HLR Section 6.8
Load Ratio Analysis
| CONTENTS
6.8.1 | Description of the Load Ratio Analysis Module
6.8.2 | Description of the Main Parameters
(1) | Standard Design Vehicles
(2) | Standard Vehicle Envelopes
(3) | Analysis Options
(4) | Span Range
(5) | Simply Supported Spans
(6) | Continuous Spans & Analysis Options
(7) | Option Buttons
| Other Links: | Main Index | Route Data | Vehicle Data | Job ID Data | Analysis Methodology |
6.8.1 Description of the Load Ratio Analysis Module
| The Load Ratios option enables comparisons to be made between the moments and shears produced by the current heavy load vehicle and those of one or more standard design vehicles and/or vehicle envelopes. Moment and shear ratios can be calculated for either a single span, a user-defined range of spans or a standard range of spans that can be stored in the system as a default set. A typical load ratio form is shown in Section 6.8.2 Ratios can be calculated either with, or without, the axle width reduction factor applied to the HL vehicle if it exceeds the nominal lane width. A number of other factors can also be applied to the calculation of ratios as described in Section 6.8.2(3). However, lateral distribution factors are not considered in this version. Ratios for continuous bridges are restricted to the negative moment at the mid-support point for symmetrical 2-span structures having a uniform cross-section along their entire length (refer to Section 6.8.2(5) for a full list of restrictions and limitations). The system will remember the settings you used during the last load ratio analysis and restore them when you next re-enter the module.
6.8.2 Description of the Main Parameters
| A typical load ratio form is shown below. It can only be accessed via the Load Ratios button on the job entry Vehicle Data form.
| (1) Standard Design Vehicles This window lists the currently selected Design Vehicles that will be used in the load ratio analysis. To remove a vehicle from the list or to add another vehicle to the list click the button labelled "Click here to add other vehicles to list (or remove from list)". HLR will show you the full list of all currently available design vehicles with check boxes highlighting the defaults that have been pre-set. Simply check/uncheck the tick boxes in order to select the ones you want to include in the analysis. A number of "standard" design vehicles have already been incorporated into the Design Vehicle database - Highways & Local Government truck (1927); the HS20 truck (NAASRA 1947-based on AASHTO); the NAASRA (1976/1992) T44 truck; and the AUSTROADS 1998 MS1600 vehicles (including lane loading). Others can easily be created and stored in the database (refer to Section 4.4 for details). For the HS20 and T44 design vehicles, Lane Loads as prescribed in the relevant codes are included in the analysis. To edit an existing Design Vehicle, first highlight it then click Edit. To create a new Design Vehicle click the button labelled 'Add a new design vehicle' then refer to Section 4.4 for instructions. (2) Standard Vehicle Envelopes This window lists the currently selected Vehicle Envelopes. To add another Vehicle Envelope to the list, (or to edit or remove one from the list), click the button labelled "Click here to add other envelopes to list (or remove from list)". HLR will show you the full list of all currently available vehicle envelopes with check boxes highlighting the default envelopes that have been pre-set (as shown below). Simply check/uncheck the tick boxes in order to select the ones you want to include in the analysis. To edit an existing vehicle envelope click the Edit button on the above form. HLR will load the envelope data into WORD as a text document. (Note: It must also be resaved in the same format!). Be sure not to change the formatting of the form; nor should additional spans be added to the end of the form. To create another vehicle envelope click the Add new envelope button. Enter a short envelope name and description then click OK. HLR will display an envelope proforma in WORD with instructions as to how you should enter information into the form. Do not modify the first two lines. Begin at the third line and enter a span length, moment and shear value and, optionally, a comment or note relating to that entry. If there are no comments leave a blank space enclosed in double quotes. It is essential that the number of span entries in this file corresponds to the number of spans in the standard span range. Finally, save the document as a text file and exit from WORD. If you choose not to save the form, or if you exit without saving, it will still be