlantaArchitect: SOMEngineer: Weidlinger Associates
2 Barcelona hotelArchitect: SOM
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 13
Barcelona Hotel Hong Kong-Shanghai Bank
Belt truss / Outrigger
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 14
Ecce
ntr
ic b
race
d f
ram
e(A
djusts
stiffn
ess b
etwee
n flex
ible m
omen
t fram
e and
stiff
brac
ed fr
ame
e = lin
k bea
m (sh
ort li
nk be
am →
stiff;
long l
ink be
am →
flexib
le fra
me
Ecce
ntr
ic b
race
d d
eta
ils
A C
olumn
; B
beam
; C
brac
e; D
gus
set p
late;
E s
tiffen
er pl
ates
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 15
Ch
ase
Ma
nh
att
an
Ba
nc N
ew
York
Arc
hite
ct/E
ngin
eer:
SO
M
Ce
ntu
ry T
ow
er
To
kyo
Arc
hite
ct: N
orm
an F
oste
r; E
ngin
eer:
Ove
Arup
Braced frame Copyright © G G Schierle, 2001-04 press Esc to end, ↓ for next, ↑ for previous slide 16
Eccentric braced frame LA courthouse @ 105 / 405
features twin link-beams