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RE: component and cladding vs MWFRS : A curious dependence of ASCE7-05 on AS1170.2 ?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: component and cladding vs MWFRS : A curious dependence of ASCE7-05 on AS1170.2 ?
- From: "Conrad Harrison" <sch.tectonic(--nospam--at)bigpond.com>
- Date: Tue, 6 Jan 2009 16:20:03 +1030
Here's a curiosity: ASCE7-05 commentary indicates that the pressure coefficients for open buildings were taken from AS1170.2 free roof pressure coefficients. Free roof has zero walls and exposed frame and roof cladding subject to frictional drag coefficients. Suggests didn't do new empirical research. AS1170.2 doesn't have MWFRS and C&C pressures. Instead we have local pressure factors kl, which magnify pressures on small areas. For an area 'a x a' typical value is kl=1.5, and for area 'a/2 x a/2' typical value is kl=2. These areas are located in turbulent zones, outside defined turbulent zones kl=1. These local pressure factors only apply for assessment of cladding and immediate supporting structure. For wind direction theta=0 (transverse) the turbulent zones are the sharp edges at wall corners, anywhere on windward wall, and sharp edges at eaves and ridge. For direction theta=90 (longitudinal) the turbulent zones are sharp edges at wall corners, anywhere on windward wall, and sharp edge at roof ends (verge of gable end). Since wind can change direction: for a symmetrical building shape, theta=0 gives similar distribution to theta=180, and theta=90 gives similar distribution for theta=270. We only get the one set of pressure coefficients: say the equivalent of ASCE7-05 Fig6-6, and Figs6-18A to Fig6-18D. To AS1170.2 the designer has to work out which direction is the most critical case for application of the local pressure factor kl, to the coefficients ASCE7-05 classes as MWFRS. As such ASCE7-05 Figs6-19A to 6-19C are some what misleading. Assuming they have been derived applying kl to the MFWRS coefficients adopted from AS1170.2. (eg. no research to determine otherwise.) To AS1170.2 for enclosed buildings kl is applied to external pressure coefficients only, for free roofs kl is applied to net pressure coefficient. Partially enclosed buildings are enclosed buildings with dominant openings: getting Cpi is more complicated than ASCE7-05: it depends on the external pressure coefficient on the surface where the opening is located and that in turn depends on the relative direction of the wind. AS1170.2 does not have a gust factor for static analysis. We have ka and kc. The area reduction factor ka accounts for the fact that those so called MWFRS pressures are extreme values and do not occur simultaneously over the entire surface area: so for large areas ka provides reduction. The authors of the timber framing code (AS1684.1) introduced kc, and it was adopted into the wind loading code in 2002. For light timber framed construction the area reduction factor is typically ka=1 : no reduction allowed. What they argued was that the pressure coefficients are extremes and they do not peak on all surfaces of the building at the same time, therefore for certain loading distributions kc<1 and provides further reduction. But ka.kc not permitted to be less than 0.8. Roughly if ka applies then kc doesn't, and vice versa. The point is the MWFRS pressure coefficients in ASCE7-05 are in the main equal to those in AS1170.2. With ASCE7-05 gust factor having default value of 0.85, whilst AS1170.2 has area reduction factor with maximum reduction 0.8. To AS1170.2 these so called MWFRS pressures are considered conservative for design of all structural elements except for: cladding and its immediate supporting structure (eg. girts/purlins/battens). Unconservative is adoption of an internal pressure coefficient of GCpi= +/- 0.18, when have no guarantee doors and windows will be closed. Since the extreme value for local pressure factor is kl=2, the typical solution for roof purlin design is to half the spacing of purlins at the ridge, and at the eaves, and in the gable end bays. For small buildings, it is not worth the effort setting out different spacings, and equal spacing is adopted throughout based on the extreme pressure zone. We do not however declare this to be the prescription for all buildings. The misleading part of the Figs for C&C pressures for open buildings in ASCE7-05 is they imply a distribution of wind loading on the building: that is pressures occurring simultaneously over the surface of the building. This is not what AS1170.2 says. The extreme pressures are randomly distributed in time. An extreme spike in pressure may occur at the ridge, but not necessarily at the same time on both sides of the ridge. A spike may occur at ridge and eaves at the same time, but low probability of doing so. And the extremes do not occur along the full length of the purlin