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Monorail Design Loads

Monorail Design Loads

Monorail Design Loads

I will be reviewing some exisitng monorails and also designing some new ones in the coming months.  I was wondering if anyone has advice or could suggest a reference that addresses impact loads.  In particular I am curious about what horizontal loads should be considered.  AISC talks about a 20% load for crane runway horizontal forces, but doesn't this just apply to larger bridge cranes, which can move in two directions, and not single beam monorails?  ASCE-7, section 4.10, seems to address vertical impact quite well.  However, when discussing lateral forces they only refer to crane runway beams with electrically powered trolleys.  Does this mean that by code I am not required to apply lateral loads to a monorail beam?  I think that it is prudent to include some load to allow for some movement of the lifted load, but the 20% seems large.  Any thoughts would be apprecitated.  Thank you.

RE: Monorail Design Loads

In the US the main documents you would design to are ASME B30.11 "Monorail and Underhung Cranes" and CMAA(Crane Manufacturer's Association of America) 74 "Top Running and Under Running Single Girder Electric Traveling Cranes Utilizing Under Running Trolley Hoists"

The 20% side load from AISC is for bridge cranes to take into effect the loads on the runway girders when the trolley is traversing and suddenly stops.  This does not apply to a monorail.  The main horizontal load on your monorail comes from picking up a load that is not directly under the crane.  It is a common practice to assume the loads are taken at 5 degrees out of plane and check for torsion and horizontal bending but I can't find the reference for that.

Since ASME B30.11 is a safety code it is pretty general but it does offer some things to consider.  CMAA 74 gives all the load factors to consider including:

dead load factor  1.1 - 1.2 depending on hoist travel speed
hoist load factor  .15 - 0.5 depending on hoisting speed
inertial drive forces
test loads

CMAA 74 also gives all the allowable stresses and load combinations.

Don't forget to check the flange bending under the concentrated wheel loads.  CMAA 74 gives some emperical formulas for this but in the past there have been some errors in their formulas.  I like to do a second check of the flange stresses going back to basics.  A good method for second checking your answers on the flange stresses is given in the Modern Steel Construction (AISC Magazine) Dec 1999 in the Steel Interchange by David T. Ricker.  The problem becomes more complicated with tapered flanges.

Good Luck

RE: Monorail Design Loads

In addition to the 2 standards mentioned above, you should also refer to the American National Standard ANSI MH27.1, "Specifications for Patented Track Underhung Cranes and Monorail Systems".  This standard (referenced by the ASME B30.11) is published by the Monorail Manufacturers Association, Inc., an affiliate of the Material Handling Industry (MHIA) out of Charlotte, NC. Their website can be found at www.mhia.org.

As opposed to the ASME B30.11 safety standard, the MH27.1 standard sets forth more "design guide" specifics, such as loading conditions, allowable stress, etc.  In particular, the impact factor is specified as 0.5% of the rated load for each foot per minute of hoist speed, with a minimum value of 15% and a maximum value of 50%.

One glaring shortfall of this standard is the treatment of operating lateral load on the monorail beam or crane runway system.  In this regard, the standard states that operating lateral load is to be "as specified by the purchaser or determined by the manufacturer..." (sounds to me like the committee couldn't agree on this issue).

In the case of an underhung crane runway where the load can travel perpendicular to the runway beam, I think you have to apply the AISC "20%" method for determining the side thrust as a minimum.

For a monorail beam with no lateral movement of the load, I would agree that prudence still dictates some consideration of side thrust.  The 5 degree angle of lift mentioned above seems reasonably conservative in lieu of anything specified by the Owner, but I've never seen that value actually referenced in any standard.

Another way of looking at the side thrust value for a monorail beam is to apply the minimum specified horizontal load that CMAA No. 74 would use in the design of an underhung girder on a crane.  At a minumum, this value would be 2.5% of the vertical load (specified by CMAA as the 'inertia forces' from the crane drive and dependent on the accel. or decel. rate of the crane).  (An earlier edition of the CMAA standard specified this value as 5% of the vertical load, regardless of the accel. or decel. rate).  In either case, the CMAA method would result in significantly less thrust than that produced from the 5 degree angle of lift.

Hope this helps.

RE: Monorail Design Loads

Some how, in my recent job changes, I lost my design notebook I had set up on monorails.  However, I have always used the 20% Lateral load and a 10% Vertical Load impact factor on the monorails.  The MHIA as listed above, if I remember right, limits your tensile stress in the flanges of the beam to 20%, and is extremely strict on the deflection criteria.  I don't remember the particulars on that though.

Also, your allowable stress is limited by your loading cycle. See Appendix K of Manual of Steel Construction, ASD, 9th edition.  This range can be extremely critical if there is a cantilevered section that will induce reverse stresses in the monorail.  

Unbraced length also plays a significant factor.  Check out this AISC Article on Monorail Design:

Hope this helps

RE: Monorail Design Loads

SEAOC had a lengty discussion about this topic in their newsgroups. Try http://www.seaint.org/mailarchive.asp
This discussion started at the end of April of 2002.

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