MWFRS or C&C 1

[cdi12]

Should wind load on purlins HSS10x4 welded to main roof trusses (see attachment) be calculated using factors for MWFRS or C&C? I thought for purlins, in this case since they are part of main wind force resisting system, should MWFRS factors used. In what part of ASCE 7 is this? Thanks IV

 

[Lion06]

I think they should use C&C pressures. They may be part of the MWFRS, but are still subjected to higher localized pressures (i.e. they have the advantage of a large area which will allow the higher pressures to be normalized by lower pressures). The idea of the MWFRS pressures is that over a very large area the pressures will equal out to the MWFRS pressures.

 

[JAE]

It is not an either/or.

In most cases you know full well if an element is part of a MWFRS (i.e. and X brace or the column that is part of the X brace).

In this case, your pulin looks from your drawing that it is definitely resisting only a fairly small area (<700 sf) of wind area so a C&C check (per StructuralEIT above) would be appropriate.

Then...if it is truly a part of the MWFRS (which I'm not sure how it is in your drawing), then you should also check it for those loads as well.

Many times the character of the wind direction and application to your member is different for the C&C wind and the MWFRS. Example: Wind on an exterior column taking load from girts could be checked with C&C wind which depends on the bending of the column perpendicular to the wall. Also, the column could be part of an X-brace in which a MWFRS wind is applied which causes axial load in the column. Two separate wind load cases to check (of course never combine the two).