– Wind

QUESTION: (SPRING2012) The User’s Guide – NBC Structural Commentaries provides external peak pressure-gust coefficient values, in Figures I-8 to I-14, for design of cladding and secondary structural members. Should these values or those tabulated in Figure I-7 be used for the design of a primary roof girder in a low-rise building?

ANSWER: The values tabulated in Figure I-7 apply to the design of lateral-load resisting systems for low-rise buildings when wind load acting simultaneously on all surfaces is considered. In the NBC Structural Commentaries, the wind effects in such loading conditions are termed “primary” structural actions whereas structural members subjected to local external pressure (or suction) are referred to as “secondary” members. Local peak gust pressure/suction values are significantly larger than the effective gust pressure acting on the whole building. Therefore, the coefficient values in Figure I-7 should not be used for the design of a roof girder unless it is a member of a rigid frame or a part of a braced frame and the effects due to “primary” wind actions govern the design.