The grillage analysis is a common form of analysis for bridge superstructure. In LEAP Bridge Concrete (IRC Code), main features of the grillage analysis include:

• It is a 2D grillage model for superstructure
• Structural behavior is linear elastic
• Beam members are laid out in a grid pattern in single plane, rigidly connected at nodes
• Longitudinal members represent composite sections (i.e., main girders with associated slab)
• Transverse members represent the slab with section properties considering tributary segments

LEAP Bridge Concrete uses STAAD Beava as the solver to perform live load analysis. The grillage model is automatically generated from the related analytical model from CIP RC/PT Girder. Beam properties of longitudinal and transverse member in the grillage model are based on the National Cooperative Highway Research Program (NCHRP) Report 620 (2008) Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. The research was sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration. Influence surface is internally analyzed and created for the related grillage model. Live loads are applied to the influence surface to obtain the controlling load cases in terms of moment, shear, axial force for each point of interest (POI) in the grillage model.

Two options (i.e., Mirrored and Floating) of lane layout are offered in the grillage analysis in LEAP Bridge Concrete. The barriers you define are ignored for both of the two options. In other words, the whole bridge deck from left edge to ridge edge is considered as the whole carriageway. This method usually produces more conservative results compared to the situations in which smaller clear carriageway width defined from curb to curb or due to layout of barriers is considered.

In the Mirrored option, the maximum number of lanes is computed from the whole carriageway. The lanes are then positioned at the left edge of the deck with the outside of the left-most lane aligned with the left edge of deck. Live load analysis is performed for this lane position as shown below.

Then the lanes are then positioned at the right edge of the deck with the outside of the right-most lane aligned with the right edge of deck. Again, live load analysis is performed for this lane position as shown below.

The maximum results considering both carriageway positions are computed and reported in the respective live load reports and diagrams.

In the Float option, the maximum number of lanes is also computed from the whole carriageway. You select whether the process starts from the left edge of deck or right edge of deck. If it starts from the left edge, the initial lane position aligns the left edge of the right-most lane with the left edge of deck as shown below.

Live load analysis is performed at this lane position. The lanes are moved (floated) to the right according to the lateral step value you enter. Live load analysis is again performed for this position, and repeats in a similar manner for each subsequent lane position until the right edge of the left-most lane aligns with the right edge of the deck as shown below.