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Light weight truss:

Light weight truss:

Light weight truss:

(OP)
I have to design/build, lift and install offshore, a trussed arrangement. The requirement is to have a box of 1.2 m wide and any suitable depth. So, I have come up with two trussess spaced at 1.2 m and depth of 400mm (something like steel joists). To minimize weight and as per requirement, the truss is composed of 2 EA 100x100x8 top and bottom members. The digonals and verticals are made up of 16dia and 25 dia steel rods.

The two trusses are separated by a transverse at every meter by 100 x 65 x 7mm. And diagonals are attached at every two meters.

Currently, I am looking at around 35m of this box section to be lifted and installed in one piece. Can somebody suggest/guide if there is any technical issue while using angles or the arrangement considered. The structure is checked with SACS considering Lb of 2m and Lx/Ly of 2 m for top and bottom members of EA 100x100x8mm.

The sections considered are

Two EA 100 x 100 x 8 mm @ top and bottom

Top transverse brace UEA 100 x 65 x 7 mm - transverse

Diagonal chords EA 60 x 60 x 6 mm

The truss diagonals and verticals are 16 dia and 25 dia round bars.

Attached sketch shows the proposed arrangement.

Thanks & Regards,

Anurag

 

RE: Light weight truss:

Have you analyzed this section?  I would expect a lot of deflection from your configuration, given a 35m span, even if lightly loaded.

Nothing wrong with your approach...it's common; however, since you have given no loading info, it is not practicable to evaluate its efficacy.

RE: Light weight truss:

(OP)
Thanks Ron.

There is no loading on the Truss. It's self weight will be only loading it will carry. For 35m, it will be around 4 T. So effectively 1kN/m self weight.

The purpose is to stick the photogrammetry stickers to carry out the survey.

Regards,

RE: Light weight truss:

You still need to analyze the frame to carry the self weight load.  Is SACS an analysis program you used?  Does it give you a deflection?  For a 35 metre simple span, I would expect it would sag about 500mm under 1 kN/m loading.  Doesn't sound stiff enough for photogrammetry, and will be very uncomfortable for anyone who has to walk on it.  Suggest making it deeper.

I would arrange the truss webs so all the diagonals are in tension and all the verticals are in compression (Pratt truss configuration).  And I would use something other than round rods, say angles, for the web members.  Makes the welding simpler.  

RE: Light weight truss:

(OP)
Thanks hokie66.

Yes, SACS is an analysis program used for analysis.

Considering lift points at 1/4th distance from each end, the deflection is of the order of 20/25 mm. which i felt reasonable for the case. The structure is modelled as per the sketch.

The spool will be carrying stickers to carry out photogrammetry, and nobody is expected to walk on it. The stickers will be installed onshore, in yard.

Thanks,

 

RE: Light weight truss:

Lifting it at 1/4 points makes a BIG difference.  I thought you were spanning 35 metres.

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