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Seismic Design of Steel Buildings

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CIE 423 Structures IIIAndrew Whittaker22. DESIGN OF STEEL BRACED FRAMES22.1 Eccentrically Braced <strong>Steel</strong> FramesObjective is to dissipate energy in the shear or moment links and to protect theremainder <strong>of</strong> 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. Thekinematics <strong>of</strong> four types <strong>of</strong> EBF are shown below:Lecture 22 Page 1


CIE 423 Structures IIIAndrew WhittakerBelow are some construction photographs <strong>of</strong> split-K EBFs.Lecture 22 Page 3


CIE 423 Structures IIIAndrew WhittakerLecture 22 Page 4


CIE 423 Structures IIIAndrew WhittakerWhat types <strong>of</strong> links are defined in the AISC <strong>Seismic</strong> Provisions?• Shear links (primary mode <strong>of</strong> deformation in the link is shear)o Link stiffened as shown above to promote formation <strong>of</strong> diagonal tensionfield and delay FLB and WLBo Also known as short linkso Link length1.6Mpe ≤Vpo Provisions based on the work <strong>of</strong> Popov and his PhD students at Berkeleyin late 1970s and 1980s.• Moment links (primary mode <strong>of</strong> deformation is flexure)o Also known as long linkso Link lengthe ≥2.6M pVpo Provisions based (loosely) on the thesis work <strong>of</strong> Engelhardt at Berkeley inlate 1980sEccentrically braced frames are designed for code-specified forces with some capacitychecks, as follows:1. Calculate earthquake loads using R=82. Impose lateral loads on the frame and size beams and columnsa. Use compact sections3. Check drifts and but typically okay with braced frames4. <strong>Design</strong> the eccentric braces and the beams outside the links to remain elasticusing “capacity” procedures using estimated material strengths and some level <strong>of</strong>strain hardening.5. Detail the links with full-depth web stiffeners per AISC; brace the ends <strong>of</strong> the linkto avoid LRB with the 6% rule <strong>of</strong> AISCSome other rules for link design include:• Sections to be compact per AISC for “seismic” designo Why?Lecture 22 Page 5


CIE 423 Structures IIIAndrew Whittaker• Yield strength <strong>of</strong> steel shall not exceed 50 ksio Why?• Link web shall be single thickness with no doublers and no web penetrations• Maximum rotation angles:o 0.08 radian for short linkso 0.02 radian for long linkso what is the drift capacity for an EBF: return to page 2 and consider thesplit-K frameOther 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 <strong>Steel</strong> FramesThe objective with concentrically braced steel frames is to dissipate energy in yieldingand buckling braces. Some sample brace configurations are shown below.Lecture 22 Page 6


CIE 423 Structures IIIAndrew WhittakerLet us first study the energy dissipation capacity <strong>of</strong> different types <strong>of</strong> braces. Threeparameters affect the hysteretic response <strong>of</strong> braces:1. slenderness ratio ( λ )2. end conditions (k)3. section shape (I, A)Braces are routinely classified as slender, intermediate, or stocky. The slenderness ratiois calculated asλ = kLAIii=kLrSlender braces have large λ , stocky braces have small λ , and intermediate braceshave a slenderness ratio between stocky and slender. Sample slenderness values are40 (stocky), 80 (intermediate), and 120 (slender). Some hysteresis loops are shownbelow for TS braces.Lecture 22 Page 7


CIE 423 Structures IIIAndrew WhittakerCompare the maximum tensile and compressive strengths <strong>of</strong> the above brace. Howrapid is the loss <strong>of</strong> strength with repeated cycling?Lecture 22 Page 8


CIE 423 Structures IIIAndrew WhittakerCompare the maximum tensile and compressive strengths <strong>of</strong> the above brace. Howrapid is the loss <strong>of</strong> strength with repeated cycling?Compare the maximum tensile and compressive strengths <strong>of</strong> the above brace. Howrapid is the loss <strong>of</strong> strength with repeated cycling?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:Lecture 22 Page 9


CIE 423 Structures IIIAndrew WhittakerConsider the change in stiffness <strong>of</strong> 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 <strong>of</strong> brace is permitted in special concentrically braced steel frames by AISC?KL 1000• ≤ , so if the yield strength is 50 ksi, the limiting slenderness ratio is 141rF y• Brace members must be compact (b/t, D/t) per Table I-9-1Connections for special concentrically braced steel frames must be designed usingcapacity principles for axial strength and detailed to permit plastic hinge formation at theends <strong>of</strong> the brace when the brace is buckling out <strong>of</strong> its plane. Also, the beams at theintersection <strong>of</strong> V-braced (chevron) frames must be designed to avoid plastic hingeformation due to out-<strong>of</strong>-balance vertical forces, as indicated by the sketch below.T yP b = 0.25T yOne application <strong>of</strong> a concentrically braced frame on the Berkeley campus is shownbelow. X braced frames were used to retr<strong>of</strong>it 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 thatdescribed above for eccentrically braced frames, except that “capacity” procedures areapplied to different components.Lecture 22 Page 10


CIE 423 Structures IIIAndrew WhittakerLecture 22 Page 11

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