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Re: Net uplift on joists and joist girders

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--- "Alden Manipula, E.I.T." <amanipula(--nospam--at)> wrote:
> Net uplift is the difference between the calculated pressures on the roof
> structure and the roof dead loads, right?

Yes, but be sure you're using your _minimum_ dead load, not the conservative
dead load used for combination with gravity load.  You can accomplish this with
load factors, but it's still good to do a reality-check on your numbers.

> 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

The problems begin when uplift exceeds download, even by a little, because this
condition will cause joist bridging to be insufficient for bracing the joists,
unless the bridging has been designed with uplift in mind.  Compression in the
bottom chord would also need to be considered, but that's "behind the curtain".

Generally, a roof uplift diagram is your best tool to communicate these uplift
forces to the joist designer/manufacturer.  Most people I've met supply a plan
of the roof with net wind uplift loads.  I prefer to supply gross wind uplift
and a detailed roof dead load summary, so the joist manufacturer can sort out
the actual loads without depending on my judgement.

Joist manufacturers?!?!?

-Keith Fix, PE
-Little Rock, AR

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