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Chapter 30 C&C Loads

  • 1.  Chapter 30 C&C Loads

    Posted 03-26-2023 05:50 PM

    New to the party here. My first post as a returning member of ASCE. If I violate forum protocols, it's purely due to my ignorance and not intentional.

    I've been wrestling with C&C wind loads for a bit and I'm hoping for some clarification.

    My first issue is that both methods referenced in Chapter 30, Part 1 and Part 2 are envelope methods. Neither are directional methods. Consider the roof layout below:

    Chapter 30 Wind Zones
    Simple roof, gable at one end and hip at the other. First of all, there are 31 (!) wind zones depicted on this plan. Before making a "business decision" to reduce the number of zones (and make the solution more conservative), I am curious how the authors intended the average engineer to model and analyze this loading pattern.

    My second issue is more relevant. When analyzing the ridge beam and hip beams (both have a tributary area < 700 SF), I interpret the chapter is that I should put all negative pressure and all positive pressure in two load cases. If a directional method were available, I would apply positive wind pressure to the windward side and negative pressure to the leeward side. The effect of putting negative pressure on both sides of the beams can easily result in a net uplift even in areas of low wind velocity. For a wood framed building, this puts the bottom face in compression and can result in a much larger beam size due to the unbraced length. This doesn't seem realistic to me.

    My next point is one of clarification. In figure 30.3-2G, in the pressure coefficients chart for overhangs, the coefficient for zone 3 is less than for zone 1 for tributary areas over 100SF. This occurs in other charts, but this one is the most dramatic illustration. Is it true that the wind load for zone 3, a corner of an overhang, is less that the interior area of the roof surface? If so, why?

    Thank you,


    Bill Allen P.E., S.E., M.ASCE
    Allen Designs
    Mission Viejo CA