Member Design Optimization

RISA-2D will optimize Hot Rolled Steel, Cold Formed Steel, Wood, and Concrete members. The criteria used for this optimization are the selected design code and the Design Rules assigned to the member. The sizes are chosen from a Redesign List assigned to the member.

The Member Design Rules Spreadsheet records the parameters for the optimization. Optimization is performed for minimum weight, taking into consideration any depth, width, rebar limitations, etc. Note that the design rules input is one large spreadsheet, thus all of your design rules will be in the same place. Note that the dimensional rules and the reinforcement rules are all a separate entity. They have no interaction with each other in the program. They are simply all input into the same location. For example, your minimum member width rules will not be influenced by concrete beam reinforcement rules.

You can assign the design rules graphically as you draw members or later as a modification. See Modifying Member Properties for more information.

For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Redesign.

Wall Design Optimization

The program will also optimize concrete, masonry, and wood walls. For Wall Design Rule information see these topics:

Note:

For the optimization procedure on concrete walls see Concrete Wall - Design.
For the optimization procedure on masonry walls see Masonry Wall - Design.
For the optimization procedure on wood walls see Wood Wall - Design.

Member Design Lists

A Design List defines a set of member sizes that are considered when the program is suggesting member sizes. You may assign a Design List to a member based on Member Type. The design lists available for Column members may not match the design lists for Beam members, because some shapes are more appropriate for use as beams than as columns. You may edit these lists or create additional custom lists of your own. For more information on these redesign lists, including file format, editing procedure, and user defined lists refer to Appendix A - Redesign Lists.

Member Design Rules – Size / U.C.

The Design Rules Spreadsheet records the limitations for the design and may be accessed by selecting Design Rules on the Spreadsheets Menu. You may create and name any number of design rules and assign different rules to various members.

The spreadsheet has three tabs: Size/UC, Deflection, and Concrete Rebar. The entries for the Size / UC tab are explained below:

Design Rule Labels

You must assign a unique label to the design rules. You then refer to the design rule by its label when assigning it to members. The label column is displayed on all tabs of the spreadsheet.

Max/Min Depth

You may enforce depth restrictions by setting either a maximum and/or minimum depth.

Max/Min Width

You may enforce width restrictions by setting either a maximum and/or minimum width.

Max Bending and Shear Check

Enter the maximum bending and shear unity checks. This should usually be specified as "1". If you desire a larger factor of safety, provide a lower factor (i.e. ".95").

Member Design Rules – Deflection

This spreadsheet allows you to define deflection criteria for beams in the model. You must choose specific load combinations from the Load Combinations spreadsheet (up to three) and give the maximum deflection L/y criteria.

Note:

In the Load Combination Generator - Gravity tab the program will create deflection load combinations for you automatically to match up with the defaults.
If you write "None" in the LC column the program will ignore this criteria when checking deflections. If all three LC columns are defined as "None" then no deflection checking will be done for beams in the model.

Member Design Rules – Concrete Rebar

The entries for the Concrete Rebar tab are explained below.

Note

If you would like to define specific flexural and shear rebar layouts for beams and columns, either use the Explict option below or see Rebar Layout Database for Custom Rebar layout.

Optimize

The options in this column are Optimize and Explicit. Optimize indicates the program will find the most optimal size and spacing of the reinforcement. Explicit indicates the exact number of and size of reinforcement bars. When Explicit is selected the additional columns are editable and allow you to enter the quantity for Top/Bottom Bars, Add'l Side Bars, and Shear Bar Spacing. These columns are greyed out when Optimize is selected.

Flexural Bars

Use the Min Flex Bar and Max Flex Bar columns to restrict bar sizes for your flexural reinforcing. Currently we support the ASTM A615 (imperial), ASTM A615M ("hard" metric, i.e. #8M is an 8mm bar), BS 4449 (British), prENV 10080 (Euro), CSA G30.18 (Canadian), and IS 1786 (Indian) reinforcement standards. You may specify your rebar set in the Model Settings - Concrete. You can force the program to analyze one bar size by setting the Min and Max values to be the same bar.

