Calculation of the load combinations for the bearing capacity, at persistent or variable situation and accidental and seismic situations, according to CTE DB SE, Art. 4 and EHE-08, Art. 13

• For the correct structure calculations, the CTE code proposes increments for each action with different factors related to the kind of action, and combination of the different actions facored with combination factors according to the kind of action and the verification considered.
  
  
• It is not easy to control at calculation prgrams if the automatic code combinations, and the "default" factors are correctly generated following the CTE. With this utility it is possible to generate the required combination for each structure from the actual loads present, and to check its generation.
  
  
• At structures with many kinds of loads a great amount of load combinations is generated, so it is reccomended to select the exact actions present at the structure.
  
  

• Its common at calculation models to define all the permanent acctions in the same load case. This simplify cannot be accurate at all cases and generate unfavorable situations respect to the combinations generated with this program. For example, at buildings with the facade at the small cantilever's extreme, this load is favorable for the beams or slabs and is being considered as unfavorable at usual cases.
  On the other side, at EHE-08, Art. 12.1, it says "...For the permanent actions, the consideration of favorable or unfavorable effect is obtained factoring all the actions of the same origin with the same factor, showed at table 12.1.a", and at CTE DB SE 4.2.2, ap. c it says "The security factors values ... are shown at table 4.1 for each kind of action, considering for resisance checkings if it effect is unfaorable or favorable, globally considered."; Those confusing paragraphs could mean that all the gravitatory loads have to be considered with syncronized effect. In author's opinion, the regulation is not clear enough in how is required to distribute the permanent loads, with the corresponding increment in the number of combinations; and in other side, the traditional calculations with the permanent actions grouped does not seem to generate mistaken or unsure results, so the groupig has to be left to the designer critera.
  
  
• In most of the calculation programs, the combinations are the same for all the structure members. This makes calculations mistaken or unoptimized at structures with steel and concrete members, because the combinations obtained with the CTE and with the EHE-08 are different. At mixed structures with programs that can't associate the combinations with the materials, it is required to make two calculations with two combination sets,one for concrete (EHE combinations) and other for steel (CTE combinatios). The points that make different combinations at each regulation are:

  	CTE	EHE
  Self weight favorable effect γQ,fab:	0.8 (4.2.3)	1 (12.1)
  Soil push favorable effect γQ,fab:	0.7 (4.2.3)	1 (12.1)
  Water pressure favorable effect γQ,fab:	0.9 (4.2.3)	1 (12.1)
  Water pressure unfavorable effect γQ,des:	1.2 (4.2.3)	1.35 (12.1)
  Dead load favorabe effect in accidental situation γQ,fab,acc:	0 (4.2.2)	1 (12.1)
  Prestress favorable effect in accidental situation γP,fab,acc:	0 (4.2.2)	1 (12.1)

  Different fators for stability checkings for dead loads at CTE.
  
  
• Most of the calculation programs let introduce user defined combinations, so with this program, will be possible to copy and paste the combinaion into a spreadsheet for it introduction into a calculation program. Keep in mind that some calculation programs let define combinations but not to add new load cases, so it is possible that all the regulation combinations couldn't be introduced correctly.
  
  
• The author didn't find any commercial program generting and applying accurately the combinations following the regulation. Usually at combination list of persistent or variable situation, the wind and the snow appears only in a few combinations, while according to the CTE or the EHE-08, the wind and the snow will have to appear the same times as any other variable load, as the formulation does not specify the kind of variaable loads..
  
  

Version 29/05/2013