Essential Design Concepts
Requires: Design Level
- Design Groups: Why? Auto-Groups, Manual Grouping
- Design Parameters: Your input data to control design checks
- Unity Checks: What is a unity ratio?
- Bracing: Important!
- Code Checking vs. Design: Automatic vs. "Design Search"
- Deflection Criteria: Total vs. Member, Load Combination Categories, Drift Checks
What is a Design Group?
Design Groups are simply an association of member elements (or plate elements) for two purposes. First, they associate the elements in VisualAnalysis with a specific material specification and its associated parameters (like code version, bracing, and deflection limits). Second, they allow you to economize your data entry and the final structure by grouping similar elements together. Design groups must "match" the shape that you are designing. You will not get unity checks if you try to design a cold-formed shape with an AISC steel design group, or visa-versa. If you change the shape to a different design category (eg. from wood to steel), you will need to delete the design group and recreate one of the appropriate type.
Design Groups are required for checks. There is no way to check members without placing them in a design group, and each member may be in only one design group at a time. You may place each member into its own "group" if you like.
When youmembers in a design group, all the members will take on the same final member properties. The software checks all the members in the group and verifies your selected shape will satisfy the stress/force (strength) and deflection (serviceability) requirements for the worst case.
By default design groups are created automatically for you whenever you switch to the. You can control this feature through the . VisualAnalysis will group members with similar shapes, materials, lengths, and orientations in a semi-intelligent way. Any time you switch to a , members that are not currently grouped are assigned a group (if possible). If you turn off this option then you can manually take charge of grouping members.
Manual Design Groups
You can manually remove members from Design Groups and Create new groups. Use themenu or the context menu in the Design View. For best results you may wish to turn-off the option in the Filter, which will allow you to select individual members rather than all the members in a group when you click. If you add or remove members from a design group, it is no longer managed by the auto-group feature.
Some Members Cannot Be Grouped
VisualAnalysis does not support design checks for all types of members. Custom blob shapes, custom materials, custom database shapes, and rigid-link members are all excluded from Design Groups. You may be able to perform custom stress and deflection checks on custom member elements.
Design parameters control how unity checks and design searches are performed for a Design Group. You enter this information in thetab after selecting a member or a Design Group in the window. While many of these parameters are specific to the type of material you use, there are some common design parameters across some of the design modules. For example, steel, wood, and cold-form steel design all use similar bracing, size constraints, and deflection criteria.
One common design criteria is limited space. If the architect has mandated a minimum floor to ceiling height, you will probably need to limit the depth of your beams and girders. VisualAnalysis supports this using a size constraints. Size constraints can also dramatically improve the performance of the software when searching for a least-weight member to meet your design criteria.
If you have specified a size constraint, it will also be checked during unity checks of existing members. If a member violates the size constraint, there will be a "flag" on the check and the member will appear differently in the Design View. You will see a note in the fly-over tip regarding the violation or a note in the report.
This section of the design parameters only effects the shapes. You specify the one category of shapes in the shape database to search. This category will be searched in a least-weight order, filtered by your size constraints, if any. Only shape categories that include shapes with the appropriate 'property sets', or required design data, are shown when you open the database to select a category.command's search for
Parametric member shapes may also be designed, when VisualAnalysis searches for acceptable sizes it will use the depth and width 'increment' numbers you specify in the Size Constraints area of the Design Group parameters. Specifying a zero increment, will effectively 'fix' that dimension.
You do not specify any design material in the Design View or Design Group parameters, this is specified before analysis in the Model View. The material used is the material for the "first" member in your design group. For best results, you should include only members with the same material within a design group.
Axial parameters provide information about the frame in which the members of this design group reside. You may ignore the axial parameters if designing members without axial loads. You have the option of overriding the effective length factors and can indicate whether the frame is braced or not. The directions referred to on this page are interpreted according to the section axes of each individual member element in the group. This means that if you have a frame that is braced in one direction and not the other, you should not place members whose orientations are different into the same design group.