Shear Ties

Use the Shear Bar column to enter the size of your shear ties. Currently we support the ASTM A615 (imperial), ASTM A615M ("hard" metric, i.e. #8M is an 8mm bar), BS 4449 (British), prENV 10080 (Euro), CSA G30.18 (Canadian), and IS 1786 (Indian) reinforcement standards. You may specify your rebar set in the Model Settings - Concrete.

Legs per Stirrup

Use the Legs per Stirrup column to enter the specific information about how may legs (2 to 6) each of your shear ties is expected to have.

Concrete Cover

The last three columns are used to specify the clear cover measured to the shear reinforcing. Note that the Top Cover is used for all sides of Column members.

Member Design Rules Editor

The Member Design Rules Editor provides a way to create or modify member design rules in a graphical way. This editor automatically updates any information input into the Member Design Rules spreadsheet.

Note:

Keep in mind that this dialog has three tabs. Make sure to fill out the information on all the tabs before clicking OK.
See the information in the Status Bar for more information on each of these entries.

The Design Rule Label is editable and will change the spreadsheet label. It is useful to name the design rule with a descriptive name like "First Floor Columns".

The Member Type listed is for reference of the picture only in this dialog. The member type will not control the Member Type of the beam or column which you will need to assign separately when the member is drawn.

The Size/UC tab:

Deflection (Beams) tab:

This only applies to Beam members.

Rebar tab:

You can select Optimize or Explict to set the rebar design parameters.

Accessing the Member Design Rules Editor

To access this editor you can click the red arrow on any tab in the Member Design Rules spreadsheet (click in the Label cell to make the red arrow appear).

Member Optimization Procedure

Member optimization is performed both on explicitly defined members and on members defined through the use of Section Sets. Members defined as part of a Section Set are checked to determine which member has the highest code check value and which member has the highest shear check value. These members are considered to be the controlling members for that section set.

The controlling forces on a member or a section set are then applied to new shapes satisfying the redesign parameters and a code check is calculated. If the calculated code check and shear check falls within the specified range the shape is considered to be an acceptable alternate.

Optimization Results

Suggested Shapes Spreadsheet

Access the Member Suggested Shapes spreadsheet by selecting the Results Menu and then selecting Members Suggested Design. Alternatively you can click the Suggested Design button on the Results floating toolbar.

These are the suggested shapes resulting from the optimization calculations. They are chosen from each member's assigned Design List. The suggested shape is estimated to most closely meet the criteria specified in the Member Design Rules without exceeding them. If the member is a "Beam" member type then the Controlling Criteria could be based on either strength criteria or deflection criteria.

Note:

The suggested shape may be larger or smaller than the current shape, except for the case of members brought over from RISAFloor, for which the program never recommends downsizing.

To confirm that these alternate shapes are acceptable you MUST adopt any changes into the model then re-solve and check the results. The suggested shapes are based on the forces for the current model. Keep in mind that the current results are based on the stiffness of the current shapes. Changing the shapes will change the stiffness, which is why the model needs to be resolved. It may be necessary to cycle through this process a few times to achieve the best shapes.

You may try the new shapes by clicking the Replace and Resolve button. The shapes listed in the Suggested Shapes column will only be used to update the model if the "Use Suggested?" box is checked for that particular member or section set.

If the message "No Shapes Found" is given, then no satisfactory shapes could be found in the Design List specified for that member or section set. This can occur for a number of reasons. Common reasons are:

The loads applied are too large for the shapes in the redesign list.
No load combinations were checked for the design of this material type. See Load Combinations - Design Tab for more information.
A code check could not be performed for a member in the section set. See the Code Check spreadsheet or the Warning Log for these members.
The member has not been assigned an initial redesign list. Check the Members and Section Sets spreadsheets to be sure they are defined with a redesign list.
On-line shapes (RE, PI and BAR) cannot be redesigned.
If you've entered a minimum code check value and the members assigned to this section set are lightly loaded, it is possible that no shape generates a code check value high enough to exceed the minimum.
A code was not specified for that material on Model Settings - Codes.

Note:

For masonry wall design optimization see the Masonry Wall - Design topic.
For wood wall design optimization see the Wood Wall - Design topic.