BRACED FRAMES
5.25 SEAOC BLUE BOOK AND THE CODE
The SEAOC Blue Books Recommendations strongly influenced the UBC and IBC provisions and continued to be a determining factor in guiding the en-gineering community on the design of better and safer structures in California and other seismic zones. The Recommendations contain design principles based on lessons learned from Northridge as well as extensive empirical re-search. The results of experimental research on braced frames in particular were primarily based on over 20 years of tests on braced frames subjected to cyclic loading. The Recommendations also summarize the results of the FEMA / SAC joint venture work on steel moment frames reinitiated after Northridge.
Apart from the above considerations the reader is reminded that, while containing recommendations that will eventually be part of the IBC, the Blue Book is not per se a legally binding document. The IBC editions, in turn, become a binding document when adopted regionally by local legislators.
Steel is quoted in the Recommendations as one of the most efficient en-gineering solutions to counter earthquakes and with regard to CBFs: ‘‘Since their adoption into seismic design codes, improvements have been made to CBFs with emphasis on increasing brace strength and stiffness, primarily through the use of higher design forces that would minimize inelastic de-mand.’’ The emphasis is on better brace performance and stiffness achieved by higher component design forces required by the special detailing require-ments of the 1997 UBC (CHAP. 22, DIV. IV). The requirement for increased brace member stiffness goes well with research findings showing that insuf-ficient brace stiffness promotes global buckling and leads to pinching—
reduction—of the area of hysteresis envelope, a measure of the energy absorption of the system.
‘‘More recently,’’ the Blue Book Commentary adds, ‘‘ductility as an essen-tial ingredient distinguishing lateral force resisting systems in seismically ac-tive areas, has been applied to the design of CBFs.’’ Ductility equates to
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5.25 SEAOC BLUE BOOK AND THE CODE 181
potential energy absorption of the structural system and is one of the most promising trends in engineering to improve structural response and minimize earthquake damage.
To maintain sufficient brace stiffness, the SEAOC Recommendations limit the brace slenderness of the SCBF to
KL 1000
ⱕ (C708.2)
r 兹Fy
As a further refinement, the 1997 UBC subsequently restricted the brace slen-derness to
KL 720
ⱕ From CHAP. 22, DIV. IV, 9.2.a r 兹Fy
The Recommendations truly reflect results of over 15 years of research work conducted chiefly at the University of Michigan as discussed earlier in this chapter. As stated in C708.1 of the Recommendations:
Actual building earthquake damage (including the 1994 Northridge earthquake) as well as damage of CBFs (Concentrically Braced Frames) observed in labo-ratory tests have generally been caused by limited ductility and brittle failures.
These brittle failures are most often observed as fracture of connection elements or brace members. Lack of compactness in braces results in severe local buckling of the brace, which leads to high concentrations of flexural strains at these locations, and reduces their ductility. . . . Extensive analytical and experimental work performed by Professor Subhash C. Goel and his collaborators has shown that improved design parameters, such as limiting width / thickness ratios (to prevent local buckling), closer spacing of stitches, and special design and de-tailing of end connections greatly improve post-buckling behavior of CBFs. A new system reflecting these developments, referred to as special concentrically braced frames (SCBFs), has been added to these requirements.
Indeed the 1997 UBC limits the width–thickness ratio of brace elements (CHAP. 22, DIV. IV, 9.2d):
Braces shall be compact or non-compact, but not slender (i.e. ⬍ r). Circular sections shall have an outside diameter to wall thickness ratio not exceeding 1,300 / Fy, rectangular tubes shall have a flat-width to wall thickness not exceed-ing兹110 / Fyunless the circular section or tube walls are stiffened.
Also reflecting test results, the SEAOC Recommendations, C708.3, ‘‘Bracing Connections,’’ states:
When brace buckling is in the plane of a gusset plate, the plate should provide flexural strength that exceeds that of the brace. This will ensure that plastic
182 SEISMIC STEEL DESIGN: BRACED FRAMES
hinges will form in the brace, rather than in the plate. . . . When brace buckling is out of plane of a gusset plate, the length needed to allow restraint-free plastic rotation is twice the plate thickness. This mode of buckling may lead to pref-erable inelastic behavior.17
REFERENCES
1. Black, R. G., Wenger, W. A., and Popov, E. P. 1980. ‘‘Inelastic Buckling of Steel Struts Under Cyclic Load Reversal,’’ Report No. UCB / EERC-80 / 40. Berkeley:
Earthquake Engineering Research Center, University of California.
