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RE: Net uplift on joists and joist girders

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In the sheltered world of the 90 mph winds, it has not been a major issue
except in the event of micro-bursts.  In the land of higher wind loads, I
have seen buckled bottom chords that resulted in a general failure in wind
uplift.  That is why Factory Mutual got so active in the roofing throughout
the US.  The insurance losses were real.

If you don't design for wind uplift and the roof is rated for Factory
Mutual, you can cause a major increase in cost to rehabilitate an existing
bar joist system.  I worked on a project 3 years ago in the Colorado Front
Range where this happened.  It was expensive for the engineer, the
fabricator, and the erector.  Factory Mutual rated roofs are very common,
and the Factory Mutual inspectors will not catch it on the plans, but they
will catch it when it is under construction.  And that is when it is
expensive to fix.

I have heard the argument of cost for the extra bottom chord bracing many
times.  I frequently call the Nucor people and ask them how much it costs.
They find that it costs them more to try to differentiate costs than it is
worth in the 90 mph wind area.  So from the supplier's view point, there is
no increase cost for the added consideration for wind uplift.  I then put
that same question to LPR a big steel erection company.  They normally don't
add cost for an extra line of bridging either.  Steel fabricators will
complain because they have a little extra paper work, but that does not
relate to any increase in real cost.  I generally run through this exercise
about once every 5 years, and the result on cost is about the same.  There
is no increase for properly designed uplift resistant roofs, but there is a
marked increase in risk if you don't.

I worked on a project recently where the net wind uplift forces were about
200 psf in the roof edge region.  The wind velocity was in the neighborhood
of 140 mph (3 second gust).  The pressures are about 2.5 times greater than
a 90 mph wind area.  We also had a major escarpment effect, coastal effect,
etc.  We had it corroborated in a wind tunnel test.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Michael Bryson [SMTP:MBryson(--nospam--at)mhpse.com]
> Sent:	Wednesday, January 16, 2002 1:03 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
> >
> >

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