braced frame design

8/12/2019 Braced Frame Design

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CIE 423 Structures III Andrew Whittaker

Lecture 22 Page 1

22. DESIGN OF STEEL BRACED FRAMES

22.1 Eccentrically Braced Steel Frames

Objective is to dissipate energy in the shear or moment links and to protect theremainder of the frame from inelastic action, including the braces. Sample eccentricbraced frame (EBF) configurations are shown below.

Of these three configurations, the split-K configuration is likely the best because largemoments (approaching the plastic moment) are avoided near the column. The

kinematics of four types of EBF are shown below:


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Lecture 22 Page 2

What construction details are employed to achieve the large shear strains shown in the

above figure?


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Lecture 22 Page 3

Below are some construction photographs of split-K EBFs.


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Lecture 22 Page 5

What types of links are defined in the AISC Seismic Provisions?

Shear links (primary mode of deformation in the link is shear)

o Link stiffened as shown above to promote formation of diagonal tensionfield and delay FLB and WLB

o Also known as short links

o Link lengthp

p

V

M6.1e

o Provisions based on the work of Popov and his PhD students at Berkeleyin late 1970s and 1980s.

Moment links (primary mode of deformation is flexure)

o Also known as long links

o Link lengthp

p

V

M6.2e

o Provisions based (loosely) on the thesis work of Engelhardt at Berkeley inlate 1980s

Eccentrically braced frames are designed for code-specified forces with some capacitychecks, as follows:

1. Calculate earthquake loads using R=8

2. Impose lateral loads on the frame and size beams and columns

a. Use compact sections

3. Check drifts and but typically okay with braced frames

4. Design the eccentric braces and the beams outside the links to remain elasticusing capacity procedures using estimated material strengths and some level ofstrain hardening.

5. Detail the links with full-depth web stiffeners per AISC; brace the ends of the linkto avoid LRB with the 6% rule of AISC

Some other rules for link design include:

Sections to be compact per AISC for seismic design

o Why?


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Lecture 22 Page 6

Yield strength of steel shall not exceed 50 ksi

o Why?

Link web shall be single thickness with no doublers and no web penetrations

Maximum rotation angles:

o 0.08 radian for short links

o 0.02 radian for long links

o what is the drift capacity for an EBF: return to page 2 and consider thesplit-K frame

Other practical issues that one should consider when designing eccentrically bracedframes include the axial forces in the links and beams surrounding the links:

22.2 Concentrically Braced Steel Frames

The objective with concentrically braced steel frames is to dissipate energy in yieldingand buckling braces. Some sample brace configurations are shown below.


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Lecture 22 Page 7

Let us first study the energy dissipation capacity of different types of braces. Threeparameters affect the hysteretic response of braces:

1. slenderness ratio ( )

2. end conditions (k)

3. section shape (I, A)

Braces are routinely classified as slender, intermediate, or stocky. The slenderness ratiois calculated as

r

kL

I

AkL

ii

==

Slender braces have large , stocky braces have small , and intermediate braces

have a slenderness ratio between stocky and slender. Sample slenderness values are40 (stocky), 80 (intermediate), and 120 (slender). Some hysteresis loops are shown

below for TS braces.


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Lecture 22 Page 8

Compare the maximum tensile and compressive strengths of the above brace. Howrapid is the loss of strength with repeated cycling?


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Very slender braces such as that shown immediately above have little stiffness in thebuckled configuration. Also, such a brace loses strength rapidly with repeated inelasticload cycles and does not return to its original geometry. Consider the axial force-versusaxial deformation relationship for a slender WF brace as shown on the following page:


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Lecture 22 Page 10

Consider the change in stiffness of a CBF with slender braces with repeated cycling:from the red line to the green line: approximately a 25-fold change in stiffness. Is thisacceptable?

What type of brace is permitted in special concentrically braced steel frames by AISC?

yF

1000r

KL , so if the yield strength is 50 ksi, the limiting slenderness ratio is 141

Brace members must be compact (b/t, D/t) per Table I-9-1

Connections for special concentrically braced steel frames must be designed usingcapacity principles for axial strength and detailed to permit plastic hinge formation at theends of the brace when the brace is buckling out of its plane. Also, the beams at theintersection of V-braced (chevron) frames must be designed to avoid plastic hingeformation due to out-of-balance vertical forces, as indicated by the sketch below.

One application of a concentrically braced frame on the Berkeley campus is shownbelow. X braced frames were used to retrofit this non-ductile reinforced concrete framedbuilding.

Why was steel used?

Why were braced steel frames used in this configuration?

The design procedure for special concentrically braced frames is similar to that

described above for eccentrically braced frames, except that capacity procedures areapplied to different components.

Ty Pb= 0.25Ty


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Lecture 22 Page 11

braced frame design

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