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Beam Splice Model

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dgkhan

Structural
Jul 30, 2007
322
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 ?
 
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Left most span is not the last one but it continues...

AhA! OK...

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

Dik
 
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
 
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.
 
"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 ?
 
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.
 
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.
 
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.

 
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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