Double angle check for flexure and axial load

[bagigio]

I am designing a steel trellis with some custom made L-shaped bent plates. The bent profile i am using is a double L-shaped profile, 5" x 28" x 1" thick, with a 3" space in between.
How do i check the flexural properties of this shape? If one end of this profile is a cantilever piece, i do i check the unbraced length?

Thank you

 

[racookpe1978]

Draw the shape please. Your description is not clear.

 

[bagigio]

Please see attached section profile. This is a 40 ft long beam, with a 18 ft cantilever at both ends.

Thank you

 

[KootK]

You might look into SJI provisions as this bears considerable resemblance to a joist top chord. The conventional strategy has been to stitch the pieces together often enough, and well enough, that the composite section member would fail before the segments of non-composite member between stitches do.

Basically, you're creating a built up WT here. And WT's have abysmal resistance to twist. As you've suggested, you'll want to play close attention to those long cantilevers.

FYI1: bending that 1" plate will be a chore an you're likely to wind up with an outside Ben radius approaching 4".

FYI2: you're span to depth ratio is 96. That's getting out there, even for a lightly loaded trellis member. There could be ice build up and potential for wind flutter.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bagigio]

Thank you for your suggestion.
How do you calculate the stitch spacing? Would you recommend a full height stitch plate or can i use a small plate at the bottom and one at the top?

Thank you

 

[canwesteng]

Check shear flow if you have any weak axis moment and design for that, and make sure the stitch spacing is such that two times the unbraced moment capacity of the angles (with the unbraced length being the distance between stitch points) is always greater or equal to the moment capacity of the composite section. Then make sure your L/r of the whole section is greater than the L/r of an individual angle, with L for the single angle being the distance between stitch points. On top of that, when using composite angles a rule of thumb at my company is no further than 4 feet apart for these connections.

 

[bagigio]

If i change my double angle profile to the attached new section (basically a WT profile with two stems), what provisions of AISC should i follow for checking of lateral torsional buckling, tees local buckling, etc.?

Thank you

 

[structSU10]

You would only be able to weld one side of one of the plates at best, which may be costly with plate bevel and whatever weld you apply.

Other than that, you can use the provisions related to local buckling and such for the stem and flanges based on compactness criteria. You could probably apply the overall WT equations, just using modified torsional properties.

There is no way to get a standard section to work? Are you trying to make this 3" gap to accommodate some specific connection?

 

[bagigio]

It is just an architectural constrain. the architect wants a 3" gap to visually distinguish the two stems from below.