Chia-Ming Uang

Establishing absorbed energy spectra—an attenuation approach

For energy-based seismic design, energy demand in the form of absorbed energy spectra was established by an attenuation relationship. The absorbed energy is proposed for evaluating the energy demand in an inelastic system because the absorbed energy is directly related to the pseudo-velocity in the elastic case. Based on a total of 273 ground motion records from 15 significant earthquakes in California, an attenuation relationship of the absorbed energy was established from a two-stage non-linear regression analysis. This relationship was established for a given earthquake magnitude, source-to-site distance, site class, and ductility factor. A similar expression for the normalized absorbed energy was also developed. This study showed that the absorbed energy for near-field ground motions can be significantly larger than that predicted by the attenuation relationship for normal ground motions. Copyright

Stress Analyses and Parametric Study on Full-Scale Fatigue Tests of Rib-to-Deck Welded Joints in Steel Orthotropic Decks

Fatigue tests of full-scale orthotropic steel decks were recently conducted to evaluate the fatigue performance of rib-to-deck partial-joint-penetration (PJP) groove welded joints. The test results indicated that rib-to-deck joints are more prone to fatigue cracks in the deck plate than in the rib wall. A shallower weld penetration (for example, an 80% PJP) also appeared to have a slightly higher fatigue resistance than a deeper one (for example, a 100% weld penetration). These PJP welds were also more vulnerable to cracking initiated from the weld toe than from the weld root. Finite-element analyses of the test specimens, using the effective notch stress method, were performed to supplement the laboratory testing and provide additional information on the behavior of these welded joints. The analysis results showed a good correlation with the observed crack patterns. A parametric study also showed that the fatigue resistance of the PJP joint can be significantly influenced by the transverse loading location, deck plate thickness, and the weld penetration ratio. Increasing the deck plate thickness was effective in reducing the stresses, while the rib wall thickness had a little effect. A shallower weld penetration at the PJP joint appeared to have a positive effect in enhancing the fatigue resistance.

Flexural Strength and Rotation Capacity of I-Shaped Beams Fabricated from 800-MPa Steel

Flexural tests on full-scale I-shaped beams, built up from high-strength steels (HSB800 and HSA800) with a nominal tensile strength of 800 MPa, were carried out to study the effect of flange slenderness on flexural strength and rotation capacity. The primary objective was to investigate the appropriateness of extrapolating current stability criteria (originally developed for ordinary steel) to high-strength steel. For comparison purposes, specimens with ordinary steel (SM490) were also tested and showed sufficient flexural strength and rotation capacity in accordance with the AISC specification. The performance of high-strength steel specimens was also very satisfactory from the strength, but not from the rotation capacity, perspective. The inferior rotation capacity of high-strength steel beams was shown to be directly attributable to the absence of a distinct yield plateau and the high yield ratio of the material. When a higher rotation capacity is required as in plastic design, the testing clearly showed that high-strength steel beams were vulnerable to brittle fracture when full-height transverse stiffeners were welded to the tension flange in the plastic hinge region. Residual stress measurements reconfirmed that the magnitude of the residual stress is almost independent of the yield stress of the base metal.

Cyclic performance of haunch repaired steel moment connections: experimental testing and analytical modeling

Abstract The use of a haunch on the bottom side of the beam to enhance the seismic performance of damaged steel moment frame connections has been proposed after the 1994 Northridge earthquake. The effectiveness of such a scheme was demonstrated through both static and dynamic testing of four full-scale specimens. Four full-scale pre-Northridge moment connection specimens were tested either statically or dynamically to failure. It was found that the performance of dynamically loaded specimens tended to be inferior to that of the statically loaded specimens. Furthermore, the fracture pattern was also different. Depending on the damage pattern, these specimens were then repaired with a haunch and retested. Static and dynamic test results showed that the cyclic performance could be significantly improved, especially when some modifications were made to the existing groove welded joint of the top flange, and the plastic hinging of the beam occurred away from the face of the column. The presence of a haunch complicates the behavior of an enlarged (or ‘dual’) panel zone. To predict the inelastic seismic response of a steel framed structure, an analytical procedure for the modeling of a dual panel zone is presented; the simplified analytical modeling is shown to correlate well with the test results.

Cyclic performance of a type of steel beam to steel-encased reinforced concrete column moment connection

Abstract Two full-scale subassemblies with steel-encased reinforced concrete (SRC) columns and steel beams were tested to evaluate the seismic performance of the connection details. For ease of construction, continuity plates were eliminated and less transverse reinforcement than that specified in the NEHRP seismic provisions was used in the connection region. The reduced beam section was introduced to reduce the shear demand on the connection. In addition, two doubler plates were placed away from the column web to enhance the connection shear resistance. This study showed that: (1) the stringent requirement for the transverse reinforcement could be relaxed; (2) the offset doubler plates were able to resist a significant amount of connection shear; and (3) the inner concrete strut could be mobilized in the two-sided moment connection, but not in the one-sided moment connection.

