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Beam Splice Model

Beam Splice Model

Beam Splice Model

(OP)
Please see attach sketch. One splice in one span works fine. But when I placed two splices, left most span, both sap and staad show instability. But it is a real life situation and a evry senior engineer is doing it manually. I just wanted to cross check and learn using s/w. Any tips ?

RE: Beam Splice Model

The first pin on the far left (just to the right of the far left support) should not be there.  You can't have a pin like that and remain stable.

Also, it appears on the far right spans that you have double pins (two small circles side by side).  A joint must have at least one non-pinned member framing into it.   

RE: Beam Splice Model

JAE is correct.  I hope it is not "a real life situation".

RE: Beam Splice Model

Quote:

But it is a real life situation and a evry senior engineer is doing it manually. I just wanted to cross check and learn using s/w. Any tips ?

What is s/w?  If senior engineers are doing this, they ought not to be senior engineers.

BA

RE: Beam Splice Model

I believe that s/w is software.  At least that is what a few other engineers at my office call it.

Joel Berg

RE: Beam Splice Model

I assumed as much.  Is it too much trouble to write the word software, particularly when you are expecting members of the forum to provide thoughtful answers to your question?  

If all supports are truly hinged, then the structure is stable, but I seriously doubt that every senior engineer is "doing it".  

My tip to you is "don't do it".  

 

BA

RE: Beam Splice Model

It's not stable.  The span on the left is the issue.  It has a mechanism in it.  If the support farthest to the left was fixed instead of pinned then it would be stable, but as it stands, it isn't stable.  It shouldn't take software to see that.

If you draw a FBD of each individual member you will see that the two left-most members are like a simple span beam with a hinge along the span - again, it's a mechanism.

RE: Beam Splice Model

Spans 2 and 4 are also problematic, not just span 1.

RE: Beam Splice Model

The thing is...with a four span cantilever and drop-in system, you can only have three hinges and remain stable.  The sketch shows six, although in two cases they are sort of double hinges, which makes no sense.  Perhaps the OP can advise what he really means.

RE: Beam Splice Model

I agree with Hokie here.  This setup makes no sense in the real world.  It is inherently unstable three times over.

Mike McCann
MMC Engineering

RE: Beam Splice Model

dgkhan,

I agree with what the previous posts have said.

Only one of those thre spans is truly stable.

In real life, a system of members with higes at each end is called a 'chain' great for tension but not much good as a beam. Three of your spans contain a chain link.

It is a worry that neither you or your senior engineer picked this up.

RE: Beam Splice Model

Could his "splices" be coded wrong?  

Assume his beam was a W12.  

Each approved splice (what we see as a single "pair" of bolt holes in the software) could be actually a pair of 10x6x3/8 thick plates with 6 bolts through each web, making a sandwhich joining the two beams.    

RE: Beam Splice Model

If all supports are truly hinged as shown, the member can have any number of hinges throughout its length and exhibit a form of "stability" similar to that of the flat arch in the recent thread "A Tricky Question" by the same OP.  The members would be primarily tension members, not bending members.  They would need to stretch and deflect enough to enable them to carry the tension.

If all but one of the supports is changed to a roller, more of a real world situation, then I agree that the structure is unstable.
 

BA

RE: Beam Splice Model

True, but in the real world, what would be the point of setting up this configuration for a tension only situation?  This scenario does not seem likely, let alone feasible to me.

Mike McCann
MMC Engineering

RE: Beam Splice Model

(OP)
I have attached a more clear sketch. My question is how to model splice in S/W (software).
Question 1. I think both beams meeting at splice release the moment therefore I showed two circles of moment release at splice location?
Question 2. When there is two splices in one span than S/w always shows instability? any way to overcome it

RE: Beam Splice Model

1.  It is just a diagram.  We were not clear about what you meant with the two circles close together.

2.  There can be two splices in one span provided the adjacent spans continue through to the next support.  That is called a drop-in span.

You cannot have two splices in an end span, so remove the splice near the leftmost support.

If you wish, you can move the next splice to the right of the second support.  You will have two splices in the second span and the structure will be stable.

BA

RE: Beam Splice Model

can he have a moment splice and then not model it?

RE: Beam Splice Model

Well, that was the intent of my question: He HAS a moment splice (or several moment splices), and HAS modeled it (them) incorrectly.


So, back to the orignal question.  Draw - not in your modelling siftware, but on a piece of paper - then scan it - exactly HOW the splice is constructed.  Not how it is modelled.  How it is built.  

