Design of Eccentrically Braced Frames

Size: px

Start display at page:

Download "Design of Eccentrically Braced Frames"

Joshua Hoover
6 years ago
Views:

1 Design of Eccentricall Braced Frames Aninda Dutta, Ph.D., S.E. What is an eccentricall braced frame? In an eccentric braced frame the braces are eccentric to the beam-column connection, i.e. the do not frame directl into the connection This results in a high shear force in the ortion of the beam between the two braces The resulting ielding of the beam in shear is what that causes energ dissiation and ver ductile behavior 1

2 Basic elements of an EBF Beam outside link Link e Column Brace Column Most Common Basic elements of an EBF Beam outside link Link e Column Brace Column 2

3 AISC Design Requirements for EBF Links Links must be of comact shae (i.e. satisf b/t f and h/t w limitations of Table I-8-1 Web of a link shall be single thickness. No doubler late or web enetrations are allowed Shear strength of a link is given b lesser of V V n n = φ 0.6F v = φ 2M v A w = F / e = Z x F ( d 2t ) / e f t w AISC Design Requirements for EBF However, in the resence of axial loads the shear strength th is reduced. d Thus if Pu > 0.15 P, shear strength is the lesser of V n = φ 0.6F A 1 v w ( Pu / P ) [ 1 P / P ]/ e Vn = φv M u / 3

4 AISC Design Requirements for EBF The length of the link e shall not exceed ' ' [ ρ ( Aw / Ag )] 1.6M / V for ρ ( Aw / Ag ) ' 1.6M / V for ρ ( A / A ) < 0. 3 In the above equation w g 0.3 ' ρ = P / V & V = 0.6F ( d 2t ) t u u f w AISC Design Requirements for EBF Link rotation angle is the inelastic angle between the link and the beam outside he link and is calculated for C d times the drift for the code level forces Link rotations shall not exceed 0.08 radians for e</= 1.6M /V 0.02 radians for e>2m /V Use linear interolation for intermediate 4

5 AISC Design Requirements for EBF Plastic Rotation Angle e γ = L/(h e) h L See AISC Seimic Commentar for link rotations of other ebf shaes Placement of Link Stiffeners Link Lengths 1.6M/V or less Sacing 52tw-d/5 30tw-d/5 0.02rad 0.08rad Link Rotation 5

6 Placement of Link Stiffeners Link Lengths greater than 2.6M/V but less than 5M/V shall be rovided with link stiffeners at a distance from 1.5bf from each end of link Link Lengths greater than 1.6M/V but less than 2.6M/V shall have link stiffeners meeting reqs. of both above Links greater than 5M/V need no intermediate stiffeners Intermediate Link Stiffeners Intermediate link stiffeners shall be full deth For link deths < 25, stiffeners are ok on one side in which case thickness of stiffeners shall not be less than tw or 3/8 (whichever is greater)and width shall not be less than bf/2-tw For link deths greater than 25, intermediate stiffeners are required on both sides of the web Fillet weld connecting stiffener to web shall develo AstF and weld connecting stiffener to link flange shall develo AstF/4 6

7 Lateral bracing of the Link Lateral bracing is required at to and bottom flanges at both ends of the link Required strength of thee bracing shall be equal to 0.06R ZF P = b h0. Design of Brace and Beam Outside Link Required combined axial and flexural strength of the brace shall be based on load combos stiulated b ASCE 7 excet E effects shall be based on actions generated b 1.25 RVn of the link Required combined axial and flexural strength of the beam outside link shall be based on load combos stiulated b ASCE 7 excet E effects shall be based on actions generated b 1.1 RVn of the link 7

8 Design of Columns 1.1R V ni 1.1ΣR V ni +Aroriatel factored Dead and Live Design rocedure for EBFs Ste 1: Al the code forces and find the shear in each link. Add to this aroriatel factored gravit load shear. Denote this b V uei Ste 2:Size the link such that V ni ~V uei. Do not oversize link since everthing else from this oint is based on link caacit Ste 3:Find Ω i =V ni /V uei Ste 4: Scale u all code level E forces in beam outside link b 1.1RΩi. Add 1.1 times gravit comonent and check beam for axial and bending 8

9 Design rocedure for EBFs Ste 4 contd: For axial caacit evaluation consider K = 1.0 Ste 5:Scale u all code level E forces in brace outside link b 1.25RΩi. Add 1.25 times gravit comonent and check brace for axial and bending Ste 6:Design columns for Σ1.1RVni lus aroriatel factored gravit loading Comuting axial loads in beam outside link and link 20 ft 30 ft 25 ft 9

10 Comuting axial loads in beam outside link 14 ft 14 ft 300 kis Code forces 15 ft 100 kis 15 ft ft ft Comuting axial loads in beam outside link 14 ft 14 ft Flow of Forces F 1 15 ft F 2 15 ft ft ft 10

11 Comuting axial loads in beam outside link 20 ft 30 ft 25 ft Inut k 4.11 k/ft k 150 k Outut t k k k Comuting axial loads in beam outside link 20 ft 30 ft 25 ft k Inut 150 k 100 k k 200 k 1.38 k/ft Outut t k 3.5 k k 11

Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi

Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi Chapter 04 Structural Steel Design According to the AISC Manual 13 th Edition Analysis and Design of Compression Members By Dr. Jawad Talib Al-Nasrawi University of Karbala Department of Civil Engineering