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VisualAnalysis 12.0 Help

Member Elements

Introduction

Member elements represent frame or truss members in your structure. They form the basis of most frame or truss models. Member properties are shown or changed in the Modify tab in Project Manager.

Member Operations

Member Loads and Results

Shape Properties

You define the size and shape of selected members through the Shape section on the Modify Tab of Project Manager. There are four types of shapes available for use in VisualAnalysis.

1. Database Shapes

The most common type of shape used by civil/structural engineers are predefined manufactured shapes. IES includes a large library of steel, wood, cold-formed, aluminum, and other shapes common in the USA and some foreign countries. The database is customizable through using IES ShapeBuilder, not directly within VisualAnalysis. Selecting a database shape will usually also define your material--most shape categories in the database have a default shape associated with them.

2. Standard Parametric Shapes

VisualAnalysis offers the following types of parametric shapes.

These shapes are defined by dimensions like width, depth, and thickness, which you provide. These types of shapes are often used for concrete (square, rectangle, round) analysis. There is an infinite number of shape possibilities so it is easier to use the parametric definition rather than pre-building a list of possible shapes.  Parametric Shapes are generally supported for design checks, however, there are some limitations.

Requires: Advanced Level

3. Custom "Blobs" (Shapeless Shapes!)

Custom Blobs are not the best approach! Use IES ShapeBuilder to create named custom shapes that "persist" for other projects, and that may also work for design unity checks. ShapeBuilder is the primary tool for IES shape and material database customization. It is also a handy utility in its own right.

VisualAnalysis allows you to enter a set of numbers to provide the mathematical properties of a cross-section without actually defining the shape of that section. You may create a "Blob" defined by Area, Principal Moments of Inertia, Section Modulus, and Torsion constants.

In this way you can analyze ANY shape in VisualAnalysis. This type of shape is created with a name and is available for use in the active project only. You cannot taper custom-blob members or design them.

Custom blobs should be used with care! The shape properties are not checked other than to ensure they are positive. We use the moments of inertia and section modulus values to "back out" a depth and width for drawing purposes. If we get unreasonable dimensions are calculated, a square section with the appropriate area will be used instead.

Requires: Advanced Level

4. Rigid Links

Rigid links are a finite element "trick". They are simply a short, stiff member element used to connect two nodes at some offset and to control the transfer of forces between elements framing into those nodes. By changing a member into a rigid link, you let VisualAnalysis worry about the stiffness of the member. The only options on a rigid link are the end-releases. VisualAnalysis does a pretty good job of "hiding" these elements from your analysis results, reports, design checks, etc, so you can focus on the real members in your structure. You can also use any member as a "rigid link" if the built-in one does not serve your purposes.

Quick Shape Pick List

The Modify tab of Project Manager shows the types of shapes available in the Source drop-down list: Database Shape, Standard Parametric, <Add Custom 'Blob'...>, followed by a list of shape names for shapes that are already in use in your project. You can normally use this list to quickly find an existing shape in your model to use for a new member element.

Material Properties

Material properties come from the IES material database, which includes most typical materials you might need. To select a material, use the [...] button in the Modify tab of Project Manager, to bring up the material selection dialog.

Select a shape before selecting a material! Most database categories have a default material that will be automatically assigned when you select the shape. If you specify the material first, your material may get replaced. To change the default material, right-click on the shape category when selecting a member's shape. Choose Default Material, select it, then right-click on the database name and use Save Database Changes..

Custom Materials

You can add new materials to your system by clicking the New Material button in the Material Selection dialog. You may also customize the material database (folders, design properties, etc.) by right-clicking on the Material Tree in this dialog box. You can now specify any "design type" for materials making it possible to define custom wood, steel, or concrete materials that will work for design checks.

Materials have certain design properties, so you should be careful to group materials in categories that make sense and be sure to provide the necessary properties. The first step is to define the basic material properties: modulus of elasticity (E), Poisson's ratio (nu, n), a weight density (gamma, g), and a coefficient of thermal expansion (alpha, a). These properties may not be zero, though you may use a very low weight density to represent a weightless material for fictitious members (e.g. rigid links). If you are defining a steel, you need to add the Fy design property to your material, for concrete you need to add f'c, and for NDS wood you will need to define a number of allowable stress values and pick the correct design type!  When you close this dialog box, any material database changes are saved.  The Shear Modulus, G, is calculated internally as: G = E / (2*(1 + nu)).

