Lane Setup Dialog
The Lane Setup Dialog Command provides the means to request analysis for Live Load Analysis Basics.
One or two travelways can be defined to set the limits of the lanes on the bridge deck. Each travelways definition requires a left and a right reference which could be the edges of the deck, alignment or any appurtenances on the deck. If a wearing surface defined previously in the main window, travelway definition could be synchronized with the wearing surface definition by checking the "Sync with wearing surface definition" box to match the wearing surface limits with the travelway limits
There are three user selectable lane position options used by LEAP Bridge Steel. They are:
Each are described below.
Fixed Option
This section describes the basic process LEAP Bridge Steel uses to perform live load analysis using grillage and FEM models and influence surfaces.
In the Fixed Option the travelways (W) are as defined in "Lane Setup" dialog. This is shown as W in the figure above. The number of design lanes is the whole number resulting from dividing W by 12. For example if W is 38, then the number of design lanes is 38/12= 3.1667, or 3.
The design lane width is computed by dividing W by the number of lanes. For example, W/3 = 12.667. LBS will then move the live load within the design lane as described below to determine the maximum effect for moment, shear and deflection.
In each design lane there are three load paths (left, center and right) for lane load and truck load (indicated by red arrows in the figure). The respective position of the load path is as specified in the AASHTO LRFD Spec. Per LRFD, the truck's outer wheel cannot be closer than 2.0' to each curb. The 10-foot lane load, on the other hand, can be right adjacent to the inside face of the curb. Thus, for a 6-foot axle truck, the load paths 1 and 3 would be located 5' from the edges of the Design Lanes.
As shown above, Design Lane 1 has load paths 1, 2, and 3; Design Lane 2, load paths 4, 5 and 4, and so-on for any additional Design Lanes. The live load analysis for a live load combination of a truck and a lane will progress as follows (illustration based on the above two-lane example:
- Lane load is computed for each of the load paths shown above (1 through 6) and the max. effects computed and stored.
- Truck load is computed for each of the load paths shown above (1 through 6) and the max. effects computed and stored.
- The overall maximum effect is determined by combining truck and lane for load positions:
- Applicable multiple presence and dynamic allowance factors are applied as specified by the AAHTO LRFD Specifications.
- Live Load distribution factors are applied to forces, moments and to deflections calculations.
- The overall maximum load effects from all of the above combinations at each Point of Interest (POI) location are collected and stored.
Mirror Option
In the Mirror Option the maximum number of 12 foot lanes is computed from the travelways defined. The lanes are first positioned at the left edge of the deck with the outside of the left-most lane aligned with the left edge of deck as shown in the figure below (referred to as Roadway 01). The lanes are then "mirrored" and placed with the outside of the right lane aligned with the right edge of deck. Last, the lanes are centered on the bridge deck Live load analysis is performed for each of the three lane positions and the maximum effect determined.
The lanes are then shift to the right edge of the deck (mirrored) as shown below (referred to as Roadway 02).
Live load analysis is performed for these lane positions. The maximum results considering all three roadway positions are computed and reported in the respective live load reports and diagrams.
Float Option
In the Float Option, as in the Mirror Option, the maximum number of 12 foot lanes is computed from the travelway width (referred to as the roadway. At the user's selection the process starts from the left edge of deck or right edge of deck. The initial lane (roadway) position aligns the left edge of the rightmost lane with the left edge of deck as shown below.
Live load analysis is performed at this lane (roadway) position. The lanes are moved to the right, or floated, according to the max lateral step value entered by the user. Live load analysis is again performed for this position and repeats in a similar manner for each subsequent carriageway position until the right edge of the leftmost lane aligns with the right edge of the deck as shown below.
The maximum results considering all roadway positions are computed and reported in the respective live load reports and diagrams.
In the mirror option there are two options for defining the live load vehicle path in each lane. The Center of lane options positions the vehicle in the center of each lane for live load analysis. The Left, center and right within lane option positions the vehicle successively in the left of lane, center of lane and right of lane as shown above in the Live Load distribution figure.