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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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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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

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6/21Braced frame Prof Schierle 6

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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2 Alcoa bui ld ing San Francisco Architect/Engineer: SOM

3 Federal Reserve Banc Boston Architect: Hugh StubbinsEngineer: Le Messurier

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AT&T tower, Seattle Architect: Basetti et allEngineer: Skilling et all

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Bank of China Hong Kong Architect: I M PeiEngineer: Ove Arup

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John Hancock Tower Chicago

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2 Barcelona hotel

Architect: SOM



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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LA courthouse @ 105 / 405features twin link-beams



LA courthouse @ 105 / 405


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09 braced frame

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