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Re: Wind loads: ASD or Strength?

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>From: Bill Polhemus <bill(--nospam--at)polhemus.cc>

>To: seaint(--nospam--at)seaint.org

>Subject: Re: Wind loads: ASD or Strength?

 

>Scott Maxwell wrote:

>> Now, the issue of C&C vs MWFRS pressures is a wee bit more messy and

>> is not suprising that many get confused...

>But it's not "confusing" for the same reason. It's a completely

>different concept than "service vs. strength." It has to do with

>statistical probabilities that a small area is equally likely to see a

>higher pressure as an entire MWFRS is to see an "average" pressure.

 

>The Commentary is your friend.

 

I agree with Scott on this one.  CC isn’t nearly as simple as you’ve stated – if it were, why wouldn’t we just use the large trib area loading based on CC for the Main wind systems?

I’ve heard an entirely different opinion of what CC means from nearly every engineer I’ve asked.  I’ve even witnessed several engineers debating this “line” for hours only to never completely agree...

However, the opinion that I’ve come across the most, and come to believe to be the most correct myself, is that CC loads are bloated loads, based on (as you said) peak loads over a smaller area – much like a wave crest can be substantially higher than mean sea level - purposed for protecting the integrity of the building envelope. i.e. anything that would change the system from, say, enclosed to partially enclosed given its individual failure - keyword here being individual.  Take residential roof trusses for example - If only truss connection fails, the envelope will not likely be compromised, thus the use of MWFRS loads is acceptable.  But if say the truss plates fail, the truss may come through the ceiling pulling the sheathing in with it – thus they are CC.  But what about wall studs? The truss itself? This is where professional opinion and local precedence starts taking over and the debates begin.  Some engineers don’t even agree that the truss clip should be MWFRS...

 

 

Richard Calvert