Dissecting a Load Combination

[jheidt2543]

From the IBC 2000, Formula 16-10 load combination:

D + (Lr or S or R) + L + (W or .7E)

Given: D = 20 psf, Lr = 20 psf, S = 20 psf,

An industrial building with a cab operated overhead crane. When looking at the load combinations that include seismic loads, we take the building weight and we take the weight of the bridge crane mechanism (trolley & bridge, but no lifted load) to figure out the seismic shear force, vertical and horizontal for the building as a whole.

Now, look at an individual column supporting the building and the crane (where in this case L = the crane loading on the column; vertical, lateral and transverse).

First, I consider the entire crane load as live load, the trolley, the bridge and the lifted capacity + impact from the single column’s point of view for vertical, lateral and transverse loads.

Secondly, when considering seismic, it seems that the column sees it’s portion of the building seismic load and, when the crane is on the column line in question, ALL of the bridge crane seismic load (trolley & bridge, but no lifted load) divided between the two columns in the line (assuming a clear span crane bay).

So, the combination for the individual column would look like:
D + .75*(Lr + S + L) + .7(E building + E crane) where E crane = total crane mechanism weight used in the seimic base shear equation.

Is this how you all look at it too?

 

[Lion06]

If the column is connected to a diaphragm, then it's seismic load will be distributed to the entire lateral system (the manner in which it is distributed is dependent on whether it is a rigid or flexible diaphragm). If it is a flexible diaphragm, then you appear correct.

 

[jheidt2543]

The building seismic loading I agree is distributed, but doesn't the seismic load from the crane go almost totally (say 90% to pick a number) directly into the column since it is so close; on the column line?

 

[Lion06]

Why would it? It gets dumped into the diaphragm with the rest of the lateral loading. The diaphragm moves as a rigid body which means that all of the lateral frames get affected the same. Well, not necessarily the same, but just like a rigid diaphragm.

 

[jheidt2543]

Now, I'm talking just the seismic component, in general a crane runway is some distance below the roof line. So, I'm thinking that the seismic load from the bridge crane weight
"hits" the column first and then is distributed down to the foundation. I guess my question is: do we dump all that seismic load from the bridge crane weight into the one column when the crane is in that column line or into two or three or what? That's what I'm questioning.

 

[Lion06]

I guess I don't know what a crane detail looks like because I've never dealt with one. If it is several feet below the diaphragm, then some of it will go up to the diaphragm and some will go down to the footing, but all of it would put that particular column into bending - from what I'm picturing anyway.

 

[DaveAtkins]

Unfortunately, the 2000 IBC does not allow you to multiply 0.7E by the 0.75 factor. This was corrected in the 2006 IBC.

To answer your question, I would put the seismic load from the crane itself into two columns, one each side of the crane bay. When the bridge crane moves laterally in an earthquake, it will distribute its load to both crane runways.



DaveAtkins

 

[JAE]

Yes, I agree with DaveAtkins - the columns (the two of them) would see the crane seismic directly.

 

[jheidt2543]

Dave & JAE:

Thanks, that is what I thought would be the maximum seismic load for an individual column too, the bridge crane stradling the column line and 1/2 of the seismic lateral load in each column at the end of the bridge.

Dave, the .75 factor is only applied to the (Lr + S + L) not the seismic load or are you saying I'd have to use the full (Lr + S + L)? That doesn't see right, I don't interpret IBD 1605.3.1.1 that way.

 

[kslee1000]

I think the column next to the trolley would take more E than the column at afar. Since the trolley (weight) bears more on it, thus the effect (trolley mass is unevenly distributed in this case).

 

[JAE]

kslee1000, I don't agree with that. A bridge crane is pretty stiff axially down its length and any lateral thrust from the bridge mass would be distributed 1/2 to each wheel truck and thus 1/2 to each column....assuming the columns have the same lateral stiffness.

 

[kslee1000]

JAE:

The bridge girders are essentially moving beams carries the trolley. Let's assume a simply supported frame with 100' span and a 10 ton load at 1' to cloumn A, 99' to column B. When earth starts to move, do you mean column A (with 9.9 ton load) has the same response as column b (has 0.1 ton load)? Interesting, I would need to think more on it. To my believe, at this very instant, column A has a greater mass than column B, with same acceleration, column A will obsorb more energy than B. Most importantantly, seismic movement reverses direction in seconds, would it be adequate time for this energy travel back through the beam to reach column B at far end, thus both take 1/2 of the total response. I sincerely doubt it is so.

 

[JAE]

kslee1000,

I think you are confusing vertical reactions with lateral response. The bridge crane is essentially a long horizontal beam. Think of the typical two-column and beam bent with a lateral point load at the top of one column. Both columns will deflect the same distance due to the stiff horizontal beam between them.

The mass at one end doesn't change the fact that the lateral seismic reaction is horizontal and dragging the bridge girder beam across the span, forcing the two columns to deflect together.

If the two columns are deflecting the same distance, they are taking the same load (i.e. basic Hooke's Law).

 

[kslee1000]

Yes, I am kind of confused on this phenomenon. Isn't lateral response in the for F=ma?

 

[kslee1000]

Also, I think Newton's "Second Law of Motion" truncks Hook's Law in this case, since the latter covers linear elastic behavior, but seismic event causing inelastic behavior to occur. I could be wrong, any comment, elaboration on this phenomenon is welcome/apprecated.

 

[JAE]

Whether the columns go inelastic or not - the effect is the same. Each column is identical. Each column is tied into an axially stiff bridge girder beam that won't go inelastic prior to the flexing columns.

So with two identical lateral supports (the columns) loaded together - they will each see 50% of the load.

 

[kslee1000]

I agree we disagree on this one.

 

[jheidt2543]

Thanks all for a very interesting discussion, it really help clarify my thinking.

JAE, your argument convinced me. Once you separate the vertical reactions from the horizontal reactions, it is plane to see that the horizontal load has no place to go except into the columns and IF the columns are identical, they share the load 50-50. And, the proof is, as you noted, the deflections are equal.

Thanks again!

 

[kslee1000]

After some search and reading, I have found a few cures to my confusion.

1. It is now recommended a dynamic analysis shall be performed on crane supporting structures. The mass to be considered are the trolley and crane bridge girders. The bridge girders should be considered as a tie in between runway supporting columns, and the trolley should be positioned in the mid-span, quarter span, and at the bridge end to generate responses.

2. The position of trolley may affect the resulting fundamental natural period.

3. The position of trolley would induce greater rotation on the colest column due to increase in P, thus P-delta effect.

Lastly, a few words from AISE regarding seismic forces:
"It is important for the design engineer to understand that real earthquakes are a dynamic displacement loading and not juat the static force loading assumed by simplified building code calculations. These assumed forces are often greatly reduced from reality and rely on structural ductility well beyond the yield displacement of the structure in order to absorb the energy of the structure's response to an earthquake. It is essential to follow the building code detailing requirements for each construction material in order to provide a safe and ductile structure."
("Guide for the Design and Construction of Mill Buildings", AISE Technical Report No. 13, 2003)

Hope the above clears the confusion I have stirred.

 

[JAE]

kslee1000...I agree that we disagree. A dynamic analysis of this type of structure is, in my opinion, not warranted or required.