Comparison of Eurocodes and NEN standards
Comparison of requirements for structural calculations (Netherlands)
NEN-EN 1990 (new)
Consequence class CC3 |
high-rise h >70 m | 1.49G + Σ 1.65ψ0, i Qk, i (a)
grandstands | i ≥1
exhibition |
halls | 1.32G + 1.65Qk, 1 + Σ 1.65ψ0, i Qk, i (b)
concert halls | i >1
Consequence class CC2 |
residential buildings | 1.35G + Σ 1.5ψ0, i Qk, i (a)
office buildings | i ≥1
public buildings |
industrial buildings | 1.2G + 1.5Qk, 1 + Σ 1.5ψ0, i Qk, i (b)
(three or more | i >1
storeys) |
Consequence class CC1 |
farm building | 1.22G + Σ 1.35ψ0, i Qk, i (a)
for agriculture | i ≥1
greenhouses |
single-family house |
industrial buildings | 1.08G + 1.35Qk, 1 + Σ 1.35ψ0, i Qk, i (b)
(1 or 2 storeys) | i >1
NEN 6702 (old)
Consequence class CC3 | 1.35G (a)
| 1.2G + 1.5Qk, 1 + Σ 1.5ψ0, i Qk, i (b)
i >1
Consequence class CC2 | 1.35G (a)
| 1.2G + 1.5Qk, 1 + Σ 1.5ψ0, i Qk, i (b)
i >1
Consequence class CC1 | 1.35G (a)
| 1.2G + 1.3Qk, 1 + Σ 1.3ψ0, i Qk, i (b)
i >1
Every structure must be able to carry both load combination (a) and load combination (b).
where
G Permanent load [kN/m²]
Q Variable load [kN/m²]
a distinction is made between the maximum variable load Qk, 1 and the permanently present
value of the other variable loads ψ0, i Qk, i, where ψ0, i is the combination factor. This differs by building type, as shown in the table below, but is the same in
NEN-EN 1990 and NEN 6702 for each type of building.
Σ, Σ Sum over i and i-1 load cases respectively.
i ≥1 i >1
This allows calculation with an extreme wind load combined with the (assumed) permanently present value of the floor load.
For wind, i = 1, and for the floor load
i = 2. The value of i corresponds to the i-th load case.
ψ0, i
residential and living areas 0.4
office areas 0.5
assembly areas 0.25
shopping areas 0.4
storage areas 1.0
roofs 0
Table: combination factors by type of use
From the governing load combinations (a) and (b), the corresponding compressive stress parallel to the grain, compressive stress perpendicular to the grain, tensile stress, bending stress and shear stress can be calculated. These stresses must be lower than the design value of the relevant strengths.
Conclusion: the new standard is sometimes stricter and sometimes less strict than the old one. For example, it would be less strict for consequence class CC1 with unchanged G and Q values, where the (often governing) load combination (b) governs over (a), and where G > 0.42 {Qk, 1 + Σ ψ0, i Qk, i},
i >1
provided the strength of the structure were unchanged between the old and new standards. Under these conditions, the difference between the old and new standards is:
0.12G = 0.05 {Qk, 1 + Σ ψ0, i Qk, i}
i >1
At the renovation fair in the Jaarbeurs on 1 April, I asked many exhibitors why 1 April was important. They themselves did not know that the requirements for structural calculations in the Netherlands had changed. The above shows that, for the buildings relevant to them (consequence class CC1, with a sufficiently large permanent load G), the change makes the new standard less strict than the old one. Under the stated conditions, this ignorance would therefore not be a disaster for these buildings. A critical consumer could, however, point out that these buildings are oversized, and therefore too expensive, insofar as they are still calculated to the old standard.
The values of the permanent load G hardly differ between the two standards. For the variable load Q, the differences can be larger.
If all Q values in the new standard rose by 20% while G and the strength of the structure remained unchanged, the new standard would only be more favourable for consequence class CC1 if:
G > 1.8 {Q(new)k, 1 + Σ ψ0, i Q(new)k, i}
i >1
For wind Qw, the new standard defines zones along the Dutch coast where wind loads more than 40% higher than under the old standard must be used. Elsewhere, the differences at various heights are at most about 10%, either favourable or unfavourable. The calculation method differs in several respects. We do not go into this further here for property developers, as the results for the structure do not differ much.
For snow Qsn and the imposed loads Qop from the use of the building, the differences between the old and new standards are negligible for property developers. Imposed loads are loads from people, furniture, movable objects and vehicles.
The resistance of a structure in steel, concrete, timber or another material differs between the new and old standards. For concrete, less of the concrete’s own shear capacity may be taken into account, so more of the required shear capacity must be provided by links. For steel, too, the shear capacity of beams is considerably limited. For steel beams, however, the unchanged bending criterion usually governs the design. The new standard is also stricter for combined compression and bending in steel members. For timber, the compressive strength perpendicular to the grain drops by a factor of 1.8. The sole plate of a timber-frame wall must therefore be larger or made of a stronger timber product. New in the standard is Laminated Veneer Lumber (LVL). This is a kind of thick plywood in which weak knots no longer extend through a large thickness, making the material more than twice as strong. It is therefore suitable as a sole plate in timber-frame construction. Many traditional timber roofing battens break, which means extra work; stronger LVL battens can save a lot of that labour. LVL also allows the rafters under the roof boarding to be spaced much further apart. It can be used to make box sections that allow floors to span up to 20 m, and T-sections through whose web services can easily pass. A well-chosen mass-spring-mass build-up also comfortably meets the high acoustic requirements of the new Dutch Bouwbesluit. The material is light, quick to install and has a high natural frequency, which also meets this often governing requirement. LVL also extends the possibilities for three-pin frames, trusses and curved roofs. At the 2012 Floriade in Venlo, a doubly curved LVL structure was built. LVL is also used for 7 m cantilevers, as at the De Oostvaarders nature centre in Almere.
From 1 April, traditional carpentry joints are no longer included in the standard. Thanks to improved CNC machines, however, these joints will become attractive again, so they will probably be included in the standard again in future.
For property developers, the other differences in structural resistance are less relevant.