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. Select the desired Roof Live Load and Notional Load options and click Generate.
  5. Click in the Wind tab, select the desired Wind Load options, and click Generate.
  6. Click in the Seismic tab, select the desired Seismic Load options, and click Generate.
  7. Modify the generated combinations and options in the spreadsheet as necessary.

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To Solve Load Combinations

Note

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

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:

Entry Description

Number

Entering a number includes that particular Basic Load Case, I.e. enter “3" for BLC 3

Category

Enter a category code such as DL, LL, etc to include all loads in that category

Lnn

Enter “Lnn” to nest the loads from another combination, where “nn” is the number of the other combination, i.e. “L3" means include all loads from load combination 3.

Mnn 

Use this entry to include a moving load where “Mnn” is the moving load tag from the moving load spreadsheet

Sn 

Includes response spectra results for the global direction “n”, I.e. enter “SX” to include response spectra results calculated in the global X direction, “SY” for global Y , etc.

Tn

Use this entry to include a time history load where “Tn” is the time history load tag from the Time History Loads spreadsheet

There are also a number of BLC Prefixes that may be included in your load combinations. You can have these generated by the LC Generator or you may click on the red arrow button in the BLC cell to select them manually. These are defined in the Seismic tab of the Model Settings. They are explained below:

Prefix Description
SF Scaling Factor - applied to RSA results
ρ (Rho)

Redundancy factor - applied to EL loads for seismic load combinations (or RSA results)

Note: If ELX and ELZ used, the ρ from the X and Z direction will be used directionally. If EL is used, the ρ from the X direction will be used and the Z direction will be disregarded. If ELY is used, the ρ from the Z direction will be used and the X direction will be disregarded.

SDS Spectral Response Acceleration Parameter for Short Periods - applied to DL for seismic load combinations
Ω (Om)

Overstrength Factor - applied to EL loads for seismic load combinations (or RSA results)

Note: If ELX and ELZ used, the Ω from the X and Z direction will be used directionally. If EL is used, the Ω from the X direction will be used and the Z direction will be disregarded. If ELY is used, the Ω from the Z direction will be used and the X direction will be disregarded.

 

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.

Notice the Seismic Options and RSA Options that are described in the Prefix table above. For additional advice on this topic, please see the RISA News website: www.risanews.com. Type in Search keywords: Seismic Factors.

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 (see Load Duration Factor). Note: this is KD for Canadian (CSA O86) wood design.

The Service check box is used in the following applications:

The next seven 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.

The last check box for Connection defines:

Load Combinations with RSA Results

The results from response spectra analyses in the X, Y, and Z 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-3D 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 and SZ for Z 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

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

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

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.

If LRFD code checking is selected, then any value of ASIF greater than 1.0 will indicate that the seismic provisions for the WF compactness checks are to be used (Table 8-1, p. 6-317 of the 2nd ed. LRFD).

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

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

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:

Gravity Options

The first tab of the Load Combination Generator is the Gravity tab. This tab contains Roof Live Load and Notional Load Options.

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.

These settings can be changed on any of the three tabs will automatically update on the other tabs. Therefore making it easy to simply move through the tabs, selecting the options you require, but not having to re-select the region and code data on each page.

Beam Deflection Options

The Generate Deflection LCs will automatically place 3 load combinations first that match DL, LL and DL + LL.

This will match the Typical design rule from the default Member Design Rules - Deflection tab inputs.

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.

Notional Load Options

The Notional Load Options allow the user to specify how complex the load combinations including notional loads should be.

When None is selected, the program will not generate any Load Combinations that include the NL load category.

The 2D Only option is used to indicate that only the most basic notional load category (NL) will be used.

The X and Z option is used to indicate that the program should generate separate notional load combinations for each horizontal direction (NLX and NLZ).

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

For additional advice on this topic, please see the RISA News website: www.risanews.com. Type in Search keywords: Notional Loads.

Wind Options

The second tab of the Load Combination Generator is the Wind tab. This tab contains Wind Load Options.

Wind Load Options

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

When None is selected as the wind load option, the program will not generate any Load Combinations that include wind load categories.

The 2D Only option generates only the most basic wind load category (WL).

The X and Z option generates separate wind load combinations for each horizontal direction (WLX and WLZ when Y is set at the vertical axis).

The X and Z w/ Ecc option generates all possible wind load combinations that include partial / eccentric wind loading (WLX, WLXP1, WLXP2, et cetera) per Case 2 from Figure 27.3-8 in the ASCE 7-16.

The X and Z w/ Ecc, Quart option generates all possible wind load combinations per Cases 2, 3 and 4 from Figure 27.3-8 in the ASCE 7-16. For additional advice on this topic, please see the RISA News website: www.risanews.com. Type in Search keyword: Quartering.

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

The Generate Roof Wind Loads option generates a roof wind load BLC for use with perpendicular roof wind loads per Figure 27.3-1 in ASCE 7-16.

The Generate Semi-Rigid Diaphragm Loads option generates wind loads for BLC's using windward and leeward direction loads prescribed by the ASCE 7-16 (WLX+Z, WLX-Z, WLZ+X, WLZ-X).

The Add Notional Loads to Wind Load Combinations option generates wind loads which include notional loads per the ASCE 7 and AISC codes.

Note:

Seismic Options

The third tab of the Load Combination Generator is the Seismic tab. This tab contains Seismic Load Options.

Seismic Load Options

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

When None is selected as the seismic load option, the program will not generate any Load Combinations that include the EL load category.

The 2D Only option is used to indicate that only the most basic seismic load category (EL) will be used.

The X and Z option is used to indicate that the program should generate separate seismic load combinations for each horizontal direction (ELX and ELZ).

The RSA? checkbox will use the spectral seismic loads (SX, SY, SZ) instead of the regular earthquake loading (ELX, ELY, ELZ).

The X and Z w/ Eccentric option is used to indicate that the program should generate all possible seismic load combinations that include eccentricities (ELX, ELX+Z, ELX-Z, et cetera).

If you have selected the X and Z or X and Z w/ Eccentric option, you will also have the ability to include non-orthogonal seismic loads. Simply check the Include Non Ortho (100% + 30%) checkbox to include these loads per the Orthogonal Combination Procedure of section 12.5.3.1(a) of the ASCE 7-16.

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.

Check the Include ρ checkbox if you want to include the redundancy factor in your seismic load combinations. This factor is set in the Seismic tab of Model Settings.

Check the Include Ev (vertical) checkbox if you want to include the vertical seismic load effect in your seismic load combinations. This value is calculated per equation 12.14-6 of the ASCE 7-16 using the SDS value set on the Seismic tab of Model Settings.

Check the Add Notional Loads to Seismic Load Combinations checkbox if you want to include notional loads applied as lateral seismic loads per the ASCE 7 and AISC codes.

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

SX, SY, SZ

Response Spectra Results

TL

Long Term Load

HL

Hydrostatic Load

FL

Fluid Pressure Load

PL

Ponding Load Category

EPL

Earth Pressure Load

OL#

Other Load Categories

Note

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

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.

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