> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/steel-tub-girder-bridge-workflow/loading-stg/loads-stg/inf-surf-based-llbrce-stg/influence-surface-highway-stg.md).

# Influence Surface (Highway) \[STG]

## General

Influence surface coefficients are the numerical values used to determine the influence of a unit load on a specific point of a structure, based on its position and orientation. These coefficients represent the contribution of a unit load at a specific point to the total response of the structure.

**Stage:** During an influence surface-based live load analysis, the stiffness matrix and active elements are captured at the selected stage, followed by the placement of a unit load on the roadway surface. The analysis then determines the influence surface coefficients that are used to assess the maximum load effect on the structure.

**Roadway:** The roadway geometry is utilized to establish the boundaries for the unit loads that are placed at equal spacing in both longitudinal and transverse directions.

**Live Load Category \[Gravity/Braking/Centrifugal]:** When gravity is selected, the unit loads are applied in the vertical (Z) direction. In the case of braking, the unit loads are applied 6 feet above the deck level in the longitudinal (X) direction along the alignment. For centrifugal or wind loads, the unit loads are applied in the transverse (Y) direction perpendicular to the alignment and 6 feet above the deck.

When the user selects the centrifugal option, it is important to specify the highway design speed and centrifugal force factor under the centrifugal tab. The library component will calculate the radius of curvature of the traffic lane at each unit load station based on the alignment data and automatically compute the C factor in accordance with AASHTO 3.6.3-1. The unit load (1) will then be multiplied by the C factor.

**FAQ:** *How do we know in which direction CE/BR unit loads will be placed? Does the program automatically align and position them correctly based on the alignment?*\
X (longitudinal) and Y (centrifugal) are in the alignment coordinate system. Each unit load is placed with respect to the direction of the curve at that specific location. You can view all unit loads by selecting the 'Loading' mode and the influence surface you are interested in. Note that these loads are individually defined in the global coordinate system, while the combined load is perpendicular to the PGL. Below is an example of a centrifugal load unit load, which applies the load perpendicular to the PGL and directs it outward from the curve.

![](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/image-20230808-094205.png?api=v2)

**Unit Load Spacing Long. :** The spacing of the unit loads in both longitudinal and transverse directions significantly affects the total analysis time. For each unit load, a finite element analysis must be conducted. For instance, if we consider a roadway that is 1000 feet in the longitudinal direction and 50 feet in the transverse direction, using a spacing of 1 foot in both directions would require 50,000 finite element analysis runs. Additionally, identifying the critical vehicle positions using these runs would also be a time-consuming task. For this reason, when considering the longitudinal direction, it is recommended to use a spacing value of 10 feet or the span length divided by 20 or 10 as it provides a good approximation.

**Unit Load Spacing Trans.:** A good approximation for transverse spacing can be achieved by starting with a vehicle width of 6 feet and gradually reducing it to 3 feet in subsequent runs.

To ensure that the model runs without any issues, it is convenient and faster to start with larger numbers for both longitudinal and transverse unit load spacing. For instance, using 20 feet in the longitudinal direction and 10 feet in the transverse direction can be a good starting point. Once you have confirmed that the model runs smoothly and have reviewed the design report, you can proceed with a spacing value of 10 feet in the longitudinal direction and 6 feet in the transverse direction to check your design. After finalizing your design, it is recommended to perform another check using a spacing of 5-10 feet in the longitudinal direction and 3 feet in the transverse direction.

![](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/path%20and%20unit%20load.drawio%20\(1\).png?api=v2)

![Unit Loads](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/UnitLoad.gif?api=v2)

**Direction \[Along PGL/Orthogonal to PGL]:**

Placement of unit loads onto the roadway should be along the PGL in most cases. This approach covers some unique situations where the traffic direction is not along the PGL but orthogonal to the roadway

## Centrifugal

**Highway Design Speed:** Highway design speed shall not be taken to be less than the value specified in the current edition of the AASHTO. 1.0 ft/s = 0.682 mph

**Centrifugal Force Factor:** 4/3 for load combinations other than fatigue and 1.0 for fatigue

![](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/image-20230308-175328.png?api=v2)

## Braking

**Braking Force Direction \[Bidirectional/One Direction]:** The influence surface coefficient calculation for braking loads in OpenBrIM is typically based on a single direction, with loads applied in the direction of the PGL upstation. However, for bridges open to two-way traffic, calculating braking loads in both directions can yield more conservative results. This approach can be set by specifying this parameter.

