Calculation of the live load, including load increment, reduction by simultaneous and combination factors, according to CTE (Spanish regulation).

• The correct definition of the actions and their coefficients is essential for the correct structures dimensioning and to obtain reliable results in verifications and appraisals of its elements.
  
  
• Usually the structures are oversized by an unaccurate definition of the actions and their coefficients. This usually happens because:
  - Unknowing of all actions that affect the structure because the architecture and facilities usually advance simultaneously (not prior) to the structure. In this situation, the designer usually considers conservative actions and factors to assume possible (and frequent) changes in architecture and facilities.
  - Lack of knowledge by the designer of the factors to be considered for the actions (and tendency to use the default options of calculation programs).
  - Difficulty that exists in calculation programs to properly consider all the required coefficients.
  - Prompt in the calculations leading to the designer to make small oversizes in turn to results in reducted time.
  
  
• Moreover, this oversize, which results in an additional cost, in some cases it is desirable to adjust, especially if you are confident that no changes will occur in the project (really utopian situation), or if it is an element of expert opinion, or verify that a calculation made works in a different situation than initially considered.
  
  
• As always that it is calculated with CTE, the scope is for building structures, being out of reach "... actions and forces acting on elements such as cranes or lifting devices, or structures such as silos or tanks".
  
  

• Usually the loads are introduced into the calculation programs so that the number of actions (or hypothesis or load cases, depending on the program) are minimized. This will facilitate data entry and checking in calculation programs reducing the number of combinations (which is already quite large), but the calculation ontained is oversized. In particular, it is common to calculate with all live loads on the same load case, but according to the CTE DB SE-A 3.1.1, p. 8, "in relation with the effects of actions combinations, the live loads of each type of use will be considered as different actions. The items within each subcategory in table 3.1 are different types". That means that in the combination in which the use is not reduced by the combination factor, is considered complete, while should only be considered complete each subcategory of use, and the rest should be affected by the combination factors. An example: let's suppose we have home use (H) and garage (G). If both are in the same hypothesis, both are considered with no reduction (H + G)* γ, while the regulation lets consider the combination factor: (H * γ) + (G * γ * ψ0), obtaining lower loads. (This can be disadvantageous on columns and foundations with very high moments, which also is not taken into account in the usual calculation and it can be unfavourable).
  
  
• In order to consider the live loads reduction in calculation programs, it is required to make several models of computation: one for the calculation of the slabs, beams, other horizontal members (where the tributary surface reductions will be considered), and the rest for the columns, walls and (in the author's opinion) foundation. In the calculation models for the vertical elements shall be applied the tributary surfce reduction only if feasible (less than 3 plants, or plants for the same use corresponding to different users), and will be required une model for each change of live load factor. Simplified, a model could be made without reductions (for higher plants) and another with the coefficients that reach the foundation (for the lower plants). The intermediate elements of reduction coefficient of 0.9 would not be considered. Always should be required to define the subcategories of use in different hypothesis. The generation of several calculation models could be avoided in programs that allow to assgn different sets of combinations to different elements, although usually the combinations are the same for all the elements of the structure.
  
  
• In the author's opinion, is a mistake to believe that a less accurate calculation provides an additional safety factor, always beneficial. The designer has the obligation to always perform a calculation with total reliability and security, which is achieved by applying strictly the regulation, but is also required to preserve the interests of the client making the lowest cost structures. Only a less accurate calculation would be justified if you have a real forecast of future modifications or changes of use, or if the deadlines are really decisive.
  
  

Version 05/03/2013