|Development of Modular Cast Steel Connections for Seismic-Resistant Building Frames
by Robert B. Fleischman, Ph.D., and Ali Sumer
Steel castings can be used in building construction to provide unique and architecturally attractive connections. They can also solve difficult performance issues by tailoring the geometry to the service requirements. For example, offshore platforms for oil and gas production have used steel castings as nodes to connect the structure together. Designers have employed steel castings to move welds to low stress areas.
With the freedom of design and robust properties of steel, castings can contribute to improving safety and reliability in building construction. This article shows another example of the use of steel castings, which exploits the ductility of steel in earthquake zones where the absorption of the earthquake energy without collapse is of primary importance.
The structures of buildings can be constructed from steels conforming to standards developed by ASTM Subcommittee A01.18 on Castings.
Malcolm Blair, Steel Founders Society of America, member Committee A01
Modular cast steel connections are being developed at the University of Arizona for use in seismic-resistant building frames. This research, led by Robert Fleischman, Ph.D., is funded by the National Science Foundations Faculty Early Career Development Program with support from industry partners: the American Institute of Steel Construction and Steel Founders Society of America. The modular cast connections are engineered to meet performance requirements corresponding to optimal seismic response.
The impetus for developing the modular connections is the recently discovered susceptibility of welded connections in steel special moment frames (SMFs) to fracture during strong earthquakes. These structures rely on welded joints to create a strong, stiff system that also allows plastic behavior in the surrounding beams to dissipate the energy of an earthquake. However, more than 100 SMFs suffered fracture at the welded joints during the Northridge, Calif. (U.S.) and Kobe, Japan, earthquakes. This poor performance has raised questions regarding the reliability of the SMF.
The consensus from a large research program on steel moment connections (the SAC Joint Venture, www.sacsteel.org) is that an effective earthquake-resistant connection design should be based on a combination of weld fracture mitigation measures and the reducing or redirecting of stress flow.
The modular connection concept attempts to address these recommendations through the creation of new connection forms. The rolled shapes used traditionally in the construction industry are structurally and economically efficient as main members, but may not necessarily provide the features needed in the connection region. Therefore, the designs rely on the versatility afforded by the casting process to create connections specifically configured for seismic performance and advancements in casting technology to ensure that the connections possess the required strength, ductility, and material isotropy.
To date, two connection concepts have been developed: a modular node and a modular connector. These connection concepts were developed by the UA research team through solid modeling and nonlinear finite element analysis using the general purpose finite element program ANSYS (provided by industry partner Swanson, Inc.). Prototypes were cast by SFSA member Varicast and fabricated by AISC member Able Steel. These connections have exhibited superior ductility, reliability and energy dissipation characteristics with respect to traditional connections in both the analytical simulations and the full-scale testing.
Material and Casting
The modular connections are created from high-strength, high-value steel using a casting process. These cast pieces are intended to serve as the special energy dissipating detail for the structure (i.e., the pieces are to act as a structural fuse by yielding, thereby limiting the seismic forces to the structure). Thus, the most important quality of the casting is ductility, that is, the ability to undergo large plastic deformation without fracture. Through the exchange of three-dimensional solid model (.SAT) CAD files, the Varicast and UA team were able to identify final forms that met both structural performance and castability requirements. Strict quality control of the casting process ensured the prototypes possessed the expected ductility. Table 1 indicates the chemical composition typical of the heats; in combination with normalizing and stress relieving this resulted in a mild steel of the desired properties (see Table 2).
The modular node (MN) is a cast piece shop-welded to the column and subsequently field-welded to the beams at a non-critical cross-section. The removal of the field weld from the cross-section at the column face represents a significant improvement over traditional construction in which the field weld acts as a potential source of brittle fracture because (1) it is difficult to control the quality of weld and thus flaws may act as the initiation of a crack, (2) a heat affected zone develops in which the careful properties of the original parent material may be compromised, (3) the weld requires an access hole in the beam web which creates strain concentrations in the beam, (4) the weld location is at a region of high triaxial restraint, which could suppress ductile yielding modes in favor of brittle fracture, and (5) high forces act in the through-thickness (weaker) direction of the column flange. Thus, the modular node design can greatly reduce the risk of brittle fracture.
Figure 1a shows the full-size MN prototype at the foundry following removal of the sand molds. This MN can be used with a large range of columns possessing the same inner dimension and is envisioned to be available in a limited set of beam depths. Full-scale test subassemblages were fabricated subject to the Federal Emergency Management Agency (FEMA)-350 cyclic testing protocol. The connection not only met the requirements (large plastic deformation and energy dissipation without fracture) but greatly exceeded them. Figure 1b shows the greatly deformed but intact MN subassemblage at a building drift demand well beyond that expected in a major seismic event.
The modular connector (MC) is a cast piece used to connect the beam to the column (see Figure 2). Typical construction will involve shop-welding the MC to beam flanges and field-bolting the MC to the column flange. The MC provides a bolted alternative for full-moment connections that eliminates the potential for brittle behavior associated with welding construction. The intent of the design is to develop the ductility within the connector itself. The MC is engineered to deliver reliable and repeatable energy dissipation through the elimination of concentrated plastic strain regions and the reduction in bolt prying forces.
To accomplish the former, the MC possesses variable cross-section arms that spread the plastic deformation along the entire length of the element (see Figure 3a). Such a distribution reduces the intensity of strain concentrations that would occur with the uniform section of traditional rolled shapes such as the tees or angles. The second requirement is achieved through a base connecting each end region that significantly reduces the demand on the bolts. Reduction of force demands on the bolt is desirable as the bolt is inherently less ductile than the mild steel detail pieces.
Half-scale MC prototypes were tested under monotonic and cyclic loads. Figure 3b shows the extremely large ductility obtained in the MC prototype; no bolt distress was observed. In comparison, bolt failure was the controlling mode in the traditional tee connection tested, and a control test of a tee with over-designed bolts failed at 43 percent of the fatigue life achieved by the MC.
Copyright 2003, ASTM