Wind Loading

[sshields]

This my sound like a basic type question...

But is MWFRS included as C&C ?? I have a 3 story building and I'm not sure if I should use the MWFRS < 60 , MWFRS all h, or the alternate with h < 60 and C&C being 60<x<90...

Please advise.
Thanks.

 

[frv]

I'm a bit confused with you question, as I don't have ASCE 7 in front of me. But you will always use MWFRS loads on your lateral load resisting system.

The C&C provisions address the probability that at very localized levels you may get significantly higher wind loads. This is why individual girts are usually designed for C&C (one girt could potentially see a very high localized load) but the X-Braces, Moment Frames or Shear walls are designed for MWFRS loads (as the "tributary area gets larger, there is less likelihood of the entire wall under consideration experiencing larger than expected wind loads).

hope this helps.

 

[southard2]

The MFWRS includes everything along the load path that collects and distributes the wind loads to the ground. So just about everything should be considered against these MFWRS wind loads. Any component or connection that has to carry loads from two directions must be designed for MFWRS loads. A roof joist connection takes wind loads from two directions (uplift and out of plane wall shear). A diaphragm also takes uplift and shear. As does a shear wall.

Components and cladding wind loads are to be applied to individual components that take only one load in one direction. Think of them as the components that collect the wind loads primarily. A roof truss or joist is only designed for uplift or gravity. Thus it is to be designed for the components and cladding wind loads. Same goes for a column footing. Roof deck and wall sheathing are also designed for the C&C loads. Note that any component with an effective tributary area greater than 800ft squared may be designed only for the MFWRS loads. Also I'd like to mention that the 3rd edition of the deck diaphragm design manual now requires deck connections to be check for combined shear and uplift. I use MFWRS loads for this. I still check the deck connections for the pure C%C wind loads but as a stand alone check.

When I do my joist to wall connections I use MFWRS loads when checking a combination stress ratio and C&C when checking stand alone uplift.

C&C loads are almost always higher than MFWRS loads. Thus I like to think that at a minimum everything must be designed for MFWRS loads for all load paths unless the C&C loads must be applied and end up controlling. Usually this only includes the roof deck, joists, trusses, walls, wall sheathing, etc.

I could go one but I think the main points have been made