Girder Load Distribution for Seismic Design of Integral Bridges Justin Vander Werff, P.E., M.ASCE 1 ; and Sri Sritharan, M.ASCE 2 Abstract: Current seismic design practice related to integral bridge girder-to-cap beam connections allows little or no lateral seismic load to be distributed beyond the girders immediately adjacent to the column. However, distribution results from several large-scale tests have shown that the distribution of column seismic moment typically engages all the girders. An approach utilizing simple stiffness models to predict distribu- tion in integral bridge structures is presented in detail; distribution predictions based on grillage analyses also are compared. The experimental results and the analytical results from the stiffness and grillage models show that current design methods related to vertical load distribution are sufficiently accurate. However, when applied to the distribution of lateral load, similarly obtained results reveal that current design practice does not appropriately account for the amount of load that is distributed beyond the girders adjacent to the column to the nonadjacent girders. The current practice leads to excessive girder-to-cap connection reinforcement, increased girder depth, unnecessarily high seismic mass, and increased construction cost. Finally, this paper makes recommendations for more appropriate distribution of seismic lateral load in integral bridge superstructures. DOI: 10.1061/(ASCE)BE.1943-5592.0000641. © 2014 American Society of Civil Engineers. Author keywords: Seismic analysis; Seismic design; Girder bridges; Load distribution; Lateral forces; Earthquake engineering; Bridge loads. Introduction Integral bridges have several advantages over nonintegral config- urations. These advantages have been well-documented in recent years (Snyder et al. 2011; Maruri and Petro 2005; Wassef et al. 2004), and have led to increased implementation of integral configurations, but design recommendations for such structures continue to be limited in some critical areas. The distribution of lateral load between girders in the superstructure is a particular aspect of integral bridge design that has not been addressed adequately. Common bridge design recom- mendations, such as the AASHTO standards (AASHTO 2010, 2009), provide very little information on the distribution of lateral seismic loads. Common standards used in seismic regions, such as Seis- mic Design Criteria (CALTRANS 2013) and Bridge Design Aids (CALTRANS 1995) also do not provide a detailed approach for seismic lateral load distribution. Investigations over the past 15 years have explored seismic lat- eral load distribution in the superstructure of integral bridge systems. Holombo et al. (2000) briefly looked at lateral load distribution alongside other issues of interest related to the use of precast con- crete superstructures in seismic regions. The National Cooperative Highway Research Program (NCHRP) Project 12-54 (Wassef et al. 2004; Sritharan et al. 2005; Vander Werff 2002) investigated lateral load distribution as part of a research effort examining seismic issues in bridges with steel superstructures and concrete substructures. These projects and others have mentioned the issues related to seismic lateral load distribution based on experimental data. However, the authors are not aware of any studies that systematically have in- vestigated seismic lateral load distribution using comparisons of experimental test data and predictive analytical models to formu- late improved design recommendations. The investigations mentioned primarily focused on the perfor- mance and sufficiency of bridge systems for high seismic regions. The studies utilized the construction and testing of large-scale ex- perimental models of prototype integral bridge structures. The first test unit modeled a bridge with a four-girder prestressed concrete superstructure (Holombo et al. 2000), using precast bulb-tee girders. This test unit is referred to as the precast bulb-tee (PBT) model. The next two test units were based on bridges with four-girder steel superstructures (Wassef et al. 2004). These units are referred to as the steel pier cap (i.e., SPC1 and SPC2) models. A more recent study by CALTRANS investigated a test unit consisting of a five-girder prestressed concrete superstructure (Snyder et al. 2011) including an inverted-tee bent cap. This unit is referred to as the inverted-tee bent cap (ITB) model. Fig. 1 provides schematic details of the prototype structures for these investigations. All of the tests had specific areas of focus; however, common areas of interest may be summarized as follows: (1) the design of a prototype bridge utilizing integral connection details capable of withstanding seismic loading; (2) the experimental validation of these details using large-scale test specimens; and (3) the formation of suitable seismic design recommendations based on the analytical and experimental findings. This paper compiles the load-distribution results from these experimental tests and compares them with predictions from grillage model analysis (GMA) and simple stiffness models (SSMs). Current Design Practice The AASHTO LRFD Bridge Design Specifications (AASHTO 2010) include a well-established procedure for using distribution factors to distribute moment and shear owing to vertical loads to interior and exterior girders with concrete decks (Section 4.6.2.2.2). The dis- tribution factors are based on the spacing, span, and longitudinal stiffness of the beams and the depth of the slab. The distribution- factor approach has been shown to be reliable for vertical live load by many studies (Zokaie et al. 1991; Kim and Nowak 1997; Mabsout et al. 1999; Barr et al. 2001; Cai 2005). Recent work as part of 1 Assistant Professor, Engineering Dept., Dordt College, Sioux Center, IA 51250 (corresponding author). E-mail: justin.vanderwerff@dordt.edu 2 Wilson Engineering Professor, Dept. of Civil, Construction, and En- vironmental Engineering, Iowa State Univ., Ames, IA 50011. E-mail: sri@ iastate.edu Note. This manuscript was submitted on November 5, 2013; approved on April 11, 2014; published online on May 6, 2014. Discussion period open until October 6, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Bridge Engineering, © ASCE, ISSN 1084-0702/04014055(11)/$25.00. © ASCE 04014055-1 J. Bridge Eng. 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