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

RE: Net uplift on joists and joist girders

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dear Michael,

I don't know where or what kind of buildings you have been designing, but
for a membrane roofed warehouse net uplift pressures can be significant (50
to 80 PSF), particularly at corner & edge zones, and can have a significant
impact on the bottom chord uplift bracing and sometimes even effect the size
of the bottom chord of the effected joists.  As far as where buckled bottom
chords go...the ones I have seen end up on the floor or storage racks, the
rest of the roof usually follows.

Matthew Stuart, PE, SE, PEng

-----Original Message-----
From: Michael Bryson [mailto:MBryson(--nospam--at)mhpse.com]
Sent: Wednesday, January 16, 2002 2:03 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Net uplift on joists and joist girders


OK, I have had some thoughts about this that maybe will sound wacko to most.

First, the net uplift forces for the most part tend to be very low, say 5 or
10psf. If you spent the time to do a very detailed analysis you might be
able to show that the beams could handle this load. Afterall, braced you are
probably designing for a minimum 30psf total load without a 1/3 increase for
wind. (Furthermore, the beams are not laterally unstable, just the
compression chord would buckle.)

But even then, as long as the roof is properly anchored down, I am not sure
it would ever fail from wind uplift. Seems to me even if the bottom chords
buckled, where would it go? I think the roof would then behave as a big
tension membrane preventing collapse. This is a justification I have used
where providing bottom chord bracing would have been at a very big cost.

Finally, has there ever been a roof failure caused by wind uplift that did
NOT involve improper uplift anchorage?

-----Original Message-----
From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
Sent: Wednesday, January 16, 2002 9:26 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Net uplift on joists and joist girders


I would check your applied loads.  I have frequently designed for a roof net
uplift.  Even in small buildings I am surprised that it hasn't governed for
you yet.

Another thing is that you need to look at reliable dead loads.  When you are
designing for gravity loads, it is conservative to have a bit larger dead
load than you anticipate.  When you consider uplift, you have to use the
minimum possible dead load.

Look at your applied wind loads.  Bar joists and purlins should be designed
as elements.  Check the trib area requirements.  Check the edge conditions.
You will frequently find that you will have to design for a net uplift.
This will put compression in the bottom of the framing element.  Check your
stresses with the unbraced lengths.  Bar joist bottom chords will increase
as will the webs at the ends and the bracing will increase.  That said, my 2
projects currently on my desk are in the big wind country in the Aleutian
Islands of Alaska and in South Florida.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Alden Manipula, E.I.T. [SMTP:amanipula(--nospam--at)novagroupinc.net]
> Sent:	Wednesday, January 16, 2002 11:17 AM
> To:	SEAINT Listserve
> Subject:	Net uplift on joists and joist girders
>
> Net uplift is the difference between the calculated pressures on the roof
> structure and the roof dead loads, right?
>
> And if so, when does it become a problem?  Most of the buildings i've done
> so far have been relatively small and the roof dead loads have always
> exceeded the pressures i've calculated.
>
> TIA.
>
> Alden Manipula, EIT
>
>

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