This paper presents all investigation of the behavior of spirally confined lap splices of deformed reinforcing bars in concrete. A total of 119 specimens mere tested in axial tension in two phases. The first phase focused on determining the best splicing detail. In the second phase, the chosen splice detail was more extensively studied. Parameters covered in this study include concrete strength bur size, number of spirals, and lap length. Maximum axial load was recorded as the failure capacity of specimens' cracking patterns zn,as also observed The results are compared with recently published research recommendations and with the ACI 318-95 provisions. The investigation showed that spiral configuration of lap splices of standard reinforcing bars can result in significant reduction of the required lap length. An equation was derived to predict spirally confined lap length. Finally recommendations were made to increase the maximum limit set forth in ACI 318-95 on the effect of transverse reinforcement on development length.

A new system of precast prestressed concrete joists has been developed for floors and roofs of commercial buildings using high strength concrete. Steel bar joists anti prestressed concrete double tees are presently popular as roof and floor elements hi commercial and multi-unit residential buildings. The new joist is a precast prestressed truss with a precast or cast-in-place slab. It combines the benefits of both the bar-joist and the double-tee into one system.The hybrid joist offers the following advantages: 1) weight of the floor system is reduced by as much as 5% compared to the double tee system, 2) mechanical and electrical utilities may pass through the webs rather than beneath them, 3) ease and flexibility of construction as well as good product quality of precast prestressed concrete construction is maintained, 4) no fire proofing is required, and 5) no corrosion protection is required. Due to the optimized shape of the joist, high tensile and compressive capacities of concrete were required to be combined with high strength prestressing strands. The fabrication and construction process of the joist system is described, and the experimental response of two full-scale specimens with 32 ft (9.75 m) spans is presented. Testing results showed superior performance of the 12,000 psi (82.74 MPa) concrete used in production. A suggested design procedure is outlined. Ductile, predictable failure was achieved. The system is in the process of being patented.

This paper is the second of two papers dealing with research conducted at the University of Nebraska on the design of double tees with large web openings. The earlier paper dealt with experimental and theoretical studies as well as initial design recommendations. This phase of the project aims to further maximize and standardize opening sizes and locations. It also aims to simplify the additional design steps required to account for the presence of web openings. The results of the study show that a significant number of large openings can be incorporated into the webs of double tees without reducing the structural capacity of the member if two main conditions are met: (1) No openings are placed within a distance equal to the strand development length from the member end, and (2) additional vertical stirrups, as detailed in this paper, are provided at the edges of the openings.

This paper describes newly developed precast concrete sandwich panels. The panels use a new system of fiber reinforced plastic (FRP) connectors, developed at the University of Nebraska's Center for Infrastructure Research, for transferring shear forces from one concrete wythe to the other. The use of FRP as the connector material increases the thermal efficiency of the panels compared to panels that contain steel at concrete connectors. The geometry and material properties of the new connector provide sufficient strength and stiffness for a significant transfer of shear between the two concrete wythes. The results of structural tests are analyzed and design recommendations are presented. Description of an accompanying thermal performance testing of these panels is also included.Four full-scale specimens were tested in a vertical position. Two of the specimens contain the new FRP connectors, and two contain steel truss connectors. Measurements of load versus panel deflection and load versus connector stress are provided. The ultimate strength of the panels containing the new connector were found comparable to the strength expected of fully composite panels.The design of panels containing the proposed FRP connecting system can be undertaken in a manner similar to that of fully composite panels. Ultimate strength of the panels can be computed assuming full composite action between the concrete wythes, if sufficient shear connectors are provided and the panels are not over-reinforced.

The use of high performance concrete (HPC) for the 120th and Giles Road Bridge in Sarpy County, Nebraska is the main focus of this paper. The bridge is one of the HPC bridge Showcases sponsored by the Federal Highway Administration under its Strategic Highway Research Program (SHRP). This prestressed concrete girder bridge consists of three spans made continuous using conventional reinforcement in the cast-in-place deck for resisting negative moment. Girders with an optimized cross section and a concrete compressive strength of 82.74M Pa (12,000 psi) are used for the design of the bridge. The cast-in-place deck is specified to have a compressive strength of 55.16 M Pa (8,000 psi) and a low permeability for improved durability and resistance to deicing salts. This bridge is to be built in the vicinity of a conventional concrete bridge, similar in dimensions and traffic volume. Both bridges are briefly described with economical comparison presented. The HPC bridge will be instrumented and planned to be monitored for several years.

The work described in this paper was undertaken to demonstrate that precast concrete systems can be practically designed to resist earthquakes, to identify areas where code provisions for seismic design need to be re-evaluated to specifically address the intrinsic characteristics of precast concrete, and to recommend improvements to these areas. The research focuses on a six-story precast concrete office building with a size and layout common in the United States. The system consists of an interior gravity load-resisting frame and a dual system for lateral load resistance consisting of interior shear walls and exterior spandrel frames. A testing program comprising panel-to-panel and beam-to-column connection tests was conducted, and an inelastic dynamic analysis of the lateral load-resisting system was performed. The study shows that the seismic performance of precast systems in resisting seismic loads can be as good as or better than that of cast-in-place systems.

A continium model for evaluating partially composite insulated concrete sandwich panel deflections with discrete shear connectors due to differential volume changes such as those caused by differences in temperature between the wythes is evaluated for accuracy by comparison with finite-element analysis. Design equations for computing these deflections are provided. Analysis of sample panels indicate that the continuum model provides acceptable accuracy for calculating thermal bowing in the panels is relatively insensitive to the stiffness of the connectors. This implies that long insulated sandwich panels with low connecting-layer stiffness will experience nearly the same amount of thermal bowing as fully composite panels.