Loads - Load Combinations

During solution the model is loaded with a combination of factored Load Categories. These combinations, load factors, and other parameters are defined on the Load Combinations Spreadsheet. Most standard load combinations are included in the program. See Solution to learn how to solve load combinations.

To Add Load Combinations Manually

From the Spreadsheets Menu select Load Combinations.
Enter load combinations by pairing loads in the Category fields with factors in the Factor fields.

To Add Auto-Generated Building Code Combinations

From the Spreadsheets Menu select Load Combinations.
Select the button from the Window Toolbar.
Select the Load Combination Region and Code from the drop down lists provided in the Gravity tab.
Modify the generated combinations and options in the spreadsheet as necessary.

Note

The generated building code combinations are made up of Load Categories and Factors. Loads that are not assigned to these categories will not be included in the combinations upon solution.
All generated combinations are added and marked for the envelope solution. You may remove combinations from the envelope after adding them.
Verify the Wind/Seismic ASIF (allowable stress increase factor), and the Timber CD settings on the Design tab for combinations after you add them.
You may save any preferred load combinations as the default by clicking the Save As Defaults button on the Window Toolbar.

To Solve Load Combinations

To solve load combinations, first mark the combinations to be solved by checking the Solve box on the Load Combinations spreadsheet. Click the Solve button on the RISA Toolbar.

Load Combinations Spreadsheet

The Load Combinations Spreadsheet records the combinations of loads for solution and may be accessed by selecting Load Combinations on the Spreadsheets Menu.

Combinations Tab

The first field, the Label, is strictly for the user's reference. Enter any descriptive label you wish and it may be displayed with the results when the combination is solved.

The Solve box is used to indicate which combinations should be included the solution. See Solution for more information.

The next eight pairs of fields (Category, Factor) are for defining what load categories are to be part of the combination, along with factors for each. Select load categories from the drop down lists in the Category columns of the spreadsheet. See Load Categories for a list of the categories available for use.

Enter in the Factor field a multiplier to be applied to the load category being included. If you are designing an unshored composite beam system, you should define at least one pre-composite load combination. If you fail to do so, the beams will not be designed to resist the effects of the pre-composite loads.

Note

Any time a load combination contains the load category DLPre, that Load Combination will be considered to act solely on the pre-composite floor system.

Design Tab

The first field, the Label, is strictly for the user's reference. Enter any descriptive label you wish and it may be displayed with the results when the combination is solved.

The ASIF column is used to set the allowable stress increase factor used for both the AISC: ASD code checking and ACI 530 ASD (masonry allowable stress design). See Allowable Stress Increase regarding its usage for Hot Rolled Steel.

The CD factor is the load duration factor and is only necessary for NDS timber design using ASD (see Load Duration Factor). Note: this is KD for Canadian (CSA O86) wood design.

The lambda factor is the time effect factor and is only necessary for NDS timber design using LRFD (see Time Effect Factor).

The Service check box only applies to RISAFloor ES concrete slab floors. Load Combinations with the Service check box turned on will be checked for the Slab Point Deflection results and will also have the plate stiffness factored. See the Elevated Slabs - Analysis topic for more information.

The next six check boxes designate which load combinations should be used for the code checking of each material type. For example when using steel and wood in your model, you may design your steel for one set of load combinations and your wood for another.

The last check box for Connection defines two things:

Whether this load combination should be used to calculate the end reactions or not for hot-rolled steel members. Only load combinations with this check box will come into play when calculating the end reactions for the End Reactions spreadsheet and the graphic viewing of the reactions for hot-rolled steel members. All other member types will be based on their own material check boxes.
If this LC should be used in designing hot-rolled steel connections with RISAConnection.

Note

Steel products are always designed to the UDL, ULL and UTL loads described in the Beam Results section of this manual. See Uniform Loads on Steel Products for more information.

ASIF Factors for Wind / Seismic Load Combinations

Entering a value/factor in the field labeled ASIF (Allowable Stress Increase Factor) indicates that the load combination should be treated as a wind/seismic load combination.

If AISC 9th Edition ASD code checking is selected, the allowable stress increase factor entered in this field is applied to this load combination.

Timber Design Load Duration Factor

For Wood design, the load duration factor (CD) is entered in the CD field on the row that the particular CD factor applies to. Different load combinations would have different CD factors. For example, per the NDS code, a load combination that had only dead load, would have a CD factor of "0.9", while another combination that was comprised of dead load plus wind load would have a CD factor of "1.6".

The CD factor will only be applied to wood code checks on wood members. "Wood" members are those members whose material properties are defined on the Wood tab of the Material Properties Spreadsheet.

