Eccentricity in shear tab design 3

[civeng]

Hi everyone,

My question is this:
When designing a shear tab (extended), can we say that the load passes through the center of gravity of my bolts and consider the eccentricity only on the welds of the shear tab to the web and flanges of the supporting member,and verify the welding with the eccentricity.
Or do I have to consider the load as passing through my welds and consider the eccentricity in my bolts.
Or do I have to consider my eccentricity in establishing my bolts AND my welds.
I have a bunch of shear tabs to do and want to make sure that I'm not going the wrong way.

Thanks

 

[271828]

Read the 13th Ed. Manual Pages 10-101 through 10-103. The answers to your questions should be there.

 

[UcfSE]

Make sure your building code references the 13th edition before you use it. Otherwise use what is referenced.

 

[civeng]

Unfortunatly, I left my manual at the office for the holidays.

 

[271828]

OK, because your Manual is away and you're curious (not just not wanting to look it up, LOL ). You'll still have to go to those pages to get a lot of info.

For tabs that fall under "conventional configurations," you don't have to consider eccentricity for most cases (depends on number of bolts and hole type).

It is actually considered for all cases, but you don't have to think about it during design--it's buried in the method. The C factor one would get from Table 7-7 is more than offset by the insane usage of 0.8 in Fnv to account for >50" end-loaded connections--tabs ain't one of these, duh.

For tabs that fall under "extended configurations," eccentricity is the distance from the weld line to the centroid of the bolt group. Note that the Manual erroneously gives e to the first row of bolts.

As for using the 13th Ed. vs bldg code adoption, that is a matter of debate, IMO. For something like this, unless somebody made me, I'd use the newer research. I worry about mixing and matching old & new mainly when it's possible to have incompatible load and resistance factors.

 

[civeng]

Thks for the response.
I read a paper entitled "Design Of Extended Shear Tabs Report", written in 2002, in which they establish the number of bolts by taking into account the eccentricity, which gives alot more bolts than when considering the load passing through the bolts (without eccentricity). The the paper verifies the weld to the web and flanges with the eccentricity. So, basically, they seem to take the eccentricity into account for both bolts and welds.
In another connection design, done by another engineer of another office, he establishes the number of bolts by passing the shear throught the bolts without eccentricity, and then designing the welds with the eccentricity. So he basically has less bolts than the research paper I read.
The other engineer's design is a little more tempting since it keeps the connection lighter (less bolts), but the research paper seems to have more security. What school of thought to use? I've spoken with another engineer who also thinks that one should choose where the load should pass (bolts or weld) and design then design the other for eccentricity.

 

[271828]

"What school of thought to use?"

Not trying to sound like a smarty-pants, but I'd use the one agree upon by folks like Bill Thornton, Larry Muir, Tom Murray, Tom Ferrell, and the rest of the AISC Manuals Committee. That school of thought is clearly prescribed in the 13th Ed. Manual. No need to fret, guess, or take sides.

 

[nutte]

The building code adopts a specification, not a manual. Thus, one could use the procedure in the 13th manual with a clear conscience, as it does represent the latest research on the issue.

The shear tab procedure is one that changes often with the new manuals. The 13th is the first one to address the extended configuration.

If you're doing connection design that will be reviewed by someone, like the EOR, you may have trouble selling the 13th manual procedure if the building code adopts an earlier specification. If that's the case, the procedure you should use may be different.

 

[Lion06]

I have a question about this. First of all, the manual tables in section 10 don't use the C factors for eccentrically loaded bolt groups unless an "e" is considered, which in most cases, it isn't.
I can almost see ignoring the "e" on the bolts if the tab is welded to something very rigid like a column flange (then it will be acting like a small cantilevered outrigger). When it is welded to something that is free to rotate, however, (such a a girder web), I don't know how to justify ignoring the "e" on the bolts. In that instance it almost seems like you are allowing 2 pins (which seems unstable - unless you consider some catenary action which is also unwarranted). The manual makes no distinction between a rigid support or a flexible support and even in the cases where it does consider an "e" for the bolts, it is not the full "e" you would expect (from the weld to the centroid of the bolt group).
It is only for the extended configuration that you would consider "e" on the bolts for all cases. It also says for the extended configuration that you can design the column without considering any "e" from the connection. I also don't understand how that makes sense. There is clearly an "e" in the application of the load (certainly if connected to the flange), how do you justify just ignoring it?

 

[271828]

nutte wrote: "The building code adopts a specification, not a manual. Thus, one could use the procedure in the 13th manual with a clear conscience, as it does represent the latest research on the issue."

A star from me. That is excellent and very true!

Now the question: will it be possible to convince an EOR (who knows about enough about conn. design to be dangerous, LOL ) that using the procedure in the 13th Ed. Manual doesn't violate the 9th Ed. ASD or 2nd Ed. LRFD Spec.!!!

 

[271828]

Hey, not meaning that comment about EORs as insulting to the EORs around here--it's all in good fun!

Most EORs don't spent a lot of time digging through conn. design research papers.

 

[271828]

StrlEIT, there are two issues, eccentricity for conn. design and eccentricity for column design. The Manual procedure is strictly addressing conn. design.

