Why is Westok cellbeam telling me my combinations do not match EN1991?

Question

I have generated my Combinations in Tekla Structural Designer. Why, when I export my beam to Westok's Cellbeam software, does it tell me these combinations do not match the Eurocode UK combinations?

 

Answer

The Westok Cellbeam expects to see 3 combinations generated using equations 6.10(a&b) from table A1.2(B) in EN1990.

These combinations are generated to 2 decimal places, so the combinations expected would be:

ULS 1.35 Dead + 1.05 Imp + 1.35 sup
ULS 1.25 Dead + 1.5 Imp +1.25 sup
SLS 1 Dead + 1 Imp + 1 sup

 
   
In Tekla Structural Designer we give users the option to generate the combinations to either 6.10(a&b) or just 6.10. If the 6.10 option is used we would get;

ULS 1.35 Dead + 1.5 imp + 1.35 sup
SLS 1 Dead + 1 Imp + 1 sup

Combo 1 (6.10) Tekla Structural Designer

 

   

The first formula is a valid STR design combination to EN1990 but is not what the Westok combination generator would use, so a warning is flagged for this.

If we use the 6.10(a&b) option in Tekla Structural Designer, the reduction factors are applied to the combinations directly without rounding so we get;

ULS 1.35 Dead + 1.05 Imp + 1.35 sup
ULS 1.24875 Dead + 1.5 Imp + 1.24875 sup
SLS 1 Dead + 1 Imp + 1 sup

Combo 1 (6.10a&b) Tekla Structural Designer

 

Combo 2 (6.10a&b) Tekla Structural Designer

 

 

Due to the direct reduction factor being applied, Tekla Structural Designer will give the a&b combinations to 5 decimal places, though only 3 are stated in Tekla Structural Designer. These are then not recognise by Westok Cellbeam, despite being ‘more’ correct.

Both the sets of combinations generated by Tekla Structural Designer are valid combinations created using methods specified in the Eurocode documentation. Due to the factors being different to what the Westok generator will use, these are flagged as being incorrect when they are merely ‘different’.

If you export a member to Westok Cellbeam and this warning is flagged, you do not need to regenerate the combinations as the combinations are correct and valid for Eurocode design.

If you have generated the combinations to 6.10 only in Tekla Structural Designer and wish to compare the results to the Cellbeam software you should not regenerate the combinations as the factors applied would be different and so the results would not strictly be comparable.

Since the difference in 6.10(a&b) would not be as great (0.001 x dead load) this would not matter as much but for comparison, it is better to not regenerate the combinations in Cellbeam.