2. Bruneau, M., and Mahin, S. A. 1990. ‘‘Ultimate Behavior of Heavy Steel Section Welded Splices and Design Implications.’’ Journal of Structural Engineering, Vol.
116, No. 18, pp. 2214–2235.
3. Goel, S. C. 1992. ‘‘Cyclic Post Buckling Behavior of Steel Bracing Members.’’
In Stability and Ductility of Steel Structures under Cyclic Loading. Boca Raton, FL: CRC Press.
4. Goel, S. C., and Hanson, R. D. 1987. ‘‘Behavior of Concentrically Braced Frames and Design of Bracing Members for Ductility.’’ In SEAOC Proceedings. Sacra-mento, CA: Structural Engineers Association of California.
5. SEAOC. 1996. Recommended Lateral Force Requirements and Commentary, Seis-mology Committee, 6th ed. Sacramento, CA: Structural Engineers Association of California.
6. Goel, S. C. 1992. ‘‘Earthquake-Resistant Design of Ductile Braced Steel Struc-tures.’’ In Stability and Ductility of Steel Structures Under Cyclic Loading. Boca Raton, FL: CRC Press.
7. Hassan, O., and Goel, S. C. 1991. ‘‘Seismic Behavior and Design of Concentri-cally Braced Steel Structures,’’ Report No. UMCE 91-1. Ann Arbor, MI: Depart-ment of Civil Engineering, The University of Michigan.
8. Tang, X., and Goel, S. C. 1987. ‘‘Seismic Analysis and Design Considerations of Braced Steel Structures,’’ Report No. UMCE 87-4. Ann Arbor, MI: Department of Civil Engineering, The University of Michigan.
9. Whittaker, A. S., Uang, C. M., and Bertero, V. V. 1987. ‘‘Earthquake Simulation Tests and Associated Studies of a 0.3-Scale Model of a Six-Story Eccentrically Braced Steel Structure,’’ Report No. EERC 87 / 02. Berkeley, CA: Earthquake En-gineering Research Center, University of California.
10. Khatib, I. F., Mahin, S. A., and Pister, K. S. 1988. ‘‘Seismic Behavior of Con-centrically Braced Steel Frames,’’ Report No. UCB / EERC-88 / 01. Berkeley, CA:
Earthquake Engineering Research Center, University of California.
11. Erdey, C. K. 1999. ‘‘Performance of Steel Structures in California Earthquakes.’’
Paper presented at the Eurosteel ‘99 Symposium, Prague.
12. Crosby, P. 1994. ‘‘Seismically Retrofitting a Thirteen-Story Steel Frame Building.’’
SEF Structural Engineering Forum.
13. Hanson, R. D. 1997. ‘‘Supplemental Energy Dissipation for Improved Earthquake Resistance.’’ Ann Arbor, MI: Department of Civil & Environmental Engineering, University of Michigan, assignment with FEMA.
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REFERENCES 183
14. Kareem, A., and Tognarelli, M. 1994. ‘‘Passive & Hybrid Tuned Liquid Dampers.’’
SEF Structural Engineering Forum, October.
15. Perry, C., and Fierro, E. A. 1994. ‘‘Seismically Upgrading a Wells Fargo Bank,’’
SEF Structural Engineering Forum, October.
16. Thornton, W. A. 1990, ‘‘Design of Base Plates for Wide Flange Columns—A Concatenation of Methods.’’ Engineering Journal, Vol. AISC 27, No. 4, pp. 173–
174.
17. Astaneh, A., Goel, S. C., and Hanson, R. D. 1986. ‘‘Earthquake-Resistant Design of Double-Angle Bracing,’’ AISC Engineering Journal, Fourth Quarter.
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