Browsing by Subject "Shear"

Now showing 1 - 4 of 4
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    Anchorage of shear reinforcement in prestressed concrete bridge girders
    (2014-06) Mathys, Brian Thomas
    The Minnesota Department of Transportation has typically used epoxy coated straight legged stirrups anchored in the tension zone as transverse reinforcement in prestressed concrete bridge girders. With the straight legs of the U-shaped stirrups anchored into the bottom flange of the girders, this configuration is readily placed after stressing the prestressing strands. American Concrete Institute (ACI) and American Association of State Highway and Transportation Officials (AASHTO) specifications require stirrups with bent legs that encompass the longitudinal reinforcement to properly anchor the stirrups. Such a configuration is specified to provide mechanical anchorage to the stirrup, ensuring that it will be able to develop its yield strength with a short anchorage length to resist shear within the web of the girder. AASHTO specifications for anchoring transverse reinforcement are the same for reinforced and prestressed concrete; however, in the case of prestressed concrete bridge girders, there are a number of differences that serve to enhance the anchorage of the transverse reinforcement, thereby enabling the straight bar detail. These include the precompression in the bottom flange of the girder in regions of web-shear cracking. In addition, the stirrup legs are usually embedded within a bottom flange that contains longitudinal strands outside of the stirrups. The increased concrete cover over the stirrups provided by the bottom flange and the resistance to vertical splitting cracks along the legs of the stirrups provided by the longitudinal prestressing reinforcement outside of the stirrups help to enhance the straight-legged anchorage in both regions of web-shear cracking and flexure-shear cracking. A two-phase experimental program was conducted to investigate the anchorage of straight legged epoxy coated stirrups that included bar pullout tests performed on 13 subassemblage specimens which represented the bottom flanges of prestressed concrete girders in a number of configurations to determine the effectiveness of straight legged stirrup anchorage in developing yield strains. Additionally, four girder ends were cast with straight legged stirrup anchorage details and tested in flexure-shear and web-shear. The straight leg stirrup anchorage detail was determined to be acceptable for Minnesota Department of Transportation M and MN shaped girders as nominal shear capacities were exceeded and yield strains were measured in the stirrups prior to failure during each of the tests.
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    Discrepancies in Shear Strength of Prestressed Beams with Different Specifications
    (Minnesota Department of Transportation Research Services Section, 2010-01) Dereli, Ozer; Shield, Carol; French, Catherine
    Although Mn/DOT inspection reports indicate that prestressed concrete bridge girders in service do not show signs of shear distress, girders rated with the Virtis-BRASS rating tool and Load Factor Rating (LFR) have indicated that a number of the girders have capacities lower than design level capacities. One of the reasons for the discrepancy was suspected to be conservatism of the rating methods (i.e., LFR). Other suspected reasons included potential flaws in the rating tools used by Mn/DOT (i.e., Virtis-BRASS software) including neglecting possible additional shear capacity parameters (e.g., end blocks). As a consequence, the rating methods have made it difficult to discern the cases for which shear capacity may be a real concern. In order to identify the reasons for the discrepancies and inconsistency in rating results relative to observed performance of the prestressed bridge girders, an analytical research program was conducted. The report provides a brief description of the models that provide the basis for the AASHTO shear design provisions and descriptions of the provisions through the 2002 AASHTO Standard specifications. This is followed by a description of the Virtis-BRASS rating tool, which was verified with example bridges provided by Mn/DOT. To investigate prestressed bridge girders within the inventory that might be most at risk for being undercapacity for shear, 54 girders were selected from the inventory for further evaluation. Some of the 54 girders were found to have larger stirrup spacings than required at the time of design. These girders were subsequently rated and evaluated per the 2002 AASHTO Standard Specifications to determine the adequacy of the designs based on the LFR inventory and operating rating methods. Potential sources for increased shear capacity were identified and reviewed.
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    Shear Capacity of Prestressed Concrete Beams
    (Minnesota Department of Transportation, 2007-11) Runzell, Brian; Shield, Carol; French, Catherine
    The shear provisions of the American Association of State Highway and Transportation Officials bridge design code have changed significantly in recent years. The 2004 Load and Resistance Factor Design (LRFD) and 2002 Standard shear provisions for the design of prestressed concrete bridge girders typically require more shear reinforcement than the 1979 Interim shear provisions. The purpose of this research was to determine whether or not bridge girders designed according to the 1979 interim shear provisions were underdesigned for shear and develop a method to identify potentially underdesigned girders. Two shear capacity tests were performed on opposite ends of a bridge girder removed from Mn/DOT Bridge No. 73023. The stirrup spacing in the girder suggested it was designed according to the 1979 Interim shear provisions. The results from the shear tests indicated the girder was capable of holding the required shear demand because the applied shear at failure for both tests was larger than the factored shear strength required by the 2004 LRFD HL-93 and 2002 Standard HS20-44 loading. The results of a parametric study showed that girders designed using the 1979 Interim were most likely to be underdesigned for shear near the support and that the girders most likely to be underdesigned in this region had smaller length to girder spacing ratios.
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    A Simplified Shear Design Method For Post Tensioned Concrete
    (2022-09) Scharmer, Dalton
    Shear forces are one of the primary forces that need to be accounted for when designing a prestressed, post tensioned, or regular reinforced concrete member. Shear forces can cause brittle, and often disastrous failure if a section is not properly designed. The ACI 318-19 code has several methods for calculating the nominal capacity of various concrete members. This research project considers the current code equations for prestressed concrete, and a new modified version of the equation for non-prestressed members that allows unification of concrete shear design as an alternate simplified method. The goal of this project is to evaluate a more straight-forward shear design approach for post-tensioned members. The detailed method used in pretensioned members models the effects of inclined and web shear types of failures reasonably well for determinate members. However, the typical post-tensioned member is often indeterminate with a series of parabolic tendon profiles and is more complicated to model with potentially little gain in reducing stirrup requirements. A basic approach that is similar to the new non-prestressed equations in ACI 318-19 is provided as an alternate method. While the proposed method would be a conservative option for pretensioned members, the likely application is in continuous post-tensioned members.