> *Note: Defining this parameter does not impact the load path of unit loads applied to influence surfaces. For additional guidance, please refer to the video below.*

![Braking.mp4](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/Braking.mp4?api=v2)

However, the definition of this parameter will directly affect the FEA results, particularly the maximum and minimum force values displayed for each node. For further explanation, refer to the example case screenshots below. For instance, **PierCap1 Spring Loop 1’s** first node shows a minimum force of **-1.16 kips** and a maximum force of **1.96 kips** for a braking load with the **Force Direction** set to **One Direction**. In contrast, for **Bidirectional Braking Load**, that node experiences a minimum force of **-1.96 kips** and a maximum force of **1.96 kips** in the **Z-axis** direction.

![image-20241113-123907.png](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/image-20241113-123907.png?api=v2)

![image-20241113-124150.png](https://openbrim.atlassian.net/wiki/download/attachments/2126151695/image-20241113-124150.png?api=v2)

## Settings

**Coeff. Tol.:** In influence surface analysis, a single unit load (1 kip) is applied on the defined influence surface to determine critical positions. These critical positions are subsequently used in LL/CE/BR load analysis. However, certain cases can be disregarded based on their force effects.

The **Coefficient Tolerance** parameter plays a crucial role here. It determines the threshold below which force effects are ignored in the analysis. Specifically, for positions where the force effect is smaller than the Coefficient Tolerance, results will not be considered.

A higher Coefficient Tolerance improves performance and optimizes resource usage by disregarding results with smaller force effects, albeit at the cost of reduced accuracy. Conversely, a lower Coefficient Tolerance enhances the accuracy of results by including a greater number of force effects in the analysis but leads to decreased performance and resource efficiency.

**Live Load Category \[Gravity/Braking/Centrifugal]:** Selects the influence-surface category used by this live load definition: Gravity for vertical design loads, Braking for longitudinal braking forces, or Centrifugal for centrifugal forces on curved alignments.

**Coeff Tol.:** Tolerance applied to influence-coefficient computations; smaller values produce more accurate load positioning at the cost of longer run times. Default 0.0001.

**Refine Unit Loads at Skewed Ends \[No/Yes]:** At a skewed abutment or pier the support line crosses the roadway at an angle, so bearing reactions and end-region forces change quickly over a short length near the ends. When set to Yes, the longitudinal unit-load spacing is refined near each skewed end of the roadway. Every interval of the regular grid that reaches into a length equal to the roadway width × |tan(skew)| from that end is refined. That length is the longitudinal distance the skewed support line covers. The skew is taken from the roadway's first and last stations. Only those end regions are refined: square ends (zero skew), the rest of the roadway, the transverse spacing and the vehicle placement stay unchanged. It applies when Direction is Along PGL. Every added unit load is one more finite element analysis, so refinement increases the run time. A roadway generates at most 12,000 unit loads. A regular grid that alone exceeds that has both of its spacings doubled until it fits. If only the refined ends would exceed it, the end spacing is coarsened instead, down to no refinement, so the rest of the grid keeps its spacing. The default, No, keeps a uniform grid.

For example, with a longitudinal spacing of 30 ft, a 45 ft wide roadway and a 20° skew, each refined end region is about 16.4 ft long. The first and last intervals of the grid therefore each get an extra line of unit loads at their midpoint. Those intervals are 30 ft long, or slightly less when the roadway length is not a multiple of 30 ft.

**Skewed End Spacing Factor (x Unit Load Spacing Long.):** The longitudinal unit-load spacing inside the refined end regions, as a fraction of Unit Load Spacing Long. Each refined interval is split into the fewest equal parts no longer than this fraction of the spacing. The regular grid divides the roadway length into equal intervals of at most Unit Load Spacing Long., so the parts depend on the interval length. For a full-length interval, the default 0.5 halves it and 0.25 quarters it. A value of 0.33 gives parts no longer than a third of the spacing: quarters of a full-length interval, but thirds of a slightly shorter one. Enter a value greater than 0 and at most 1; 1 leaves the ends at the normal spacing. This parameter is shown only when Refine Unit Loads at Skewed Ends is Yes.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.openbrim.org/templates/steel-tub-girder-bridge-workflow/loading-stg/loads-stg/inf-surf-based-llbrce-stg/influence-surface-highway-stg.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.