RE: Beam Splice Model

Is the support at the left a wall of another building with, perhaps, the left most beam actually continuing further to the left to another support?  If so, then this span would be stable.

Mike McCann
MMC Engineering

RE: Beam Splice Model

(OP)
I do not know the splice as-built details. It is existing structure. Ignoring splice, beams start to fail in moment if treated as simple supported. Obviously, it should not be the case. So by looking closely at vague drawings, I figured out splices. In my understanding, purpose of this mechanism is to reduce moment and make this beams work. Left most span is not the last one but it continues. Probably I will increase my model and see the effect. I can do it manually but will loose money on it. It is a large frame.400 x 400 feet. So wanted to use s/w(soft ware) to run down time. Thanks for all the help.

RE: Beam Splice Model

Left most span is not the last one but it continues...

AhA!  OK...

Mike McCann
MMC Engineering

RE: Beam Splice Model

No.  The splice details ARE what you need to know.  Get a photo of the beam.  Get a ladder and go MEASURE the dumb thing.   Get the diameter of the actaul bolts (or rivets.)  Find out the distance between rivets/bolts/length of the welds around the splice plates.  Thickness of the splice plates.  

More "analysis" of a WRONG connection definition inside a problem will only waste your time, your company's time, and give you additional WRONG answers.    

RE: Beam Splice Model

If the beam is continuous beyond the leftmost support, the structure appears to be stable, at least the portion that we can see.  

It is probably easier and faster to check this beam by hand than to use S/W (software).  The original designer would likely have considered each splice to be a point of zero moment.

BA

RE: Beam Splice Model

Numbering them 1 to 4 from the Left, splice 1 has to be a moment connection.  Why the splice? I dunno.

Dik

RE: Beam Splice Model

Dik,

The beam to the left of Splice #1 continues left beyond the column to another support.

dgkhan,

As stated earlier, it should be easy to perform all calculations using hand methods because each span is statically determinate.

If you want to model only four spans of a multi-span  structure, move your left support to Splice #1.  The leftmost span is a simple span, sometimes called a drop-in span because it rests on the ends of a cantilevered beam at each end.

What is the purpose of this exercise?  Are you simply checking an old design or are you proposing changes to it?

I don't know which code you are using, but be sure to check for patterned loading in accordance with the code.  When the cantilever portion of a beam is fully loaded and the span is unloaded or partially loaded, the bottom flange has a large unsupported length and probably requires lateral bracing.

BA

RE: Beam Splice Model

dgkhan, I remember several decades ago that engineers would use erector sets to visualise stability of structures.  May be you should invest in such set.
 

RE: Beam Splice Model

"I do not know the splice as-built details."  as other posters have said already, you need to know the details of the splices; critically, are they moment capable ?
"Ignoring splice, beams start to fail in moment if treated as simple supported."  do you mean analyzing each span indiviaully, rather than as a continuous span beam with several supports ?
"So by looking closely at vague drawings, I figured out splices. In my understanding, purpose of this mechanism is to reduce moment and make this beams work." A splice cannot reduce moment; now it is possible i guess that if the spilices intentionally don't react moment, that the design thought was to create a structure that reacted the applied loads as axial load in the members, which would mean that the supports react transverse load (assuming vertical loads).  this would probably cause very large deflections ... not sure why you'd have two hinges close together.
"It is a large frame.400 x 400 feet." ... is the structure a frame or a beam ?
 

RE: Beam Splice Model

If your only issue is one of stability then either continue the modeling to a point where it is stable or just fix the left end of what you have shown.  That will give you almost the same effect as continuing the modelling to the left.  I would also like to say that this is different than any other "drop-in framing" that I've seen.  It is typically a beam over two columns with a single drop-in beam between the ends of two cants - similar to your third span.  I've never seen it where a beam is supported on one column and the end of a cant.  

RE: Beam Splice Model

SEIT,

While not as common, some engineers do (or did) it that way.  Some prefer it, as erection can be from end to end, reducing the amount of temporary support required.

RE: Beam Splice Model

hokie66, I've also usually done it the way you describe, with beams starting at one end and cantilevering over the next columnn, then the next beam extending from cantilever end to next column, then forming the next cantiliver.  

No problems to date but one erector one time pointed out that if the first beam gets damaged or looses support, the whole beam run becomes unstable - sort of a progressive collapse mechanism.

 

RE: Beam Splice Model

JAE,

That would be true, while doing it the other way, the collapse mechanism would be confined to at most 3 spans.

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