User Area > Advice
How smart combinations are calculated
The smart combination facility allows a user to select a number of loadcases from the available results, and associate two load factors with each: "permanent" and "variable" factors.
If "Loadcases to consider" or "Variable loadcases" are checked, a text box is made available where the user may enter a number. Only numbers less than the total number of loadcases selected for use in the combination are valid.
The principles underlying the calculations can
be described in terms of the following 4 cases:
-
Smart combination (Max) will assemble results from the loadcases selected using just the permanent factors given for negative load effects and using permanent + variable factors for positive load effects.
More on this calculation
-
Smart combination (Min) will assemble results from the loadcases selected using just the permanent factors given for positive load effects and using permanent + variable factors for negative load effects.
More on this calculation
-
If "Loadcases to consider" is checked and an appropriate number entered, the smart combinations will be
assembled in the manner described above, but the number of load effects summed is restricted to the number of loadcases specified by the user. The loadcases used are the most adverse viz. most positive for max combination, most negative for min combination. All other load effects assembled are discarded.
More on this calculation
-
If "Variable loadcases" is checked and an appropriate number entered, the smart combinations will be assembled in the manner described above, but two further processes are invoked. Firstly, the max combination will include only positive load effects, all negative load effects are discarded. Likewise, the min combination will include only negative load effects. Secondly, the maximum number of variable load factors used will be the number specified by the user.
More on this calculation
The same processes are repeated for each location in the structure independently. Users should note that "adverse" loadcases may be different for adjacent locations, and will normally be different for Max and Min effects.
The processes specific to the four cases are described more fully
below:
A. Smart combination (Max), results are assembled at each location in the structure:
-
For loadcases that produce nominal load effects of positive sign at a location, the permanent load factor is added to the variable load factor. For loadcases that produce load effects of negative sign ("relieving effects"), the variable factor is discarded.
-
The "nominal" load effect from each loadcase is multiplied by the associated factor calculated in A1.
-
These factored load effects are summed for the location under consideration to give a total factored load effect (Max) at that location.
B. Smart combination (Min), results are assembled in a similar manner:
-
For loadcases that produce nominal load effects of negative sign at a location, the permanent load factor is added to the variable load factor. For loadcases that produce load effects of positive sign ("relieving effects"), the variable factor is discarded.
-
The "nominal" load effect from each loadcase is multiplied by the associated factor calculated in B1.
-
These factored load effects are summed for the location under consideration to give a total factored load effect (Min) at that location.
C. When a combination of loadcases is generated and "Loadcases to consider" is activated, with 'p' entered into the text box, results are assembled at each location such that:
-
For loadcases that produce nominal load effects of positive sign at a location, the permanent load factor is added to the variable load factor. For loadcases that produce load effects of negative sign ("relieving effects"), the variable factor is discarded.
-
The "nominal" load effect from each loadcase is multiplied by the associated factor calculated in C1.
-
The resulting factored load effects are compared and the most positive 'p' load effects selected from all the loadcases available. Note that "most positive" may include some negative load effects if there are less that 'p' positive effects available.
-
The 'p' load effects selected are summed to give the total load effect (Max) at that location. All other load effects are discarded.
For the "Min" combination, the least positive 'p' load effects are selected in C3 above.
D. When a combination of loadcases is generated and "Loadcases to consider" is activated with 'q' entered into the text box, and "Variable loadcases" is activated with 'r' entered into the text box, results are assembled at each location such that:
-
All loadcases that produce load effects of negative sign are discarded.
-
For the remaining loadcases, the permanent load factor is added to the variable load factor.
-
The "nominal" load effect from each loadcase is multiplied by the associated factor calculated in
D2.
-
The resulting factored load effects are compared and the most positive 'r' load effects selected from the loadcases available.
(Note that "most positive" will not include any negative load
effects).
-
Those loadcases selected in D4 are also
discarded (for the time being).
-
For the remaining loadcases, the "nominal" load effect from each loadcase is multiplied by the permanent factor.
-
The resulting factored load effects are compared and the most positive
(q-r) load effects selected from
those available.
-
The factored load effects selected in D4
and D7 are summed to give the total load effect (Max) at that location.
For the "Min" combination, the positive load effects are discarded in D1. The least positive effects are selected in
D4 and D7 above.
If "Loadcases to consider" is not activated, all loadcases included in the combination are considered by default.
|