Note

See Table 2.3.2 in the NDS 2018 code for the CD factors to be applied for typical loads. Note that the CD factor used for a load combination should be for the load with the shortest load duration in that load combination.
When the Canadian CSA O86 Wood design code is selected, these entries will all change to "KD". See Table 5.3.2.2 of the CSA O86-14 code for direction on what factors to enter.

Generating Building Code Combinations

Major portions of the load combinations that are specified by building codes are included and may be applied to the model for solution. These combinations may be inserted by selecting the button from the Window Toolbar once the Load Combinations Spreadsheet is open and active. This will activate the Load Combination Generator Dialog shown below:

The LC Region refers to the various regions supported by the program (U.S., Canada, India, British, et cetera).

The LC Code refers to the actual code used to build the load combinations. For the United States, there are a number of different codes that could be used to build load combinations. If the only option is Sample, that means that no load combinations have been input for that region. See Customizing the Load Combination Generator for more information on how to add or edit combinations for that region.

Roof Live Load Options

The Roof Live Load Options allow the user to specify how complex the load combinations including roof live loads should be.

Placing a check in the box next to RLL (Roof Live Load), SL (Snow Load), and/or RL (Rain Load) will indicate that the selected load categories are to be included in the generated load combinations. If the box next to SL (Snow Load) is checked, the load combinations will include the Non-Shedding Snow Load category, SLN, as well as the SL load category.

The Composite Options allow the user to specify whether Pre-Composite and/or Post-Composite loads are to be included in the generated load combinations.

If the Pre-Composite checkbox is checked, load combinations containing the DLPre and LLConst load categories will be generated based on the selected code.

If the Post-Composite checkbox is checked, 'traditional' load combinations containing the DL, LL, LLS, and any of the categories included in the 'Roof Live Load Options' section above will be generated based on the selected code.

For more information on any of the load categories mentioned in this section, see Loads - Vertical Loads.

The Save as Defaults button may be used to establish the current load combination generator settings as the defaults for future use. Clicking the Generate button will generate load combinations in the Load Combinations Spreadsheet based on the selected options.

General Notes on the Load Combination Generator

The following loads are not generally included in the standard combinations but may be added by editing the combinations in the spreadsheet or by modifying the source document itself. For more information, see Customizing the Load Combination Generator below:

Category Code	Description
TL	Long Term Load
HL	Hydrostatic Load
FL	Fluid Pressure Load
PL	Ponding Load Category
EPL	Earth Pressure Load
DLConst	Construction Dead Load
OL#	Other Load Categories

Note

The standard combinations are made up of Load Categories and Factors. Loads that are not assigned to these categories will not be included in the combinations upon solution.
Some load categories do not occur in all of the design codes. Loads placed in categories that are not part of the standard combinations will not be included in the solution of these combinations.
Verify the Wind/Seismic ASIF settings for combinations after you add them.
After you add the combinations, verify the settings for the wood design Load Duration factor, CD/KD.
See Editing Spreadsheets - Inserting, Deleting and Clearing Cells for quick ways to edit or add combinations.
When a new installation is performed the program will back-up any existing XML files that contain load combination data rather than overwriting them. The back up copies will be given the extension BAK.

Customizing the Load Combination Generator

The Load Combinations for each region are contained in an XML file which can be opened and edited using a standard spreadsheet program. An example of on of these XML files is shown below:

The name of the file itself becomes the name of the Load Combination Region that appears in the Load Combination Generator. Each XML file has a series of "worksheets" and the name of each worksheet will be the name of the Load Combination Code that appears in the Load Combination Generator. The user may add, edit or modify these documents to completely customize the available load combinations.

Note

When the program is updated for new versions, the existing XML files will be "backed up" and saved with a '.bak' extension while the new XML files in the update will replace them. If any customizations were made to the XML files, they can be retrieved from the back ups.

The first row of each sheet is reserved for the column headers. The recognized column headers are as follows: Label, Solve, ASIF, CD, Hot Rolled, Cold Formed, Wood, Wood Products, and Concrete, Masonry, Footings, Aluminum, and Stainless,. In addition, there will be multiple pairs of Category & Factor headers.

Other than Label and the Category / Factor pairs, the user may omit columns. If a column is omitted, the default values will be used for those entries. The order of the column labels is optional except for pairs of Category and Factor. For the program to correspond the Factor with correct Category, the user should always provide Category label PRIOR to the corresponding Factor. The user may insert blank columns or columns with other labels than described above. In those cases, the program omits those undefined columns.

Note

The program will read these files from the directory specified in Tool - Application Settings - File Locations. See Customizing RISA for more information.
SX, SY, and SZ are not valid BLC that can be used within the Load Combination Generator spreadsheet. A response spectra analysis must be done first within RISA-3D.