Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: C & C pressures, trusses

[Subject Prev][Subject Next][Thread Prev][Thread Next]
But, that fastner would NOT be designed to MWFRS pressures...but a C&C
loading for the wind trib area of THAT fastner (i.e. much smaller than 700
sq ft unless it is one MONSTER fastner).

You need to be careful that you don't infer Australian code mentality into
the ASCE 7.  While they may be based upon the same theoretical stuff and
base methodologies, they appear to have differences in how they practically
work.

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 and thus the
resulting average wind pressure that results from the methods.  Since MWFRS
pressures are always assumed to be much larger wind areas than any C&C
element, they will have lower average pressures as there are more low local
pressures to average out possible high local pressures.

Now, this does not mean that there are not other differences.  You certainly
pointed out on major difference...the fact that MWFRS pressures are applied
to multiple surfaces with potentially different directions of application,
while a C&C pressure is basically applied to in one consistant direction
even if the element has multiple surfaces.  Thus, a truss with a peak in the
middle with enough slope will still need to be looked at for MWFRS pressures
as it can have the windward slope with a positive pressure and the leeward
slope with either a positive or negative pressure depending on slope...but
then that potential net horizontal pressure/force is something that the
lateral system will have to deal with and thus the truss in effect would be
part of the lateral system (that horizontal load must somehow go through
that truss out to the lateral system).  But, this is to some degree no
different that than a flat truss still having to be designed to handle the
diaphragm force from the MWFRS pressures.  The point is that the truss would
STILL have to be checked/designed to handle pure uplift from C&C pressures
in ASCE 7, whether it is peaked with a slope or flat...because the C&C
pressures would still be conservative for that situation compared to uplift
pressures from MWFRS pressures (unless the truss has a large enough wind
trib area...such as maybe a girder truss).

The key point is that many people believe that they can magically designate
something as part of the lateral system and then only design it for MWFRS
pressures.  To my knowledge, that is an incorrect belief.  There are lots of
elements of the lateral system that must still be checked/designed with C&C
pressures.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Conrad Harrison [mailto:sch.tectonic(--nospam--at)bigpond.com] 
Sent: Tuesday, May 13, 2008 3:06 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: C & C pressures, trusses


I don't agree with ASCE7-05 clause 6.5.12.1.3, which allows MWFRS pressures
to be used for C&C with large tributary areas. A large area heavy concrete
panel, or other rigid plate, may experience a high localised pressure with
no significant effect transferred elsewhere. But continuous span steel
sheeting 100 metres long over arches and the likes is flexible between
supports, and high localised pressures can fail the fasteners. That then
sets two spans of the cladding oscillating, which in turn fails more
fasteners, increasing the length of cladding in motion, until the whole
length of cladding is eventually detached.

 



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 




******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********