Truss internal diagonal effective length 3

[MIStructE_IRE]

Someone please refresh my memory..

A standard steel warren truss supporting roof decking - I would typically take effective lengths of internal diagonals as 1.0L. Can this legitimately be taken as 0.7 or 0.85L?

(Trying to get an old truss to work on paper!)

 

[rb1957]

you're talking about effective length, ie accounting for partial fixity at the ends.

1) is the end more than a single fastener ? if no, then 1

2) is the end multiple fasteners, or one of those mesh plate thingies ? if yes then maybe 0.85

3) 0.7 seems optimistic (but what do I know)

another day in paradise, or is paradise one day closer ?

 

[dik]

Alcan used to publish a design handbook for aluminum and the effective length for truss diagonals was 0.9 x length.

Dik

 

[KootK]

Need more info:

1) what kind of chords.

2) what kind of webs and how are they oriented with respect to their local axes?

3) what kind of connection.

4) Are you studying buckling in the plane of the truss or perpendicular to it? Liberalization is usually more appropriate in the plane of the truss.

 

[MIStructE_IRE]

This relates to my other thread on the failed truss.. I have other (non failed) trusses within the building to either condemn or advise what load they can take.

Top/bottom and internals are all welded steel angle sections. Out of plane of truss I feel 1.0L is realistic as the bottom chord is only restrained at intermediate points. Top chord is restrained by the deck.

I’m considering

 

[sbisteel]

Might want to take a look at table 4.3-1 of the SJI specification to see what they typically use.

https://steeljoist.org/wp-content/uploads/StandardSpecs_K_LH_DLH-Series_0820.pdf

 

[r13]

IRE,

Have you found out what was the cause in 0your case?

 

[KootK]

Cudos to SBI. I wanted to recommend the SJI values as both reasonably liberal and nicely defensible as well. I just didn't know where to find them in short order. In my Googling, I stumbled upon a pretty spiffy thesis from Bucknel that would suggest that even the SJI recommendations are quite conservative: Link. I only read the conclusions section, some of which is repeated below.

Caveat: all of the research involved symmetrical, double web setups.

 

[MIStructE_IRE]

Retired13 - the jury’s still out on that one. The truss appears to be overstressed - BUT it looks like it took a bang also. So hey, we were ALL correct!

Interestingly we found buckling of another truss during further survey works today so I think there’s a bigger issue here.

 

[r13]

IRE,

Thanks for the update. It is quite intriguing event, wish to know more.

 

[MIStructE_IRE]

Yeah, I intend adding to the other thread once I know more. So I won’t leave you all hanging!!