Crane Runway Girder 1

[WARose]

I've been handed the task of designing a new crane runway girder. This will kind of be a open ended thread as I may have more questions as I go. (I haven't done anything like this in about 15 years....so lots of rust.)

For right now, 2 questions:

As far as the lateral load on the runway beam goes.....I seem to recall that you could deal with that as a torsional force in the girder, or you could take care of it by bending in the cap channel + top flange combination. Is that how you do it? That seems to be how they do it in AISC Design Guide 7.

In CMAA 70, among the loads, the following statement is made:

"The twisting moment due to the starting and stopping of bridge motors shall be considered as the starting torque of the bridge motor at 200 percent of full load torque multiplied by the gear ratio between the motors and cross shaft."

I have no clue how to get a load out of that....does anyone know what that means?

 

[TehMightyEngineer]

1) I've seen it done both ways. In the past I've designed lateral loads on runway beams as weak axis bending of the runway beam with a torsional moment.

Edit: CMAA 70 3.3.2.4.3 seems to state this is the correct way.

2) I've also having some trouble parsing that. I've not had to deal with this as the bridge and gantry cranes I've design or modified all had direct drive motors on the end trucks.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Shu Jiang PEng SE]

Lateral support the top flange of the crane beam at specific spacing and the top part of the W section (or plus channel if it has) take horizontal bending
Start or stop motor may introduce lateral load on crane beam

 

[WARose]

Thanks for the help TME. I just hope the lateral 20% covers me for whatever they are talking about.

 

[TehMightyEngineer]

WARose: I assume you're referring to the ASCE-7 loading requirements? Maybe I've not been conservative but my understanding of CMAA 70 is that the primary design loads for the crane design per CMAA 70 does not include the dynamic load factors from ASCE 7. CMAA does include inertia forces and so on; which can cause torsion and lateral load but adding the ASCE 7 horizontal loads isn't the intent in my reading.

Thus, I've used ASCE 7 loads for design of the building and crane rail supports and designed the bridge cranes themselves to CMAA 70 only.

Do you concur?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

I'll take a stab at Question 2, although I could be way off. Presumably, the 'bridge' portion is a frame assembly with 2 rails riding on the runways at the ends. When starting and stopping the motor, inertia generates a moment on the frame proportionate to the torque capacity of the motor. This moment resolves into a force couple at the ends of the frame where it transfers those forces into the runway beam. Or am I understanding/visualizing this all wrong?

 

[WARose]

WARose: I assume you're referring to the ASCE-7 loading requirements? Maybe I've not been conservative but my understanding of CMAA 70 is that the primary design loads for the crane design per CMAA 70 does not include the dynamic load factors from ASCE 7. CMAA does include inertia forces and so on; which can cause torsion and lateral load but adding the ASCE 7 horizontal loads isn't the intent in my reading.

Thus, I've used ASCE 7 loads for design of the building and crane rail supports and designed the bridge cranes themselves to CMAA 70 only.

I'v never been 100% sure. To be sure, I've always met the ASCE requirements......but some of CMAA's requirements also seem to me to be applicable to runway beam. (For example, the collision force for the bumper stop is given in Sect. 3.3.2.3.2)

 

[TehMightyEngineer]

True, I've also carried over some things like collision. Generally my process has been to model the crane and rail supports, building columns, and other interactions in one structural model. I'll then have ASCE 7 load cases and CMAA load cases. This way I see interaction of everything and generally feel covered. It's definitely a gray area.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[FLCraneBuilder]

AISC has a free download spreadsheet that does a good job at this
assuming you are designing beam only & not the supports

 

[TehMightyEngineer]

Wait, really? I've been doing it by hand all this time! Oh well...

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/