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RE: Flat roofs in coastal areas

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Warren,

The only time you should consider wind sheltering effect is if you do a wind
tunnel test.  When you take any structure and do a wind tunnel test, you
will see some areas that will have higher local wind effects and areas where
they will be lower.  The coefficients contained in the ASCE 7 were developed
from a battery of wind tunnel tests.  In the real world, there will always
be configurations that will either add wind pressure or lower local wind
pressures.

Regarding ballast.  
I would check with Factory Mutual.  For years FM would not certify a
ballasted roof.  They allow ballast now, but it is not off the shelf gravel.
Gravel will become projectiles in high wind events.  And as the projectiles
are blown off of your roof, you will get scour, loose the counter balance
effect, maybe loose your roof, and break windows neighboring buildings.  For
high wind areas, you will be required to use very large aggregate (1" or 2"
diameter, "well rounded", with a density of 165 pcf).  FM also requires a
larger "corner" condition.  A better consideration for ballast may be
pavers.  Check the FM Property Loss Prevention Data Sheet 1-29 and 1-28.

You may want to consider requiring a FM roof.  Their suggested details and
Loss Prevention Data Sheets have been developed over many years.  

Dade County has its own requirements for roofs.  Their requirements were
upgraded after Hurricane Andrew.  The FM I-90 requirements were also
upgraded and are now very close to the Dade County roof construction
requirements.  

When it comes to critical roofs, consider FM and dual / redundant roofs.
There are some critical nuclear structures that I have worked on that went
to the dual / redundant roof.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Foy, Warren [SMTP:Warren.Foy(--nospam--at)mhgrp.com]
> Sent:	Thursday, August 29, 2002 12:10 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	RE: Flat roofs in coastal areas
> 
> Bill,
> 
> Thank you for your reply.  You understood the situation perfectly.  One
> thing I forgot to add is that it is a Building Category IV, essential
> facility.  You are also correct in that roof and deck fasteners need to be
> designed using component and cladding wind pressures.
> 
> Thanks
> 
> -----Original Message-----
> From: Bill_Coulbourne(--nospam--at)URSCorp.com [mailto:Bill_Coulbourne(--nospam--at)URSCorp.com]
> Sent: Thursday, August 29, 2002 12:41 PM
> To: seaint(--nospam--at)seaint.org
> Cc: seaint(--nospam--at)seaint.org
> Subject: Re: Flat roofs in coastal areas
> 
> 
> 
> Warren:
> If I understand the sketch and the explanation - here are some thoughts.
> I believe SC has adopted the 2000 IBC code and so the requirements of
> ASCE7-98 can be used for wind design. The primary design loads that will
> govern the roof covering are those for components and cladding. Using
> Figure 6-5B and the smallest effective area, the external pressure
> coeeficient is 1.8; when you combine that with the internal pressure
> coefficient, the uplift load on a fastener or other attachment device will
> be more than twice what the velocity pressure is. So if the velocity
> pressure on the roof is 44 psf, the uplift pressure on a compenent could
> be
> 88 psf +/-.
> 
> I believe the cornice will act like a parapet so the parapet attachment to
> the building will need to resist windward and leeward horizontal pressures
> exerted on it. However, if the cornice is 3' or more high, the corner
> zones
> of the roof can be treated like edge zones and thus the uplift pressures
> in
> the corners are somewhat lower.
> 
> It sounded from your description, that the roof will be a metal deck with
> some sort of insulation to which the built up roof will be laid on top of
> then covered in gravel. You could use the roof surface to help offset some
> of the uplift but in order to make that work, the roof surface and
> covering
> need to be installed such that they will still be there during the
> duration
> of any potential high wind event. Gravel usually won't stay on a roof like
> that during a hurricane. The attachment of the assembly is usually
> builtup
> roof layers mopped to the insulation mopped or mechanically fastened to
> the
> metal deck with the dack mechanically fastened with screws or puddle welds
> to the framing members such as steel bar joists. That means depending on
> the effective area of each of the fasteners, each fastener will need to be
> able to resist on the order of 88 psf x 10 sf effective area or 900 # of
> uplift (minus whatever credit you want to give the weight of the roof). Of
> course, this all assumes I understand the situation. Hope this helps.
> *************************************
> Bill Coulbourne, P.E.
> Principal Structural Engineer
> URS Corp.
> 200 Orchard Ridge Drive
> Suite 101
> Gaithersburg, Md 20878
> 301-670-3344 (Office)
> 301-869-8728 (Fax)
> 703-850-2891 (Cell)
> bill_coulbourne(--nospam--at)urscorp.com
> 
> 
>  
> 
>                       "Foy, Warren"
> 
>                       <Warren.Foy@mhgr         To:
> <seaint(--nospam--at)seaint.org>                                
>                       p.com>                   cc:
> 
>                                                Subject: Flat roofs in
> coastal areas                        
>                       08/29/02 10:31
> 
>                       AM
> 
>                       Please respond
> 
>                       to seaint
> 
>  
> 
>  
> 
> 
> 
> 
> 
> I have a project on the South Carolina coast that will have a flat roof.
> The design wind speed is 130 mph resulting in a wind load of 44 psf
> uplift.
> The roof has a standing seam roof with cold-formed metal framing cornice
> around the perimeter of the building so the flat portion of the roof is
> somewhat sheltered.  Below is my first attempt at test graphics to
> describe
> the roof.
> 
> 
>         /|                                   |\
>        / | Cornice                           | \   Cornice
>       /  | framing                           |  \  framing
>      /   |                                   |   \
>     /    |                                   |    \
>    /     |              flat roof            |     \
>    -------------------------------------------------
>   ||                                               ||
>   ||                                               ||
>   ||                                               ||
> 
> I have two questions (so far) regarding this roof.
> 1.  Could the actual uplift pressure on the roof be higher than the
> standard flat roof wind load due to the cornice roof?
> 
> 2.  The roofing is to be a gravel ballasted built-up roof.  Does the
> weight
> of the ballast need to offset the wind uplift?  If not, what is the
> mechanism that holds the roofing down?
> 
> Thanks in advance,
> 
> Warren S. Foy, P.E.
> Structural Design Manager
> The Mason & Hanger Group Inc.
> 
> 
> 
> 300 West Vine Street, Suite 1300
> Lexington, KY 40507-1814
> v 859.252.9980 x221
> fax: 859.389.8870
> web: http://www.mhgrp.com
> 

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