Live Load Reduction for Connections 3

[TidePoolJunkie]

Hi all,

What are engineers out there doing for long span situations where a girder is eligible for a live load reduction (floor live load, trib area>400sf--ASCE 7), but the trib area for the connection at either end of the girder is <400sf (ie 700sf for the girder, 350 sf for the connection).

Thanks,

Kyle

 

[dik]

Generally the tributary area is too small for it to matter. I would resist using reduction factors for connections... not a good idea.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Aesur]

I typically don't see much that qualifies for live load reduction; however because interior columns have a KLL = 4 and interior beams KLL = 2, I would think the appropriate way would be to use the beam reaction including it's LL reduction.

 

[driftLimiter]

Its not the tributary area that has to be > 400 sf its the influence area. The influence area for a shear tab of a simple beam is the same as the tributary area of the entire beam. However the influence area for a beam is the full area between two adjacent joists. (i.e. 2*Atrib, Kll = 2 for interior beam) See ASCE 7-16 Commentary on chapter 4.

Although its a conservative assumption to not reduce the live load at the connection, I don't know that the code necessarily requires it.

IMO it makes sense to reduce the load at the connection for the same reason it makes sense to reduce the load over the beam. The code has indicated there is a low probability of seeing large areas of floor loaded to the maximum live load.

 

[OldDawgNewTricks]

As you have probably found, the code does not explicitly address this and therefore some engineering judgment is required. One could make a reasonable argument either way, (A) to use a different live load reduction for the connection than the girder, or (B) to use the same live load reduction for the connection and the girder:

(A) Say you have a steel girder with a discrete connection element to the steel column such as a sheartab. The connection is not technically a part of the girder, so it has its own tributary area and its live load reduction will be different than the girder if you follow the letter of the code.

(B) Now consider a cast-in-place concrete girder connected monolithically to a concrete column. Since there is no discrete connection element, the design of the "connection" would typically just use the girder reaction which includes the girder's live load reduction.

It doesn't seem entirely rational to use a different factor of safety for the connection design in cases A and B. So that's where the engineering judgment comes in. Some would argue that connections are critical elements and deserve some additional conservatism. But it would require more bookkeeping to keep track of separate live load reductions for the connections rather than just using the girder reaction load combinations, so I assume that many engineers just use the girder reactions as Aesur said.

 

[human909]

I find this discussion a little odd and perverse. Though it is likely it is just different codes.

As far as I'm concerned a connection doesn't HAVE a tributary are. It has loads that are imparted directly by the attached members, that is the reality as well as the model as far as I'm concerned. Thus the forces are directly those imparted by the member.

(A) Say you have a steel girder with a discrete connection element to the steel column such as a sheartab. The connection is not technically a part of the girder, so it has its own tributary area and its live load reduction will be different than the girder if you follow the letter of the code.

Why does the connection have its own tributory area? Surely the only tributary that matters is the gider?

 

[271828]

(deleted. Upon further thought, I'm not sure about this one.)

 

[human909]

I agree with the above comment both because it aligns with logical sense. For what it is worth this is the pertinent section in the AS code. (I realise that only a small percentage of readers here are using the AS code, nevertheless seeing other code approaches can give insight.)

I see the minimum shear values as simply a robustness requirement, it never really affects connection design in my experience. The minimum moment value does turn out to be a pretty harsh requirement in my experience but it makes sense because rigid connection are normally associated with indeterminate structures which can readily result in high than anticipated loads if build tolerances are different to the model.

 

[JoshPlumSE]

This is a perverse conversation.

Connections can ABSOLUTELY be subject to live load reduction. Let's say that a simply supported beam has a tributary area of 400 SF, uniformly distributed along it's length. Then the connections at the end have a Tributary area of 200SF. Let's then say that the simply supported beam also supports a column reaction at the mid-length of that beam. And, the column has a Tributary area of 1000 SF. Then each connection at the end of the simply supported beam would have a Tributary area of (400/2 + 1000/2) = 700 SF and you could take any live load reduction effects based on that tributary area.... Understanding that the connection supports more than one floor level (which is important depending on which code you are looking at).