in the ridge zone, only at certain points. Further more the extremes at eaves and ridge are for transverse wind flow, whilst spikes in pressure at the gable end roof edge occurs for longitudinal wind flow: thus not at the same time. How do you apply those C&C pressures from ASCE7-05? AS1170.2 approach is that the fabric of the building should be resistant to the direct action of peaks in wind action. That is high pressures on small areas which can lead to a breach of the building fabric and generate dominant openings and high internal pressures. The local pressure factor kl is there to provide added resistance to the building fabric. For example do not want to loose the ridge cap, and then have the wind working away at the free edge of the roof cladding, and tearing a dominant opening in the roof, creating an internal suction of say Cpi=-0.9, to be combined with windward wall Cpe=+0.8. We don't want small concrete roof tiles torn free which then put a hole through a window, generating Cpi=+0.8. We want to maintain the integrity of the building fabric, so that assumptions about the primary structural elements remains valid. So when the older engineers have an intuitive feel that the traditional loading of 15psf was adequate for most buildings, they are potentially correct. And when the manufactured timber truss manufacturers oppose applying ASCE7-05 C&C pressures over entire truss span they also potentially correct. >From a perspective of AS1170.2. I would say for ASCE7-05 that gust factors needs to be G=1 to be conservative for static analysis of small buildings, not G=0.85. Secondly the ASCE7-05 commentary advises that internal pressures may be higher than those put in the code: so a more rigorous assessment of GCpi and adopting values greater than GCpi=0.18, would improve tie-down requirements and strengthen/stiffen cladding and battens. Thirdly if the C&C pressures are applied as spikes in loading in the zones considered but not along entire length then the influence on over all structure is diminished. For example a purlin in the ridge zone does not experience the pressure from zone 1 along its entire length at the same time, nor those peaks from zone 2 and zone 3. Therefore consider pattern loading of the purlin, with peaks on individual spans, the rest loaded with the so called MWFRS pressures. If only considering as single span then the peaks control, but if consider as continuous span, then can gain some benefit on long structures: note the longitudinal distribution of loading Fig6-6. Similarly, if must apply to the truss, the peaks most certainly do not appear on both roof planes at the same time. So once again consider pattern loading combinations in the peak zones: with MWFRS elsewhere. Conservative prescriptions are good, but not surprising housing industry has some opposition if C&C pressures are being used across the board. Also when I refer to high wind load tie-down, I typically mean a requirement for tie-down rods from the roof to the footing: or more substantial structure. In Australia we don't have any framing brackets as substantial as those by Simpson strong tie: and if we did I believe carpenters would oppose installing all the fasteners required. In any case steel straps for rafters, and wall studs is a typical wind loading requirement: unless have heavy concrete roof tiles in wind class N1 (qzu=0.69kPa), which permits traditional skew/toe nailing. From which perspective the toe-nailing of rafters, for light timber framing, in IBC:2003 for Vb=100mph seems highly unconservative. Further to which the reference pressure qz, would be a better constraint than regional velocity. There is more than one way to get a given value of qz: vary Vb, or kz, or kzt. Building designed for given qz should have same resistance irrespective of how qz arrived at. As for minimum load, AS1170.2 has a minimum design velocity Vzu=30 m/s, which would give qzu=0.54kPa, that is for ultimate strength limit state. Wind class (AS4055) N1 has Vzu=34m/s (qzu=0.69kPa), this wind class does not affect size of members for light timber framing, where wind class N1 and N2 (qzu=0.96kPa) span tables are combined. N1 and N2 only affect lateral bracing and tie-down. Wind class N3 (qzu=1.5kPa) affects member sizes. Basically I am saying there appears to be a misunderstanding in the interpretation of the ASCE7-05 C&C pressure coefficient diagrams. Plus the cause of failures seem to be unwarrantedly associated with misuse of MWFRS and C&C pressures, rather than the most likely cause of failure being under estimating the value of GCpi: which affects the result of both MWFRS and C&C pressure states. Regards Conrad Harrison B.Tech (mfg & mech), MIIE, gradTIEAust mailto:sch.tectonic(--nospam--at)bigpond.com Adelaide South Australia ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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