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RE: C&C vs MWFRS pressures

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Please excuse a delayed jump-start of this thread. This is not a defense
or rebuttal of anything anyone has said except as noted. Also please
excuse if I quote briefly and if context is lacking. No malice intended.
Peace.

Regarding the decision to design a structural member for MWFRS and/or
C&C pressures...

>>As Scott suggests it is the size of the thing, that determines the
choice, >>but not entirely.

I disagree; but not entirely ;). It is the function of a structural
component that determines which pressures should be used. There is
nothing in the Standard that requires this decision to be made based on
size or tributary area or effective wind area. There is only an
exception in the Standard which allows Components and Cladding elements
to be designed using MWFRS pressures if they are over a certain
effective wind area. The existence of this singular stipulation in the
Standard proves this point. 

>>I have not followed this from its beginning.  But the definition for
C&C >>contained in the ASCE 7-05 Commentary may shed a little light.
Roof >>trusses are specifically cited as an example.  

Again, this may seem a bit pedantic, but the text referred to here is
not a definition but is commentary on the definition found in the
Standard. The Standard is state law and the Commentary is not. The
Commentary "is included for information purposes" and is a guide to the
professional and code official. I only say this to defend the authority
of the professional to make the determination we are discussing, not to
undermine the importance and utility of the Commentary. But to address
the specifics of your point, trusses are listed in the Commentary as
examples of components just as they are listed as examples of MWFRS
elements. This does not mean that trusses are always components just as
beams are not always part of the MWFRS such as when a beam is used to
support a glass storefront or wainscot wall.

>>I would design a roof truss for C&C loads, particularly for wind
pressures >>wanting to blow a roof off.  The wind pressures act on the
roof sheathing, >>then the roof truss top chord, and finally the truss
bearing.  C&C loads >>are about 25% (<--- guess) greater than those
pressures due to main system >>forces...

The same could be said of any element or connection in the MWFRS. Why
stop at the truss bearing? If your answer has something to do with
"size" consider that many wall, beam, and foundation elements commonly
have tributary areas no greater than the common truss they are attached
to. Do you feel similarly obligated to check an artificial load case
where C&C pressures are applied to the entire wall-truss-wall frame?
This might be conservative but doesn't replicate any real-world loading
scenario. BTW, to prove this to yourself, tell me how you would
distribute C&C uplifts to the hold-downs on a 3-point bearing truss? If
you think that distribution by effective wind area based on span is
conservative then you do not understand that reactions on 3-point
bearing trusses are "highly" dependent on the internal stiffness of the
truss and have nothing to do with effective wind area. These are all
MWFRS considerations.

>>But, that fastner would NOT be designed to MWFRS pressures...but a C&C
>>loading for the wind trib area [read "effective wind area"] of THAT
>>fastner

I disagree since the tributary or effective wind area of a roof truss
hold-down fastener has no meaning for a 3-point bearing truss.

>>As I said, the primary difference in practical terms between the MWFRS
and >>C&C pressures in ASCE 7 is the effective wind area of the two

I disagree. The primary distinction of MWFRS pressures is that they
generate a conservative real-world, albeit simplified, load
configuration on the MWFRS. The redundancy, strength reserve with size
degeneracy, and alternate load path available to MWFRS elements and
connections are the reason that MWFRS pressures are appropriate
regardless of "size". Using the typical truss hold-down as an example,
there is no reason to believe that a truss hold-down experiencing a
localized pressure spike would not distribute load to a hold-down with
reserve strength 24", 48" or 72" away rather than be jerked bodily from
the system.

>>The C&C pressures will be greated than MWFRS pressures since for all
>>things being equal, C&C pressures will be greater than the MWFRS
>>pressures.  Thus, for a pure uplift case (call it a flat truss to keep
it >>simple), C&C pressures will produce a more overall severe load
reversal on >>a truss then MWFRS pressures.

This is an unnecessary and unrealistic simplification and may not always
be true. The truss hold-down connector is the quintessential example of
a MWFRS element since it is designed using the interaction equation for
uplift and in-plane and out-of-plane shear and sometimes gravity loads.

>>C&C have nothing to do with 'a' distances in ASCE 7 (i.e.
>>localized higher pressures are edges and such)...at least not
directly.

I don't follow you here. The pressure on a component or cladding element
is directly related to it's location in relation to the pressure
coefficient zone 'a'.

>>There are lots of elements of the lateral system that must still be
>>checked/designed with C&C pressures.

I don't disagree. The state of the practice for the typical residential
roof truss is to design truss tails, top chords and gable end webs for
C&C pressures. Hold-downs are designed for MWFRS loads. This decision is
based on the function of the element and not its size.

Christopher Banbury, PE
President

Ark Engineering, Inc.
PO Box 10129, Brooksville, FL 34603
22 North Broad ST, Brooksville, FL 34601
Phone: (352) 754-2424
Fax: (352) 754-2412
www.arkengineering.net



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