Daniel Miller and Dr. Paul Richards, Civil and Environmental Engineering

Introduction

Since the early twentieth century, engineers have recognized the suitability of steel construction for design of earthquake resistant structures. Beginning in the 1990’s, building codes have provided specifications for seismic steel design that are intended to allow inelastic behavior while preventing catastrophic collapse (Hamberger, 2009). However, this inelastic response leads to permanent structural deformation which can require costly building repair or rehabilitation for continued use (Pettinga et al., 2007). A study conducted by Christopoulos and Pampanin (2004) proposes the use of a flexible secondary structural system to act in parallel with the main system in order to limit permanent deformation. The proposed secondary system is intended to remain elastic and provide additional stiffness once the primary system has yielded. Subsequent analytical work has confirmed that a dual structural system, with moment-resisting fames (MRFs) as a secondary system, might reduce residual drift by as much as 50% (Kiggins and Uang, 2006).

While these studies suggest a promising possibility for mitigating excessive drift, the design of elastic secondary systems has remained essentially hypothetical. Traditional MRFs attempt to maximize strength using welded connections for all joints between beams and columns. Very little consideration has been given to MRFs with hinge connections because these tend to have lower yield strengths. However, the increased flexibility obtained from hinge connections may be a key to maintaining elasticity.

This research investigated alternative configurations for two-story MRFs that can provide much greater elastic displacements than conventional MRFs. The goal was to establish the feasibility of designing secondary frames capable of remaining elastic under expected earthquake drifts, while maintaining equal strength with frames as they are currently designed. Case studies were also used to demonstrate the economic viability of these new secondary frames compared to conventional secondary frames.

Methods

Elastic analysis was used to develop closed-form equations for the strength, yield drift, and connection efficiency of five moment-resisting frame configurations. The five configurations (shown in Figure 1) each use a different combination of welded and hinge beam-to-column connections. Frame M4 represents an MRF under current design practices with only welded connections.

Results

Typically it was found that frames with greater flexibility also suffered a decrease in strength. However, Frame M1-B was found to have a unique combination of high flexibility and high strength per number of welded connections used. These initial findings were used to develop two case studies, both of which compared a conventional back-up system (frame M4) to a back-up system using an M1-B style frame. The first case study compared two-story systems and the second case study considered six-story systems. In both case studies it was found that the alternative system was capable of achieving the large drifts that would be expected during a seismic event while remaining elastic. Additionally, these alternative systems required only a slight increase in material weight, while using only half as many welded connections as a conventional MRF. Due to the high labor costs associated with welded connections, this reduction makes the alternative systems economically competitive with the conventional system.

Discussion and Future Work

The results from both case studies provide a strong motivation for further investigation of frame M1-B as an economical and more effective back-up frame. Results from this research were presented at the American Society of Civil Engineers (ASCE) student conference at Utah State University in March 2013. The presentation was awarded first place among ten technical presentations. Additional simulation studies are being conducted to further verify the results from the case studies mentioned above. These simulations will be included with the results and submitted for publication in the journal Engineering Structures. Furthermore, a recent proposal has been submitted to seek funding for experimental work based on this foundational investigation.

Selected References

• Christopoulos, C. and Pampanin, S. (2004). “Towards Performance-Based Seismic Design of MDOF Structures with Explicit Consideration of Residual Deformations”, ISET Journal of Earthquake Technology, Vol. 41, No. 1, pp. 53-73.
• Hamburger, R. (2009). “Facts for Steel Buildings—Earthquake and Seismic Design”, American Institute of Steel Construction.
• Kiggins, S. and Uang, CM. (2006). “Reducing residual drift of buckling-restrained braced frames as a dual system”, Engineering Structures, Vol. 28, pp. 1525-32.
• Pettinga, D., Christopoulos, C., Pampanin, S. and Preistley, N. (2007). “Effectiveness of simple approaches in mitigating residual deformations in buildings”, Earthquake Engineering and Structural Dynamics, Vol. 36, pp. 1763-83.