necessary to delete the newly created envelope from the list. Use the Delete button to do this. (3) Analysis Options Use Reduced (Width-Modified) HL Vehicle Loads This option allows you to indicate whether the reduced (width-modified) HL vehicle axle loads are to be used in the analysis. If left unchecked the full axle loads will be used. Refer also to Section 4.2 for further information regarding reduced axle loads. Include DLA Factors in the Analysis This option allows you to indicate whether DLA factors are to be applied to both the HL Vehicle and the Design Vehicle(s) during the analysis. If the check box: "Include DLA factors in the analysis" is ticked, the Set DLA Factors button will be activated. Clicking this button will bring up the form shown below. Select the DLA options you require then click OK. DLA values for the HL Vehicle are based on the default DLA-Frequency curve as shown on the Options tab. You can, however, specify your own unique value of DLA that will be applied to all span lengths in the range. DLA values for the T44/W7 design vehicles can be selected from either the NAASRA 1976 formulation or the AUSTROADS 1992 DLA-Frequency curve or both. NAASRA values will be truncated to 1.1 at the lower-bound or 1.3 at the upper. Note that you can elect to have the analysis performed for both code conditions. In this case, only one of either the T44 or the W7 vehicles can be included in the Design Vehicles list, not both. In the case of the M1600 loading (and any other that has a uniformly distributed component), the DLA factor will also be applied to the UDL component. Most of the other Standard Design Vehicles in the list will be analysed using the NAASRA formulation for DLA [viz: 40 / (L + 40)], with the exception of the S1600 loading, for which no DLA will be applied. When analysing Standard Vehicle Envelopes, DLA will not be applied when calculating the M-V ratios, either to the envlope or to the HL Vehicle (since it will effectively cancel itself out). Output reports will indicate if DLA has been applied, and the vehicle names in the table header will be annotated with a number that indicates the type of DLA formulation used. Include Dead Load Effects This check box allows you to indicate whether dead load effects are to be included in the calculation of load ratios for design vehicles (not envelopes). The method is based on the assumption that for older bridges the simple ratio approach takes no account of the effect of dead load and consequently can be overly conservative. (Refer to Section 6.3 for a detailed description of the methodology). The relationship used in HLR can be expressed as follows:
| Mdv . (1 + DLAdv) . Span / Phi + Mhl . (1 + DLAhlv)
R = |
| Mdv . (1 + DLAdv) . (1 + Span/Phi)
where:
| Mdv | = | Moment or shear due to the design vehicle
| Mhl | = | Moment or shear due to the HL vehicle
| DLAdv | = | Dynamic load allowance for the design vehicle
| DLAhlv | = | Dynamic load allowance for the HL vehicle
| Span | = | Span length
| Phi | = | Phi factor as defined in Section 6.3
The Phi factor is based on a Girder Type 1 (beam or girder) structure. If the check box "Include DLA factors in the analysis" is toggled off then the Dynamic Load Allowance factors DLAdv and DLAhlv will be set to zero. Dead load effects are not applied to ratios of standard envelopes. Use Two HL Vehicles in the Analysis This check box allows two HL vehicles to be used in the analysis. The trailing vehicle can be identical to the main HL vehicle (as shown in the screen dump above) or, if the right-hand tick box is unchecked, HLR will allow you to select a different vehicle from the HL vehicle database (the check box will be replaced with a browse button). The headway between the two vehicles can be changed from the default value of 17 metres by entering an appropriate value into the field at the bottom of the Analysis Options frame. For symmetrical 2-span continuous structures the headway can be progressively increased in increments of Headway/No vehicle increments to obtain the largest negative mid-support moment (refer to Section 6.8.2(6) below). Include Vehicle Reduction Factor Indicate whether the vehicle reduction factor shown on the panel is to be applied to the vehicle loads during the analysis. If it is, enter an appropriate value in the field immediately below the check box (the default is 1). (4) Span Range Calculation of moment-shear ratios will be done for the selected span range (or a single nominated span length). Select from one of the three available span range options then enter values if appropriate. All span lengths must be specified in metre units.
