Crane Runway Lateral Bracing Requirements

[levey12]

I am in the preliminary design phase for a 50 ton crane system with crane runway beam spans varying from 35ft to a max. of 82ft. I would appreciate any advice for the lateral bracing requirements for both the top and bottom flanges of the crane runway beams. The rail beams are simple span.

For the compression flange it seems that lateral bracing should be provided to account for the effects of LTB. However, I have some questions pertaining to the bottom flange bracing requirements.

Per AIST Technical Report No. 13, there is a requirement for all crane runway girders with spans of 36ft and over in building classifications A, B, and C or runway girder spans of 40ft in class D buildings to have bottom flange bracing. Is this requirement included to address web sidesway buckling? If the design of the crane runway beams includes the proper checks (serviceability, biaxially bending of top flange, web yielding, web crippling, sidesway buckling, etc.) does the bottom flange still have to be braced? I know that this is a requirement per AIST Tech. Report No. 13 and not for some other codes, including Canadian standards.

Thank you in advance for any helpful information

 

[DaveAtkins]

The only thing that causes a tension flange to buckle is sidesway web buckling, so if you design you crane runway beam for this, you are OK.

I always design a crane runway beam as unbraced along its full length for both flanges, anyway--I have never seen an industrial building where you can brace a crane runway beam.

DaveAtkins

 

[apsix]

The shorter span runway beams usually don't have lateral bracing along the length.
Larger span beams can have a horizontal truss at top flange level, or a box section with trusses on 3 sides and the runway beam as a welded girder.

 

[B16A2]

I have the exact same question, except my 50T cranes span up to 100'!

The issue is not web sidesway (though it sidesway should be checked), it is torsion from forces being applied off the shear center. ie, the forces from crane thrust and rail misalignment. HUGE. If your girder rotates too much, it will throw off your top flange alignment, rail alignment, and you get a stuck crane.

The problem is that AIST, AISC, and CISC all have conflicting approaches, with AIST being the most conservative. What is the correct way? Deflection problems are a PITA after the fact and I want to make sure my approach is solid.

 

[hokie66]

For your 82', 100', etc. runway girders, the approach suggested by apsix is the way I have seen it done. Never designed one that long myself.

 

[paddingtongreen]

The deflection limits will force some hefty members, let alone the bracing.
I wonder if normal limits will do the job, or if the cranes will be running up and down the hills instead of on the level.
It might be possible to use heat straightening techniques to get some exact cambers into the beams, regular cambering is too inexact.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[B16A2]

Good point, most of the literature says for spans over 75' to camber out at least the dead load. Anyone know how cambering plate girders with fracture critical welding is done?

 

[ToadJones]

Hoping I'm not walking the plank here...

If you have an 82' to 100' span girder as has been mentioned, your side-thrust loading and lateral deflections limits will probably dictate that you have a horizontal truss at the top flange level, or, at bare minimum, a horizontal thrust plate connecting back to a back-up truss or beam spanning between columns on the building column line. This combined top section will most likely (in almost all cases I have investigated and designed) be very stiff.

On Girder spans this long, you will most certainly have to have intermediate columns and/or wind columns as having a 100 foot span for cladding members is not realistic. Therefore, a back-up truss is the only practical way to go. This back up truss serves the dual function of supporting intermediate roof columns and providing horizontal stiffness to the runway. Usually, a pinned-pinned wind column is used from the bottom of the back-up truss to the ground provided with vertical slotted connection to allow for deflection of the truss.

In checking the top flange section for combined stress (strong axis bending, weak axis bending, and axial load from tractive forces) you will use the combined top flange section for weak axis bending as allowed in AIST 13, the full section of the girder + some portion of the horizontal thrust plate of truss again, as allowed by AIST 13, for the strong axis bending.
In doing the combined stress check, AIST 13 allows you to consider the top flange as fully braced by the horizontal system if provided. This can be seen in 5.8.2.2 in the paragraph below equation 5.2.

Finally, I have had plate girders cambered by having the webs cut on arc or segmented arc.
Be careful of the bays ooposite the long spans have shorter spans. This can result in differential deflections side-to-side that may not me CMAA allowables.

Good rule of thumb here....there probably isn't an architect to yell at you, and the owner will want this building running around the clock. DO NOT skimp. Make the girders VERY stiff. This mitigates many of the numerous fatigue problems that occur in heavy industrial buildings.

 

[B16A2]

Thank you Toad, your comments were very helpful. Yes, no skimping here on deflection.

Do you know if CMAA deflection limits apply for very long spans? They dont put a cap on their numbers, so L/400 would be huge.

 

[ishvaaag]

Just a comment on the fact of that feeble members don't go well with very strong ones; when these go, temperature, force or whatever, the weaks go ruined. Hence precise determination of what expected is required, and likely also specific actions to limit this kind of problem through the connections.

 

[ToadJones]

CMAA deflection of L/400????
You referring to lateral deflection??

 

[B16A2]

Yep, lateral. L/600 for vertical on my particular crane.

Posed the question to the crane supplier (sales middle man) and they claimed all that's required is CMAA's requirements. I think CMAA is only good for normal spans so obviously I'm designing stiffer.

 

[msquared48]

If lateral bracing of the top flange is the issue, in the older AISC manuals there were tables for a composite section of a channel over a W shape to accomplish exactly this, with the crane rail attached to the top of the channel.

My older copies are all packed away...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Mike,
He's spanning 100'. The channel he needs will be called a truss.

 

[msquared48]

Details, details...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Actually, if the crane trolley itself was designed to ride on both the top of the top flange and under the bottom flange of the beam, the crane trolley itself could provide lateral support to the top flange via developing a lateral capacity to the wheels. If the top flange wanted to rotate, it would have to transfer a moment to the crane trolley. Moving lateral support - would be an interesting design project.

I think I'll patent the method... or not...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

B16A2-
What are you designing too?
CMAA is a specification for the cranes themselves and only has limited input on the runway design.

If you read section 1.4.6 of CMAA 74, it specifies that the runwya be "designed with sufficient strength and rigidity to prevent detrimental lateral and vertical deflection" so L/400 is not a catchall.

It also states that L/400 lateral deflection should be based on 10% of the max wheel load without impact, BUT, this does not necessarily match AIST 13 or ASCE 7 requirements for determining side thrust forces.

You need to pick a spec or at least tell us what this building is for. If using AIST 13, building class is very important.

 

[B16A2]

The owner doesn't care, but I've designed to AIST with a C Building Class per what they've described their daily routine.

As for deflections, I've designed far far stiffer because nobody seems to know the magic number that causes "detrimental operation".
One would think the supplier would be responsible for dictating what deflections are detrimental to their product. The crane rep I talked to didn't even know cranes had horizontal thrust!

And correct, there's no top channel on this little beam, lol.

 

[ToadJones]

AS the designer of the building, you need to be concerned with the movement that will be detrimental to your building.

 

[B16A2]

The crane is a pre-engineered component. They should have an idea of what tolerances are required for their component to operate.