Judy Liu

Behavior and design of single plate shear connections

Abstract Steel shear connections are primarily used to transfer the reaction of a simply supported beam to its support, normally a column or a beam. Currently, the most common shear connection in North America is a single plate connection consisting of a plate fillet welded to a supporting column or girder and bolted to the web of a simply supported beam. A shear connection should be strong enough to be able to transfer the shear force, yet, it should be sufficiently flexible and ductile to allow the end of simply supported beam to rotate with ease and accommodate the rotation demand of the beam. This paper summarizes a number of research and development projects conducted at the University of California, Berkeley to study behavior of single plate shear (shear tab) connections and to develop design procedures and guidelines, both for gravity and lateral load (seismic and wind) effects. The connections were sufficiently ductile to accommodate end rotation demands of simply supported beams under gravity load and drift rotations under lateral load effects. Design procedures developed and proposed and currently used in design of single plate connections are strength-based procedures that ensure occurrence of ductile and more desirable failure modes, such as yielding of the steel plate prior to occurrence of brittle and less desirable failure modes such as fracture of bolts and welds.

Large-scale experimental evaluation of steel gravity framing structural integrity

An ongoing collaborative research project is studying the structural integrity of steel gravity framing systems composed of steel beams and girders with composite slab on metal deck. A major contribution of this project is large-scale testing of critical components in gravity systems subjected to severe demands consistent with column loss scenarios. Detailed component tests of the steel-concrete composite floor slab and simple beam-column connection are complemented by complete floor system tests. Component tests on the concrete slab on metal deck include uniaxial tests to evaluate behavior in both orthogonal directions with respect to the deck corrugations, and shear and tension tests of the sidelap splices. Parameters include metal deck thickness and type of sidelap (e.g., button punch, screw, weld). The full-scale beamto-column connection tests consider a wide range of parameters, including connection type, number of bolts, edge and gap distances and slab effects. The floor system tests are being conducted at half scale on a three-bay square configuration that considers interior, exterior and corner column loss scenarios.

Long-Term Effects of Super Heavy-Weight Vehicles on Bridges

A permit truck which exceeds the predefined limit of 108 kips is defined as a superload in Indiana. This study was conducted to examine the long-term effects of superload trucks on the performance of typical slab-on-girder bridges and to assess the likelihood of causing immediate damage. Typical steel and prestressed concrete slab-on-girder type bridges were analyzed using both beam line analysis and detailed finite element models. Furthermore, one prestressed concrete bridge and one steel bridge were instrumented using more than 50 sensors each. Strains and deflections were measured during a live load test, and each bridge was monitored for more than six months. Capacities of the investigated bridges were calculated and compared with the demands generated by various groupings of typical superload trucks. Analysis of the steel and prestressed concrete bridges demonstrated that typical superload trucks up to a gross vehicle weight of 500 kips are not expected to cause any damage or impair the long term performance of the investigated bridges. Serviceability limit states of the prestressed concrete bridges controlled the rating, and the bridges had adequate strength to accommodate all superloads included in the database. However, strength limit states controlled the rating of steel bridges. Long term monitoring of a continuous and a simple span bridge indicated that strains comparable to those of a 366-kip superload truck can be generated by regular truck traffic. The field measurements also demonstrated that the in-service behavior was different than the design assumptions. Fixity due to integral abutments, effectiveness of the continuity joint in continuous prestressed concrete bridges and contribution of the secondary members lead to notable differences between the expected and the anticipated behavior. Furthermore, the AASHTO girder distribution factor equation was found to be conservative for the investigated bridges. Use of a more accurate method such as FEA or the spring analogy method is recommended for the evaluation of bridges traversed by very heavy superload trucks.

Experimental Characterization of a Composite Slab Subjected to Simulated Column Removal Loading

AbstractSteel gravity frames are inherently simple systems, which has led to questions regarding their structural integrity or robustness. The concept of structural integrity is often tied to column removal scenarios in which the structure must develop alternate load paths. The mobilization of the composite slab with ribbed metal decking as a part of this behavior, theorized to be via membrane action, shifts the loading in the slab from bending to primarily tension. This study consists of composite slab component tests that include uniaxial tests in both orthogonal directions with respect to the deck corrugations, and shear and tension tests of the side lap splices. Also included are transverse uniaxial slab tests with additional reinforcement, and tension tests of the support fastener connections between the metal deck and beams and girders. The test results show the composite slab to be weak in the transverse direction regardless of side lap engagement and strong but susceptible to debonding in the long...

Application of Experimental Results to Computational Evaluation of Structural Integrity of Steel Gravity Framing Systems with Composite Slabs

AbstractThe simplicity of steel gravity framing systems in their load paths and construction has led to questions regarding their structural integrity or robustness. When considering structural integrity, the problem is often framed around the concept of column-removal scenarios, in which the structure must develop alternate load paths. The sometimes prohibitive space and resource requirements for experimental evaluation of steel gravity framing systems subjected to column-removal scenarios have resulted in the development of computational approaches to the problem. Recent computational research on this topic has highlighted the importance of considering the contribution of the composite slab to system robustness. A shell-based composite layup strip approach is the current state of the art for representing the composite slab in computational models. However, this approach has not been fully validated with experimental data and contains inherent assumptions, such as continuity of the metal deck between bay...

