Specimen S-HA

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.

在美国钢结构建筑的常规设计实践中，侧向地震荷载由少量抗侧力框架（弯矩框架、支撑框架、钢板剪力墙等）承担。其余结构体系仅承担竖向荷载，通常由柱、梁以及组合楼盖系统中的主次梁构成，主次梁通过简抗剪连接（simple shear connections）与柱相连。尽管重力承重框架并非专门为抗侧荷载设计，但实际上可为钢结构建筑提供可观的侧向刚度、承载力与变形能力。既往研究结果表明，当抗侧框架发生延性不足的破坏（如老旧建筑中常见的破坏形式）时，重力承重框架的作用尤为关键。此前的研究还证实，在强震作用下，这类建筑是否发生倒塌往往取决于重力承重框架的表现。即便对于新建建筑，重力承重框架作为“备用抗侧体系”，在极端强烈的地震动作用下也能有效防止结构倒塌。尽管已有大量研究探讨了重力承重框架在钢结构建筑抗震性能中的作用，但仍有诸多空白有待填补。因此，本项目的总体目标是深化对重力承重框架在钢结构建筑抗震性能中作用的认识，并建立可量化该作用所需的数据集、模型与分析工具。
本项目的一项核心工作是开展一系列梁柱子结构（beam-column subassemblies）的大型低周反复加载试验，试验中梁通过简抗剪连接与柱相连。所有试验均采用双角钢组成的简抗剪连接节点——这是美国建筑施工中常用的节点构造形式。试验分别针对带有组合混凝土楼板与不带组合混凝土楼板的试件开展，以明确楼盖系统对节点刚度、承载力与延性的影响。这类大型试验获取了大量珍贵数据，为揭示简抗剪重力连接在侧向低周反复荷载作用下的力学性能提供了全新的认知。同时，本试验也极大丰富了全球范围内带组合混凝土楼板的简抗剪重力连接节点的试验数据库。
本项目还针对钢结构建筑中的重力承重连接节点开展了建模研究，开发了适用于简抗剪连接的组件式模型——即结合刚性连杆与非线性弹簧的单元模型，可用于模拟整个建筑体系在地震荷载作用下的响应。此外，本项目还提出了一种全新的金属材料延性断裂（ductile fracture）起始预测理论。与所有钢结构节点一样，重力承重钢连接节点的最终破坏通常由节点部件（螺栓、焊缝、角钢、梁体等）的断裂所控制。利用高精度有限元模型（finite element models）准确模拟节点性能，断裂预测能力是至关重要的前提。本项目提出的断裂起始预测理论，可有效提升这类模拟分析的能力。