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RE: Net uplift on joists and joist girders

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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.

Harold O. Sprague

> -----Original Message-----
> From:	Alden Manipula, E.I.T. [SMTP:amanipula(--nospam--at)]
> 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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