If you customize the material database you should take ownership of any modified database files. See preferences, files for the locations of the .dbm (material database) files.

Taper

VisualAnalysis supports a single or double linear depth variation along the length of a member. The defined shape of the member is the starting depth and the Depth2 value you enter is the depth at the other end or at the "end" of the taper offset. The stiffness of the member is calculated based on the depth of the member at any point along the length of the member. The built-in taper is more accurate than using 100 "pieces" of member shapes!

Single Taper Bottom Double Taper

You may optionally enter 'start x' and 'end x' offsets (measured from the starting node for the single-taper, and from both ends for the double-taper) for the variation. By default the offsets will be zero and the tapered Depth2 will be at the far end of the member. This will allow you to create non-prismatic members with many profiles. The "bottom" taper can be used so that the members draw differently in the graphics, but will not affect the numerical stiffness significantly. Shown below are the "bottom" taper variations:

Tapering Limitations

Not all member shapes can be tapered. Standard parametric shapes and most database shapes that correspond to a standard parametric profile can be tapered. Cold-formed shapes, custom blobs, and double angles are examples of shapes that do not allow tapering.

Combined members cannot be tapered. You must split combined members into individual elements in order to use tapering features.

There is no provision for tapering the width or flange-thicknesses of the shape, you would need to manually split member elements and give each different shapes or properties to accomplish that.

End Connections

Member connections in a space-frame FEA structural model are infinitely-rigid by default. In other words, full force and moment transfer exists between the member, the joint itself, and therefore to all other members framing into this joint. In some situations this is not realistic. A common situation in steel is the clip angle connection where little or no moment transfer exists. In this case a moment end release should be placed at the joint.

On the Modify tab of Project Manager you may quickly define the most common end conditions using the Connection Type drop-down list. A type like "Simple-Rigid" means the 'start end' of the member is simple, the other end is rigid. View the End Releases for the actual condition at each degree of freedom at each end. We use this terminology for rotational connection types and end-releases:

For more sophisticated analysis, using the advanced-level of VisualAnalysis, you can also create Semi-Rigid connections between members for strong or weak bending, but these are rarely used or necessary.

If you wish to have a truss member in a frame model you can use the "Simple Connect" connection type. This will release the end moments at both ends of the members so that no moment will be transferred in or out of the member. The member can however still carry a moment due to applied loads or self-weight. The simple connections are typical for cross bracing members in steel construction. Realize that the convenient connection types may release weak-axis moments, making 3D models less stable. If you have stability issues you may want to remove the weak-axis releases in beam members.

Other situations involving slotted holes may require force releases in the direction of the slot. Again, member end releases are used for this application. To create advanced member end releases you may need to use the Connection options of the Modify Tab.

Member releases are always specified in member local coordinates. In the most realistic connection situations there is a partial release of force transfer between the member and its joint. This situation is commonly called a partially restrained (PR) joint. End zones described below may help you model this situation more accurately.

End Zones

Members connections in VisualAnalysis are modeled at their centerlines, which is normally accurate enough for most problems. In some cases however, you may wish to be more precise in your modeling. Member end zones allow you to account for a number of different situations you may encounter.

In structural steel connections, the beam to column joint is not totally rigid or totally flexible. The column may have a thin, flexible web that will allow some additional rotation.

Flexible Panel Zone

In other situations, say when a concrete beam frames into a very stiff wall or column, you may want to assume there is no rotation between the face of the support and the centerline.

Rigid End Zone

In an attempt to approximate this behavior, a special end zone may be specified at the end of the beam element. The end zone allows different member stiffness over a short region to linearly approximate the moment-curvature relationship. Internally, a short member element is inserted with modified properties.

To specify end zones enter the width, w, (along the length of the member) of each end zone and a percentage of stiffness. This can be done on the Project Manager, Modify tab for a member. This percent factor multiplies the modulus of elasticity of the member when determining overall stiffness characteristics of the end zone member. The reduced EI and EJ values have the effect of creating the partially restrained joint. If you select a rigid end VisualAnalysis uses a multiplier of 1000 on the member modulus to determine the end zone member stiffness.

