09 braced frame
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Braced frame Prof Schierle 2
Eight-story steel braced frame AssumeHigh strength steel (yield strength Fy = 50 ksi ) Fy = 50 ksi
Average wind pressure P = 30 psf Gravity 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 klf Beam loadw = 95 psf x 30/1000 w = 2.85 klf
Base shear V = 30 psf x 45 x 90/1000 V = 122 kOverturn moment MM = 122 k x 51 M = 6222 k
Column P lateral = M/30 P gravity = n w A tributary Pa + d P = 0 P = 8x2.7x15 = 324 k 324 k
h =
9 6 - 6 =
9 0
5 1
Brace force (see vectors) P = 78 k
Note: brace alters from tension to compressiondesign for compression, due to buckling
V=122 k
LL 50x0.4 = 20 50x0.5 = 25 50Total 90 95 120
b + c P=6222 k/30= 207 k P = 8x2.7x30 = 648 k 855 k
V=122 k
P=78 kP=78 k
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Braced frame Prof Schierle 3
Column P KL length Use P all 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)
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Braced frame Prof Schierle 4
Bending momentM = wL2/8 = 2.85 klf x 30 2/8 M = 321 kSection ModulusS = M/Fb = 321 kx12/ 30 ksi S = 128 in 3
Use W18x76 146 > 128Deflection = (5/384) W L 3/ (E I)
= (5/384) (2.85x30) (30x12) 3/ (30,000x1330) = 1.3allowable = L/240 = (30x12)/240 = 1.5 1.5 > 1.3, ok
Note: while the center bay beams are supported by braces,IBC requires such beams to be designed without brace supportB
m d
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Column P KL length Use P all 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
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First interstate Bank Los Angeles Architect: I M PeiEngineer: CBM
Combines framed tube exterior with braced core
Moment frame Braced frame Combined
drift drift drift
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1 IBM build ing Pi ttsburgh Architect: Curtis and DavisEngineer: Worthington & Skilling
2 Alcoa bui ld ing San Francisco Architect/Engineer: SOM
3 Federal Reserve Banc Boston Architect: Hugh StubbinsEngineer: Le Messurier
1 Onterie Center Chicago Architect/Engineer: SOM
2 Proposed 142 story tower Chicago Architect: Kay Vierk Janis E x
p r e s
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John Hancock tower Chicago Architect/Engineer: SOM
Braced tube
AT&T tower, Seattle Architect: Basetti et allEngineer: Skilling et all
Braced superstructure with4 columns of 12 ksi high strengthconcrete in 10 steel tubes
Bank of China Hong Kong Architect: I M PeiEngineer: Ove Arup
Braced superstructure definesarchitectural form
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Alcoa Building San Francisco
Architect/Engineer: SOM
John Hancock Tower Chicago
Architect/Engineer: SOM
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1 Georgia-Pacific tower, Atlanta Architect: SOMEngineer: Weidlinger Associates
2 Barcelona hotel
Architect: SOM
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Large drift(unglued boards resist independently)Small drift(glued boards resist in synergyshear joins tension & compression)
Gue-lam beam anlogy
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Vi sc o el a s t i c b r a c i n ghttp://ascelibrary.org/emo/resource/1/jenmdt/v129/i6/p597_s1?isAuthorized=no
www.dist.unina.it/proc/1998/ECEE11/ECEE11/T5/PAOODO.PDF
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Chase Manhattan Banc New York Architect / Engineer: SOM
Century Tower Tokyo Architect: Norman Foster Engineer: Ove Arup
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c c e n t r i c b r a c e d f r a m e
LA courthouse @ 105 / 405features twin link-beams
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LA courthouse @ 105 / 405
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LA courthouse @ 105 / 405
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