Scope of Parametric Studies

This section includes a discussion of the parameters and cases considered for parametric studies, various geometric configurations, the loading configurations adopted for bridges,

computation of load distribution factor for girders, and a comparison of the results obtained from the two finite element models. The results of interest in this study are strain at the girder bottom, girder deflection under live loads, and the load distribution factor (LDF) at the mid-span.

The parameters adopted in this study were the type of girder, girder spacing, span length, ID type, skew angle, number of spans, and compressive concrete strength of girders. All these parameters were varied to observe the influence of each of them on load distribution and on the effectiveness of diaphragms. For a successful study, numerous cases of bridges and loading configurations are required. The parameters in this study were suitably chosen from a reasonable range of these variables so as to quantitatively represent bridges of as many configurations as possible in the defined range.

6.5.1 Geometric Configuration of Bridges

Typical two-lane highway bridges with two shoulders were considered in the parametric study. The width of the bridge was taken as 50 ft, with each lane, shoulder, and cantilever being 12, 10, and 3 ft, respectively. In order to place the loading system close to the edge, an 18-inch thick barrier was assumed along the edges. These barriers, however, were not considered in the actual analyses of the studied bridges. The slab thickness was taken as 8 in. and the compressive strength of concrete for slab and diaphragm was taken as 3,500 psi. Parameters involved in the study are

a. Four types of girders, AASHTO Type II, III, IV, and Bulb T, were chosen, as these are

the predominantly used prestressed concrete girders in Louisiana.

b. Normal concrete compressive strength in the girder was taken as 6,000 psi, and for high

strength concrete, this was taken as 10,000 psi. For all the configurations of bridges, an analysis was performed using normal concrete compressive strength while the study on the influence of using girders of high strength concrete was limited to a few cases.

c. Girder spacings of 5 and 9 ft were chosen; these are the minimum and maximum

spacings specified by the LADOTD Manual (2003).

d. Minimum and maximum values of the span length for each type of girder were chosen as

specified in LADOTD Manual with slight modification.

e. All bridge configurations were analyzed without IDs and then with IDs. The number of

IDs was chosen based on LADOTD specifications.

f. In addition to analyzing right bridges, skew bridges with skew angles of 30o and 50o were also analyzed. Bridge continuity was also investigated. Some of these results, however, are presented but not extensively discussed; they can be found in detail in LTRC Project No. 03-3ST (2006).

g. For a limited number of cases, an analysis was performed for bridges with different steel

diaphragm configurations.

6.5.2 Diaphragm Configurations

At the locations of supports for all the bridges considered in the parametric study, end diaphragms were provided parallel to the direction of support. The end diaphragms extend from the bottom of the slab to the bottom flange of the girders.

Intermediate diaphragms type, number, spacing, and location were provided as per the LADOTD specifications (2002). All RC diaphragms were considered to be 8-inch thick.

For bridges with a single diaphragm, the ID is provided at the midspan and for bridges with two diaphragms, these are located at the third points of the span length. The current practice in Louisiana is to connect girders with IDs at the girder web; this was adopted modeling the studied bridges.

In the case of skew bridges, ID construction is a difficult task and there are various possible geometric configurations of IDs in skew bridges. The diaphragms can be parallel to the support,

perpendicular to the girder line, or perpendicular to the girder line but discontinuous with the staggered IDs to maintain equal distances from the support. The third type of configuration described above is predominantly used in Louisiana; hence this configuration of IDs has been used for modeling diaphragms in skewed bridges. For small skew angles, the orientation of IDs does not influence the results since the distance between the positions of IDs for different configurations would be small.

One of the objectives of this study was to search for alternative steel configurations which could replace RC diaphragms. A parametric study was made by analyzing bridge configurations where appropriate steel diaphragms were chosen for the corresponding bridges.

6.5.3 Loading Configurations

A HS20 standard truck that is a common truck used for design loading was used to load the bridge. The lane loading was not considered in this study, since the difference between the load distribution of lane and truck loading is insignificant, as observed by previous researchers (Chen, 1995, Chen, 1995, and Chen and Aswad, 1996). Meanwhile, Barr et al. (2001) concluded that using truck load distribution for lane load is more conservative. Therefore, only the effect of truck loading on the bridges was studied. This is also consistent with the methodology used in developing the AASHTO LRFD Code Specifications (2004) where only truck loads were considered in determining the load distribution factors (LDFs).

As to be described in Chapter 8, the truck loading used in the finite element models have two units and three axles. The front axle weighs 8 kips, while the other two weigh 32 kips each. The wheels on each of the posterior axles bare the same load. A minimum spacing of 2 ft was provided between the curb and the wheel line of the truck, and the closest wheel lines of the two trucks were placed no closer than 4 ft. as per AASHTO specifications.

throughout. The truck was moved parallel to the direction of the bridge. The spacing between the second axle and third axle was taken as 14 ft for all cases because this configuration of truck generated the maximum load effect for all bridge configurations. The loading was intended to generate maximum straining action at the mid-span section of the bridge; this was achieved by placing the middle axle of the truck at the mid-span for right bridges. Two kinds of loading positions were adopted to obtain the maximum straining action in exterior girders and in an interior one.

To obtain the maximum straining action for the exterior girder, the trucks were placed as close as possible to the exterior girder. Unless specified, the distance between the exterior girder and the edge was taken as 30 in. The maximum straining action for an exterior girder may be the case where the wheel line of the first truck is applied on the exterior girder. However, since the minimum spacing between the curb and the wheel line must be 2 ft, the first wheel line was placed 42 in from the bridge edge (18 in of barrier width + 24 in of minimum distance of barrier to the wheel line) by default. In order to obtain the maximum straining action for the interior girder, the second wheel line of the first truck is placed above the innermost girder (third girder in the case of 9-foot girder spacing and fifth girder for 5-foot girder spacing) and the first wheel line of the second truck is placed 4 ft away from the first truck (figure 6.7).

The loading configuration was the similar for skew bridges, except that both wheels of an axle could not be at the mid-span since the sections under consideration were not in line with the loading axles. Hence, only the first wheel of the second axle of both trucks was placed at the mid- span, as shown in figure 6.8.