Sidesway y: Choose yes if the member is part of a sway frame in the plane formed by its section z and local x axes. Choose no if it is part of a braced frame in this plane. If the automatic K factor calculation is on for Ky, the sidesway y parameter will effect the calculation of the Ky value.
Sidesway z: Choose yes if the member is part of a sway frame in the plane formed by its section y and local x axes. Choose no if it is part of a braced frame in this plane. If the automatic K factor calculation is on for Kz, the sidesway z parameter will effect the calculation of the Kz value.
Manual Ky: Ky is the effective length factor for buckling when the member kicks out in its section z direction (normally weak axis buckling). If Set Ky is left set to no, the effective length factor will be calculated automatically based on the relative rigidity of the members framing in at its end points. If Set Ky is set to yes, the Ky value must be entered.
Manual Kz: Kz is the effective length factor for buckling when the member kicks out in its section y direction (normally strong axis buckling). If Set Kz is left set to no, the effective length factor will be calculated automatically based on the relative rigidity of the members framing in at its end points. If Set Kz is set to yes, the Kz value must be entered.
A unity check is simply the ratio of actual demand over design capacity. In some cases the unity check represents a stress ratio, in others it is an ultimate force ratio. The unity check might also represent a deflection ratio or something else entirely. All of the design load cases are checked for the member and the worst-case value is stored. These checks encompass all types of checks pertinent to the material and according to the assumptions and limitations of each design material module and your settings.
Another way to think of the unity check is as a "Percent Material Utilization" factor. If you have unity checks in the range of 0.01 to 0.25 your elements are over-designed: there is a lot of wasted material. On the other hand if your unity check values range from 0.95 to 0.99, your elements (not necessarily the model as a whole) are optimized for least weight.
Unity checks are normally greater than zero and less than five. A value greater than one indicates a failure or that some design check has been exceeded. If you have done a good job of initially sizing your model, your unity check values will not exceed 1.0 by a great amount. If you see a unity check value that is negative, or very large, that could be an indication of an unreasonable model.
Unity check values and errors and warnings are displayed in the double-click a member in the Design View to get detailed design reporting.window and the . You can hover over a member to see quick information in the pane or
VisualAnalysis assumes that the length of a member element is equal to the unbraced length. It is up to you when you create a design group, to specify the actual unbraced length for members in the group. This is one of the key criteria that you will use in determining which members to group together. Bracing is not automatic in VisualAnalysis, you must pay attention to this important detail. The terms top and bottom relate to the member's section axes, and may not refer to the actual 'top' of the member! To see the location of bracing on a member use the Brace Locations checkbox found in the Member Details section in the tab while in the .
Three Directions to Brace:
- Lateral Top Bracing (at the +y face of the member), for strong flexure checks
- Lateral Bottom Bracing (at the -y face of the member), for strong flexure checks
- Strong Bracing (z), for axial compression checks.
VisualAnalysis does not provide a way for you to enter any torsional bracing information. The software makes the assumption that the unbraced length for torsional buckling is the minimum of the strong and weak flexural unbraced lengths, i.e. KLx = Minimum(KLy, KLz).
Ways to Specify Brace Positions
These are brace locations along the length of each member element.
- Bracing Patterns (e.g. midpoint, third point)
- Bracing Fractions
- Secified Unbraced Length
- At Interior Nodes for Combined Members
By default, each member element is braced at its end points and unbraced along its length. Bracing patterns allow you to define intermediate bracing points that are not part of the VisualAnalysis model.
You may specify a bracing pattern like continuous, mid-point, third-point, or quarter-point. This means that for each member element, a brace is assumed to restrain the member against buckling at these specific positions along the member. This is a fast and easy way to specify the bracing and it can work well even if the members are of different lengths. You may also restore members to the default unbraced state using patterned bracing.
By default, each member element is braced at its end points and unbraced along its length. You may use fractional bracing to define the locations of intermediate brace points along members.