Centerline Offsets

A common situation arises when plate elements are combined with member elements. The plate usually is placed on top of the member and thus the beam centerline is offset below the plate element. Floor slabs, metal deck, and wood sheathing supported by beams are all good examples. The sketch below shows beams and girders aligned at the top flange. In order to model these effects you may use centerline offsets. In many cases the effects of offsets are negligible and the performance cost can make this feature undesirable.

Centerline offset lengths are specified in a local coordinate y or z direction, so a beta angle rotation will change the direction of the offset.

Member centerline offsets

Beam Top @ Nodes

This feature allows you to align all beams (members orthogonal to the specified vertical axis) with the associated nodes.  The benefit to using this feature is you can specify node elevations as they might be called out on drawings such as a Top of Steel Elevation = 100'-0".  In order to do this, VisualAnalysis offsets the member using a Rigid Link just like using a Centerline Offset (above).  Keep in mind that this will affect model statics and subsequently member moments and forces, depending on specified boundary conditions.  Specifying "Beam Top @ Nodes" and testing the two cases of a simply supported beam with end nodes pinned-pinned versus end nodes pinned-roller ( X-displacement "Free") will demonstrate the behavior of the rigid link used in this feature.

Optional: When you enable this feature you may optionally ignore any previously specified manual centerline offsets for beams. This insures all beams are aligned at the top. You can disable this option to allow your manual offsets to be superimposed with the automatic beam offsets.

One-Way Behavior

Many situations exist where either supports or members are capable of having a force in one direction only. VisualAnalysis offers tension-only or compression-only elements to model these conditions. This setting is found under "Action" in the member's Options, and the default is "Normal (two-way)". A tension-only member is different from a cable element, which sags and can have pretension.

One-Way Option Disabled?

If this option is disabled, it is because (a) The member is Combined, (b) The member lies in the plane of an auto-meshed area, or (c) There is another nonlinearity in the project (e.g. P-Delta, Semi-rigid ends, etc...).

One-Way Examples

For example, soil is typically able to produce a compressive reaction only. When a footing uplifts from soil, the supporting effect is gone. In this case you may use a compression-only spring support to model the soil. The stiffness of the spring is normally calculated using the soil subgrade modulus multiplied by the area the spring supports.

Another example is a slender bracing member such as a rod that will buckle under a small axial compression, yet carry a large tensile force. In this case you may use a tension-only member to model this behavior.

Combined Members (Physical Members, Girders)

It is common to have girders that support multiple members framing in along their length. In any finite element program, including VisualAnalysis, distinct elements and nodes need to be created along the length of the girder for connecting these supported members. This necessitates dividing the girder into pieces. For reporting and design purposes, you may want to treat the girder as one single combined (continuous) member. 

You can convert a 'chain' of member elements into a combined member using Model | Combine Members. We recommend that you first build your model to the point where you have members that are all connected. Next, locate the members you want to combine and combine the individual elements. Finally you can assign member properties, apply loads, and complete the analysis.

When you create a combined member, the first member in the chain defines the shape, material, orientation and other properties. The end releases are taken from the first element and the last element for the respective ends. If you try to combine a group of tapered members, only the taper settings for the first element in the chain are used and will be applied over the entire combined member.

The combined member feature allows you to revert back to the original member chain at any time. Once a group of member elements is marked as combined, you can go back to the individual elements using Model | Split Combined Members. 

Controlling Combined Members

Members are automatically split and combined when drawing. If you draw a new member that crosses an existing member or members in a frame model type, the existing member or members are automatically combined back into a single piece. You can prevent the automatic splitting by holding the Alt key while sketching members. Use Edit | Preferences, the Desktop tab to set the option "Always ask to Split or Combine". 

Advanced Editing: You are allowed to edit the location of both interior and exterior nodes that belong to continuous members. Use this feature with caution! If the nodes don't line up, the model may still analyze producing erroneous results. The project manager may be of considerable help when editing nodal locations. Exact values can be entered for each nodal coordinate. If you use the project manager to edit the location of a continuous member's end node, the interior nodes are brought with automatically. This is not the case if you use one of the other methods for moving a node.

