HSS Roof Truss Bottom Chord Compression

[Galambos]

I have a 70ft long hss roof truss that goes into uplift, causing compression in the bottom chord hss6x6 members.

i see hss trusses with no bottom chord bracing all the time, and for my span, it would take an hss12x6 member to get a fully unsupported section to meet kl/r<200, which seems excessive.

I am trying to minimize the amount of bottom chord bracing and i came across the new hss design guide.

on page 95, the author makes the statement, "The effective length of such laterally unsupported truss chords can be considerably less than the unsupported length, and design guidance is provided by Packer and Henderson (1997) and Galambos (1998)."

yes, i know i can buy the book, but does anyone have any insight as to what this is referring?

thanks in advance.

 

[BAretired]

I couldn't find much design guidance in Packer and Henderson. Under 2.3.1 Simplified Rules, they say:

For HSS chord members

In the plane of the truss,
KL = 0.9L where L is the distance between chord panel points.

In the plane perpendicular to the truss, KL = 0.9L where L is the distance between points of lateral support for the chord.

I don't believe that omitting bottom chord bracing can be justified with 6x6HSS chords on a 70' span if wind uplift causes a net compression in the bottom chord.

Perhaps the examples you have seen were designed on the assumption that dead load exceeded wind uplift.

BA

 

[Galambos]

BA, thanks for looking into this. You're probably right about the net uplift, too.

It's good to have some agree that they're too slender for a completely unbraced span.

Though, I wonder if the design guide was implying that the trusses could be designed as some sort of inverted pony truss.

hmmm...

 

[hokie66]

You could brace the bottom chord with a bending member like in a pony truss, but you would need a moment connection at the top to something stiff enough in the roof. Another approach is to add ballast, but for a big roof, that is probably out of the question.