Eurocode design factors and limits

National Annex to BS EN 1990 – Draft for Public Comment. Charles King reports.

BSI has published the draft National Annex to BS EN 1990, Eurocode – Basis of Structural Design. BS EN 1990 gives information required for design in all materials covered by the Eurocodes. The National Annex

• defines which load combinations should be used for Ultimate Limit State and Serviceability Limit State
• gives values for partial factors for loads
• gives values for load combination factors ψ (“psi”)
• gives “guidance” for deflections

Steel designers may find the biggest surprises in Table NA1, “Examples of limiting values of vertical deflections”, and in Table NA2, “Examples of limiting values of vertical and horizontal deflections”. The National Annex states that “the serviceability criteria should be specified for each project and agreed with the client. Tables NA1 and NA2 are given below for guidance.” Although only given for guidance, such values tend to become the limits used in almost all specifications.

Vertical deflections

Vertical deflection limits are given in Table NA1, Examples of limiting values of vertical deflections. The values from a few categories in Table NA1 are given in Table 1. The values in this table are for the total deflection from dead plus live loads (“permanent plus variable loads” in Eurocode terminology). They are clearly more onerous than the “suggested limits” given in Table 8 of BS 5950-1: 2000 which are calculated for imposed load only, three of which are given in Table 2.

Vertical deflections in longer span floors

For longer span structures, the proposed Table NA1 might cause less concern because the dynamic response of a floor is often the governing design criterion. Although minimum natural frequency alone cannot guarantee acceptable dynamic response, it has been common practice to design composite floors to a minimum natural frequency of 4Hz. Natural frequency is often calculated using SCI P-076, Design Guide on the Vibration of Floors, which recommends that the loading considered for calculation of floor frequency is (Dead Load + 10% Imposed Load). In many floors, (Dead Load + 10% Imposed Load) is approximately equal to 0.5 (Dead Load + Imposed Load).

If an individual beam with a span of 10m were designed for a deflection of L/300 under (Dead Load + Imposed Load), then the deflection under (Dead Load + 10% Imposed Load) would be approximately 0.5 (10,000/300) = 16.7mm. Therefore, the natural frequency, f, of the beam is f = 18/√δ = 18/√16.7 = 4.4Hz. If the beam were part of an internal floor bay of 10m x 10m with both primary and secondary beams, the mode shape would be similar to Mode B of Design Example 2 in SCI P- 076. Allowing for the combined affects of the flexibility of the primary beams, secondary beams and the slab, the resulting frequency of the entire floor would be 4.0Hz, which is the design value commonly used in practice for the design of a composite floor. Therefore, a limit of L/300 would not have affected the design of this floor. For a bay in which Mode B could not be assumed, the span at which the limit of L/300 affects the design would be even less.

Horizontal deflections

Horizontal deflection limits are given in Table NA2, Examples of limiting values of horizontal deflections. This contains no explicit exclusion of portal frames as in Table 8 of BS 5950-1: 2000. However, the condition of “no cracking in bearing walls without reinforcement” and “no cracking in partitions” are not relevant to normal portal construction or detailing.

Conclusions

The differences between the National Annex to BS EN 1990 and BS 5950-1 are a cause for concern because they could result in increased cost of structures without good reason. Therefore, the SCI encourages member companies to study the Draft for Public Comment and respond to BSI. (The SCI would also be glad to receive feedback either positive or negative feedback from members on this topic.)

The document: Draft BS EN 1990 National Annex to BS EN 1990: Basis of Structural design. DPC: 03/700353 DC

It is obtainable from: BSI, 389 Chiswick High Road, London W4 4AL Telephone +44(0)20 8996 9000 Fax +44(0)20 8996 7001. The latest date for comments is 31 August 2003. The responsible committee secretary is given as Mr M.K. Greenley. The document contains guidance on the acceptable format for the submission of comments on the second page. Charles King, is Senior Manager – Standards, at The Steel Construction Institute.