Live Load Dialog Box
This dialog box allows you to edit a Live Load. To add a Live Load to the project loads or to a combination, select the Loads/Analysis tab, select the analysis/design case from the drop-down list, and then right-click the Live Load Group and select the Add Live Load command. To edit an existing Live Load, either double-click the name of the Live Load or right-click it and select the Edit Load command. You can add any number of Live Loads to the project loads and to a combination.
A similar looking dialog box is used to view Library Live Loads. Library Live Loads displayed on the Loads/Analysis tab are obtained from the Live Load Library file and can only be viewed. The name and location of this file can be set using the Setup option from the Libraries menu. If you need to make any changes to a library live load, use the Vehicle menu item from the Libraries menu. The Library Live Loads in the Load Tree will be rebuilt after you have made changes to the vehicle library. A Library Live Load that is dragged into the project loads (or into a combination which automatically adds it to the project loads) can be edited as a normal Live Load.
A live load consists of any number of design trucks and a single lane loading. The design truck can have any number of axles. If the design truck is specified as Variable, then one axle can be designated as a variably spaced axle that has a minimum and maximum spacing. If you are doing an LRFD design, you can direct CIP RC/PT Girder to associate the lane loading with all design trucks in the live load or just specific ones. If you are doing an LFD design, then the lane loading is treated as another design truck. Each design truck can have its response determined using LRFD special enveloping algorithms for moment responses in regions of contraflexure and axial force responses at interior supports. For Cooper train loadings, you can specify a trailing uniform loading spaced a specific distance from the rear of the Cooper loading. Finally, you can associate a separate live load to be the side-by-side live load.
Responses due to raw (unfactored) design trucks and lanes, influence lines (displacement, force and reaction), and live load results are available in the Analysis Results screen that displays after performing an analysis.
Adding a Design Truck to the Live Load
- Select one of the existing rows in the grid by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- Click the Add Truck button. This adds a new design truck after the selected one.
- Enter the truck definition values in the grid. A definition for each parameter is available below.
Copying a Design Truck to the Live Load
- In the grid, select the design truck you want to copy by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- Click the Copy Truck button to copy the selected row. This adds the new row after the selected one.
- Enter the truck definition values in the grid. A definition for each parameter is available below.
Deleting a Design Truck From the Live Load
Adding an Axle to a Design Truck Definition
- Select one of the existing rows in the Design Truck Definitions grid by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- Select one of the existing rows in the Axle Definitions grid by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- Click the Add Axle button. This adds a new axle after the selected one.
- Enter the axle definition values in the Axle Definitions grid. A definition for each parameter is available below.
Deleting an Axle from a Design Truck Definition
- In the Design Truck Definitions grid, select the design truck you want to delete an axle from by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- In the Axle Definitions grid, select the axle you want to delete by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
- Click the Delete Axle button to remove the selected axle.
Including Lane Loading
Click the check box next to the Include Lane Load option to include the lane loading in the live load. You can then selectively associate the lane loading with the defined design trucks if you are doing an LRFD design, otherwise, the lane loading is treated as another design truck.
Defining Load Intensity
Define the lane load intensity by entering the value in the Load Intensity field.
Defining Concentrated Moment Rider (LFD Only)
Define the concentrated point load for moment lane responses by entering the value in the Concentrated Moment field.
Defining Concentrated Shear Rider (LFD Only)
Define the concentrated point load for shear lane responses by entering the value in the Concentrated Shear field.
Defining Side-by-Side Option
Define the side-by-side live load by selecting one of the following from the drop-down list. The contents of the drop-down list consists of the None option plus all the live loads from the Library Live Load Tree and all the live loads in the project loads, except the live being edited in the Live Load dialog box. The names of the live loads in the drop-down are preceded with the "Lib-" to indicate that the side-by-side live load came from Live Load Library.
The full name of the side-by-side live load will be the name of the live load being edited + ":" + the name of the side-by-side live load + "*". For example, "Permit:LIB-HS-20*" is the formal name of the side-by-side live load when you are editing a permit live load and have selected the Library HS-20 live load. The "*" gets added to end to indicate that live load used as the side-by-side live load might have been edited and changed using the Edit button next to the side-by-side drop-down list. Select None to remove any currently selected live load as the side-by-side live load.
Grid Definitions
| Setting | Description | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Truck Name | Enter a unique name for the design truck. | ||||||||
| Truck Type | Select the truck type from the drop-down list.
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| Include Lane | Each design truck can be enveloped with the lane loading. This option has no effect for LFD designs and the option is disabled. | ||||||||
| Notional | The response for each design truck in a live load can be obtained using LRFD notional loading concepts. | ||||||||
| Special Envelope | For LRFD designs, a design truck can be restricted to have only certain responses considered when constructing the live load: in regions of negative moment due to lane loading, 0.9 of the negative moment due to the truck is the only response considered; for interior supports, 0.9 of the vertical reaction is the only response considered. Usually the LRFD dual truck is the only truck that has this attribute set. | ||||||||
| Special Env Factor | Factor to be used for special enveloping. | ||||||||
| Following Load | For Cooper train loadings, this field specifies the intensity of the following distributed load. | ||||||||
| Following Distance | For Cooper train loadings, this field specifies the distance from the rear of the Cooper training loading to the start of the distributed loading. |
Axle Definitions
| Setting | Description |
|---|---|
| Axle Number | This is a read only field which lists the axle number. |
| Intensity | Define the axle weight. |
| Spacing | Defines the spacing to the next axle. This field is disabled (and blanked out) if the variable spacing field is set to variable. In this case, the Min Variable Spacing and the Max Variable Spacing fields are enabled. |
| Variable Spacing | Select the spacing from the current axle to the next axle to be Fixed or Variable. Only one spacing can be made variable. |
| Min Variable Spacing | (Variable-Axle Design Trucks only) Defines the minimum spacing to the next axle. |
| Max Variable Spacing | (Variable-Axle Design Trucks only) Defines the maximum spacing to the next axle. |
Technical Discussion
| Setting | Description |
|---|---|
| LFD | LLRsp = TrkOrLaneRsp * (1.0 + IM) * DF / 2.0 |
| LRFD | LLRsp = [TrkRsp *(1.0 + IM) + LaneRsp] * DF |
Moment impact factors are used to factor axial and moment responses, and shear impact factor is used to factor shear responses. Single lane Moment DF is used to factor axial and moment responses, and shearLDF is used to factor shear responses.
| Setting | Description |
|---|---|
| LFD | The raw truck response is obtained for each design truck in the live load, impact and distribution factors are applied. The largest positive and largest negative responses among all design trucks are found. If a lane loading is part of the live load, it is considered as an additional design truck and the same process is used. |
| LRFD | The raw truck response is obtained for each design truck in the live load and the impact factor is applied. If a lane loading is included in the live load, and if it is to be considered for the design truck, its response is added to the impact factored truck response. This response is then factored by the distribution factor. |
If special enveloping rules are not to be used for the design truck, then the largest positive and largest negative responses are found. If a design truck has the special enveloping attribute set (usually only the dual truck for LRFD has this attribute set), then two different methods of enveloping are used depending on the location of the response:
If the location is an interior support, 0.9 * the vertical reaction (positive and negative) is the only response to envelope with the other design trucks. If the location is not an interior support, then a temporary lane response if obtained and if the moment due to the lane loading is negative, then 0.9 * the negative moment is the only response to envelope with the other design trucks.
Since DF1 for LRFD includes a multiple presence factor, it needs to be removed; the Z factor accomplishes this since it is 1.2 for LRFD and 1.0 for LFD.