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RE: ASCE 7-05 Wind

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Actually, ASCE 7 now also has corner zones for MFWRS.  It used to only be for C&C (in older ASCE 7 editions and BOCA codes).  This is the one area where I think that the ASCE 7 went overboard.  In general, having ONE uniform pressure (or increasing pressure with height on the windward side) makes a lot more sense.  The corner zones for MWFRS seems to be an over complication to me.  I suppose there might be situations where accounting for the increase in wind pressure at corners might have some effects (i.e. non-symmetrical buildings that are torsion sensative), but by and large, it should have a rather insignificant effect.  Like Jordan, I tend to just take the corner pressures (which are the "worst" case) and use it for the whole building (or use something between the main portion pressures and the corner pressures if I don't want to be WAY conservative)...but that can be conservative.
 
Regards,
 
Scott
Adrian, MI
-----Original Message-----
From: Neil Moore [mailto:nma(--nospam--at)omsoft.com]
Sent: Wednesday, November 21, 2007 6:27 PM
To: seaint(--nospam--at)seaint.org; seaint(--nospam--at)seaint.org
Subject: Re: ASCE 7-05 Wind

Corner zones is in the UBC Section 1622 when you are designing your walls for out of plane loads and are areas of discontinuities and areas not in areas of discontinuities.

In ASCE 7 they are talking about Components and Cladding or C&C's.   Deflection ratios in the CBC will require anywhere from L/240 for brittle finishes to L/120 for flexible finishes per Table 1604.3.  

Actually there is a whole host of wind items that should be considered.   We don't hear much about roof's blowing off, but they do happen.  Sometimes you can look at a scalloped stucco wall and wonder if they built it that way!    If you research some of the big wind storms in Northern California, you'll find some Category 2 wind records.  I can remember the big wind storm that trashed Sacramento in 1938.   I was about six and my dad was later in the backyard cleaning up the mess.

Sometimes we have little tornadoes like the one that touched down in the Natomas area north of Sacramento a couple of years ago.   Watched the cloud for an hour coming up Highway 5 and the big dark cloud finally let a little funnel snake down.  Wife took a picture through the windshield!

Lately I've been using the wind program from MECA.   It's even got a section in there for Kzt.



Neil Moore, SE, SECB


At 01:50 PM 11/21/2007, WISH DENNIS wrote:
Jordan,
Help me to understand "corner zones"?  This is what seems to be confusing me. I have not yet paid close enough attention to comparing the zones on the various model analysis I've seen, so please bear with me. Regardless of the shape of the building (with or without reentrant corners and interior shear walls) can the engineer simplify the MWFRS down to a uniform load applied tributary to each diaprhagm and then compared to wind OR is this too conservative?
Maybe if you could just explain briefly what the code is attempting to do different from the UBC conventional methods that those of us only know? For example, simply start with some rhetoric about corner zones and idea behind breaking the building down into zones to start with. This might help some of us who have the codes but do not fully follow the logic.
 
Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant


----- Original Message ----
From: "Jordan Truesdell, PE" <seaint2(--nospam--at)truesdellengineering.com>
To: seaint(--nospam--at)seaint.org
Sent: Wednesday, November 21, 2007 9:15:58 AM
Subject: Re: ASCE 7-05 Wind

You are absolutely correct. As a practicing engineer being held to the standards commonly exercised by others who practice in my field, I would request that you continue to ignore that particular section of ASCE-7.  If we all ignore it, then we are all acting in a common way.

That said, this is an area where engineering judgment has to control. I would suggest that any building which does not resemble an airfoil is going to be regular, regardless of the definition proposed by the non-practicing professor who decided it would be a good idea to wind-tunnel test every L shaped strip mall. I typically analyze non-rectangular structures by taking corner zones at every corner which could reasonably result in turbulence, and fudge the data for rounded areas. Maybe I'm cavalier because of my aero background, or maybe I'm just naive; if I'm ever confronted, I will happily pass on the building official's requirement that wind tunnel testing be done.  I'll either end up with an enormous fee, or the official will be run out of town by builders with pitchforks and torches - I win no matter what.

What you will like in ASCE 7-05 provisions is that they have greatly expanded the types of conditions for which equations and drag factors area available.  It will take a little bit to go through, but if you've ever been frustrated by the lack of guidance on odd shaped areas in previous editions, you'll find that many questions are answered in 7-05 (even if there are still many gaps).

Jordan


Doug Mayer wrote:

I am making the transition to the new code (CBC 2007) and being confronted with the ASCE wind provisions.  They seem astoundingly complex and I have a couple of questions that desperately need some answers. 

 

First off, what is a “regular-shaped building”?  ASCE defines this as “a building or other structure having no unusual geometrical irregularity in spatial form.”  To me, this sounds like any non-rectangular structural is irregular.  Is this true?  This is very important because it leads into the next question of which analysis method to use.  Both Method 1 and Method 2 require “the building or other structure…” to be “… a regular-shaped building or structure as defined in Section 6.2.”  This would seem to indicate that you have to use Method 3 (wind tunnel testing!) for an non-rectangular building.  I can’t believe that is the case, so I think the definition of a regular-shaped building is different than what it seems to be.

 

Any help would be greatly appreciated.  Also, I apologize if I am asking questions that have already been asked.

 

TIA,

 

Doug Mayer, SE

Structural Engineer

 

 

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