Technical: Wind Action part 2

1010NSCOct2010

Technical Blowing - our way The second part of a review of the impact of Eurocode wind action on the UK, in comparison with BS6399-2, by Alastair Hughes of the SCI. Part One appeared in the September issue Part Two: Interaction of wind and building factor. The overall wind force calculated this way (q c × shadow area) will p f generally be lower than that from summation of pressures. A more slender Introduction building must, and a less slender one may, use force coefficients from Code EN1991-1-4’s velocity pressure q , based on its new wind map and the factors clause 7.6 (not forgetting that the NA substitutes Table 7.16). p discussed in Part One, is practically identical to dynamic pressure q of There is no mention in EN1991-1-4 of the ‘enclosing rectangle’ concept BS6399-2. Though calculated for a height, and perhaps for directional sectors, for irregular shaped buildings, but it is as valid as ever and short of a wind to suit the building, it is an attribute of the incident wind. Effects on the tunnel there is no practical alternative. Not all architects are content to building will also depend on its own size, shape and dynamic character. As in design shoeboxes to suit the Code! For circular and polygonal shapes force BS6399-2, factors apply to qp to account for size and dynamism, so new qp cs cd coefficients are available. substitutes directly for old q C (1 + C ). Overall forces are calculated either s a r directly using force coefficients or indirectly (as in BS6399-2) using pressure External pressure coefficients coefficients together with a lack-of-correlation factor. Pressure coefficients, in principle the same as before, are surprisingly Separate size and dynamic factors are used in the UK. Combining them into controversial. At the eleventh hour (April 2010), the EN has been amended to a ‘structural’ factor c c , though not ruled out, is discouraged. Dynamic factors allow national determination of roof pressure coefficients. Consequently the s d apply to overall effects on the building; surface pressures for (e.g.) roof uplift or revised NA will replace Recommended Values with coefficients almost (not cladding design would be calculated without them. However size (of the area quite) identical to those of BS6399-2. Designers are rescued from legal limbo whose wind contributes to the effect being designed against) is always an (an ‘advisory note’ at the back of the original NA recommends continued influence. adherence to BS6399-2), but disharmony remains. Lack-of-correlation factor now varies with the slenderness of the building. NDP status allows the NA to set 1 m2 values for roof pressure coefficients The traditional 0.85 only applies up to h/d = 1. Unhelpfully, upwards of h/d = 5 equal to 10 m2 values, an indirect abolition. The slate could have been wiped the factor is 1 (linear interpolation intermediately). These values, given in a clean by doing likewise for walls, but it seems that Table 7.1’s values for 1 m2 are NOTE, do not have normative status (nor a symbol, though c could serve) but to remain. loc are endorsed by the UKNA. Zones A, B, C into which side faces are subdivided are not to be confused with the similarly labelled zones for size factor in Table NA.3. The PD includes Size effect advice for zoning of some building shapes not in the Code. Table NA.3 gives the size factor, varying with size (expressed not as a diagonal Calculation of lateral force by vectorial summation of surface pressures will dimension but as b + h for the building or portion in question) and also with still be called for where roofs are not flat (unless a force coefficient is applied height (above displacement level) and zone. There are three zones A, B or C, as to an enclosing cuboid). The NA clarifies that the lack-of-correlation factor may in BS6399-2, but now assigned in Figures NA.7/8. be applied to all of the lateral force, not just the component from the walls. The base Code has a separate localized allowance for size effect, wherever Another candidate for the pressure summation approach is a building different pressure coefficients are presented for loaded areas of 10 and 1 m2 subdivided by movement joints between front and rear faces, with respectively. The higher 1 m2 values are only relevant to cladding design, if apportionment of wind force dependent on internal pressure. that, because the NA declares the 10 m2 values valid for any loaded area in ‘Division by parts’ is permitted, but for windward face only, which limits its excess of 1 m2. One of the reasons given for this bold simplification is that the appeal. UK cladding and glazing industry preferred it, which seems refreshing. Internal pressure Dynamic effect For virtually all buildings, computation of internal pressure is necessary for The NA includes graphs for dynamic factor, depending on height, aspect cladding design. For some, it will also influence structural design. The new ratio and assumed damping. Table F.2 of the Code informs us that damping regime has little new to say on the subject; its declaration that the internal values, expressed as logarithmic decrement, may be approximated as 0.05 pressure coefficient can be taken as the more onerous of +0.2 and –0.3 (in the for steel buildings and 0.08 for ‘mixed structures concrete + steel’, which absence of a dominant opening) echoes that of BRS Digest 119 in 1970, and could be interpreted to mean either composite construction or steel frame designers of portal frames will look in vain for the relief offered by BS6399-2. with concrete core. Unfortunately no graph is provided for 0.08. Damping values are, in a sense, anybody’s guess, but Table F.2’s recommendations look Funnelling hasn’t gone away distinctly cautious compared to traditional assumptions. Locally increased side face suctions where wind is funnelled between adjacent The NA’s treatment is presented as a simplification. The Code’s more buildings receive scant attention in the Code, but the NA corrects this elaborate one, 6.3.1 in conjunction with Annex B, is not overruled and ‘should deficiency with notes under (but rather unrelated to) Table NA.4. be more accurate’. However its Annex C (in lieu of Annex B) and Annex D (bundling together size and dynamic factor) are rejected by the NA. Comparison of design wind forces A taxpayer-funded calibration study commissioned by CLG is freely Force coefficients downloadable at http://www.communities.gov.uk/publications/ Force coefficients experience a resurgence. The ‘net pressure’ coefficients planningandbuilding/calibrationeurocodewind given in Table NA.4 (for rectangular buildings up to 5d in height) can readily Conclusions are that the new regime will generate slightly lower design be transformed into force coefficients by application of the lack-of-correlation wind forces than BS6399-2 for low rise buildings, but that high buildings 32 October 10NSC


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