Limitations of Combined Members

This feature is a convenience that can shorten reports, simplify design and editing. However, it has some draw-backs. Combined members cannot have different properties along their length, cannot be tapered and cannot be marked as one-way elements. You must split combined members into simple member elements to use these features.

Creating X Braces

X-Braces are best created by crossing, but not connecting two member elements. There are other situations where you may desire to model crossing, but not connected members. To create a crossing member, use Alt+Drag in the Model View key and it and any crossing members will not be split. If the end points of the new member fall on existing members, these members still will automatically split.

Local Coordinate System

Member elements each have a local coordinate system that is defined by their connectivity and orientation in the model. Local coordinates are always represented with lower case letters {x, y, and z}. The local system is assumed to align with the shape's principal axes. (Asymmetric members, like L and Z shapes, show a non-zero alpha- angle which is the rotation from the geometric axes.)

The local system is used to define loads applied in the member's local directions. A force applied in the local x direction is always an axial force. End releases are also oriented according to the direction of the local coordinate system. Finally, member local forces and displacements are reported with respect to the local coordinate directions.

The local x-axis is directed from the start node to the end node. The local y-axis originates at the start node perpendicular to the x-axis and will lie in the plane formed by local x and a vector parallel to global Y. For plane frame structures, the local z and the global Z will always be parallel. The picture below clarifies this idea:

Member local coordinates

In the case when the member's local x-axis is parallel to the global Y-axis, the plane for the local y-axis is undefined. This can happen in both the positive and negative global Y directions. The picture above shows the defined orientation for these two situations.

You can reorient the local coordinate system in a space frame model by using the Beta Angle. The Beta Angle is available for space frame structures only. Please see the sketch in the next section for a picture of how the Beta Angle rotates the local system.

Shape Orientation: Alpha and Beta Angles

Alpha Angle

Alpha is zero for symmetric cross sections. For asymmetric shapes, like single-angles, zees, or spandrels, the shape's Principal axes do not line up with the Geometric axes. The angle between them is called alpha, and is a function of the shape's dimensions. We display this in the Modify tab for these shapes. All shapes in the shape database are oriented according to their Principal axes, so you may need to adjust the beta angle (-alpha) to orient the member with the Geometric axes.

Beta Angle

A Beta angle changes the default orientation of the local coordinate system and rotate the section properties of a member in a Space Frame model. Positive rotation follows the right-hand-rule.

Historical Note: The old Theta Angle is now gone. It was formerly available in a Plane Frame model and rotated the section axes with respect to the local axes. To rotate members about their own axis, you MUST use a Space Frame structure type and the beta-angle.

Framing Type

Each member can be designated as one of three framing types { Beam, Column, Bracing }. These are interpreted in various places in the software in different ways. They are initialized automatically based on the vertical axis direction and the orientation of the member in space. You may override the setting at any time. Currently this setting is used to control area loads on members, column drift reports, and design options as follows:

IES will likely expand on the use of this setting in future versions of VisualAnalysis.

Load Types

Members may be loaded at discrete points or with distributed loads. The loads may be forces, moments, or thermal changes.

Concentrated loads are applied at a specific point along the member's length. You do not need to split members to get a node at a concentrated load point. When multiple concentrated loads exist you may specify their starting offset and spacing, and the sequence can be generated automatically.

Distributed loads can cover the length of the member or just a small portion of it. Uniform and linear loads are treated separately in the software. They are reported using separate tables in a report.

Temperature change loads cause a member to shrink or expand along their length. Gradient temperature causes a member to expand on one side and shrink on the other to produce a bending effect. For more information on applying loads and load types refer to the Loading section of this document.

Load Directions

The choice of directions depends on the structure type and the load type. In general, loads may be applied with respect to the global coordinate system or with respect to each member's own local coordinate system. Member loads are always assumed to pass through the centroid (or shear center) of the member. 

Working with the local system is usually preferred, as it is much easier to be certain of the direction and type of load.

