Link to Latest Report: September 2023 Progress Report
Background:
In recent years, public concern about road closures resulting from new construction, replacement, or retrofit of bridges has been on the rise. The consequences of these works could be economic losses, security concerns at the construction site, costs and delay time suffered by the users, and in general problems that worsen the public perception of transportation agencies. At the same time, due to current environmental awareness, there is a concern about unnecessary use of vehicles operating on fossil fuels, in this case due to detours or traffic congestion. To reduce the impacts on the driving public and the environment, accelerated bridge construction (ABC) techniques have been gaining popularity.
In ABC projects, bridge elements or entire systems are prefabricated and erected to expedite construction (Culmo, 2011). Examples of such prefabricated elements include deck panels (Garber and Shahrokhinasab, 2019) and columns (Shafieifar et al., 2020). Prefabrication of beams and girders has been an integral part of bridge construction in the U.S. for many years (Culmo, 2011). Precast prestressed concrete (PC) girder bridges comprise a large percentage of the National Bridge Inventory (NBI). In PC bridges, end diaphragms are used to transmit loads—mainly transverse in the case of earthquakes—from the bridge superstructure to the substructure. Typically, these end diaphragms are cast-in-place concrete. Culmo (2009) notes, “The time for forming and curing of [these] connections can be significant,” motivating the need for prefabricated diaphragms for use in ABC projects. According to the investigators’ knowledge and extensive literature review, both experimental work and seismic design provisions for end diaphragms on PC girder bridges are limited despite their abundance in practice. The 2010 Chile earthquake came to demonstrate the importance of end diaphragms and the need for developing and understanding a viable and clear seismic load path in bridges (Yen et al., 2010; Marsh et al., 2015). Furthermore, for regions located in high-risk seismic zones, great care must be taken in the way the connections between precast elements are made (Marsh et al., 2011; Culmo, 2009).
In the case of steel bridges, some important distress suffered by the superstructure and mainly by the substructure during the most significant earthquakes during the last three decades that occurred worldwide has been identified (Zahrai and Bruneau, 1999a). As a proposal to solve these problems through retrofit, Zahrai and Bruneau (1999a) developed a system of ductile end- diaphragms for slab-on-girders steel bridges. They tested three types of diaphragms based on three successful bracing frames systems for steel buildings (Zahrai and Bruneau, 1999b). Furthermore, they proposed a simplified design procedure based on analytical evidence from 2-D and 3-D computational models. The solution has evolved until it became the Type 2 Global Seismic Design Strategy (GSDS) of the AASHTO Guide Specifications for LRFD Seismic Bridge Design (2011) that applies only to steel superstructures, and likewise it forms part of other important seismic design and retrofit codes in the U.S. Therefore, following the concept proposed by Zahrai and Bruneau (1999a; 1999b) for steel bridges, it would be important to develop guidelines on the behavior and detailing of precast concrete ductile end-diaphragm elements for seismic resistance. With this regard, the use of concrete ductile diaphragms as fuses (Type 2 GSDS) should be explored for the seismic lateral resistance of slab-on-girder concrete bridges. This diaphragm system should be developed to be part of ABC solutions for design of new bridges, has potential as an ABC solution for retrofitting of old infrastructure, and even could be used for a combination of both, in the case of simply supported PC girder bridges located in high-risk seismic regions.
(a)
(b)
Figure 1. (a) General view of proposed system in a prototype bridge; (b) Schematic view of the expected rocking behavior of the system under lateral seismic load
In a preliminary study (Villalobos-Vega and Santana, 2022), the cyclic lateral load behavior of a ductile precast end-diaphragm system was proposed and modeled (see Figure 1(a)). The system, whose intention is a global low damage behavior even under extreme events, is comprised of (a) diaphragm-to-girder connections based on the concept of the conventional hybrid system with unbonded post-tensioning and (b) girder-to-slab connections with the use of steel angles for partially restricted (PR) or semi-rigid connections. In that study, a simplified method of design and step-by-step analysis was developed for both the components and the ductile end-diaphragm system, by means of which the force-drift relationship and the dynamic and seismic demand parameters of the bridge were obtained. To get the experimental behavior of the proposed concept, a length corresponding to 24 in. (0.60 m) of one of the ends of a prototype bridge was considered, and it was tested in real scale in the laboratory subjected to pseudo-static cyclic lateral loading (see Figure 1(b)). The main objectives of the preliminary study were to propose the main characteristics and components of the system, determine the feasibility of its construction process, establish a first trial of its theoretical behavior, and test just a portion of the bridge to obtain a reasonable response of the system to have a parameter for comparison. Overall, these goals were met, but additional questions resulted. In particular, the experimental result of overstrength in one direction of load needs to be addressed, besides to determine if the connections’ performance can be improved by using other known or novel cost-effective technologies adapted to the ABC market in the U.S. Additionally, it is necessary to improve and extend the analysis to the overall behavior of the bridge—not just at the ends—and at the same time parameterize variables such as the length, depth, and number of girders, to be able to better understand the interactions between the girders, ductile diaphragms, and deck under seismic actions. Finally, a second test is necessary to incorporate the improvements identified in the completed preliminary study and/or through modeling and verify experimentally its performance subjected to seismic demands. The proposed research aims to address these questions.
Objective:
The objectives of the proposed research are to: (a) calibrate and validate computational models for the behavior of the proposed precast concrete ductile end-diaphragm elements and rocking girder-to-slab connections, (b) synthesize analytical evidence of the enhanced seismic performance of these prefabricated ABC elements through computational modeling, and (c) develop guidelines for computationally modeling the proposed prefabricated ABC elements for use by practitioners and researchers.
Scope:
Task 1 – Computational Modeling
Objective: To model the holistic behavior of a bridges equipped with the proposed ABC system.
Task 2 – Design Optimization
Objective: To fine tune the detailing of the ductile diaphragm system and connections.
Task 3 – Experimental Testing
Objective: To calibrate the computational model and to validate the predicted load-drift behavior
Task 4 – Guideline Preparation
Objective: To promote the uptake of the proposed ABC system by providing guidance to the design and modeling of these systems.
Research Team:
Principal Investigator: Philip Scott Harvey Jr.
Co-Principal Investigator: Royce W. Floyd
Student Assistant: TBD