- Standard range of simply supported spans | The standard range of spans will be displayed
- User-defined range of simply supported spans | Enter a start and end span length and a span increment
- Single simply supported span only | Enter a span length
To change the default range of standard spans use a text editor, such as Notepad, and edit the file LoadRatio Standard Spans.txt located in the ..\Database folder. Be sure to retain the format and layout of the file and ensure that each span length is separated by a comma. (5) Simply Supported Spans Both moment and shear ratios are calculated for the nominated span or span range. For the HS20 and T44 design vehicles, Lane Loads as prescribed in the relevant codes are automatically included in the analysis as a separate design vehicle load case. viz:
HS20 | For spans > 8.53m (28') a UDL of 9.35 kN/m plus a concentrated load of 80.2 kN at midspan are applied.
T44 (1976 NAASRA) | For spans > 10.0m a UDL of 12.5 kN/m plus a concentrated load of 106 kN at midspan are applied.
T44 (1992 AUSTROADS) | For spans > 10.0m a UDL of 12.5 kN/m plus a concentrated load of 150 kN at midspan are applied.
Maximum vehicle moments (for either the HLV or the Design Vehicle) are obtained by analysing five sections in the range 0.4 - 0.6 span length (using increments of Span/20). (6) Continuous Spans & Analysis Options This option is only intended to be used as a broad filter to isolate a range of potentially problematic spans for the heavy load vehicle under investigation. Once a critical span or span range has been identified, structures having span lengths lying within that range will need to be properly assessed using conventional analytical methods. This feature has the following limitations and restrictions: It can only be used to check 2-span continuous bridges with equal span lengths Section properties must be uniform along the entire length of the structure Only negative moment ratios are calculated at the mid-support point Shear ratios are not calculated Vehicle movement increment = Span length/No. vehicle movement increments Headway increment = Specified Headway/No. vehicle movement increments Variable axle group increment = Specified axle group spacing/No. vehicle increments Standard Loadings are: HLG, HS20, A14, T44 (1976), T44 (1992), A160, MS1600 Code-specific Lane Loads are included for HS20, T44 (1976), T44 (1992) loadings Austroads UDL is included in the M1600 and S1600 loadings Calculation of negative moments at the mid-support point for all vehicles is based on an influence line that was derived by moving a unit load in increments of Span/100 along a symmetrical 2-span continuous beam, where Span was set to 1 metre. Therefore, the contribution to the negative moment at the mid-support point of an arbitrarily placed axle on a bridge span is equal to: Influence Value * Axle Load * Span Length. The maximum negative moments generated by each of the Standard Design Vehicle loadings is determined as follows:
HS20 | Either the truck or, for spans > 8.53m (28'), a UDL of 9.35 kN/m plus a roving concentrated load of 80.2 kN in each span.
T44 (1976 NAASRA) | Either the truck or, for spans > 10.0m, a UDL of 12.5 kN/m plus a roving concentrated load of 106 kN in each span; or a roving point load of 140 kN.
T44 (1992 AUSTROADS) | Either the truck or, for spans > 10.0m a UDL of 12.5 kN/m plus a roving concentrated load of 150 kN in each span; or a roving point load of 140 kN.
A160 (1998 AUSTROADS) | Single roving 160 kN axle load. DLA (if applied) is fixed at 0.3
M1600 (1998 AUSTROADS) | Truck plus a UDL of 6 kN/m
S1600 (1998 AUSTROADS) | Truck plus a UDL of 24 kN/m (note: DLA is not applied to this design vehicle, even if selected by the user).
