Need a book? Engineering books recommendations...

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

Re: Flat roofs in coastal areas

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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

******* ****** ******* ******** ******* ******* ******* ***
*   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 
******* ****** ****** ****** ******* ****** ****** ********