Column Splice Criteria
Column Splice Criteria
(OP)
I have a simple pin/pin column that needs to be installed in two pieces. What would be the criteria for the splice. Obviously I don't want another pin cnx at the splice, but I'd rather not develop the full moment capacity either.
I realize that I'll need to put some nominal load laterally just for things leaning on the column. Also, I'll Fix the base as much as possible since that will make installing the upper sections easier. But, I'm uncertain if there's a rational method for the spice without other considerations.
Would using 2% of the axial force apply? (in my particular case I'd use more)
It's more of a post than a column . . about 14' high, maybe 6x6 tubes with less than 10 kips. But it is repetitive so I'd like to be economical.
Thanks,
Rich
I realize that I'll need to put some nominal load laterally just for things leaning on the column. Also, I'll Fix the base as much as possible since that will make installing the upper sections easier. But, I'm uncertain if there's a rational method for the spice without other considerations.
Would using 2% of the axial force apply? (in my particular case I'd use more)
It's more of a post than a column . . about 14' high, maybe 6x6 tubes with less than 10 kips. But it is repetitive so I'd like to be economical.
Thanks,
Rich






RE: Column Splice Criteria
I'm not sure what criteria ought to apply to the joint but joint flexural stiffness will be every bit as important as strength. In a non-welded scenario, I'd lean towards thick end plates with a copious amount of pretensioned bolts.
I bet some criteria for this exists out in the industrial world somewhere...
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
RE: Column Splice Criteria
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
I have some minimum loads I want to apply laterally . . maybe 400#. So I can easily design the splice for a moment generated by the 400# lateral load. But is that enough for stability? It would be more than 2% of the axial load, but I'm not sure if that's the right criteria.
I've spliced multi story columns often enough, but I think this is the only time I've spliced a single story column.
Thanks,
Rich
RE: Column Splice Criteria
I get where you're coming from with the 2% business but I don't think that it's applicable here. The 2% stuff is about nodal bracing forcing a second mode buckling shape on a continuous column. The problem here is about reconstituting flexural stiffness across the joint. Whatever the solution is, connection stiffness has to factor in somehow.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
Then check what moment shows up at the connection point and make sure your system's stable.
RE: Column Splice Criteria
RE: Column Splice Criteria
This has me seriously rethinking tiered column buildings where the splices are made 4' above every other floor. I've always assumed that the splices provided flexural continuity across the splice joint. Is that not the case? Should column splices be treated as flexural pins? I've worked on buildings with 12' floor heights where a typical 4' splice is pretty darn near mid span.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
RE: Column Splice Criteria
RE: Column Splice Criteria
What are you doing to transfer vertical load? If these are heavy columns they should be machined, and then an AISC minimum tension applied via welds or splice plates. If they are lighter weight columns, then the load can be transferred via weld or splice plates. These connections, to the best of my memory, have always been considered sufficient for stability.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Splice Criteria
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Splice Criteria
Suppose it's a 6"x6"x3/8" tube, you check your numbers and conclude a 6"x6"x0.035" tube would actually work, so you design that splice for the moment capacity of the 6"x6"x0.35" tube. You're just neglecting the excess material for that part of the check.
This assumes that load is independent of stiffness (should be, for pinned-pinned). And assumes that actual load is a lot less than capacity, or you wouldn't need to bother.
RE: Column Splice Criteria
Nice find StructSU10. It even covers exactly the case that I mentioned above (tiered columns). Unfortunately, the criteria proposed still requires one to know the connection stiffness. Based on the testing, however, the right bolted detail seems apparent: thick end plates and tight bolts.
It's also interesting to consider that the flexural stiffness of the splice essentially matches the un-spliced section until the prestress effect of the axial load is overcome. If only we could count on that from a reliability perspective. I'm sure that it helps many column splices to work in real life.
I disagree with this statement strongly. Deflection is not impacted substantially by spot reductions in flexural stiffness. However, stability is a different animal. Euler column buckling, at its heart, is a mathematical statement that, at all sections along the column length, restoring moments develop faster than P-Little Delta moments develop. As an extreme case, consider a column splice with no flexural stiffness at all.
And that's just elastic column buckling theory. If you consider inelastic buckling theory as proposed by Shanley and incorporated into modern code provisions, the importance of splice connection stiffness becomes even more apparent.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
The diagram you show is so overstated as to be nonsense, we are discussing reduced stiffness, not zero stiffness.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Splice Criteria
If you read through the paper that StructSU10 posted, you'll see that they also claim that localized stiffness reduction due to column splices has a significant impact on stability.
I contend that the sketch that I posted is quite sensible and germane to our discussion. Columns buckle when their flexural stiffnesses, including little P-delta effects, approach zero. If we're not going to discuss the attainment of the zero stiffness condition, then we're not discussing buckling.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
I've come around, partially, to your way of thinking on this one Paddington. I'll try again. more carefully:
The paper above says otherwise. We're kind of stuck with that.
Nope, it's not. I was wrong about this. P-delta moments grow as the column deflects laterally and the entire length of the column contributes to the flexibility that leads to that lateral deflection. So then, why does having such short lived increase in flexibility at the splice seem to matter so much? More on that in a moment.
I think that this would be a true statement were the splice an element that could be said to have a radius of curvature. A welded splice, as you suggested above, would be an example of this.
However, when the splice is modeled as a spring element as was done in the paper above, the radius of curvature is effectively zero. Any rotation within the spring results in rigid body rotation of the segments of the column above and below the spring. And that leads lateral deflection to accrue fast. I think that this is why splice stiffness has such a significant impact on buckling load. Of course, whether a column splice ought to be modeled this way is another question altogether.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough that I want to either change it or adopt it.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Splice Criteria
The model is of a strut with an axial load and which has been given a lateral push in the middle. The critical load is one that can just support and remain in equilibrium. On the application of a small increase will cause the strut to fail. Big deal, we already can get that load easily. But we need to know the lateral deflection at that point to know the design moment, and shear if the splice is anywhere but in the center. OP suggest he will have some fixity at the base so I am not sure where the "center" is although full fixity would be covered in the method.
By applying the conjugate beam method we can calculate both PEuler and the offset and thus, the design moment.
I am a generation or more removed from many of you and I am often puzzled by your nomenclature until I figure out that you are simply using a different name for something I know, E.G. when I started out in the UK, we simply had "long columns" and "short columns" and the implications were well understood. Other times, I stay puzzled.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin