Seismic considerations for monorail beam. 2

[bsmet95]

My company manufactures patented track monorails and cranes. A customer has a rather unusual request for some calculations. Our track is to be installed in a building in a seismic zone, and in case of seismic activity he wants the track to fail before the building does. According to ANSI spec's., each span of the track is to be considered a simple beam. The track is composed of a top flange and web of A36 steel, with the bottom flange a special steel section. Allowable top flange compression stress is determined by Fb=12x10^6/L(d/Af) to be less than/equal to 0.6Fy. The end user wants an estimate of lateral force which will cause track failure, which would be top flange buckling. In this case, I'm not sure what stress should be used for buckling. Would it be Fb, Fy, or something else?

See the attachment.

Thanks.

 

[phamENG]

A few questions:

1) What does the customer define as failure? Do they want the track to separate from the building to reduce the participating seismic load and lower the overall lateral force the building is experiencing? Do they want it to generate a plastic hinge (or hinges) in strategic locations to increase flexibility and alter the overall behavior? True failure analysis can be complex and the goals need to be clearly defined.

2) Special steel section - just in terms of shape or material also?

3) Back to the failure analysis thing - it's important to use real yield/ultimate stresses if known, and if not then they need to be approximated with some level of accuracy. AISC Seismic Provisions provide modification factors for various steels. I believe they tweaked these in the 2016 version, but 2010 lists expected strength of A36 plate as 1.3F_y and 1.2F_u. If you have a mill custom rolling those shapes for you, you're going to be more in the neighborhood of 1.5F_y and 1.2F_u. But...since you're working for the manufacturer...check the mill certs. Does the mill supplying your material have a consistent yield strength and ultimate strength? The mill will have the real test data. Use some judgement and determine a likely set of stresses to consider.

4) I don't see how top flange buckling would be caused by a lateral load. How is the monorail crane mounted to the track? Based on the connection scheme I would assume it's under-hung (though I'm not sure how with that tiny little bottom flange). If that's the case, the inertial loading from the crane will be applied laterally to the bottom flange, producing torsion and weak axis bending.

 

[bsmet95]

Good question. Thus far all I know is that the track is to fail before the building does. I'll net to get more info.

The crane is underhung. The special steel section is approx. 3-1/4" wide and our trolleys are made to run on it.

 

[phamENG]

Whatever their answers, I suspect you have a pretty calculation intensive job in front of you. There's not going to be a "standard" equation to give you the right answer. Unique loading on a non-standard section will make it interesting. Understanding their definition of failure will help direct the design to achieve the desired performance.

It's important to remember that most code checks don't predict failure - they're intended to preclude failure.

On the other hand, it seems an odd and inadvisable situation. The owner should have a structural engineer investigating the structure to support your crane and, if necessary, giving you lateral accelerations to design your crane to withstand the seismic event. The building should be strengthened to support the loads added - a falling crane is nearly as likely to kill somebody as a falling building.

 

[HTURKAK]

A customer has a rather unusual request for some calculations. Our track is to be installed in a building in a seismic zone, and in case of seismic activity he wants the track to fail before the building does. According to ANSI spec's., each span of the track is to be considered a simple beam.......

It is hard to estimate the probability that the earthquake will occur when the monorail /overhead crane is loaded..However, The LL suspended to monorail will not vibrate together with the building structure. But, monorail beam and travelling trolley will vibrate , that is the beam wt and trolley wt with crane should be added to seismic wt.

I have designed acc. to various codes in past and remember that ,only Russian Code SNIP includes the LL for monorails or overhead cranes for only VERTICAL SEISMIC EFFECTS..

I will say with using engineering judgement, the weight increase (in the range of %30 ) due to seismic event will not cause the lateral buckling of the monorail beam..


ASCE 7 'C12.7.2 Effective Seismic Weight. During an earthquake, the structure accelerates laterally, and these accelerations of the structural mass produce inertial forces. These inertial forces, accumulated over the height of the structure, produce the seismic base shear. When a building vibrates during an earthquake, only that portion of the mass or weight that is physically tied to the structure needs to be considered as effective. Hence, live loads (e.g., loose furniture, loose equipment, and human occupants) need not be included. However, certain types of live loads, such as storage loads, may develop inertial forces, particularly where they are densely packed. Also considered as contributing to effective seismic weight are the following:
1. All permanent equipment (e.g., air conditioners, elevator equipment, and mechanical systems);
2. Partitions to be erected or rearranged as specified in Section 4.3.2 (greater of actual partition weight and
10 lb∕ft2 (0.5 kN∕m2) of floor area);
3. 20% of significant snow load, pf > 30 lb∕ft2 (pf > 1.4 kN∕m2) and
4. The weight of landscaping and similar materials.

