Crane Building 3

[SteelPE]

I have a project that has been subjected to crane creep.... That is, when the project was proposed we were dealing with a 7.5 ton crane..... then it went to 12 tons and now it is at 15 tons. I have a few issues that I am dealing with at the moment. The crane is listed as a CMAA Class C with is equivalent to an AISC Loading condition 2.

1) Originally we had designed the crane girder as a WF beam w/o a channel cap. Now that we have the final wheel loads and it appears as if we are going to need a channel cap. How do the channel cap and the WF get attached together. The fabricator is saying that they use stitch welding...... I don't think that is going to work and we are going to need a continuous fillet weld. The problems is that it is a 350' long run with the crane.... lots of welding here.

2) My second issue has to do with the crane creep. Originally when we were designing the building we were planning on using brackets off of the side of the columns. Now with the larger crane we are going to violate the 50k bracket requirement slightly. Depending on the weight of the crane girder, we are looking at 55.7 kips. Is this going to be a problem? I don't think so, but I know it is in violation with AISE TR 13 (which I can seem to find anywhere). We are kind of forced to use brackets because of the space/permitting requirements of the structure (building size is set with the town).

 

[SrVaro]

For item #1; I have always used stitch welding in the past with a "long" (as designed) continuous weld at each end of any girder section.

For items #2; Even if the 50kip is a "rule of thumb" I would be concened, not so much because of the 10% overage, but more what will happen next year when they want to increase the capacity once again. Also how sure are you that you have the actual data (ie weight) of crane being purchased. If you know the actual type or have used a standard/typical on the heavier end, you have less to be concerned about. But I have had a cranes come in for final coordination much greater than the estimated. You will need to explain to your client that they doubled the original loading and therefore the original system will not work. (Granted this is easier said then done.) In addition to the higher gravity laods there are the lateral and longitondual load that are more difficult to deal with on the bracket than if an auxillary columnn.

 

[SteelPE]

SrVaro,

I'm not sure the exact reason for the creep but I can tell you that my former mentor designed the building for the company originally in 1997. We are now doing an addition to the facility. We submitted preliminary engineering for them on an addition last May based upon the same design information used in the 1997 building which used a 7.5 ton crane. In August they changed it out to 12.5 tons and then somewhere between August and now, someone thought it would be a good idea to use 15 tons.

The building has now been approved and we are starting the final engineering design of the building. We requested the actual crane weight from the manufacturer which we have now received from the client. We would not design the building with out this information as the entire structure revolves around the crane. If they decide to increase the weight of the crane, then they will need someone to approve the increase or face the consequences. Notes about the crane design will be plastered on our drawings..... and we may even plaster the information I received from the manufacturer on my general notes page.

So, I am not concerned about the client changing things around.

 

[dik]

If your crane is in frequent use, I would avoid stitch welding... it has poor fatigue resistance. Crane creep happens, and with most new projects, we advise our client about this and make sure the crane is sufficient... sometimes 'creep' still happens...

Dik

 

[SteelPE]

Lets see if I can properly portray the calculation.

CMAA class C = AISC Loading Condition 2 = 500,000 cycles over the life of the structure:

Using table A-3.1 section 3.4 (Cf=11x10^8) gives Fsr = 13ksi.

Ix = 5437 in^4
Q = 106 in^3
V = 46.8 kips (crane near support)
q=VQ/2I = .456 k/in/weld = 5.47k/foot/weld

Welding with 3" of 1/4" fillet weld would produce a stress of (5.47/(.25*3))=7.3 ksi which is less than 13ksi therefore it should be OK.

 

[dhengr]

SteelPE:
A .25" fillet, 3" long, at 12"o/c, stitch weld is a crappy fatigue weld however you cut it or make it. All the starts and stops are very fatigue sensitive. And, the layout, and then the over welding and doing the starts and stops right are very time consuming from the welding hours standpoint. Use a 3/16" fillet, continuous, and be done with it. This provides a better fatigue detail, not much more weld metal used, and considerably less layout, welding and inspection time. Do these welds in some predetermined lengths, not just from one end to the other, to control distortion.

 

[SteelPE]

I understand it stinks.... but isn't the calculation in accordance with the requirements of the AISC? or did I do the calculation wrong towards the end? I know the fabricator is the same fabricator from the original building and they have all of the original details.... and they know how the existing building was done.

I'm going to push for continuous welds but I'm sure there will be a ton of push back. I will let you know how I make out.

 

[dik]

Thanks dh...

