LRFD Bridge Design Software - VBridge Program Features

Key Program Algorithms

A sample of features and algorithms within VBridge that assist the bridge engineer with simplifying the design process include:

· AASHTO Specifications – Either AASHTO Load Resistance Factor Design (LRFD) or the Standard Specifications for Highway Bridges (LFD) can be selected for analysis. In addition, since many states customize their approach to superstructure design, a Locality setting has been built into the program. This allows users to customize program behavior to match local practice. For example, California adjusts the allowable stresses in prestressing steel. The Locality option (i.e., choosing US, CA, or PA) provides the user an option to easily set all VBridge behavior to match local practice.

· Bridge Model Generation – In VBridge, the user describes the bridge using bridge terminology. A graphic window exists within each geometric component dialog to provide real time feedback of the data being entered in three dimensions; there is no need for a graphic “generate” or “update” button. As the model is developed, the described bridge is shown in the model view. This process quickly and accurately guides the user through the model generation.

· Load Generation – In VBridge, when loads are defined or requested, these are automatically placed on the bridge. For example, if the user desires a wearing surface load, VBridge automatically computes the load from the described bridge geometry. VBridge also automatically adds the load to the appropriate load combinations. The user only has to request the load be applied to the bridge. However, in order to maintain full control of the model, the user has the option of overriding the default load parameters and AASHTO load combination specifications.

· Create and Solve Finite Element Model (FEM) – After the model has been described, VBridge automatically generates the FEM. The user never needs to see (unless you want to) or manipulate the creation of nodes, elements, loads, or the FEM results. The FEM model uses a 3D space frame to represent the three dimensional model described by the user. If defined, horizontal and vertical curves, skews, rotated and offset columns, will be included in the FEM model.

Results are presented to the user in bridge terminology, such as the dead and live load reactions at an abutment. The FEM results are only available if specifically requested.

· Integral (Monolithic) Bents or Non-Integral Bents –The longitudinal moment connection between the superstructure and substructure can be modeled as either integral or non-integral (non-integrals are sometimes referred to as drop-caps). The cap connection to the bridge can be fixed, pinned, or set on rollers.

· Live Load – Longitudinal live load forces are automatically computed based on the live load vehicle types selected by the user. Design, fatigue and permit vehicles can be analyzed. VBridge has extensive vehicle libraries (i.e. AASHTO design vehicles, California P15, etc.) as well as the ability for the user to define vehicles. Results can be requested anywhere along the span, as well as at abutment and bent supports.

VBridge internally computes live load wheel load distribution factors for box girder and slab bridges based on the described bridge geometry. Moment and shear factors are computed, and separate superstructure and substructure factors are defined. The user has the option of overriding the program computed factors.

· Post-Tension Prestress Algorithm – One of the premier features of VBridge is the design of P-Jack for post-tensioned, prestressed bridges. For post-tensioned bridges, a default cable path is created that reflects the typical path, but tendon paths are easily modified by the user. VBridge can then design the P-Jack, or analyze the bridge for a specified P-Jack. The P-Jack design is completely automatic, with no cumbersome user driven iteration required. Friction losses due to horizontal curves are automatically included. After the required P-Jack force is determined, the required initial and final concrete strength is computed.

P-Jack can be designed for both box girder and slab bridges.

· Output Reports – VBridge output is very flexible and extremely easy to navigate. Upon analysis, VBridge automatically opens a standard set of reports summarizing input data and specification results. Then, at the user’s option, detailed reports can be requested for nearly every computational result, along with intermediate values computed by VBridge. This provides an exceptional insight into the computations performed by VBridge. For example, live load influence lines, unfactored load results, and post-tension prestress computation information, can be requested. Output navigation is made simple by finding the desired output report in the Table of Contents and clicking on the report name.

· Execution Time – All the above computations, and more, are performed in an extremely efficient and fast manner. A typical two span bridge takes less than 3 seconds to analyze with a P5 3.0 GHz processor. A seven span bridge takes approximately 20 seconds to analyze.