 

[EDub24]

It is hard to estimate the probability that the earthquake will occur when the monorail /overhead crane is loaded.

That was my thought. The code requires the runway beam to be designed for a lateral load of 20% of the rated capacity of the crane which is probably a way to take into account the overall weight of the system even when it's not loaded.

This is an odd request from a client. I guess you could design the rail with no (or minimal) lateral support but that's not something I would be comfortable doing.

 

[bsmet95]

Having discussed it with the project manager we determined we will not be able to give the info that was requested.

 

[RPMG]

The EOR is asking for a fuse that has a reliable failing point to support the monorail.

It has nothing to do with load combinations or ANSI monorail stresses. The monorail failure load isn't predictable, so it can't be the fuse. It has to be a hanger, connection, or other element that fails and no longer loads the building.

 

[dhengr]

Bsmet95:
I think you are kinda taking the wrong tack on this problem. What the people above have posted all sounds pretty much on the money to me, and should be woven together to educate your customer. I think your customer wants to know that your crane system is not going to be a significant contributor/inducer to a building failure during an EQ event. But, they don’t know how to ask that question or have asked it in an uninformed fashion. Your beams and trolly are rigidly connected to the bldg. so, even though a small percentage of the total, they must be included in the bldg. mass as relates to EQ analysis. Any crane load acts as a pendulum, completely out of sync with the bldg., but I suppose it could start to induce torsional problems in the crane beams or their connections. As mentioned above this is a very difficult analysis, and is not normally considered in the bldg. design, or as a requirement for this type of crane system design. There are some types of crane systems which more rigidly connect to and lift a load which might require consideration of the lifted load induced affects, in some concentrated fashion.

Explain your normal system design process; simple beams with load at mid span and at the connections for max. bending, deflections and max. connection loads, etc. You do have longitudinal and lateral components for the normal operating conditions, so your system withstands normal daily usage, and you’ll stand behind that, or under it. Then, the weaving of the above and a little textbook time on your own, to explain how your system would act and contribute to the bldg. as a whole during an EQ, as the posts above have started to eluded to. Your educational effort is to convince the customer that your crane system loaded, or not, will not adversely affect the bldg. during an EQ event, and it does not have to fail first for this to happen. It is just a fact of the matter in the way these systems act and interact, in normal use or in an EQ event. Don’t tell them you won’t do the calcs. Show them that you have given this some considerable thought. At first, this is in the way of a few phone calls with your customer and the EOR, whoever prompted the request for calcs., etc. It may end up as a short report, explaining the above, which is made part of your delivery package.

I could write a book on the crazy questions/requests from various customers/clients, in good part they had a concern, but didn’t know how to express it. So, they came up with some crazy questions or attempted means of reassurance. They needed some reassurance. And, with my neck sticking out sometimes, but mostly good engineering judgement, I could talk them down, and explain why that wasn’t a concern, or why that couldn’t be done that way, etc.

 

[HTURKAK]

That was my thought. The code requires the runway beam to be designed for a lateral load of 20% of the rated capacity of the crane which is probably a way to take into account the overall weight of the system even when it's not loaded.

Your comment is regarding the horizontal forces developing during operation of the crane. ASCE 7 suggests :
Lateral Force,20% of rated capacity, Longitudinal force on crane runway beams, 10% of the maximum wheel
loads of the crane and vertical impact ,25 % of the maximum wheel loads of the crane...

The concern at this thread is,

A customer has a rather unusual request for some calculations. Our track is to be installed in a building in a seismic zone, and in case of seismic activity he wants the track to fail before the building does.

In my previous post, I implied that the crane LL will not induce seismic loads to the main structure.

AISC Design Guide 7 suggests: Quote(Although cranes do not induce seismic loads to a structure,
the crane weight should be considered in seismic load determination. The seismic mass of cranes and trolleys that
lift a suspended load need include only the empty weight of the equipment. The designer should carefully evaluate the location of the cranes within the building in the seismic analysis....)