Loads - Load Combinations

During solution the model is loaded with a combination of factored Load Categories and/or Basic Load Cases, both of which are defined on the Basic Load Cases Spreadsheet. 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

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

To Add Auto-Generated Building Code Combinations

1. From the Spreadsheets Menu select Load Combinations.
2. Select the button from the Window Toolbar.
3. Select the Load Combination Region and Code from the drop down lists provided in the Gravity tab.
4. 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.
• You may specify P-Delta options and SRSS combinations for each combination after you have added them.
• Verify the Wind/Seismic ASIF (allowable stress increase factor), the Footing ABIF (allowable bearing 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 a single load combination, click the Solve  button on the RISA Toolbar. Select the load combination from the drop down list and click the Solve button.
• To solve multiple 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, and choose the Envelope or Batch option before clicking Solve.

Note

• The Envelope solution is where all combinations with a checkmark in the Solve field are solved simultaneously. The Maximum and Minimum results of these solutions are listed, along with the number of the controlling load combination for each solution result.
• The Batch solution is where all combinations with a checkmark in the Solve field are solved simultaneously. All results of these solutions are listed.

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 Description, 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 next three fields are for options that apply to each load combination.

The Solve box is used to indicate which combinations should be included in batch or envelope solutions. See Solution for more information.

The P-Delta entry is used to enable an analysis of member secondary effects. See P-Delta Analysis for more information.

Note

• Per Section C1 of Chapter C (Design for Stability) of the AISC 15th Edition steel code, a factor of 1.6 will be applied to all load combinations for which a P-Delta analysis is to be performed (and for which the Hot Rolled box is checked on the Design tab) whenever 'AISC 15th: ASD' is selected on the Codes tab of the Model Settings Dialog. This factor is applied prior to conducting the P-Delta analysis and the results are subsequently divided by a factor of 1.6.

The SRSS entry is used to combine response spectra results by the Square Root of Sum of Squares. See SRSS Combination of RSA Results for more information.

The next eight pairs of fields (BLC, Factor) are for defining what loads 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. The following are the entries you can use in the BLC field:

Enter in the Factor field a multiplier to be applied to the loads being included.

You may also use the button in the BLC cell to help you specify the loads. Choose from load categories, basic load cases, or spectral results or moving loads from the drop-down lists.

Design Tab

The first field, the Description, 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 is used in the following applications:

• Cracked Concrete Section Properties: Per the provisions of ACI 318-14 Section 6.6.3.2.2 (ACI 318-11 Section R10.10.4.1) the cracked section properties of beams, columns and walls may be multiplied by a factor of 1.43 for service level loads (<= 1.0*Ig). All concrete members and walls apply this factor when the Service box is checked. Note that this 1.43 factor does not apply to masonry walls. Masonry wall stiffness is NOT dependent on this check box.
• Story Drift: The story drift results are broken up by each lateral direction into Service and Strength load combinations. These tabs can be seen in the Story Drift results spreadsheet.
• Member Deflections: The first two tabs of the Member Deflections spreadsheet provide deflection and deflection ratio results for Service and Strength load combinations.

The next check boxes designate which load combinations should be used for the code checking of each material type. The example shown uses one set of Load Combinations for the NDS wood design, and another for the ACI concrete design. Member results (Forces, Stresses, Torsion) will only be reported for a members if it’s material type has been checked for that load combination. Member results for general material will be shown for all load combinations. 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.

Load Combinations with RSA Results

The results from response spectra analyses in the X and Y direction may also be included in the load combinations. Remember, when you perform a response spectra analysis (RSA), you specify in which global direction the spectrum is applied. RISA-2D will retain the three RSA solutions (one for each direction) simultaneously.

To include the RSA results for a particular direction in a load combination, enter "Sn" in the BLC field, where n is the global direction. Suppose you wanted to include X direction RSA results, you would enter SX in the BLC field. You would enter SY for Y direction RSA results. Also be sure to put the RSA Scaling Factor for the RSA results in the Factor field. You can have more than one RSA entry in a load combination.

