General Properties

In typical U.S. design practice for steel buildings, lateral seismic loads are resisted by a small number of lateral force resisting frames (moment frames, braced frames, steel shear walls, etc.). The remainder of the structural system is designed to resist gravity loads, and normally consists of columns, beams, and girders with a composite floor system, wherein girders and beams are connected to columns using “simple shear” connections. The gravity framing, although not specifically designed for lateral load resistance, can in fact contribute significant lateral stiffness, strength, and deformation capacity to a steel building. Results of previous research have shown that the role of gravity framing is most important when the lateral frames experience non-ductile failures as may occur in older buildings. Previous work has also demonstrated that the gravity framing can often make the difference between collapse and survival of such buildings in a large earthquake. Even for new buildings, the gravity framing plays an important role as a “back-up” system that can prevent collapse under very large earthquake ground motions. While a considerable amount of previous work has been done to examine the role of gravity framing in the seismic performance of steel buildings, much information is still lacking. As such, the overall goal of this project was to develop a better understanding of the role of gravity framing in the seismic performance of steel buildings, along with the data, models and tools needed to quantify this role.

A major activity of this project was a series of large-scale cyclic loading tests of beam-column subassemblies, where the beams were connected to the column using simple shear connections. All tests were conducted with simple shear connections consisting of double angles; a common detail used in U.S. building construction. Tests were conducted on specimens with and without a composite concrete floor slab, to better understand the role of the floor system in affecting the connection’s stiffness, strength, and ductility. These large-scale tests generated a wealth of valuable data that provided new insights into the behavior of simple gravity shear connections under cyclic lateral load. The tests also significantly expanded the world-wide experimental database on simple gravity shear connections with a composite concrete slab.

This project also advanced modeling of gravity connections in steel buildings through the development of component type (nonlinear springs combined with rigid links) models for simple shear connections that can be used for simulating the response of entire building systems under earthquake loading. This project also developed a new theory for predicting the initiation of ductile fracture in metals. The ultimate failure of steel gravity framing connections, like all other steel connections, is typically controlled by fracture of a connection component (bolt, weld, angle, beam, etc.). The ability to predict fracture is critical for accurate simulation of connection performance using high fidelity finite element models. The new theory for predicting fracture initiation developed in this project advances such capabilities.