Authored by Mehrdad Aghagholizadeh*
Abstract
A comparative analysis of two bridges constructed with the most commonly used girder types in Florida is carried out. The girder types that the bridges are employed for this study are AASHTO Type III (American Association of State Highway and Transportation Officials) and Florida I-Beam. Two bridges that have exactly the same specifications but with different girder type are analyzed under baseline state and different prestress loss cases. Finite element models of these bridges are utilized, and the FE models are subjected to two types of virtual load tests of Florida legal loads namely C5 and SU4. Florida I-Beam tends to have higher load carrying capacity, higher lateral stiffness, cost efficiency and better element level reliability when compared to AASHTO Type girders.
Keywords: Florida I-Beam; AASHTO Type girder; Prestressed concrete; Load rating factor; Bridge reliability; Prestress loss; Virtual load test
Characteristics of HSLA Steel
Bridges are important and inevitable components of transportation networks; designed and constructed with the aim of achieving safe; efficient; cost-efficient and time saving transportation. Throughout the years; in return for answering these aforementioned needs; bridge engineers have explored new design philosophies; high technology materials; real life issues and practices as well as the needs of general public. Type of the bridges may differ based on the material or design technique. Concrete or steel girder bridges could be given with respect to material difference whereas prestressed or posttensioned bridges might be related to design philosophy. Concrete girders allowed engineers to construct bridges for centuries whereas pre-stressed bridges rendered new designs possible with multiple and longer spans. Nowadays; pre-stressed concrete bridges constitute approximately 50% of newly built structures in US [1]. Use of prestressed members has many advantages due to their increased strength and durability. Excessive deflection of long spans is also a critical issue to be considered for such bridges. However; prestressed members not only provide reduction in deflection since they would already be bent the opposite way that bridge loads will mainly be applied; but also endure greater loads due to increased internal compression that will compensate a big portion of tension that will be induced by downward loads. Concrete girders together with pre-stressing application let clear majority of geometric forms be designed. AASHTO (American Association of State Highway and Transportation Officials) I-beam and bulb T-beam have been widely used for concrete girders bridges across the US. Subsequently; ascent in traffic flow rate; need for reduced construction time and cost drove officials to create a more economic; efficient design. Consequently; Florida I-Beam (FIB) was created with the partnership of Florida Department of Transportation (FDOT) and academic researchers to meet these requirements [2]. In comparison to AASHTO Type beams; FIBs show consistent and stabilized characteristics during site placement; easy manufacturing with only-web height adjustable forms and identical flange sizes. Besides; FIBs have larger sections both in web and flanges that allow higher number of strands to be placed in (up to 72–0.6 in diameter strands) and also higher strength (8–10 ksi); thus; carrying the same or higher magnitudes of loads with less number of beams and with higher clearance. This design eventually results in cost effective construction by saving of about 24%; which was given in a study by FDOT [2, 3]. These findings and studies led FDOT to decide using FIBs instead of AASHTO beams [2]. Several studies were performed by FDOT on cost analysis of these two different bridges indicating FIBs are much more effective than AASHTO Type girders. In addition; one recent study outlined the development of 3D FE (finite element) models and their results for standard AASHTO based analysis and evaluation [4].
Another study stated the results of a comparative evaluation of the AASHTO Type III and FIB bridge [5]; in this study authors also considered additional permanent load other than dead loads. However; although there is some documentation in the literature about load carrying capacities from a comparison point of view; there is still need for deeper investigation on capacity; strength; reliability and durability to explore the new FIB designs compared to commonly used AASHTO Type girder bridges. In this study; a comparative evaluation of two bridges; one built with AASHTO Type III girders and the other with FIBs; is carried out in terms of load rating factor and reliability indices under different structural conditions and virtual load testing scenarios. The first as-is condition represents the newly designed bridges and can be thought as the baseline case. Study on other cases gives information about how the load rating factors and reliability indices will change when there are different scenarios applied on the bridges such as pre-stress loss in both interior and exterior girders. In the emerge of studies on model updating to have reliable finite element model [7]; this study will help to better understand how these property uncertainties effect the load carrying capacity of the structure using virtual load testing.
Bridge Specifications
Cross sections of the bridges with AASHTO Type III girder and FIB girders are presented in Figure 1. This section is comprised of six girders and the second bridge with the same load-carrying characteristics. The critical details for the appropriate FE modeling of the prestressed sections are presented with the necessary assumptions made for this study. Two different models subject to this study are three span (90 ft each) bridges supported with three circular columns at the end of each section and with 41.5ft long beam cap on their top. Two cross sections that belong to two different models have the same section widths 43’-1” each but with different girder spacing that is 7’6’’ for AASHTO Type III and 12’6’’ for FIB as can be seen in Figure 1. Both types of girders have the same 45 in. depth. Each AASHTO Type III girders constitute 26-0.6 in low-relaxation prestressing strands whereas FIB girders contain 42- 0.6 in low-relaxation strands (Figure 1).
Method and Evaluation
Finite element model description
A finite element model using CSiBridge (ver. 15.2.0)[7] is used to model and analyze the two types of Florida I-Beam and AASHTO Type prestressed concrete girder bridges shown in Figure 2. To define Florida; I-Beam section; CSiBridge Section designer is used [7]. Slab thickness is assumed to be 8 inches with 2 inches of haunch. The deck and columns concrete is cast in place (CIP) with compressive strength of 4 ksi. The same CIP concrete is used for abutments and beam caps. Girders are prefabricated and made with normal weight; 8.5 ksi compressive strength concrete. Cross diaphragms are used in every one third of the spans with depth and width of 19 in and 12 in respectively. The deck is modeled using shell elements with six degrees of freedom at each node. Girders; columns and beam caps are modeled using frame elements in the software. The first 3-span bridge model is defined with 12 FIB-45 girders and 168 tendons; then the other mode with l8 AASHTO Type III girders and the total number of 156 tendons are defined for the entire bridge model (barker and Puckett; 1997; Nilson et al.; 2010). The flexural capacity of the sections is computed, and it is seen that the cross-section capacity of FIB bridge is 17% higher than AASTO Type III girder bridge. The cross-sectional properties are given in Table 1 (Figure 2).
To read more about this article....Open access Journal of Current Trends in Civil & Structural Engineering
Please follow the URL to access more information about this article
To know more about our Journals....Iris Publishers
No comments:
Post a Comment