Define Load Combinations dialog

Used to specify a load case that combines results of analysis performed for different primary load cases.

Opens when the Combination Load Case tool is selected from the Loading Specifications group on the Loading ribbon tab.

Tip: To automatically generate multiple combinations, use the Edit Load Rules for Auto Load Combination Generator dialog.

For more information about Load Combinations, refer to TR.35 Load Combination Specification.

Setting

Description

Load No.

Specify an integer value to define a new load case combination. This number is automatically incremented.

Name

Specify a Name for the load case combination.

Type

There are three types of combinations possible:

Normal - A pure algebraic load combination in which the results of all constituent cases will be algebraically added. To accomplish this, select each case individually from the left pane, specify the value by wish we see its results factored, and move it over to the right pane by clicking on the > arrow. Do not check any of the boxes Combine Algebraically, SRSS or ABS.
SRSS - The square root of the sum of the squares method of combination.

This enables one to achieve a combination such as L1 * f1 + r1 * [√(f2*L2² + f3*L3²)]. The sequence of operations will be,
1. Select the SRSS check box. The Combine Algebraically check box will become active.
2. Select L1. In the box above Combine Algebraically, specify a Factor, (term f1 in the equation above), place a check against the Combine Algebraically check box, click on the > arrow.
3. Select L2. In the box above Combine Algebraically, specify a Factor, (term f2 in the equation above), un-check the Combine Algebraically check box, click on the > arrow.
4. Select L3. In the box above Combine Algebraically, specify a Factor, (term f3 in the equation above), keep the Combine Algebraically box un-checked, click on the > arrow.
5. In the box called Factor which appears to the right of the expression SRSS, specify the value for r1.
ABS - Algebraic combination of absolute values.

This enables one to achieve a combination such as f1*ABS(L1) + f2*ABS(L2). The sequence of operations will be,
1. Switch on the ABS check box.
2. Select L1. In the box above Combine Algebraically, specify a Factor, (term f1 in the equation above), click on the > arrow.
3. Select L2. In the box above Combine Algebraically, specify a Factor, (term f2 in the equation above), click on the > arrow.

Factor

The load factor coefficient, b, used for the square root value of the sum of the squares.

Default

Indicates the default value of the load factor, a (or c, in the case of a SRSS combination), for any load case moved from the Available Load Cases to the Load Combination Definition.

Load factors may be changed from the current default by selecting the entry in the Load Combination Definition.

SRSS Component

(Active only when the SRSS type is selected) The SRSS load combination type allows a mixture of algebraic combination with an SRSS combination. Select this option to for the load to include it as a SRSS in lieu algebraic addition.

Generate Combination

(Active only when either the SRSS or ABS type is selected) Se this option to generate 1 - 64 load combinations for each combination of degrees of freedom at member ends to create load envelopes for positive and negative analysis results for SRSS and Absolute load combinations.

Available Load Cases list

Lists all of the previously created Primary Load Cases or Load Combinations in the current input file.

Tip: Double clicking any load case or combination will add it to the Load Combination Definition list.

Load Combination Definition list

Entries here represent the Primary Load Case, load factors, and SRSS component (if any) which will make up the Load Combination.

List Operators

Click this button…	to…
>	Add the selected load case or combination to the Load Combination Definition list
>>	Add all load cases and combinations to the Load Combination Definition list.
<<	Remove all entries in the Load Combination Definition list.
<	Remove the selected entry from the Load Combination Definition list.

Example

Thus, for example, if you want to create a combination case called 101, which combines the absolute values of the results of cases 7 and 15, factored by 1.5 and 1.64 respectively, the command would be

LOAD COMBINATION ABS 101
7 1.5 15 1.64

From the Create New Load Combination dialog:

Select the ABS option.
Select load case 7 in the Available Load Case list.
Specify a Default factor of 1.5 and move it to the right pane by clicking on the > button.
Repeat this process for case 15 with a factor of 1.64.
Click Add.

The table shown below illustrates the Load Combination logic for Algebraic and SRSS types assuming that the Primary Load Cases are L1, L2, and L3.

Primary Load Case	Factor	Combine Algebraically?	SRSS?	SRSS Factor	Resulting Combination Formula
L1	0.75	Yes	No	n/a	0.75(L1) + 0.75(L2)+1.33(L3)
L2	0.73	Yes	No	n/a
L3	1.33	Yes	No	n/a
L1	1.0	No	Yes	1.2	$1.2 \sqrt{1 {(L 1)}^{2} + 0.4 {(L 2)}^{2} + 0.4 {(L 3)}^{2}}$
L2	0.4	No	Yes	1.2
L3	0.4	No	Yes	1.2