For connection design (conventional config.), eccentricity isn't being neglected and does exist. The effect of eccentricity is to decrease the bolt strength by some amount (i.e. C factor). However, the Manuals Committee recognized that Fnv has a 0.8 factor in it that should not be there. See the new DG on high-strength bolts for more info. Several experts have recommended removing the 0.8 factor to no avail because it absolutely does not apply to this kind of connection. If this 0.8 factor is deleted, then Fnv goes up by as much or more than the effect of eccentricity pushes the bolt strength down. Therefore, one can design the conventional connections considering no e and be safe.

For the extended config. e is in there for conn. design, so I don't think you have an objection.

For member design, it is up to the EOR to offset the load a little to account for eccentricity. For example, the RAM system provides a variable for this offset and then computes moments to apply to the column. Nobody ever said not to do this--to my knowledge anyway (impossible to prove non-existence, LOL).

Let me know if that helps. I have a lot more info on this subject.

 

[Lion06]

I don't have my manual in front of me, but I believe at the end of the extended configuration discussion, it explicity states that designing by the extended configuration allows you to NOT consider eccentricity in the column design. It is page 10-102 or very close to it. I'll check when I get in tomorrow and report back.

 

[271828]

Holy crap, you're right! Right near the bottom of 10-103. I'm sure they had their reasons, and will have to look into this for my own curiosity.

One good question is why such a statement would be in a connection design section. Seems like a subcommittee stepping onto another's toes.

 

[WillisV]

The manual is allowing the designer to choose how to handle the eccentricity. The second part of the statement regarding how to handle eccentricity is "in some cases, economy may be gained by considering alternative design procedures that allow the transfer of some moment into the support...".

Really all you are doing is shifting the moment diagram of the member. For the method in the manual, where you are considering eccentricity at the bolts and no moment applied to the column, the point of zero moment is being considered at the face of the connection (imagine the moment diagram starting at zero at the column and smiley-facing from there - you are designing the bolts for the moment at the bolt location).

Alternately, you can say that the point of zero moment is located at the bolts, design the bolts for straight shear, and then design the weld as well as the column for the moment due to the eccentricity of the reaction at the bolt line. This is the same as thinking of some "fixity" pushing the moment diagram up until the point of zero moment is at the bolt line and designing the column for the "negative" moment.

Either way is acceptable.

 

[Lion06]

I am not sure I necessarily agree with that philosophy. I know I read somewhere that you should ALWAYS consider the eccentricity on welds, the main reason being that a weld is not susceptible to slip like bolts and are inherently stiffer than bolts.
Additionally, even if I could convince myself that you could ignore the "e" on the weld and just design for it in the bolts (which wouldn't be hard to do if I sat down and thought about it a little - I would be more inclined to go that route if it were welded to something that is more likely to rotate such as a girder web instead of a column flange) and say that the weld is applying a simple shear to the column flange, it is still applying it at an "e" of 1/2 of the "d" of the column. How do you just ignore that? It is clearly not applied at the centroid of the column.

 

[WillisV]

StructuralEIT -

For a connection welded to the column web, the eccentricity is zero. For the connection welded to the column flange as you are talking about, you are correct that there is still an eccentricity of d/2 of the column even when using the method of designing for the moment at the bolts. This eccentricity is typically ignored primarily on the basis that the moment produced is generally negligable compared to the strong axis capacity of the column (not always - but usually). Also, in most interior cases some portion of this moment would be canceled out by the reaction on the other side.

For further discussion on the effect of connection eccentricities see p10-133 thru 135 of the manual.

 

[nutte]

271828 - Thanks for the star. I appreciate that coming from a prominent member such as yourself.

The part about convincing the EOR of this is the tricky part. If they're making the calls, sometime you have no choice.

StructuralEIT - I believe the eccentricity on the bolts, column, and supporting member (column or beam) has been neglected because of prior investigation and succesful use. These connections have performed well in the past, so within some certain parameters, assuring the connection is "normal," the effect of this eccentricity has shown over time to be so small that it can be safely neglected. I would call this a triumph for those who would like to see procedures get simpler rather than more complex.

Again, this procedure gets updated with nearly every manual. Sometimes it gets more complicated, but this time it has gotten simpler. We don't have shear tabs falling off all around the country, so perhaps we (the engineering community) have been trying to make it too difficult.

 

[271828]

"271828 - Thanks for the star." --> My pleasure. That is a very astute observation and very useful.

This is a little off the subject, but it's Friday afternoon, so here goes. Pardon me while I philosophize.

I find it strange, in a way, that there is interest in shear connection eccentricity for gravity column design. There have undoubtedly been millions upon millions of square feet of steel floors supported on columns designed for axial load only up until about 1995 +/-. I dare guess that folks started considering them when the option was put into Ram Steel (mid-90s?). Now it seems weird to some to NOT consider those eccentricities. It's like the software drove the design practice. I'm not sure why, but I find that to be a little disturbing.

FWIW, I've worked in three strl firms, the last two being very sizeable and respectable. At those last two, we worked on some huge projects and the principals had combined couple hundred years of experience. We did not have Ram Steel and just designed typical columns for axial load (gasp!!!). Weird conditions got special treatment.

Anyway, just some random random thoughts.

 

[nutte]

I'm with you. By letting the software dictate our design process, we are losing touch with how the structure behaves. Hopefully this slippery slope won't lead us astray.

I work for a small firm. We do a lot of connection design, so I believe that the engineers in our office know a lot more about complicated steel happenings than the average engineer.

For the typical case, we don't put any eccentricity on gravity columns. Maybe we're crazy.