COMBINATIONS:
Purpose
The COMBINATIONS section of each template is used to explicitly define the load combinations. The load factors associated with each load case in each load combination are specified.
Format
COMBINATIONS: Flag Combination1 Flag Combination2 Flag Combination3 Flag Combination4 . . . Flag Combination n END:
The Combinations section begins with the keyword COMBINATIONS: .
Each subsequent line consists of a flag and a mathematical expression describing a single combination, as described below. Each combination must be listed.
The Combinations section ends with END: .
Flag is either T: or blank. It indicates whether the combination is to be included when the Tension-only option is invoked in the analysis performed by RAM Frame.
Combination1 , Combination2 , etc, are mathematical expressions consisting of Factors, Load Case Symbols, Operators and Variables that define a single combination.
Factors can be any number, positive or negative. Generally there should be one combination with a positive factor on the Wind and Seismic load cases and another with a negative factor, to account for the reversal of those load cases. If the factor is 1.0, it can be omitted.
Variables can also be used, either instead of or in conjunction with Factors. The variables and their definition were described previously in the CONTROLS section. Variables should only be used if the corresponding Control has been activated and defined. Acceptable variable names are V10 through V19 (from Control 1), V20 through V25 (from Control 2), V30 through V39 (from Control 3), V40 (from Control 4) and V50 through V55 (from Control 5). No other variable names will be recognized. The program will automatically assign to the variable the value entered by the engineer in the dialog box.
The Load Case Symbols are given in Table 1 in Chapter 2.1. They are used to specify the general Load Case type. In RAM Frame, there may be multiple Wind, Earthquake, Dynamic or Other load cases; each of them are of the general Load Case type W, E, Dyn, and O, respectively. Any Load Combinations containing those load cases need only be generically defined once; the program will automatically generate actual load combinations for each load case of that type.
Operators can be + , - , * (multiply), and / (divide). Parentheses can also be used. Operators must be used between Factors, Variables and Load Case Symbols.
An example of a Combinations section is given in the following:
COMBINATIONS: T: 1.0 * D T: 1.0 * D + 0.75 * Lp + 0.75 * Rfp T: 1.0 * D + 0.75 * Ln + 0.75 * Rfn T: 1.0 * D + 0.75 * Lp + 0.75 * Sp T: 1.0 * D + 0.75 * Ln + 0.75 * Sn . . . T: (0.75 + 0.375 * V10) * D + 0.75 * E (0.75 + 0.375 * V10) * D - 0.75 * E T: (0.75 + 0.375 * V10) * D + 0.75 * E (0.75 + 0.375 * V10) * D - 0.75 * E END:
An example of how a Control works with the Combinations section is the SBC '99 requirements for concrete design combinations, which call for Av to modify the Dead Load in certain combinations. To accomplish this, Control 1 of the template is used to assign a value to a variable for Av. It looks like this:
CONTROL1: T V10, Av, 0.05, 2.0, 0.1
Av is assigned to variable V10
This control will appear in the Load Combination dialog box, providing a means for the Engineer to specify a value of Av. In the combination template, V10 is used to modify Dead Load.
(1.1 + 0.5*V10)*D + 0.5*Lp + 1.0*E
When the combinations are generated, the value that the Engineer enters for Av will be substituted for V10 to calculate the correct factor for Dead Load.