Fractional bracing is similar to Bracing Patterns but allows an essentially unlimited number of patterns. You simply specify that braces exist at specific fractions of the member element length. These fractions are defined from the start-end of the member element. You create a list of fractions to locate braces at arbitrary positions along each member element.
This method also works if member elements have differing lengths. Fractions should be listed in increasing order in the range of zero to one. For example, "0.5, 0.625, 0.79"
Specified Unbraced Length
When you specify a specific unbraced lengths, these braces are assumed to exist at fixed distances from the 'start' end of the member element until the end of the member is reached. You may specify an unbraced length that is longer than the member-element length to model things like a chain elements that is not exactly straight and where VisualAnalysis will not allow a 'Combined Member'
Using the specified unbraced length can result in an incorrect calculation for the Cb value and other possible errors in checks--please use wisely.
At Interior Nodes
VisualAnalysis provides a convenience feature that lets you combine member elements into a single composite member for design or reports. Behind the scenes the member consists of multiple elements with interior node points. The design software does not automatically "see" the interior node points as braced points, but you may specify this option in the bracing.
Checking vs. Design
VisualAnalysis distinguishes between code checks and design.
Checks: A check of your model "as is" using the Design Group options that apply. Each member in a Design Group may be a different shape and is checked using that shape. Checks are generally automatic when you switch to the Design View and analysis results are available.
Design: When you explicitly tell the software, using (or similar) to search for a shape that works for all members in the group; each member is then checked as if it were the chosen shape. When you design a group, all the members in the group take on the same final shape.
Deflections are checked for load combinations that have been marked as "Deflection" or "Allowable and Deflection "in the Load Case Manager.
You must choose the method for checking deflections! For cantilevered beams you generally need to check "total" deflection (described below). Depending on how your model is constructed (e.g. combined members or separate member-elements), or how members are grouped together, you may need to change the type of displacement check! The measured displacements can change dramatically (and even reverse direction!) depending on the method chosen.
Span Ratio vs Actual Deflection
You may specify a limit based on a span ratio (like L/360), or a specified displacement limit (like 0.25 inches). The span ratio approach allows you to group members that may not have the same span length, yet still provide a consistent deflection limit. You may disable specific limits by setting a zero value (for a displacement limit) or a small value (for a span-ratio limit).
The span-length is calculated as the member-element length, or for a 'combined' member it is the total length of the combined member--regardless of the types of internal connections or supports! Span-ratio checks may not make sense for combined members that are supported in their interior. Span-ratio checks generally do not work for cantilevered members.
Member vs Total Deflection
You will also need to choose between two methods of measuring the actual deflection. VisualAnalysis uses the terms "Member" and "Total" to distinguish between bending deflection in a member element and the total movement of a member including the nodal displacements.
In the picture on the right above, the member deflections are calculated from the green-line between the deflected end-points to the red-line (the deflected beam), while the total deflections are calculated from the dashed-blue line.
In many cases, the member and total displacements will be identical. This is true for a simple span beam, or (approximately) a beam framing between two columns. In the case of a beam split into multiple elements you will need to make sure that you are checking a total displacement. In the case of a beam spanning between two girders, and the girders are also deflecting, then you will want to check member displacements for this beam. Finally in some situations you will simply need to check absolute displacements that you have pre-calculated. Using the span-ratio limits does not work for all situations in modeling, though it is most convenient for typical beams.
Load Combination Deflection Categories
You can view deflection categories in the, on the combinations page for any load combinations marked for "Deflection" or "Allowable and Deflection" checks. See loading for design for more details.
You can specify different deflection limits for each category in a Design Group's parameters.
Column Drift Checks?
VisualAnalysis does not directly check drift requirements. There are special Reports available to show building or member drift that you may use to check your drift requirements and make frame-wide adjustments as necessary. The "Complete Member" report item will display the actual drift extremes calculated for a column member.