Question on Crane Runway Girder Design

[kirei17]

Hi,

I Initially searched for a topic related to my question (this exact question has been asked before), but it is closed now. So, i decided to create a new one.

Here's the question. In AISC Design guide 7 Industrial Building Design 3rd edition, about design of crane runway girder example 14.1.1 (ASD) there are some things i like to question:

1. Why lateral load distributed equally to the number of wheels on each end of the crane , when there is Maximum Wheel Load where the position of load, hoist and trolley are positioned as the worst possible, does the movement of trolley itself across the bridge, while in this position, not induce any lateral impact force when it stopped?

2. When i looked into the data itself; crane capacity= 40 kips, bridge= 57.2 kips, combined trolley and hoist= 10.6 kips and MWL = 38.1 kips (ignored the accessories weight). From there my assumption on the way they got the MWL is like this:
- divide bridge weight by 2: 57.2 kips/2 = 28.6 kips
- multiply MWL with 2, assume equal load on each 2 wheels on one end of the bridge= 38.1x2 = 76.2 kips
- reduce MWLx2 with bridge/2= 76.2-28.6 = 47.6 kips
- divide result above with sum of (crane capacity + combined trolley and hoist), so 47.6/(40+10.6)= 0.94 ~ 0.95
- so 0.95 is the left-right proportion i get when the position of the entire live load (trolley, hoist, hook and load) is on one end of the crane bridge
Any comment ?

3. On calculating Mnx/ omegab the writer refer to AISC Manual (15th edition) table 6-2, and i looked into the resource, it indeed pointed to 605 kip-ft for the certain size of the beam and length of unbraced. However why do they not include the Cb value? As in previous part of the manual, the book permitted to multiply of the value to Cb if Cb larger than 1. From my calculation, I got Cb x Mnx= 1201.9 kip-ft instead of Mnx 1010.35 kip-ft (1.67 x 605 kip-ft). Or are they just being on the conservative side of things?

4. Does anyone know why the concentrated load check is only for web sidesway buckling? Isn't web local yielding and web local crippling also need to be checked because the girder 'sits' on the column bracket for crane girder support?

Also on another note, one other question is whether any of you understand the concept behind the combination of Mnx and Mny on why the vertical impact load is not included in calculation of Mnx if Mny (weak axis moment) is included in combination and only when Mny is not included in the combination that the effect from vertical impact in Mnx can be included? Perhaps it's because that vertical and lateral impact can not happen at the same time? But it's just a guess.

Actually the reason i'm asking is that i'm creating this simple (or not very simple) excel sheet based on this AISC Design Guide 7 and I'm currently testing my excel with the sample question from the journal/ code.

Thank you in advance,
Rio

 

[canwesteng]

1. This is a judgment call by yourself. Distribute the lateral load according to the stiffness - usually this means for side thrust each wheel sees an equal load. For tractive forces, I usually assign 100% to each side, in case only half the bridge is driven. The rest of the question I don't follow.
2. Not sure I follow the question. Anyway, the crane supplier will give a maximum wheel load.
3. Using the Cb value is not that common in design offices when designing with the manual. Also, Cb could be considered one conservatively since the load is on the top flange (consistent with the approach in the canadian code).
4. It is likely assumed there are stiffeners at the ends. It is also assumed the bearing length is sufficient for wheel loads such that is ok by inspection, once the additional bearing length provided by the crane rail is accounted for.
5. Vertical impact and side thrust are not taken at the same time.

I don't recall if DG7 accounts for torsion as well, but it is something I would look to address. Maybe it just lumps all the lateral load at the top flange and neglects the contribution of the rest of the section?

 

[kirei17]

Thanks for the quick reply.

1. Okay i got it. That's why i'm a bit in doubt with the design example in question because it divide the load equally to every wheel point of the crane (that's 2 x 2, 2 wheels on each end). I agree with your "100%" take as a conservative approach.
2. I'm aware the manufacturer give MWL, but as your explanation in point 1, there could still some approach like the one you stated (the 100% part). But of course the stiffness of the crane system is not something the structure designer aware and hence it's pretty recommended if to take the manufacturer recommendation. Anyway, here i'm creating this template so i provided the 'coefficient' cell input to modify the number corresponding the manufacturer recommendation. Case closed here.
3. Just a side question. What's the relation between the position of load in top flange and Cb ? As i know it's just coefficient that reflect the variations in points within an unbraced segment as a factor to calculate LTB. Can you enlighten me in this?
4. Okay i understand, kinda.
5. Perfect, just as my assumption then.

About the torsion part, i only just start read it recently. I think it also aware of that certain issues regarding torsion and that Mx and My combined formula doesn't consider torsion but in the article it only stated like this at one part of the text (14.1):
"AIST TR-13 requires that the lateral force be increased because the force is applied to the top of the rail. This eccentricity of lateral load increases the magnitude of the lateral force to the top flange and requires consideration of a corresponding bottom flange lateral force and bending moment in the opposite direction". Maybe what the text means in this part is the application of multiple criteria in the calculation of lateral load for crane according to AIST TR-13 as DG7 mentioned in table 12-2. And so i already considered this part in my spreadsheet like atttached.

Thanks again,
Cheers,
Rio

 

[canwesteng]

3. There is no relation between Cb and the lateral load, it's just a recommendation in CSA S16. The AISC commentary has something else as well for load applied above the top flange, but DG7 doesn't consider this.