Assigning a force to a truss

[Anonymous]

Good Morning.

I'd like to knowif there is a way to assign a force of compression or tension to a truss element.

Thank you,

Daniel

[StefanoPasquini6790]

Hi, what do you mean exactly???

Can you send a scatch of your needs. There are many ways to assign such kind of force in a truss elements, it depend by your needs...

Waiting for more specs, cheers

[Anonymous]

I have to replace the braces with the axial forces that develop in tension and compression (for capacity design).

I want to run a static analysis in which these forces to check braces. It is similar all'assegnare a thermal deformation element, but I would avoid this solution so inelegant. There is no option to assign to upwind some sort of force to pre-voltage or pre compression?

Devo sostituire dei controventi con le forze assiali che sviluppano in tensione e in compressione (per il capacity design).

Voglio girare un'analisi statica in cui assegno dette forze ai controventi. É simile all'assegnare una deformazione termica all'elemento, pero vorrei evitare tale soluzione cosí poco elegante. Non c'é un'opzione per assegnare al controvento una sorta di forza di pre tensione o pre compressione?

[StefanoPasquini6790]

Hi DTHepta,

Answering to your question, you can assign a precompression load to your bracinga, following this two simply steps:

But, if you are working in capacity design (gerarchia delle resistenze) for steel structures, I strongly suggest you to take a look to the Italiy NTC2008 Steel tools, 

with this you can manage all the requirements of the NTC2008 cap.7.5 (and of EC 8). You need only to have a little bit of training with this tool.

Hope I help you, ciao.

[Artur.Kosakowski]

I'm not sure if I understand you correctly but if you want to design a bracing against axial force you may do this in the following way:

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[Anonymous]

Thank you,
the problem is that I'm working with the US standard.

I have to calculate a model in which replace the braces with their maximum forces in tension and compression.

[Artur.Kosakowski]

The middle model is without bracing so that you can see that the forces I applied at the ends of "dummy" cantilevers are equivalent to the complete (left) model. In the right model I deleted all loads but the "maximal" tension force in bracing. I hope it is what you need. 

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[Anonymous]

Last night I also thought of using a dummy beam splitting into three parts braces and erasing the central part (the two sides have to be small).

Doing that the system can become unstable. We must put external supports. To do that we have to use a good way, so is not discharged a greats forces to this supports.

What do you think?

[Artur.Kosakowski]

For such models (e.g. a frame with pinned supports and pinned connections between columns and beams with an inclined bracing) you will need to use either temperature or dilatation load applied to the bracing (or just the forces from my "cantilevers") but actually keep the bracing in its position. I'm not sure what the US code approach is for such structures but I don't think you can simply replace (= remove) bracings by forces.

Can it be that that this provision applies e.g. to moment resisting frames only ?

[Anonymous]

I share the standard AISC 341-10. Page 52 in the pdf.

https://www.dropbox.com/s/jgyfjgtc3zmad65/AISC%20341-10%20Seismic%20Provisions%20for%20Structural%20...

[Artur.Kosakowski]

If I understand the concept of SBCF correctly you should not have a situation when such frame is unstable after removing bracing. What is more I think that actually the approach we discussed previously is the correct one however again I apply my common sense to this topic rather than my knowledge of AISC code which I'm not familiar with. 

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