IntroductionAccording to the recent Indian standard code on earthquake resistant design of structures, more than 60-65% of the area of our country falls under seismic zone III or above. This underlines the importance of seismic detailing. In any structure, the joints assume more importance and have to be detailed carefully so that they are able to withstand the inelastic joint rotations (in the order of 0.04 radians) and drift that may result during an earthquake. The detailing of reinforced concrete structures have been covered adequately in the Indian codes. However, until recently such detailing of joints in steel structures was not covered in the Indian code on steel structures. Though the recent version of the code, IS 800:2007, contains provisions for design and detailing for seismic loads, it does not suggest the type of connections which are suitable for high or intermediate seismic zones.
Comparison of the damages in recent earthquakes in Haiti and Chile has shown that strict adherence to codal provisions and quality constructions could prevent excessive damages to constructions and human loss. The Northridge earthquake in California during January 1994 and the Kobe earthquake in Japan during 1995 have shown that even well designed connections are susceptible to severe damages. Based on the research that followed these earthquakes, The American Institute of Steel Construction (AISC) has stipulated that connections in special moment frames (SMF) or Intermediate moment frames (IMF) should be qualified for use by testing. However, as testing of connections is time-consuming and expensive, it has also specified a few pre-qualified connections, which have been tested and found to be satisfactory (FEMA 355D, 2000), Gross et al, 1999). These connections may be adopted in India also for better performance in strong or intermediate earthquakes.
Damages to Beam-column Connections During EathquakesSubsequent to the Northridge and Kobe earthquakes, it was determined that some damage to moment – resisting frames occurred at the beam-column connections. These connections experienced rotation levels well below the plastic moment capacity of framing members. Failures included non-ductile fractures of bottom beam flange-to-column flange complete-joint-penetration (CJP) groove welds, which propagated into the adjacent column flange and web and into the beam bottom flange. This failure was accompanied in some instances by secondary cracking of the beam web shear plate and failure of the beam top flange weld. The factors that contributed to the damage include the following (FEMA, 2000):
- Stress concentration at the bottom flange weld, due to the notch effect produced by backing strips left in place,
- Poor welding practices, including the use of weld metal of low toughness
- uncontrolled deposition rates
- The use of larger members than those previously tested or the use of higher strength girders,
- Less system redundancy and higher strain demands on connections,
- Lack of control of basic material properties (large variation of member strength from the prescribed values)
- Inadequate quality control during construction, and
- The tri-axial restraint existing at the center of beam flanges and at the beam-column interface, which inhibits yielding.
Pre-qualified Moment ConnectionsAISC 358-2010 gives the following pre-qualified connections:
- Reduced–Beam section connection
- Bolted un-stiffened and Stiffened Extended End-Plate Moment Connections
- Bolted Flange Plate (BFP) moment connection
- Welded Un-reinforced Flange-Welded Web Moment Connection (WUF-W)
- Kaiser Bolded Bracket (KBB) Moment connection
The effect of dogbone is similar to that of cover plate connections. With cover plates the connection is made stronger than the beam by strengthening the connection. In the dogbone, the connection is effectively made stronger than the beam by weakening the beam. While producing the same effect of cover plates, the dogbone connection can be constructed with relatively simpler details, resulting in a more reliable and economic solution. Moreover, the strong-column and weak-beam design can easily be achieved. The earliest application of dogbone connection was made in 1969 (Iwankiw and Carter, 1996, Engelhardt et al, 1998).
However, the reduction in flange area may reduce the stiffness of beam flange and may increase the susceptibility of lateral torsional buckling in the reduced section. Hence additional lateral bracings may be provided in these locations. Note that various shaped flanges such as straight cut, taper cut, arc cut, and drilled flanges have been tested and the arc cut was found to provide favorable results.
Several tests on this type bolted bracket connection were conducted at Lehigh University (Adan and Gibb, 2009).Then it was patented with the United States Patent and Trademark Office by of Steel Cast Connection LLC. The advantage of using casting is that it will not have HAZ issues or residual stresses that would be found in welds.
This bracket is available in USA, in various sizes and bolt patterns to match the demand required for strength and ductility. The bracket is shop welded to the beam and field bolted to the column. The brackets used in Wasatch property Management Corporate Headquarters building in Utah, USA are shown in Fig. 6. The use of these cast steel brackets resulted in a saving of $3,000 per joint, due to the avoidance of complete penetration field welding, doubler plates, continuity plates and ultra-sonic testing and also from the reduced beam tonnage (Cartwright, 2006).
The design procedure and detailing requirements for these connections are given in AISC 358-2010.
A similar, but a bit complicated, field bolted cast modular connector is under development and is shown in Fig. 7 (Sumer et al, 2007).
ConclusionThere seams to be an increasing earthquake activity throughout the world. The recent earthquakes have demonstrated that the damages and loss of lives will be extensive if the buildings are not designed and detailed properly. Though the recent version of steel code contained provisions for seismic design and detailing, designers are not given guidance to choose proper beam-to-column connections, in SMF and IMFs. After 10 years of extensive research, initiated by Federal Emergency Management Agency, USA, the AISC has developed a few pre-qualified connections, which have shown to provide the required amount of ductility. A brief description of such connections is given to aid the designers. The design methods are discussed in AISC 385-05.
- FEMA 355D, State of the Art Report on Connection Performance, prepared by the SAC Joint Venture for Federal Emergency Management Agency, Washington, D.C., 2000.
- Gross, J.L., Engelhardt, m.D., Uang, C.-M., Kasai, K., and Iwankiw, N.R., Modification of Existing Welded Steel Moment frame Connections for Seismic Resistance, STEEL Design Guide series 12, American Institute of Steel Construction, Inc., Chicago, IL, 1999.
- ANSI/AISC 358-05, Pre-qualified connections for special and intermediate steel moment frames for seismic applications including Supplement No.1, American Institute of Steel Construction, Inc., 2005. and Draft AISC 358-2010.
- Carter, C.J. and Grubb, K.A., Prequalified moment connections (revisited), Modern Steel Construction, Vol.50, No.1, Jan 2010, pp. 54-57
- Iwankiw, N.R., and Carter, C.J., The Dogbone: A new idea to Chew on, Modern Steel Construction, AISC, Vol. 36, No.4, April 1996
- Engelhardt, M.D., Winneberger, Zekany, A.J., and Potyraj, T.J., Experimental Investigation of Dogbone Moment connections, Engineering Journal, AISC, vol. 35, No. 4, Fourth Quarter, 1998, pp. 128-139
- Hamburger, R.O., Krawinkler, H., Malley, J.O., and Adan, S.M., Seismic design of steel special moment frames: A guide for practicing Engineers, NEHRP Seismic design technical brief no.2, National Institute of Standards and Technology, Gaithersburg, USA, June 2009,33pp. (http://www.nehrp.gov/pdf/nistgcr9-917-3.pdf)
- Adan, S.M., and Gibb, W., Experimental Evaluation of Kaiser bolted Bracket Steel Moment-Resisting Connections, Engineering Journal, AISC, Vol. 46, No.3, Third Quarter 2009, pp. 181-196
- Cartwright,C., Wasatch Property Management Corporate Headquarters, The Newsletter of the Structural Engineers Association of Utah, Vol. XI, No.1, Sept 2006, pp.2-3
- Sumer, A., Fleischman, R.B., and Hoskisson, B.E., Development of a cast Modular Connector for Seismic-Resistant Steel Moment Frames, Part 1:Prototype development, Part 2: Experimental Verification, Engineering Journal, AISC, Vol. 44, No.3, Third Quarter, 2007, pp. 195-231