braced frame

8/9/2019 Braced Frame

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Braced frame Copyright Prof Schierle 2011 1

Brac ed f ram eA ColumnB Beam

D Brace

E Gusset plate


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Eight-story steel braced frame

Assume

High strength steel (yield strength Fy = 50 ksi ) Fy = 50 ksi

Average 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 klf

Beam load

w = 95 psf x 30/1000 w = 2.85 klf

Base shearV = 30 psf x 45 x 90/1000 V = 122 k

Overturn moment M

M = 122 k x 51 M = 6222 k

Column Plateral = M/30 Pgravity = n w Atributary Pa + d P = 0 P = 8x2.7x15 = 324 k 324 k

h=96-6=

90

51

Brace force (see vectors) P = 78 k

Note: brace alters from tension to compressiondesign for compression, due to buckling

3045

V=122 k

LL 50x0.4 = 20 50x0.5 = 25 50

Total 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 Pall vs. P

a + 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 moment

M = wL2/8 = 2.85 klf x 302/8 M = 321 k

Section Modulus

S = M/Fb = 321 kx12/ 30 ksi S = 128 in3

Use W18x76 146 > 128Deflection = (5/384) W L3/ (E I)

= (5/384) (2.85x30) (30x12)3/ (30,000x1330) = 1.3

allowable = L/240 = (30x12)/240 = 1.5 1.5 > 1.3, ok

Note: while the center bay beams are supported by braces,

UBC requires such beams to be designed without brace supportB

m

d

g


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Design 4th story columnsAssume

High strength steel (yield strength Fy = 50 ksi ) Fy = 50 ksi

Average 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 klf

Beam load

w = 95 psf x 30/1000 w = 2.85 klf

4th floor shear VV = 30 psf x 45 x 3.5 x 12/1000 V = 56.7 k

Overturn moment M

Lever arm = 3.5x12/2+6 = 27

M = 56.7 k x 27 M = 1531 kColumn Plateral = M/30 Pgravity = n w Atributary P

27

Brace force (use vectors) P = 36 k

LL 50x0.4 = 20 50x0.5 = 25 50

Total 90 95 120

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


6/21Braced frame Copyright Prof Schierle 2011 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



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First interstate Bank Los Angeles

Architect: I M Pei

Engineer: CBM

Combines framed tube exteriorwith braced core

Moment frame Braced frame Combined

drift drift drift



1 IBM bui lding PittsburghArchitect: Curtis and Davis

Engineer: 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


2 Proposed 142 story tower ChicagoArchitect: Kay Vierk Janis

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John Hancock tower Chicago


Braced tube

AT&T tower, Seattle

Architect: Basetti et all

Engineer: Skilling et all

Braced superstructure with4 columns of 12 ksi high strength

concrete in 10 steel tubes

Bank of China Hong Kong

Architect: I M Pei

Engineer: Ove Arup

Braced superstructure definesarchitectural form



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Alcoa Building San Francisco


John Hancock Tower Chicago




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1 Georgia-Pacific tower, Atlanta

Architect: SOM

Engineer: Weidlinger Associates

2 Barcelona hotel

Architect: SOM



Large drift

(unglued boards resist independently)

Small drift

(glued boards resist in synergy

shear joins tension & compression)

Gue-lam beam anlogy



Barcelona Hotel Hong Kong-Shanghai Bank

Bel t t russ / Out r igger



Visc o elas t i c b rac inghttp://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 YorkArchitect / Engineer: SOM

Century Tower TokyoArchitect: Norman Foster

Engineer: Ove Arup



cc en t r i c b raced f r am e

LA courthouse @ 105 / 405features twin link-beams



LA courthouse @ 105 / 405


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braced frame

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