AS1170.2 Combination Factor for Roof

[sdz]

5.4.3 Action combination factor (Kc)
Where wind pressures acting on a combination of surfaces of an enclosed building...
A surface shall be ..., a roof (the upwind and downwind roof shall be treated together as a single surface),...

First Kc only applies to enclosed buildings (roof and full perimeter walls (nominally sealed) from floor to roof level.) which excludes pitched free roofs.

From this clause I would say that rafters on one side of the roof would not have Kc applied, but that a roof truss would.
From this it would appear that if you cut the building at the ridge line to get a monoslope roof then Kc would not apply to that either, even though we now have the full roof. The labelling in Figure 5.2 would seem to support this interpretation.

What about a roof pitch of less than 10 degrees, where the ridge has no effect, and you don't have upwind and downwind slopes? Should Kc be applied? Again it seems to me that it should not.

 

[rowingengineer]

Depend on whether you have internal pressure above 0.2, but assuming less than 0.2, I would say Kc = 1.00 for all the cases including a truss Table 5.5 (e) supports my interpretation.

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[sdz]

@rowingengineer
Table 5.5(f) shows Kc,e=Kc,i=0.9 for 2 effective surfaces, Roof pressures in combination with internal pressures.

 

[rowingengineer]

Within an internal pressure coefficient of greater than +0.2, if the internal pressure coeff is less than 0.2 refer to (e) Hence we need to know if you talking about with or without internal pressure coefficients greater than +0.2.

Page 35 1'st paragraph last sentence explains this as to what can be considered an effective surface.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."