Note

• Remember that RSA results are typically unsigned (all positive) and you should provide some means of accounting for this. The figure above uses two combinations for each RSA result, one with a positive factor and the other with a negative factor to capture the maximum deflections, stresses and forces. See Unsigned (All Positive) Results for more information.
• If you have to combine 2 or 3 different spectra results with many static load combinations, it is convenient to put all the spectra results (SX, SY, and SZ) and factors in one load combination and nest that combination in other combinations. You must set the RSA SRSS flag on each load combination for it to be performed.

SRSS Combination of RSA Results

This is used to cause orthogonal RSA results in the combination to be summed together using an SRSS (Square Root of Sum of Squares) summation. This gives a good approximation of MAXIMUM responses but it also causes all the RSA results to be positive.

You may choose from the options by clicking on the button. The entry is “+” or “-” to indicate whether the combined RSA results are to be added (+) or subtracted (-) from the other loads in the combination.

Note

• This flag is used to combine different spectra that are acting in different directions. This is different than the modal combination method that combines results for one direction and is specified in the Response Spectra settings.
• This method was only approved for use in the 1997 UBC and is no longer noted as an appropriate method in the current codes.

Load Combinations with Moving Loads

Moving loads are included in your analysis by referencing them on the Load Combinations spreadsheet. For example, to reference moving load number “n” you would enter “Mn” in one of the BLC fields, and then also enter the corresponding BLC factor. The moving load numbers are shown for each moving load on the Moving Loads spreadsheet.

Nesting Load Combinations

You are allowed only 10 BLC's per load combination, which may not be enough. For this reason you can define "combinations of load combinations". This means if you need more than 8 BLC entries in a single combination, you can define the needed BLC's and self-weights over several load combinations and then pull these combinations together into another load combination.

Entering "Lnn" in the BLC field means include all the BLC entries (with their factors) from Load Combination "nn". For example, say Load Combination 4 has "L1" entered for one of its BLC's. This specifies to include all the BLC's (with their factors) entered in Load Combination 1 as part of Load Combination 4 (this includes self-weight and RSA entries as well). The flags for Load Combination 1 (i.e. Solve, PDelta, and SRSS entries) apply only to Load Combination 1 and will not be used when Load Combination 4 is solved.

Also, the factor we enter with the "Lnn" entry will be applied to the BLC factors entered for LC nn. Thus, if we enter "L1" with a factor of "0.9", we're including 90% of the BLC entries of Load Combination 1.

Note

• These “combinations of load combinations” can only be nested to one level; i.e. the load combs referenced with the Lnn entries may not themselves have Lnn entries.

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.

P-Delta Load Combinations

The P Delta field is used to perform a P-Delta calculation for that load combination. You may choose from the options by clicking on the    button. A blank field indicates no P-Delta analysis, Y indicates that a P-Delta analysis is to be performed for the combination.

You may also perform a compression only P-Delta analysis. Invoke this option by putting a C in the P Delta field. See P-Delta Analysis for more information.

Note

• P-Delta analysis is normally required for LRFD based code checks. If this is not desired, the user may change the setting by selecting Application Settings on the Tools menu and clicking the Solution and Results tab.
• P-Delta analysis is not performed on plates.

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.

The Wind Load Options specify how detailed the generated wind load combinations should be.  

The Reversible option generates two combinations for each wind load, one with positive load factors and one with negative load factors.

The Seismic Load Options specify how complex the generated seismic load combinations should be.

The X and Z RSA option will use the spectral seismic loads (SX, SY, SZ) instead of the regular earthquake loading (ELX, ELY, ELZ).

When the Reversible box is checked, the program will generate every seismic load combination twice. Once with a positive load factor, once with a negative load factor.

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:

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.
• All combinations added from the drop down list are added to the envelope solution. You may remove combinations from the envelope after adding them.
• 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.
• You may specify P-Delta options and SRSS combinations for each combination after you have added them.
• 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, pDelta, SRSS, ASIF, CD, ABIF, Service, Hot Rolled, Cold Formed, Wood, Concrete, Masonry, Footings, Aluminum, and Stainless,. In addition, there will be multiple pairs of BLC & Factor headers.

Other than Label and the BLC / 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 BLC and Factor. For the program to correspond the Factor with correct BLC, the user should always provide BLC 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

• As shown in the above example, the wind and seismic loads should be entered as WL and EL in order for them to be "expanded" using the Wind Load Options and Seismic Load Options.
• 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.