# Combination of Variable Loads in Structural Design
## 4 Discussion
### Codes Become Simpler and the Design Work Lessens  
When codes are simplified, it can impact reliability and the costs associated with design outcomes, either negatively or positively. A pertinent question is what harmful consequence is justified if one load case is removed from the codes. The current load combination of the Eurocodes provides an example. In most European countries, combination rule (8.12) is applied, which includes only one permanent load safety factor and omits at least one load case in the entire design domain in the ULS regarding rules (8.13a,b) and (8.14a,b). Rule (8.12) results in approximately 10% excess safety and about 5% excess material usage based on current understanding. Code writers in most European countries prefer simplicity and consider one extra load case harmful despite a 10% excess reliability.  
Implementing combination factors for variable loads results in an overall reliability gain of 2-3% but introduces many additional load cases in virtually all design tasks. The current Eurocodes are deemed complicated, and the multiple load cases induced by combination factors are a major reason for this complexity. The author estimates that removing the combination factors would decrease the overall numerical calculation work by at least 25% and reduce the overall design work by at least 15%.  
Simplification and user-friendliness are major targets in the development of the Eurocodes [4]. Considering the example above, the proposed amendment of removing combination factors is realistic.  
The proposal eliminates one abstraction--combination factor--from the codes, which is an essential step towards code simplification.  
The author previously suggested some other simplifications for the Eurocodes [25].  
* The current characteristic load for variable loads, based on a 50-year return period, leads to unequal load factors \(\gamma_{\text{G}}\
eq\gamma_{\text{Q}}\). A suitable alternative is to set the load factors the same (\(\gamma_{\text{G}}=\gamma_{\text{Q}}\)) and preferably at unity (\(\gamma_{\text{G}}=\gamma_{\text{Q}}=1\)), while keeping the characteristic load for permanent loads unchanged at the 0.5 fractile. The characteristic load for variable loads should be increased by 15-30, i.e., adjusted to a 300-400-year return period to match the characteristic load of the permanent load and account for the actual variability of the variable loads.
* Additionally, removing the material safety factor, i.e., setting \(\gamma_{\text{M}}=1\), can be achieved by changing the characteristic load, the 0.05 fractile, which is currently the same for all materials, to a much lower value specific to each material that fulfils the target reliability in the design equation.
* Using a truncated Gumbel distribution in reliability calculations can also be beneficial, as the basic Gumbel distribution results in unrealistically high safety factors for materials of low variation.
* Currently, all variable loads have the same characteristic load, i.e., the 50-year return load, which results in excess safety for loads with low variability. This can be largely avoided by setting characteristic variable loads tailored to each type of load. This amendment would not increase code complexity or design work but would require code writers to adjust the characteristic values for each variable load.
* The rounding rule for material safety factors introduces about a 2% excess error, which can be avoided by removing the material safety factors altogether.  
In the author's opinion, structural codes should include no load safety factors, \(\gamma_{\text{G}}=1\), \(\gamma_{\text{Q}}=1\), no material safety factors, \(\gamma_{\text{M}}=1\), and no combination factors, either \(\psi=1\) or \(\psi\) = 0. Such a change would necessitate modifications to the load tables for variable loads and alterations to the characteristic load values for material properties. Additionally, some design equations especially for concrete that currently use factored parameters would need to be revised to use unfactored parameters.