The new AASHTO code for the load and resistance factor design of bridges introduced a so-called empirical method for designing deck slabs of reinforced concrete. The reinforcement ratio is constant, and it does not depend on the girder spacing. The objective is to verify whether the empirical method is adequate for wider girder spacing [∼3 m (∼10 ft)]. Bridge behavior is analyzed by an advanced finite element method. The developed procedures are applied to two structures: a steel girder bridge and a prestressed concrete girder bridge. Strains and corresponding stresses due to dead load, live load, and shrinkage effect are determined. The analytical model is calibrated with the use of field test data. Stress distribution is then investigated. Field test results indicate a considerable difference in live load distribution factors between steel and prestressed concrete girders, primarily because of stiffness differences between the girders and the slab. Prestressed concrete girders are considerably more rigid than steel girders and therefore have very limited load sharing. For bridges designed by the empirical method, expected extreme values of the stress caused by live load (heavy trucks) were observed to be lower than the critical (cracking) values. However, dead and live loads combined with shrinkage (particularly restrained shrinkage) can lead to cracking and eventually even failure of the deck and the bridge. According to the AASHTO load and resistance factor design (LRFD) code (1), a reinforced concrete deck slab is designed by using two design procedures: standard bridge slab empirical design and stan- dard bridge slab load factor design. Research has shown that the pri- mary structural action of a concrete deck is not flexure but internal arching, which creates an internal compressive dome. Therefore, only a minimum amount of isotropic reinforcement is required for the local flexural resistance and global arching effects. As a result, the empirical method specifies the same reinforcement ratio regardless of the girder spacing, which gives a lower percentage of reinforce- ment than that calculated according to the load factor design method. There is a need to verify whether the amount of reinforcement is adequate for bridges with a wider girder spacing. The objective of this study is to analyze the behavior of isotropic decks with empirical reinforcement supported on girders with wider spacing. A computer procedure was developed for the stress–strain analysis of isotropic decks of reinforced concrete supported on gird- ers spaced up to 3 m (10 ft). The analytical model was verified and calibrated experimentally. Two bridges were tested: one with steel girders and one with prestressed concrete girders. FIELD TESTS Selected Bridges Two bridges with a girder spacing of ∼3 m (∼10 ft) were selected in collaboration with the Michigan Department of Transportation (DOT). Both are simply supported spans; the first has steel girders spaced at 3.12 m (123 in.), and the other has prestressed concrete girders spaced at 3.17 m (125 in.). The steel bridge, built in 1974, has two spans. Each has a length of 43.89 m (144 ft) and a cantilever of 3.66 m (12 ft); only one span was tested. The total length of the bridge is 87.78 m (288 ft), with no skew. The bridge has five steel girders spaced at 3.12 m (123 in.), and each girder has a depth of 1.52 m (60 in.). The concrete deck slab, which carries one lane in each direction, is 24.13 cm (9.5 in.) thick. Figure 1a is a cross section of this superstructure. The prestressed concrete girder bridge, built in 1995, has three simply supported spans. Each span is 13.72 m (45 ft) long, and only one span was tested. The total length of the bridge is 41.16 m (135 ft) with no skew. The bridge has seven AASHTO Type II girders spaced at 3.17 m (125 in.). The concrete deck slab, which carries three lanes (all in one direction), is 22.86 cm (9 in.) thick. Figure 1b is a cross section of this superstructure. Load Testing Procedure All girders were instrumented so that the strain transducers were attached to the lower surface of the bottom flange of the steel and prestressed girders. Measurements were recorded at midspan on the prestressed concrete girder bridge and at approximately one-fifth of the span on the steel girder bridge because of limited access to the girders under the bridge. Strain values caused by a moving test truck were recorded simultaneously on all girders. The test vehicle for both bridges was a Michigan three-unit, 11-axle truck with a total weight of 145 kips. The actual weights of test trucks were measured at weigh stations before testing. For both tested bridges, the test truck was driven at a crawling speed to sim- ulate static loads and at a regular speed to obtain dynamic effect on the bridge. Several runs were completed, each at a different trans- verse position. FINITE ELEMENT ANALYSIS Results of the field test were compared with finite element (FE) ana- lytical computations and used to determine the actual boundary con- ditions. The analysis was performed with the use of the ABAQUS software suite for FE analysis. Material and other structural param- Field Test and Finite Element Analysis of Isotropic Bridge Deck David Ferrand, Andrzej S. Nowak, and Maria M. Szerszen D. Ferrand and M. M. Szerszen, Department of Civil and Environmental Engi- neering, University of Michigan, 2340 G. G. Brown Building, 2350 Hayward, Ann Arbor, MI 48109-2125. A. S. Nowak, Department of Civil Engineering, Univer- sity of Nebraska, W181 Nebraska Hall, Lincoln, NE 68588-0531. 153 Transportation Research Record: Journal of the Transportation Research Board, CD 11-S, Transportation Research Board of the National Academies, Washington, D.C., 2005, pp. 153–158.