Bracing advice needed

[trazimcalvina]

Hi all,

Recently a customer came to me with a sketch of a steel construction based around secondary bolted trusses onto a main truss (Sketch below). The main fixed demand is that the depth of the truss is 0.65 meters.
I personally found the truss depth of the primary truss (8 meters) to be too small at first glance but I am even more worried about the very real possibility of the main truss tipping over due to all the trusses being bolted into it. What's worse I don't really see a way to brace it against such an eventuality.

Am I missing some obvious solution or is this just hopeless?


I have attached the sketch below, please note that no secondary steelwork is included including ofcourse bracings.

https://files.engineering.com/getfi...8-e1a2-4892-8f49-0baa341881be&file=Image2.JPG

 

[1503-44]

I generally estimate minimum required truss depth as 10% of span length. Note that is for the entire length of the span. I note that you have a sloping top chord, mansard trusses, so that will not translate into much effective depth across the entire 6m span. Your deflections and vibration potential and rotation of the end joint are likely to be high towards and at the 0 depth end.

For the mansard trusses you generally need a lot more depth on the high side to make up for the lack of depth on the other end. Probably a minimum of 25% of span would be a good place to start. 33% might be more typical. It depends on load though. With a light load, you might get away with 25%.

The mansard trusses should be cross braced between themselves at the bottom chords.

The mansard trusses should be connected to the main truss at both top and bottom chords to keep the main truss upright and to provide lateral bracing for the bottom chords of the main truss.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[trazimcalvina]

Thank you very much for the swift reply.

I agree on the 10%, and I have found that lower percentages can work given light enough loads, which is definitely not the case here.
Here we have a truss that needs to carry the weight of entire seven additional trusses in addition to the normal roof loads (Dead load plus snow and wind). That is what made me anxious about its lack of depth.

Regarding the mansard trusses I honestly did not find them to be troublesome due to the relatively low load that each would carry (50 mm aluminum panels + snow and wind). I could be wrong though.

Agree on the cross bracing, however I am not convinced that simply connecting the mansard trusses to both chords of the primary truss would suffice to keep it upright and prevent rolling. I can't really explain why, maybe its my lack of practical experience, but I just find it unsafe to introduce such an eccentric load to one side of the truss. Please feel free to let me know I'm being paranoid for no good reason.

Regards.

 

[1503-44]

That just depends on the integrity of the mansards. It's impossible for the main truss to overturn as long as the mansards remain intact. You don't want them going into lateral buckling.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[human909]

I don't see a problem. A 650mm deep truss can readily span 8m. Of course the loads matter, so just size the chords accordingly.

And like 1503-44 says twisting of the main truss isn't of concern. LTB of the minor truss could be an issue under uplift if the bottom chord isn't restrained.

 

[trazimcalvina]

Thank you both for the advice.
I made some adjustments by eliminating unnecessary mansard trusses and adding some cross bracings to the lower chords.
As for the chord sizing 100x80x4 mm rectangular HSS seems to be able to handle the loads fine.

https://files.engineering.com/getfi...-54dc-4efc-92cb-e0658baf1f02&file=Image_3.JPG

 

[driftLimiter]

Why crossbrace the lower chords??

 

[human909]

Agreed what are you attempting to do but using cross bracing in that location?

It seems like you are just throwing members at a picture that your customer gave you without consideration of their structural role.

 

[driftLimiter]

Some more background on the actual structure, its loading, and any lateral requirements I think need be hashed out. Also don't forget the role of details in your modeling and design. The details of how these parts are will effect the strength and stability of your structure.

What code provisions are you using to check your members?