There are more sophisticated ways to calculate Tributary area. But, the basic part I start with is applying a 1psf load across every area that has live load and seeing where that load is carried. If you get 500 lb reaction in a connection, then it has approximately 500 sf of tributary load.

Caveat: You have to come up with some way to manually manipulate this for cantilevers and possibly continuous beams. But, the basic premise is sound.

Note: I think this is VERY common practice in the design of multi-story buildings. Just take a look at what the major programs (ETABS, RAMSteel, and RISAFloor) do to handle this.

2nd Caveat: I was part of the original development staff at RISA when RISAFloor was being developed. If my memory serves me correctly, I don't believe the original program was capable of doing live load reduction with member reactions. Which led to a lot of complaints from ETABS and RAM users who had switched over. For the life of me I don't know if this was corrected or not before I left RISA in November of 2017 (when it was taken over by the evil empire).

 

[phamENG]

It's not that it's odd or perverse...it's that the whole question is moot. Though the confusion comes from the code itself and the annoying reliance on tributary area. As driftLimiter pointed out, it's all about influence area. Unfortunately, however, the gods of ASCE 7 have decreed that we shall not consider influence area, we'll consider tributary area modified by some arcane factor that, when applied properly, is incidentally identical to influence area. (Alright, so it's a little easier to do it that way when you start dealing with continuous beam systems, but they should still address it directly as influence area.)

I disagree with drift's assessment of the influence area of the shear tab, though. Just as the resulting shear in the beam is subject to an influence area equal to the full length of the girder multiplied by the full distance from the girders on either side, so too is the influence area of the connection. That's because the influence area is the area in which a change in the applied load cause a change in the resisted load of the member under consideration. Any change in loading within the girder's influence area will cause a change in the loading on the girder and, subsequently, the moment, shear, and reactions of the girder. So 271828, you can undelete your post - I think your initial intuition was correct.

Remember, the uniform loads we apply aren't just about a uniform load. It's about applying an approximate load for analysis that will result in a sufficiently strong, stiff, and resilient structural system to resist the real loading that causes the worst probable conditions. Live load reduction is just a statistics game. What are the odds that we'll see XXpsf over YYYsf all at once? The bigger the YYY gets, the lower those odds and the less approximate loading we need to consider.

 

[271828]

Revisiting after spending some time thinking about this.

I think it's OK to use the member reduced live load for connection design. My initial hang-ups were that ASCE 7 calls it "member live load reductions" and that I've heard of prohibitions on live load reduction for flat slab checks at columns.

[ul]
[li]The ASCE 7 commentary, C4.7.1, includes the statement "The use of influence area, now defined as a function of the tributary area, A_T, in a single equation has been shown to give more consistent reliability of the various structural effects." (emphasis mine) Presumably, "structural effects" doesn't just mean moment at midspan, but also would include shears and other force effects.[/li]
[li]For a simply supported beam, the influence line for shear at one end of the beam is a triangle that covers the entire beam. For a floor beam, the influence area for shear at the end would be the same as the influence area for moment at midspan or anywhere else.[/li]
[li]I looked everywhere I could think of and couldn't find a mention anywhere about live load reduction and connections. Amazingly, all AISC system-level examples I could find went around this question. I found examples where the same Mu was used for the member and connection design, but it didn't say whether the member Mu included LL reduction.[/li]
[/ul]

Unless someone can cough-up a reference that says LL reduction is not OK for connections, I'll certainly be using it.

 

[dik]

Just because you can, doen't mean you should... connections are the last place I'd want to save moneyk, unless you have a kazillion of them.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JoshPlumSE]

Just because you can, doesn't mean you should... connections are the last place I'd want to save money, unless you have a kazillion of them. pipe

Yeah, that was my impression too. I think the issue that RISAFloor ran into was that many RISAFloor beta users were using RISA-3D for smaller projects maybe up to 5 stories. But, when larger firms (that had been using ETABS and RAM Steel) checked out the program, they were disappointed with the lack of LL Reduction for connections. Those connection costs really add up when you're doing a 30 story building!

