Need a book? Engineering books recommendations...

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

Re: ground snow v. roof snow - which is it?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I don't disagree with you, but you have to realize that not all states
adopt the codes for required statewide use.  In some states, codes are
adopted (and potentially modified/added to) by local jurisdictions
(counties, cities, etc).  That _WAS_ the case in Michigan until recently.
It was not until a handful of years ago that Michigan went to adopting
a code (which they DO modify some), the IBC and IRC, for state wide use.
It used to be that each local jurisdiction would adopt what they wanted.
Lansing, MI had adopted a version of the UBC.  Most jurisdictions in the
SE Michigan area adopted a version of the BOCA, but one city might use
1993 while another used 1996, etc.

And the point is that the jurisdiction (state, city, county, etc) that
legally adopts the codes can modify it as they see fit...and most do so.

And beyond that, even in some states where there is a state-wide code
adopted, it is a "minimum" code.  In otherwords, local jurisdictions
(cities and/or counties) can still modify the state code _IF_ they make it
more conservative (and requiring the use of full ground snow load rather
than roof snow loads per ASCE 7 is more conservative).

And like it or not, by and large the local code official is the
"interpreter' of the code.  While it is certainly possible that they could
interpret it wrong, it becomes a hassle (at minimum) to argue against such
misinterpretations at times, to (at the extreme) bad thing for the well
being of your project (the good old phrase of "I fought the law and the
law won" comes to mind).

Scott
Adrian, MI


On Wed, 21 Sep 2005, Jordan Truesdell, PE wrote:

> I believe that the requirement to use ground snow would have to be
> proposed and included in the legislation adopting the code, or in
> subsequent published addenda.  Code officials, as far as I know, do not
> have the right to change the building code, though some feel that they
> do. That's the whole idea behind using model codes. Otherwise, we're back
> to pre-national code times when every jurisdiction makes it own rules. It
> may not be worth the fight - its not our money we're spending by upsizing
> roof structures - but the courts would likely side with the letter of the
> law, and in this case the building code is extremely clear. "Shall be
> determined in accordance with" doesn't leave much wiggle room.
>
> Jordan
>
> Scott Maxwell wrote:
>
>  Jordan,
>
> Not so quick.
>
> You have a point, but then Jim does as well as local code officials or
> states can "modify" the plain vanilla IBC (i.e. adopt it with
> modifications/additions).  From my experience, some local juridictions
> will require the use of the full ground snow load as the roof snow load
> (i.e. no modifications to flat roof snow load or sloped roof snow load).
> Now, it is my belief that this is largely due to the IRC (which I believe
> REQUIRES the use of ground snow load and does not PERMIT modifications per
> ASCE 7) and local code officials not realizing that such it not "required"
> if the design is an engineered design (i.e. IBC is used rather than IRC).
>
> Scott
> Adrian, MI
>
>
> On Wed, 21 Sep 2005, Jordan Truesdell, PE wrote:
>
>
>
>  Jim, I think you've got is bass ackwards ;-)
>
> I quote, "Design snow loads shall be determined in accordance with
> Section 7 of ASCE 7..." This is the first sentence of section 1608 of
> the 2000 IBC (Snow Loads).  The ellipses referes to the requirement that
> the roof load must not be less than the roof live loads specified in the
> Live Load section, 1607.
>
> That's pretty cut and dried - engineers (actually, anybody) are
> specifically allowed by the code to use any and all reduction (or
> amplification factors) they deem appropriate, as listed in ASCE 7.  The
> code writers appear to have specifically allowed the reduction of loads
> where allowed by the ASCE.
>
> As has been mentioned, the span tables are probably based on ground snow
> loads, and should not be altered without analysis by a registered
> engineer.  Ground loads are convenient, because that's what's in the
> table.  I would suspect that the tables also assume partial exposure,
> ventilated roof over thermally isolated living area, Terrain Categories
> A, B or C, an importance factor of 1.0, and no drifting conditions or
> sliding surcharges, and - of course - no unbalanced loads.  Prescriptive
> design has its place, but can always be overridden by proper analytical
> techniques. (That doesn't always mean reduced loads, btw)
>
> Jordan
>
> Jim Wilson wrote:
>
>
>
>  It seems as if the code writers have eliminated the
> option of reducing snow load when using prescriptive
> design.  Subsequently, it is up to the local enforcers
> to decide if an engineered design is permitted to
> include the reduction factors.  Does that sound
> correct?
>
> Jim
>
>
>
>
>
>
>
>
>  ******* ****** ******* ******** ******* ******* ******* ***
> *   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
> ******* ****** ****** ****** ******* ****** ****** ********
>
>
>
>
>
> ******* ****** ******* ******** ******* ******* ******* *** * 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 
******* ****** ****** ****** ******* ****** ****** ********