The maximum negative mid-support moment for UDL's is given by the relationship: Mmax = 0.125 * w * Span * Span (where 'w' = UDL in kN/m) while for a concentrated (point) load the maximum negative mid-support moment occurs with the load at the 0.58Span point and produces an influence line value of -0.0962. The influence line is stored in the text file 'LoadRatio-Moment Infl Line-Mid Supt.txt' and is located in the ..\Database subfolder. Analysis Options Clicking this button will bring up the following dialig box: Axle number 'n' wrt the start of the variable axle group spacing: This parameter is only required for Design Vehicles that are not on the Standard Design Vehicles list described under restrictions and limitations (see above). (Standard Design Vehicles have already had this information 'hard-wired' into the program code). As the diagram shows, it specifies the axle number that defines the beginning of the variable axle group spacing 'S'. During the analysis HLR will increase dimension S in increments up to the value specified in the next field (Maximum variable axle group spacing). Unless all non-standard vehicles have the same axle group configuration, only a single non-standard vehicle can be analysed at a time. Maximum variable axle group spacing 'S': This parameter is only required for Design Vehicles that are not on the Standard Design Vehicles list described under restrictions and limitations (see above). As the diagram shows, it specifies the minimum (or starting) axle group spacing 'S'. During the analysis HLR will increase dimension S in increments up to the value specified in this field. If left at zero the axle group spacing will not be varied. For the HS20 vehicle the maximum spacing is 9.14m, for the T44 it is 8m while for the MS1600 there is no fixed maximum. The HLGD vehicle is fixed - no variation occurs. Unless all non-standard vehicles have the same axle group configuration, only a single non-standard vehicle can be analysed at a time. Number of vehicle movement increments along the bridge: This parameter is used to determine a number of different movement increments:
| The increment by which all vehicles are moved along the bridge i.e. by a distance equivalent to Span/Specified number of vehicle movement increments (e.g. Span/20).
| The increment by which the headware between two vehicles varies. This is used for determining the headway for maximum moment. Headway is progressively increased by a distance equal to Initial Headway/Specified number of vehicle movement increment s. During the analysis HLR will increase the headway by this increments until the maximum moment has been found.
| The increment by which the specified axle group spacing varies i.e. by a distance equal to S/Specified number of vehicle movement increments. During the analysis HLR will increase dimension S by this increment up to the value specified in the second field (Maximum variable axle group spacing).
(7) Option Buttons Analyse This button performs the analysis and displays the results in tabular form. Both the Design Vehicle(s) (if selected) and the heavy load vehicle are moved across each span in the specified span range in increments of 1/20th span length. The maximum moment and shear is determined for each vehicle and for each span, and the ratios Mhl/Mdv and Vhl/Vdv are then calculated. Results are written in HTML form and displayed through Internet Explorer. They can be viewed and printed but not edited. Note that if more than four design vehicles and/or envelopes have been selected for analysis the results will be spread over two separate reports. Display Reports This button will display the ratios in tabular form. Note that if more than four design vehicles and/or envelopes have been selected for analysis the results will be spread over two separate Internet Explorer (HTML) reports - results are not written to the same file. Both will have to be closed if you intend performing another run. Reports can be viewed and printed but not edited. They include: The HL Application (Assessment) ID details The span lengths (each span on a separate line) The unfactored moments & shears due to the HL vehicle for each span in the range The moment and shear ratios, (HL Vehicle/Design Vehicle), for each vehicle & span in the range. The moment and shear ratios, HL Vehicle/Envelope Value, for each envelope and each span in the range.
| Export to EXCEL This button will export the complete load ratios report into the EXCEL spreadsheet. All titles will be placed into the first column, but the table headers and load ratio values will be inserted into separate columns.
| Ad-Hoc Results This button has been implemented to allow a number of user-specified ad-hoc spans to be analysed for the design vehicles/analysis options currently selected. Click the button labelled Ad-Hoc Results to bring up the following form. Enter a span length then click the Calculate Ratios button. The moment and shear values due to the HL vehicle will be displayed in the first column followed by the moment and shear ratios for all selected design vehicles and/or standard vehicle envelopes. If dynamic impact is included, the respective DLA formulations used in the analysis will be included in the table heading (as shown in the example above).