Le-Wu Lu

Seismic behavior and modeling of high-strength composite concrete-filled steel tube (CFT) beam–columns

Abstract The behavior of square concrete-filled steel tube (CFT) beam–columns made from high-strength materials was investigated experimentally. The effects of the width-to-thickness ratio, yield stress of the steel tube and the axial load level on the stiffness, strength and ductility of high-strength CFT beam–columns were studied. Sixteen three-quarter scale CFT specimens, which included eight monotonic beam–column specimens and eight cyclic beam–column specimens, were tested. The experimental results indicate that cyclic loading does not have a significant influence on the stiffness or strength of CFT beam–columns. However, it causes a more rapid decrease of the post-peak moment resistance. The moment capacity of high-strength CFT beam–columns can be predicted with reasonable accuracy using the American Concrete Institute (ACI) code provisions for composite columns. Fiber-based models were developed for the CFT beam–column specimens. The uniaxial stress–strain curves for the fibers were derived from three-dimensional nonlinear finite element analyses of the CFTs. The results from the fiber analyses of the monotonic and cyclic beam–column specimens compare favorably with the experimental results.

Development of improved welded moment connections for earthquake-resistant design

Abstract A comprehensive research program was conducted at Lehigh University to investigate the causes of brittle failures observed in numerous welded connections during the 1994 Northridge earthquake and ways to improve their ductility. In this program, tests on pre-Northridge connection details demonstrate that several weak links in the connection design and fabrication resulted in their premature failures. To develop improved details for welded unreinforced connections the effects of weld metal, weld access hole geometry, beam web attachment, continuity plates, and panel zone strength on cyclic ductility were investigated using non-linear finite element models and full-scale connection tests. The results from these studies are presented, along with recommendations to insure ductile connection performance.

Ductile details for welded unreinforced moment connections subject to inelastic cyclic loading

Abstract The results of a 3-D finite element study of welded unreinforced flange beam-to-column moment connections in steel special moment resisting frames are presented. Computer models of connection subassemblies were developed using the general-purpose nonlinear finite element program abaqus . Several issues were addressed in the study: (1) geometry and size of the weld access hole; (2) benefit of a welded beam web; (3) control of inelastic panel zone deformations; and (4) effectiveness of continuity plates in reducing local demand on the connection. The analytical results provided information related to basic performance and the effects that these connection parameters have on inelastic cyclic performance, thereby furthering the current understanding of welded moment connection behavior under seismic loading conditions and leading to improved design criteria. Based on the results of the analytical study, recommendations for the seismic design of connections are given. The incorporation of the recommendations into an experimental test program showed good connection ductility in the test specimens.

Critical issues in achieving ductile behaviour of welded moment connections

Abstract A comprehensive research program on large-size, welded moment connections conducted after the 1994 Northridge earthquake in California has identified three critical issues that could significantly affect the strength and ductility of the connections. They are: fracture toughness of weld metal, geometry and size of weld access hole, and control of panel zone deformation. Experimental and analytical studies on each of the issues have been performed at the Center for Advanced Technology for Large Structural Systems at Lehigh University. This paper presents brief descriptions of the studies, key results obtained and recommendations made to insure ductile performance of the connections.

Lateral Load Tests of Unbonded Post-Tensioned Precast Concrete Walls

The paper discusses the behavior of unbonded posttensioned precast concrete walls subjected to static monitonic and cyclic lateral loads. Dynamic analysis results indicate that unbonded posttensioned precast walls have large flexural ductility and self-centering capacity without sustaining significant damage and excessive drift under moderate to severe earthquakes.