I think the AISC as well as the CISC had information regarding the preference of using continuous fillet welds for crane beam construction...

Dik

 

[fabtank]

I disagree that stitch welding is more time consuming.
From someone who has personally welded many of these channel caps before I can say that I have rarely seen continuous welding.
Also, it is much much faster to stitch weld.
To layout the stitch welding only takes the time to walk the length of the beam and back.
I would say stitch welding is about 1/4 of the time continuous welding would take.
Continuous welding would be better for fatigue, but if the calculations support stitch welding it will definitely save cost.

 

[hokie66]

Agree with dhengr. A 15 ton crane needs continuous welds, and it also needs a separate crane column or a stepped column. This is not from any crane code, but from the "Bible" of a major industrial company which was my employer for a long time.

 

[rowingengineer]

Hokie,
Sounds like a good referance, did they publish? We normally would change at 20t to 25t but this is probably because we are in a high wind region thus strong columns.

We would normally change to continous welds at 10t.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[JAE]

The attached is from AISC's design guide 7 - note underlined portion regarding the channel cap welds:

 

[dik]

JAE... thanks for the clarification...

Dik

 

[SteelPE]

I have an older version of DG 7 which does not have that paragraph. That is interesting and I guess it does clear up some confusion as I am a CMAA class C. This building is in a relatively high wind region as well.... 110mph so the columns are pretty large already.

 

[hokie66]

RE,
No, it was an in-house Central Engineering design guide. Nothing secret, but somewhat proprietary in nature.

 

[amec2004]

The followings are my suggestions to you.

1. AIST suggests if the crane reaction load >50 kips = 178 kN, bracket column shall not be used as bracket is vulnerable to fatigue. If the crane reaction load >50 kips, use stepped column or composite column instead.

2. Fatigue check is only required when crane CMAA class >= D
Class of crane determines the number of lifts per day, which is used for fatigue design. Only consider fatigue check of runway beam and tie back for crane with CMAA class >=D

3. For the intermittent fillet weld between channel cap and wide flange section

If your design is per AISC/AWS code (see attached PDF file for code reference)

* For crane class <=C, using intermittent fillet weld is permitted as per the latest AWS code and fatigue check shall be performed.
* For crane class >=D, using intermittent fillet weld is permitted as per the latest AWS code and fatigue check shall be performed.
It’s highly NOT recommended although AWS D1.1 takes out the specific prohibition after 2000 revision while CSA W59 still has such prohibition.

If your design is per Canadian CSA code

Using intermittent fillet weld is NOT permitted for any crane class as per CSA W59-2003 (see attached PDF file for code reference)

For dynamic or cyclically loaded weld connection, intermittent fillet weld has much lower fatigue strength because each weld end forms a stress concentration and the fatigue cracks initiate at the weld ends.

For dynamic or cyclically loaded welds or structures

* AWS D1.1-2000 Section 2.27 (see attached PDF file) provides the specific prohibition on using intermittent fillet weld in the cyclically loaded connection as the code recognizes that these types of welds are not adequate to perform satisfactorily under cyclic or dynamic loading, but, [red]AWS D1.1 takes out the prohibition on intermittent fillet weld starting from 2002 revision[/red]

* CSA W59-2003 Section 12 Cyclically Loaded Structures clause 12.4.14 prohibit the use of intermittent welds except for "members always in compression", and "interconnection always in compression of two or more rolled sections or plates"

Please refer to attached PDF file for all the code references.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[dirtsqueezer]

You're OK.

 

[dik]

AMEC... 50K is approx 220 kN

Dik

 

[amec2004]

>>AMEC... 50K is approx 220 kN

Sorry 50 kips shall be 222kN. I couldn't figure out why I hot 178kN at the first time.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[SAIL3]

some good advice has been given by experienced engineers using good engineering judgement....the OP devoted most of the post addressing crane creep .....it is unrealistic to expect or eliminate the possibility of this occuring again in the future....intermittent welding is a red flag to me in a fatique situation for the reasons spelled out in some of the responses...if I had an iron-clad guarantee that crane creep will not occur in the future, I might consider it....a practicing engineer is supposed to interpret the codes and use them as a guide...they are not a substitute for sound engineering judgement, unless, you are a cookbook engineer....crane supporting structures are among the few structures that experience design loads and beyond, repeatedly, day and nite, change of ownership,etc...the expectation that the crane operator or other workers present are continually checking if a particular lift falls outside the design class of the crane is not realistic.....in any event, a 3/16" continous weld would be more cost effective than a 1/4" intermittent weld, IMO...