Types of linear load case combinations

SCIA Engineer offers the following types of combinations:

Envelope - ultimate	This combination defines a base for automatic generation of ultimate combinations.
Envelope - serviceability	This combination defines a base for automatic generation of serviceability combinations.
Linear - ultimate	This combination defines one particular user-specifies ultimate combination.
Linear - serviceability	This combination defines one particular user-specifies serviceability combination.
Code dependent combinations	According to the active code set for the project the program may offer a set of other combinations based on the particular technical standard.

Envelope

An envelope contains all the load cases specified by the user and combined in all possible ways according to defined Action type, Load type and Load group of individual load cases inserted into the combination. Usually, more than one linear combination can be generated from the envelope.

IMPORTANT NOTE: This type of combination was called User combination in previous releases of SCIA Engineer

What happens if this combination is exploded?

If a envelope is exploded to all possible, a set of linear combinations is generated.

Envelope ultimate

The user can enter the multiplication coefficients for individual load cases. The program generates several combinations for the inserted load cases if there are any variable load cases. All possible combinations of specified load cases are generated.

A small example will show the difference between Linear-ultimate (see below) and User-ultimate combination:

A project contains two load cases: LC1 - type permanent and LC2 - type variable.

The linear-ultimate combination with the contents LC1/ coefficient1 and LC2/ coefficient2 will give the following combination:

C1 : coefficient1 * LC1 + coefficient2 * LC2

The user-ultimate combination with contents LC1/ coefficient1 and LC2/ coefficient2 will give the following combinations :

C1 : coefficient1 * LC1 + coefficient2 * LC2

C2 : coefficient1 * LC1

(LC2 is a variable load: both situations (with this load and without this load) are considered by the program).

Ultimate combinations are used for a strength check (steel code check, reinforcement calculation).

Note: See also chapter Exploding the load case combination.

Envelope serviceability

This type of combination is similar to User-ultimate. Serviceability combinations are used for a serviceability check (deformation check).

Linear combination

A linear combination is a combination of load cases in which the user explicitly specifies which particular load cases should be included in a specific combination. The result is exactly what the user does, nothing less and nothing more.

Linear ultimate

The linear-ultimate combinations are the combinations known from other programs: the specified load cases are multiplied by the given coefficients and the total sum is then made. Nor additional combinations are generated.

Ultimate combinations are used for a strength check (steel code check, reinforcement calculation).

Amplified Sway Moment method

In case the combination Type is set to Linear – ultimate an additional check-box is displayed: Amplified Sway Moment method. By default this check-box will be de-activated.

In case the check-box is activated the following actions are taken:

1) The combination coefficients in the Contents of combination are disabled.

2) A new subgroup Amplified Sway Moment method is added in the dialogue.

Within this subgroup the following items will be shown:

α_cr = input field with the default value 3.00 and limited to values bigger or equal to 3.00 and smaller or equal to 15.00.
Amplification factor is greyed out.
Load case list where the user can select individual load cases that will be affected by the amplification factor.

The Amplification factor is calculated using the formula below:

Example:

A Linear – ultimate combination is defined with the following load cases and coefficients:

Load case	Coefficient
G –Self weight	1.35
DL – Dead load on roof	1.50
LL – Live load	1.05

If the user activates the Amplified Sway Moment method check-box, all combination coefficients become greyed out and a new Amplified Sway Moment method subgroup is shown.

The value of α_cr is by default set to 3.00 so the Amplification factor becomes (1 / (1 – (1/3))) = 1.5

In addition the new subgroup lists all load cases and by default all check-boxes are de-activated so no change is done to the combination coefficients.

After the user activates the check-box for “LL – Live load”, the content of the combination looks as follows:

Load case	Coefficient
G –Self weight	1.35
DL – Dead load on roof	1.50
LL – Live load	1.575

This setting is only available for EC-EN

Purpose of this setting:
"For the global analysis the different Eurocode parts allow the use of the so called 'Amplified Sway Moment method'.
See for example EN 1993-1-1 & EN 1999-1-1 art. 5.2.2(4) and following.
Using this method the user can indicate which relevant action effects (load cases) should be amplified. The main advantage of this method is that it accounts for 2nd order
effects during a 1st order analysis. This provides a fast and convenient way to account for these effects, especially for larger structures."

Linear serviceability

This type of combination is similar to Linear-ultimate. Serviceability combinations are used for a serviceability check (deformation check).

Code-related combination

A code-related combination is an extension of the envelope. Once again, all the load cases specified by the user are combined in all possible ways according to specified Action type, Load type, Load group of individual load cases and with respect to regulations of the particular technical standard (code).

Usually, more than one linear combination can be generated for the code-related combination.

What happens if this combination is exploded?

If a code-related combination is exploded, a set of envelopes is generated.

If a code-related combination is exploded into all possible combinations, a set of linear combinations is generated.