An Experimental Evaluation of High-Strength Square CFT Columns

The behavior of concrete filled tube (CFT) columns made from high strength materials was investigated experimentally. The effects of the width-to-thickness (b/t) ratio, steel tube stress-strain characteristics, and axial load on the stiffness, strength, and ductility of CFT beam-columns and stub columns were studied. Twelve experiments, which included four stub tests (monotonic axial load) and eight beam-column tests (constant axial and monotonic axial load) and eight beam-column tests (constant axial and monotonic flexural load) were conducted. The CFT specimens were 305 mm square tubes, made from either conventional (A500 Grade-B) or high strength (A500 Grade-80) steel with nominal b/t ratios of 32 and 48. The CFT specimens were filled with high strength (104 MPa) concrete. Experimental results indicate that the concrete infill delays, the local buckling of the steel tube, and that for lower levels of axial load and smaller b/t ratios the steel confines the infill concrete, thus increasing its ductility. Comparison of the experimental results with predictions based on current code provisions indicates that the axial load capacity of the high strength CFT stub column specimens can be predicted with reasonable accuracy by superposition of the yield strength of the steel tube and 85% of the compressive strength of the concrete infill. The moment capacity of the high strength CFT beam-column specimens can be conservatively estimated using American Concrete Institute provisions for conventional strength CFT beam-columns. The initial and serviceability-level section flexural stiffness of these specimens was predicted with reasonable accuracy using the uncracked transformed and cracked transformed section properties, respectively. The experimental results indicate that the curvature ductility of a high strength CFT beam-column decreases significantly with an increase in the axial load or the b/t ratio of the steel tube.

Steel moment frame connections that achieve ductile performance

Publisher Summary Several investigations were undertaken after the Northridge earthquake in order to examine various aspects that were believed to be associated with the failures observed in the pre-Northridge connection and to improve connection performance. Two full-scale beam-and-column assemblies with a “pre-Northridge” connection detail were dynamically tested. The column was a W14x311 made of A572 Gr. 50 steel. It was supported by a pin at its bottom and a roller at its top. The beam was an A36 steel W36x150 section with flange yield strength equal to 262 MPa. The first specimen tested was specimen A-1, which was typically pre-Northridge practice. The second specimen, specimen A-2, was the same as specimen A-1, but with the backup bar and weld tabs removed and a 9.5-mm reinforcing fillet weld (E71T-8) added to its weld root. The removal of the backup bars in specimen A-2 slightly improved the connection performance, but brittle fracture of the flange welds again led to failure. Other than the limited yielding at the weld access holes, the connection behaved elastically when the welds of the top and bottom flanges fractured almost simultaneously during reversed loading.

Lateral load behavior of unbonded post-tensioned precast concrete walls

Publisher Summary This chapter investigates the experimentally and analytically observed lateral load behavior of unbonded post-tensioned precast concrete walls. Each wall is comprised of six one-storey precast panels that are connected along horizontal joints using unbonded post-tensioned steel, which is anchored at the roof and within the foundation. The bottom panel has regions that are confined and this confinement enables the base panel to sustain the large compressive strains that develop as a result of gap opening displacements that develop along the base of the wall due to lateral loads. The limit states characterizing the lateral load behavior occur as anticipated in the design of the walls and at force and drift levels predicted by the analytical model, except that the experimentally observed drift capacity significantly exceeds the drift capacity predicted by the analytical model. The results demonstrate that unbonded post-tensioned precast walls can be designed to undergo significant nonlinear lateral drift without significant damage, and to retain their ability to self-center, thereby eliminating residual lateral drift. The cyclic lateral load behavior of the walls is nearly nonlinear elastic, with only a small amount of energy dissipation per cycle of loading. As a result, larger lateral drifts can be expected under earthquake loading. However dynamic analysis results show that these walls can be designed to avoid sustaining excessive drift under moderate-to-severe earthquakes.

High-Performance Steel Structures: Recent Research

Publisher Summary High-performance steel is defined as steel that has the combined characteristics of high strength, good ductility, high toughness, good weldability, and fabricability. These are the properties essential for successful construction of high-performance structures in a civil infrastructure system. For exposed structures, such as bridges and ships, good corrosion resistance is also necessary. From metallurgical composition and processing point of view, yield strength of above 450 MPa is considered as high strength. The fracture toughness, weldability, and formability of the steels should be significantly better than those of the conventional steels. Microalloyed steels with low carbon content, high manganese levels, and microalloy carbide and nitride formers have been available for sometime in construction of structures that require high strength, high fracture toughness, and good weldability. The low-carbon, age-hardenable steels have gained increasing usage in shipbuilding, heavy-vehicle manufacturing, and offshore structure construction because of their excellent weldability and fracture toughness. These steels have become known as high-strength low alloy steels, although their total alloy content is generally around four percent.