 

[KootK]

1) I am 100% in favor of using live load reduction for most connections and feel that it's somewhat irrational to do otherwise. Modern structural design is governed by reliability theory where we, quite rationally, split our attempts to be appropriately conservative into:

a) Stuff we do on the load side and;

b) Stuff we do on the member design side.

As such, if live load reduction is justifiable for a member coming into the connection, then it's justifiable for the connection as well.

2) I suspect that ASCE doesn't specifically address live load reduction for connections because they always intended for live load reductions to apply throughout the entire load path because of #1. They saw no need to explicitly say that some parts of the load path qualify and others do not precisely because that would be irrational.

3) The way that the setup works out for ASCE's live load reduction procedure is such that, if a member qualifies for live load reduction then, by definition, it's connections will also qualify for at least that much of a reduction. OP's located in British Columbia however and, therefore, likely working with the National Building Code of Canada. That will change the details but, in my opinion, not the overarching concept that any live load reduction applicable to a member should also be applicable to its connections.

 

[driftLimiter]

I disagree with drift's assessment of the influence area of the shear tab, though. Just as the resulting shear in the beam is subject to an influence area equal to the full length of the girder multiplied by the full distance from the girders on either side

Agreed I left after this post thing, nah the influence area of the shear tab is the same as the beam. The influence area is the area which any applied load causes a force effect on the element in question.

Also I don't think Kll factor is arcane lol. It literally just accounts for the difference in tributary area and influence area in a hidden way. If you read the tributary and inspect the KLL factors its actually pretty straightforward.

 

[phamENG]

I agree with KootK, and to his point number 2, we should look at places where the code does explicitly require connections to be stronger than the member - seismic. In capacity based seismic design, we have extra requirements for the connections. So that sets a precedent in the code for such cases - in other words, it's not saying that we should always over design connections, it's saying that when you need to over design the connections, the code will tell you.

Of course engineering judgement comes into play - particular circumstances may require you to be more conservative or design for a higher load, but that should also apply to the beam. So, outside of seismic design, designing the connection for more shear/moment than the same location of the beam doesn't make much sense.

 

[phamENG]

Also I don't think Kll factor is arcane lol. It literally just accounts for the difference in tributary area and influence area in a hidden way. If you read the tributary and inspect the KLL factors its actually pretty straightforward.

I understand, but unless you fit nicely into one of the general categories in the little table in ASCE 7, to use the equation as written (which is important when your calcs are being reviewed), you have to determine your influence area and divide it by your trib to get K_LL. So what's the point? I get it that more people are familiar with tributary area, but the answer isn't to make it "easier" by just telling them to multiply it by such and such a factor (that may not be quite accurate for a specific situation based on framing layout, transfer girders, etc.)...it should be to educate people about the concept of influence area.

 

[driftLimiter]

@phamENG Agreed Ill admit it was several years before I actually realized what Kll was doing. When I finally understood it correctly I felt like I was betrayed by the code writers lol. Influence area isn't hard to understand they shouldn't need to hide it behind Kll factor.

While I still understand the sentiment of @dik, don't skimp on your connections I think @kootK gets it best with 1). The other thing I think about too is how much we underestimate the nominal strength of shear tab connections already. Live load reduction is one of those few opportunities the code gives us to reduce loading.

 

[271828]

...The other thing I think about too is how much we underestimate the nominal strength of shear tab connections already. Live load reduction is one of those few opportunities the code gives us to reduce loading.

I'm against anything that makes it harder to use shear tabs. LOL. Long live LL reduction!

 

[cliff234]

In the case of a simple span beam, the influence area for computing the reduced live load on a uniformly loaded beam is the same as the influence area for computing the reduced live load on the connection.
Therefore, the reduced psf live load for design of the beam is the same as the reduced psf live load for design of the connection. There is no need to do additional calculations to compute the required strength of the connections.

We always consider reduced live loads on connections where permitted by the building code.