With global loads, the actual effect on the member depends on the orientation of the member. For example, a concentrated force in the global X direction might act on a beam as an axial load, while the same load applied to a column could be a transverse force. Directions are shown with coordinate letters (X, Y, or Z for global coordinates and x, y, or z for local coordinates) and labeled as a shear force, axial force, or moment.

Use a negative magnitude to apply a load in the opposite direction. For example, a negative axial force might indicate compression while a positive force means tension. For the other directions, a negative value means the load acts opposite to the coordinate system direction. For rotation, a positive load follows the right-hand-rule, meaning that a counter-clockwise rotation occurs on an axis that is pointed toward you.

Global distributed loads may be applied directly on the member or they may be distributed over the projected length of the member. The total value of the load is reduced if it is over a shorter projected length.

Load Offsets

Member loads are located from the starting node of the member. This is defined by node 1, or more simply, by the direction in which the member was sketched. For distributed loads, both the starting offset and the ending offset are measured from the starting node. A full span load has a starting offset of zero and an ending offset of L, the span length.

Multiple distributed loads may be applied on a member, but for display purposes they may not overlap.

Results

Local Forces

Member local forces use the strength-of-materials sign convention. Tension is positive. Positive bending moment about the z-axis causes tensile stresses on the negative y side. Positive bending moment about the y-axis causes tensile stresses on the negative z side. Positive local displacements and rotations are in the same direction as the positive local coordinate directions or rotating about a local axis according to the right-hand-rule, respectively.

Member Local Forces

Local Deflections

Member deflections are reported with respect to the local axes (Dy is in the same direction as y, Dz is in the same direction as z). Analysis deflections include the displacements of the end-nodes, if any!

Internal Stresses

Axial stresses are calculated assuming that the axial force, Fx, is applied at the centroid. Therefore fx = Fx/A where A is the cross sectional area. Axial tension force and stress are always positive, while compression forces and stresses are negative.

Shear stresses are calculated as a simple average over the entire cross section: fvz = Vz/A, fvy = Vy/A. The actual shear stress distribution or extreme stress is not calculated by VisualAnalysis. (You could manually estimate maximum shear values using formulas based on the shape profile: fvy_max = 1.5*Vy/A for a narrow rectangle, or Vy/Aweb for a wide flange, for example. Or you could use IES ShapeBuilder to determine shear distribution on a cross section.)

Bending stresses are calculated and reported with respect to the local axes, which are assumed principal axes.

Stresses are reported separately for bending (fb) in each direction and for axial stress (fa). The bending stress is simply M/S, and the axial stress is simply P/A, where M is the moment, S is the section modulus to the extreme fiber, P is the axial force, and A is the cross-section area. Similarly shear forces are a strict 'average' V/A, without consideration of the variation of shear on the cross section. For a detailed stress analysis on a cross section, you may wish to use our ShapeBuilder product.

Member Local Stresses

The software will also calculate a corner or combined stress, fc, using a simple algebraic sum of the bending stress for each direction and then superimpose the axial stress.

Be aware that if your shape is not rectangular or does not have physical fibers in the corners of the bounding rectangle (like a wide flange does), the corner stresses reported in VisualAnalysis can be very conservative, because they are calculated blindly at 'fictitious' corners for L, T, Pipe, and other shapes.

Finite Element Formulation

The finite element used for member elements is a 2-noded prismatic (or linear depth-tapered) line element. Member elements can be offset from their centerline and this is handled directly in the stiffness matrix. The axial displacements are based on a linear displacement assumption and the transverse displacements are based on a cubic displacement assumption. Shear effects are included only for user-defined sections with nonzero shear areas. The stiffness matrix is the standard found in most finite element textbooks. Special cases such as end zone are all handled internally using multiple member elements (creating an internal 'Combined Member').

Important Limitations: Member elements are a 1D element (a "line" without thickness) connections to elements are made at the centerline, not at the "face"!  Torsional stresses are not available, as part of the finite element analysis. However, if a member is twisted a torsional force is generated in the member, and nodal rotations can be used to determine the twist. Warping behavior of thin-walled open steel cross sections can be analyzed ( with idealized boundary conditions ) during the advanced steel torsion design process.  VisualAnalysis does not perform 'buckling analysis', or 'plastic analysis' on member elements.