Reinforcement of existing Truss frame 3

[zbinova]

Dear all,
I am doing a project which involves reinforcing existing truss frame.. I have never done this before and personally do not know anyone who has done this. So if anyone having some experience in subject work can guide… refer to any book… preferable Code…
Please note that, the already constructed frame is 3 years old and has undergone deformation due to wind load.
I don’t know what is the code allowance for such structure?
Design Methodology?
Methodology to stress relieve the structure?
Do we have to tack weld the new structure initially?
Proposed methodology and sequence?
Any other step/sequence or precaution needed to be taken during execution?

 

[Ron]

You have not given sufficient information to allow us to help you. Where are you located? Have you ever analyzed trusses for code-applied loading?

I would suggest that you get a local structural engineer involved. To repair a truss damaged by wind loads implies that you had a significant wind event that perhaps exceeded code levels and thus the truss failed by overloading or you had a wind event that was less than code level and you had a deficiently designed, fabricated or constructed truss that must now be analyzed for its deficiencies. This is not something that you should do if you are not familiar with structural analysis and code requirements.

 

[MiketheEngineer]

Ditto to Ron

 

[zbinova]

Thanks for your reply Ron. I am located in Pakistan. And Yes i have not done this before . I have analyzed and designed trusses for code-applied loading but have no experience in reinforcing and already constructed truss.
This is a mistake on my part I should have been more clear. By deformation I did not meant failure, just that structure is in usage for designed load and have encountered some wind so I assume the structure must be stressed to some extend and may have deformed under allowable limits.
I was not able to find any methodology for reinforcement of such structure in AISC, ASME or any other code.
Any detail you require I will be happy to provide. Please bear with me.

 

[apsix]

Photos, sketches, diagrams etc would help.

At the moment we don't know what the truss looks like and don't know the nature of the 'failure'. In fact, it's unclear if there is a failure, quoting; "I assume the structure must be stressed to some extend and may have deformed under allowable limits".

You need to define the problem before attempting to find a solution.

 

[Ron]

zbinova...yes, sketches and photos will help.

If you have had a wind event and it was insufficient to overstress the truss system, then it should not have permanent deflection or deformation as a result. It sounds more as though you have a construction deficiency, a fabrication deficiency or a general underdesign.

Wind loads are temporary loadings and usually result in elastic response from the structure.

 

[zbinova]

The already constructed truss has D.L of 40 Kn, now we have to add new platform to it. the Dead load will be double i.e. 80 KN.. we have checked that the existing truss cannot take this load. That is why we have to reinforce this truss.... I hope now it is clear. I am trying to upload the sketch but its not uploading...

Now going back to original post... if i add new sections .. how to do it.. should i jack the whole truss..??? ..any ideas guys....

 

[Ron]

What is the truss made of? Is it wood? Is it steel? How are the joints constructed? Have you checked the shear in the connections? If steel, do you have on-site welding capability?

Also, how is the primary load applied to the truss? Is it bottom chord dominant or top chord dominant? Is the truss flat or pitched?

 

[apsix]

And how long is that piece of string?

 

[BAretired]

What piece of string?

BA

 

[zbinova]

It is Steel Truss. Welded Connections , yes we have on site welding capability. We used Staad Pro to design the truss.
Plz find attached sketches

 

[zbinova]

file 2... i don't seem to be able to upload all files in one post....

 

[zbinova]

existing truss .. previous file was proposed strengthening...

 

[apsix]

That's certainly a substantial truss!

1. Is it possible to drop in an extra 1 or 2 identical trusses rather than strengthening the existing?

2. The support strengthening changes the support condition fron pinned to (partially?) fixed. Has the consequences of this been assessed?

3. It will be wise to specify intermittent welds only up to the capacity required, as the strengthening shown requires a lot of welding.

4. Propping to eliminate the dead load deflection while strengthening is recommended.

 

[zbinova]

Thanks Apsix,

No we cannot drop any extra trusses. we already have 24 truss frame.

Yes done that i fixed it in Fz and Fy but still not much difference.. however i will try to fully fix it and check the foundation for the support reactions.

3. That is a good idea.

4. Yes, i am particulary looking for a methodlogy to execute such work . I have to submit a report and methodlogy for the execution... and i am stuck with methodlogy.. any code... Book... paper which can assist me ???

 

[Ron]

zbinova...that's a substantial repair to a substantial truss! You are adding significant dead load to the truss, with lots of welding. I agree with Apsix on the intermittent or stitch welding.

How are you modeling the connections in the truss? Are they pinned or fixed?

It appears that the top and bottom chords were underdesigned.

One fabrication problem I see is that of keeping the additional angles aligned in the inverted position. That will slow your production down. Tube sections would be easier to work with.

 

[MikeHalloran]

>>> The already constructed truss has D.L of 40 Kn, now we have to add new platform to it. <<<

I'm not seeing where the platform connects to the truss.

Or did I misunderstand?



Mike Halloran
Pembroke Pines, FL, USA

 

[BAretired]

The bottom chord braces nearest the center of the truss intersect the bottom chord half way between panel points. Can you lengthen and lower these braces so that they intersect at a panel point? See attached for this suggestion.

BA

 

[zbinova]

Thank you all for your input.

BAretired ... the panel point you indicated is the very place the new platform would be fixed. but i will still check that if there is headroom I will definitely try out your suggestion.

Mike ..... I think that answers your question too.. The platform is not in the drawing but I have applied the load of the platform in my model.

Ron.....I have assigned the member truss members in Staad model, therefore there is no moment at the connections …

 

[Ron]

zbinova....you might run your model with the connections fixed and see what it does to your stress distribution. After all, the connections are welded and they will transfer some moment.