Fiber Reinforced Polymer Bridge Decks

The overarching goal of this study was to perform a comprehensive evaluation of various issues related to the strength and serviceability of the fiber reinforced polymer (FRP) deck panels that are available in the industry. Specific objectives were to establish critical limit states to be considered in the design of FRP deck panel, to provide performance specifications to designers, and to develop evaluation techniques for the deck panels in service. Two different FRP panels were studied during the research project: a sandwich panel and a pultruded panel. The sandwich panel was initially selected for the rehabilitation case study bridge. However, for a variety of reasons outside of the scope of this study, both the sandwich panel and the initial case study bridge were dropped from consideration. A new case study bridge was selected, and new proposals from FRP deck manufacturers were solicited. At that time, the pultruded deck was selected. Analysis and experimental results related to both FRP deck panels are included in this report, as information from both decks is relevant to the overarching goal of this study. In November 2009, Sugar Creek Bridge became the first bridge in Indiana to be rehabilitated with an FRP bridge deck. An extensive study, including literature review, analysis, and load tests, suggest that the installed deck should perform well, with web buckling as the ultimate failure mode at a factor of safety of 5. Deflection limits, generally an issue with FRP decks, are satisfied with the installed deck. Meanwhile, some combination of acoustic emission methods, infrared thermography and a newly developed traveling truck deflection method show promise for non-destructive evaluation of the deck in-situ and identification of damage such as delamination of the wearing surface or web buckling. However, such methods have shown variability and could be prohibitively labor-intensive. Therefore, further evaluation is needed if such methods are to be pursued.

Design and Analytical Validation of Post-tensioned Column Bases

A post-tensioned (PT) column base connection for use in a seismic resistant PT frame is presented. The PT column base connection allows for self-centering behavior of columns (i.e., returns to its original position with negligible residual drifts). The PT column base connection consists of PT high strength bars, buckling restrained steel plates, reinforcing plates, keeper plates and shim plates. This paper will discuss design considerations for the PT column base connection based on a performance-based design approach, and evaluate the PT column base connection using validated finite element (FE) models.

Analysis and Instrumentation of a Steel Bridge for Investigation of Superload Effects

The number of superload trucks, which often carry loads three to 15 times those of common design vehicles, has increased in recent years. The increased frequency of occurrence has prompted concerns about the ability of highway bridges to accommodate such loads and remain in serviceable condition. To address these concerns, an analysis of effects of superloads has been conducted for a case study bridge. A detailed finite element (FE) model of the bridge, including all primary and secondary members, was created to evaluate the behavior. The FE analysis results were compared with results from a controlled load test of the bridge. The girder distribution factors determined experimentally and analytically were found to be significantly lower than comparable design values. Furthermore, from the load tests, the rotational restraint at the girder ends due to the abutments was determined to be as high as a fixed support. The FE model was then used to predict the effects of various superload trucks. The analysis results and field data suggest that some of the stiffener plates and cross-frame members of the analyzed bridge would be overstressed by certain superload truck configurations.

Fiber-Reinforced Polymer Decks for Bridge Rehabilitation A Case Study

Fiber reinforced polymer (FRP) bridge decks are used in bridge rehabilitation projects, often because of their relatively low self weight and high durability. Related benefits of FRP include rapid construction and advantages in terms of life cycle costs (e.g., corrosion resistance). A case study bridge in Tippecanoe County is the first in Indiana to be rehabilitated with a FRP deck. Among the bridges evaluated, County Road 900E over Wildcat Creek is a three-span continuous steel stringer bridge with two concrete approach spans. The FRP deck replacement would only take place on the three main spans. This bridge rehabilitation project presents a unique challenge in that the deck would be widened with no changes to the superstructure. The resulting overhang would also be the longest seen for this application. Since the properties of FRP are such that serviceability issues govern deck design, requirements for the overhang dominate decisions related to deck type and geometry, and choice of barrier or guardrail. These and other project details will be discussed. The choice of deck is affected by deflection limits of the overhang at the exterior stringer. Based on American Association of State Highway and Transportation Officials (AASHTO) span-to-deflection ratio recommendations, a deck produced by hand lay-up rather than pultrusion is selected. The ability to tailor the deck cross section geometry to satisfy serviceability limits motivated this decision. The FRP deck consists of a honeycomb core sandwiched between two faces, or structural surfaces. This bridge design also involves a trade-off between a deeper section to satisfy deflection criteria and overall cost. Specifically, a deeper section will increase material cost for FRP and the cost of building up the concrete approaches to match the deck height on the main spans. The deck geometry also affects the decision regarding proposed stiffener brackets, which would supply discrete support points to the overhang at each diaphragm location. Finite element analyses show that the deck is significantly more flexible in its transverse direction, affecting distribution of load and resulting in absolutely no reduction in deflection or stress with the bracket supports. The brackets are therefore excluded from the design. Finally, the choice of vehicular rail/ barrier is also affected by the overhang. Both concrete barrier and steel guardrail options are evaluated. AASHTO allows for a reduction in overhang loading for concrete barriers due to the structural continuity provided to the deck. However, finite element analyses demonstrate that this provision is

Applicabiliy of the simplified load distribution factor equation to PSC girder bridges

AASHTO LRFD specifications first introduced a new load distribution factor equation as a result of the NCHRP12-26 project. However, this equation involves a longitudinal stiffness parameter, which is not initially known in design and thus introduces an iterative procedure. Practicing engineers perceive this need for an iterative design procedure as the major impediment to wides pread acceptance of the AASHTO LRFD equation. Recently, a non-iterative simplified equation based on the AASHTO LRFD formula was developed for steel girder bridges. The applicability of this equation to Prestressed Concrete (PSC) girder bridges is investigated in this paper. A total of 17 PSC girder bridges are selected and analyzed using a sophisticated finite element model. It has been found that the new simplified equation produces LDF values that are always conservative when compared to those obtained from the finite element analyses and are generally greater than the LDF obtained using AASHTO LRFD specification. Therefore, the simplified equation provides a simple yet safe specification for LDF calculation.