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RE: C & C pressures, trusses

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I wasn’t suggesting not to use C&C pressures for truss design. But advising that whilst C&C pressures are conservative in magnitude they are not the correct distribution of forces on the building surfaces: and where the distribution of forces is important the C&C pressures may give unconservative results. Further that the C&C pressures are highly localized and intermittent: and have more influence near the surface of the building, than within the body of the primary structure.


So I would agree that the eaves over hang and hip corners, which cause turbulence and which would be fully encompassed by an (a x a) high pressure zone, would have member size determined by C&C pressures. But the preliminary sizing of the truss elements and internal connections should come from MWFRS pressures and then the local influence of C&C pressures should be taken into consideration. The C&C pressures are really about detailing, all those problems areas that you mentioned. The truss manufacturer plugging C&C pressures into their software instead of MWFRS pressures isn’t really doing that detailing. In the hip corner the C&C pressures may push up the size of top and bottom chords of hip trusses to improve connection requirements for small jack trusses and rafters. The C&C pressures may also increase tie-down of truss at wall due to small cantilever eaves overhang. And the member sizes of the elements in small jack trusses may also be increased by localized pressure effects. Then the practicalities and economics of fabrication come into consideration and all elements may increase in size: and the available resistance re-checked for compliance. All very time consuming.


So my post was really a cautionary note to make sure are erring on the save side, when C&C is only conservative regarding magnitude. Here not so much of a problem, because we apply local pressure magnification factors to the MWFRS pressures, so we maintain the windward and leeward distribution of wind loading. Here however many object to using k[l]=2, but that’s because they double the load on the entire purlin or other element, (a/2 x a/2) seldom loads the full length of element, and for large elements has minor influence. ASCE7-05 however appears to have averaged pressures for full element.


I recollect last year some debate about tie-down straps for trusses versus skew nailing and industries resistance. Here the nominal connection for trusses is steel straps or framing brackets, skew nailing is not acceptable. With our simplified wind classification system (AS4055) and timber framing code (AS1684), builders and carpenters can work out for themselves. So there is less argument about tie-down and bracing requirements. Also trusses can be ordered for say wind class N2, which in many cases would be conservative, because the actual design wind speed for the site would be much lower, but too high to make site class N1.

More conservative design is better obtained by boosting the reference or velecity pressure qz (ASCE7 clause 6.5.10), and otherwise playing around with extreme value pressure coefficients, wind directions, location of openings, and seeking worst case scenarios.

It wouldn’t make much difference to internal pressure coefficients using ASCE7. But to AS1170.2 architects placing windows and roller doors, further away from the windward edge of side walls can reduce the expected internal pressure coefficient. For example the common 600mm (say 2ft) from the corner, places the opening within 1h of the windward edge with Cp=-0.65, if pushed beyond 1h then can get Cp=-0.5. Possibly not practical for a house, but for an industrial shed, getting roller doors clear of the end could push internal pressure coefficent down to say Cp=-0.3 for longitudinal. For transverse still get Cp=+0.7. Doing so can change the critical load case.

From that perspective ASCE7 Figure 6-5 potentially underestimates the internal pressures resulting from an opening in a windward wall, or sidewall, or even a roof level window. Refer to MWFRS pressure coefficients for each of the surfaces: an opening in these surfaces potentially generates that as an internal pressure. For example an open dormer window could produce a internal pressure coefficient of +0.8, rather than +0.55.

So just be cautious that you are actually erring on the conservative side.



Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust



South Australia