Cyclic Testing and Modeling of Cold-Formed Steel─Special Bolted Moment Frame Connections

A cyclic testing of nine full-scale beam-to-column moment connection subassemblies was carried out to support the development of design provisions for a lateral load-resisting system in the AISI S110: Standard for Seismic Design of Cold-Formed Steel Structural Systems—Special Bolted Moment Frames. This type of one-story framing system features C-section beams connected to hollow structural section columns by bearing-type high-strength bolts and is commonly used in industrial platform construction. The test results showed that these specimens had an interstory drift capacity significantly larger than 0.04 rad. The cyclic behavior was characterized by a linear response, a slip range, and a significant hardening response due to bearing at bolt holes. The conventional strong column-weak beam design philosophy is not appropriate for this system. The inelastic action through the bolt slip and bearing in the connection is a ductile yielding mechanism. Beams and columns should be protected to remain elastic by th...

Seismic Response of an Instrumented 13‐Story Steel Frame Building Damaged in the 1994 Northridge Earthquake

This paper summarizes a case study of a 13-story welded steel moment frame (WSMF) building affected by the 1994 Northridge earthquake. The building, which was instrumented, sustained extensive damage to its welded connections. Ground motion records from the basement and response records from the sixth and twelfth floors were available. Damage data was collected with post-earthquake inspection and testing of each joint. The primary objective of the study was to compare modeled behavior with recorded response in order to assess the value of present analytical tools and modeling techniques for predicting the distribution and severity of connection failures. Calculated elastic time-history displacements matched well with recorded displacements in the E-W direction, less so in the heavily-damaged N-S direction where the elastic model was unable to simulate fractured moment connections. In the elastic analyses, joint demand was represented by beam demand-capacity ratios (DCRs). The highest beam DCRs were concentrated between the second and seventh floors; these locations correlated strongly with observed damage. Inelastic time-history analyses improved the displacement match in the N-S direction. They also indicated that panel zone yielding would have controlled the intended ductile response. This study suggests that for a regular structure, current modeling and analysis tools for both elastic and inelastic analysis, while unable to simulate premature brittle fractures, can be useful for predicting in a probabilistic way the intensity and distribution of damage expected in moderate seismic events.

An evaluation of two-level seismic design procedure

The two-level design philosophy is recognized by modern seismic codes. When this philosophy is implemented in the code, the intensities of the two design earthquakes, the structural performance criteria, explicit versus implicit design approach, and the effectiveness to achieve the performance criteria vary considerably from one code to the other. For the ultimate limit state, the UBC was compared with seismic codes of Canada, Japan, and Eurocode. It was found that a trend to deviate from the UBC approach of using a single seismic force reduction factor (i.e., R w ) is apparent. Instead, an approach using a compound force reduction factor which considers the contribution of structural ductility and structural overstrength is preferred. For the serviceability limit state, a comparison of the level of design earthquakes and performance criteria of the UBC, Tri-Services Manual, and the Japanese code indicates that the UBC produces the most flexible structure, and that UBC does not control structural damage. It is suggested that the UBC adopts an explicit serviceability design procedure.

Seismic Performance Factors for Cold-Formed Steel Special Bolted Moment Frames

The American Iron and Steel Institute (AISI) recently issued a standard on the seismic design of cold-formed steel special bolted moment frames (SBMF). It is expected that ductility is provided through bolt slippage and bearing in the moment connection region; both the beams and columns are to be designed based on the capacity design principles. This paper provides background information for the proposed seismic performance factors. Based on cyclic testing results, a value of 3.5 was proposed for the response modification coefficient, R . A statistical evaluation through nonlinear time-history analysis, which considered the unique structural characteristics of this framing system, showed that the Newmark-Hall ductility reduction rule is conservative. Based on the analysis results, a revised rule was proposed, which resulted in a deflection amplification factor, Cd , of R/1.2 ( ≈3.0 ) . A design procedure in the AISI seismic standard allows the designer to directly calculate the maximum seismic force in th...

A FEMA P695 Study for the Proposed Seismic Performance Factors for Cold-Formed Steel Special Bolted Moment Frames

The objective of this study was to verify the adequacy of the proposed Seismic Performance Factors for the newly developed Cold-Formed Steel Special Bolted Moment Frames in the AISI S110 Seismic Standard. The FEMA P695 methodology, Qualification of Building Seismic Performance Factors, was used for this purpose. A total of 13 archetype designs, representing two seismicity and two gravity load levels, were designed and analyzed. The computed results from all individual archetypes and four performance groups showed that the proposed seismic performance factors met the acceptance criteria and could provide a sufficient margin against collapse under the maximum considered earthquake.