Defining the Applied Loads

The Load Control Toolbar

Sub-menus are also available to define or modify various nodal and member loadings. These sub-menus appear along the side of the screen. The types of loads available are the following: concentrated loads at nodes, moments applied at nodes, moments applied at the ends of members, uniformly or linearly distributed loads along members, variations of temperature applied to members, and global load-trains (live load for bridges).

The Load case and Load combination menu buttons are only active in the Advanced Edition (see Advanced Edition).

General Information

• Load Definition
  The loads definition system follows the same procedures as the one that defines member properties. A type of load associated with a user-supplied name is created and added to the corresponding load list. The figure below shows a drop-down list of distributed loads defined by the user in a specific analysis. The values of load associated with the selected name are automatically displayed in the fields of submenu and can be edited.

  

  The icons shown in the figure below are used to manipulate the load in a list:

  

  The current load will be applied to the selected elements (members or nodes). The members of interest must be selected and the load is applied through the button for members, or the button for the nodes.

• Coordinate System in Ftool
  There is a system of structural global axes in Ftool and a system of local axes for each one of the members. In the global system, the global X-axis is horizontal and positive from left to right; the global Y-axis is vertical and positive from the bottom to the top; and the global Z-axis is always positive outward from the display. In a member’s local coordinate system, the local x-axis coincides with the longitudinal axis of the member, with the positive direction following creation of the member; that is, from the initial node to the end node. The local x-axis direction can be displayed by selecting the Member Orientation option in the Display menu (see Visualization Controls - The Display Menu). The local y-axis is perpendicular to the x-axis. The z-axis for a member is always positive outward from the display. The positive direction of local y then follows the right-hand rule of vector cross products: y = z × x.

• Application of concentrated loads
  Concentrated loads (forces and moments) can be applied only on nodes of the structure. Of course concentrated loads could be applied along the span of a member. However, for user-interface simplicity, it was adopted a police of only applying concentrated loads at nodes. If it is necessary to apply a concentrated load on a member, insert a new node at the desired position, thereby dividing the member into two members. Concentrated loads are always applied in directions of the global axes of the structure, positive when the forces have directions of the global axis, and negative when they have the opposite direction. Positive concentrated moments are applied in a counter-clockwise orientation.

• Orientation for distributed member loads
  Distributed loads along a member may be specified in the framework of global coordinates or in the framework of member local coordinates. The loads are positive if they coincide with the direction of the global or local axes.

• Partially distributed member loads
  New nodes can be introduced along a member to apply distributed loads that act on a portion of the member length. As for concentrated loads (see above), this is done for user-interface simplicity.

• Load removal from nodes and members
  To remove a load from from selected nodes or members, select the first element (NONE) of the corresponding load drop-down list and apply this to the selected entities. In other words, "to remove a load from selected entities in Ftool is to apply nothing".The user may also use the button.

Nodal Loads

This sub-menu allows the user to define the concentrated loads on structure nodes. It uses the global coordinate system.

Moments Acting on Member Ends

This sub-menu allows the user to define concentrated moments at the ends of members. Moments applied in a counterclockwise orientation are positive. "Ma" denotes the moment applied at the "initial" node of the member, while "Mb" is the moment applied at the "ending" node of the member.

Linear and Uniform Distributed Loads

This sub-menu allows the user to define linearly varying or uniformly distributed loads over a member. The user can specify the global or local member coordinate system for the loading direction.

Thermal Loads (Temperature Changes)

This sub-menu allows the user to define a linear temperature gradient over the member depth. The user specifies the temperature on the section’s top edge (i.e., on the positive side of local y-axis) and on the bottom edge (i.e., on negative side of local y-axis). Ftool must have access to the section depth to impose this loading – even for "generic" sections.

Load-Trains (Vehicle Live Loads for Bridges)

This sub-menu allows the creation of live loads (on bridges, for example) that are used to calculate envelopes of internal forces (see Results - Load-train Envelopes Toolbar). A load-train is composed of concentrated forces, uniformly distributed forces and live loads (representing the population of small vehicles on a bridge). Concentrated and distributed loads are assumed in the vertical top-bottom direction. Therefore, according to the sign convention of Ftool, all load values are negative. In case the user does not enter a negative sign for a load value, the program automatically changes the sign of this value.

The concentrated and distributed load matrices resize themselves automatically: once the user begins to fill out the last row of each matrix, a new row is created below it. Loads can be deleted by either setting the load values to zero or by selecting the desired rows and pressing the Delete key on the keyboard. The matrix will then also resize itself accordingly.

The currently selected load train may be defined with the dropdown list on the top-right corner of the program’s window. This dropdown list is only enabled when in either Influence Line or Load Train Envelope result modes and/or if in the load train definition menu.

• Impact factor
  This is an amplification factor that multiplies globally all the effects of a load-train and allows the consideration of a dynamic effect on the structure. The value of this factor should be always greater than one.

• Load-train length
  This extension length limits the application of concentrated, distributed and internal live loads.

• Concentrated forces
  Load-train concentrated forces are specified using a matrix with two columns, with the following parameters:

  • x - position of concentrated force in relation to load-train beginning;
  • P - value of a concentrated force.

  It is not allowed to create more than one concentrated force at a single position, or outside the limits of the load train (specified by its length). To add a concentrated force to the current load-train, first enter the force position then its value. As other concentrated forces are inserted, they are ordered according to their position.

• Uniformly distributed forces
  The matrix of distributed forces varies according to the type of load-train. In case of a load-train that has single values of distributed forces, the matrix has three columns for the following parameters:

  • xa - initial distributed force position in relation to load-train beginning;
  • xb - end distributed force position in relation to load-train beginning;
  • q - value of a distributed force.

  In case of a load-train that has full and empty cars, q becomes the value of the distributed force for a full car and the matrix has an additional column for the following parameter:

  • q’ - value of a distributed force for na empty car in a railroad load-train.

  Overlap of distributed forces is not allowed. Initial and end positions of distributed forces must be within the load-train extension (length). To add a distributed force to the load-train, first enter its initial and end positions then the load value(s). In the case of a railroad load-train, the first load value to be inserted is q and then q’. As other distributed forces are inserted, they are ordered according to their initial and end positions. When xa is greater than xb, or q is less than q’, these values are automatically inverted. It is possible to modify the type of load-train, even after distributed forces have been inserted. When transforming a load-train with single value distributed force to a load-train with double distributed load value, q’ is set equal to q.

• Live loads
  There are two types of live loads that represent the population of small vehicles on a bridge:

  • Exterior: is applied outside the limits (length) of current load-train;
  • Interior: is applied within the limits (length) of current load-train.

  Live loads may be applied partially along the load-train path on the structure. The portions of the path on which live loads are applied depend on influence lines. These portions are defined to maximize or to minimize a target effect. The maximum value of an effect is obtained applying the live load only on the positive portions of the influence line of that effect; and the minimum value is obtained applying the live load only on the negative portions of the influence line.