Elevator Rail Stop Analysis
Elevator Rail Stop Analysis
(OP)
Have a bit of an unusual little project to analyze elevator rail stops, which are used as a barrier for the elevator techs to test the pressure in their hydraulic system.
Attached is my analysis attempt. It is the impact analysis only, not the analysis of the connection strength. Any critique is welcome, as I have not performed a calc like this before - well maybe since dynamics course 6 years ago.
I do have one question. I assume that St. Venant's principle is very important to note here; therefore, the first through-bolt will absorb a majority of the loading, in my opinion. To make life easier, I propose to transfer 3/4 of the impact force to the first bolt and conservatively 1/2 to the 2nd bolt. Thoughts?
Attached is my analysis attempt. It is the impact analysis only, not the analysis of the connection strength. Any critique is welcome, as I have not performed a calc like this before - well maybe since dynamics course 6 years ago.
I do have one question. I assume that St. Venant's principle is very important to note here; therefore, the first through-bolt will absorb a majority of the loading, in my opinion. To make life easier, I propose to transfer 3/4 of the impact force to the first bolt and conservatively 1/2 to the 2nd bolt. Thoughts?
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RE: Elevator Rail Stop Analysis
RE: Elevator Rail Stop Analysis
In Russia building design you!
RE: Elevator Rail Stop Analysis
RE: Elevator Rail Stop Analysis
You have to know much more about what the elevator people are trying to do with this test. Why don’t they have a std. detail for this, they know how their equipment works and is designed. What pressure are they trying to test to? What size ram or rams push this elevator? This should give you a force, less the weight of the cab. And, your design force should be some reasonable multiple of this, 2, 3, maybe 4, but not 78 or whatever. Certainly, it can’t be more than the strength of the tee rails as columns or the strength of their attachment to the bldg. or you’ll be fixin that too. I would think that 2.083 ft/sec is mighty fast for a hydraulic elevator, that’s 4-5 sec., floor to floor.
Who suggested you use 1/2"x2" bars and 3/8" all thread for the bolting? What is the shear and bearing cap’y. of the 3/8" all thread in this application? Do they really expect an instantaneous stop, in zero distance? Therein lies part of the problem with getting such high forces. These just don’t make any sense, you will rip the elevator cab and its guide hardware apart, you will literally push the tee rail through the roof, you certainly will shear the 3/8" all thread. And, this assumes they/you have enough hydraulic pump volume and pressure capacity to make things happen that quickly. Finally, normal tolerances in drilling the tee rails and the stop bars almost guarantees that you won’t load all four stop bars or all four bolts at once, and you won’t know which bolt is going to go first, that’s a tolerance problem. You will more than likely just cleave one bolt at a time in quick succession.
This type of problem isn’t my daily cup of tea either, so I’ll have to think a bit about this problem and your calcs. so far. I think your calcs. essentially represent a perfectly elastic system, and you don’t have that, can’t have that. You should be thinking in terms of impulse and momentum, not kinetic and elastic energy. In fact, the stopping force (impact force) is actually a function of the time to stop, and distance traveled in stopping. Remember the example of the car hitting a bridge abutment, and the damage done and impact forces, vs. the car at the same speed hitting a cable net type barrier, and the lesser forces involved over longer time and distance. And, your design has to manipulate t & d, and absorb some energy, to bring the magnitude of the stopping force down, or the forces go through the roof, with the tee rails. There are a couple people on E-Tips who have a better handle on these types of problems than I do, but until they come along, the above is my two cents worth.
RE: Elevator Rail Stop Analysis
If you can't find any other references (and I'd look hard for some), at the very least you can use a more reasonable stopping distance, based on perhaps the bending of the bolt.
Brian C Potter, PE
http://simplesupports.wordpress.com
RE: Elevator Rail Stop Analysis
Trust, me I do not agree with that number the least bit, guys. That is why I was concerned about my procedure. A perfectly elastic response doesn't seem right at all. But, as my first 'stab' at the calc, I wanted to be as swift and conservative as possible, as I will tell you what the "real" problem is - it is being asked to complete this task in 2 hours. That is nearly as unreasonable as the 480 kips. So, dhengr, I will tell you that often times, I will ask all of the sorts of questions you describe - they are excellent. However, I often get squashed by the time frame to get it done. I don't like it, but I am not the PE and I am not the project manager determining how much we will bill.
On a side note: I had thought about combining the stiffness of the rail stops and the stiffness of the elevator cab. But, given no information about the cab, the best I could do was assume the same stiffness as the rail stop - by combination of springs in series, that only brought the total force down to three hundred or so kips - still no good.
I am going to check out some elevator-specific references and ASME ones as well. Workin off the clock again...
In Russia building design you!
RE: Elevator Rail Stop Analysis
In Russia building design you!
RE: Elevator Rail Stop Analysis
RE: Elevator Rail Stop Analysis
The change in length of bars is probably insignificant when compared to the elongation of the rails where they are attached (area of 4 plates = 4in²; area of 2 15#Ts = 8.81 in²). Without guidance from the manufacture or elevator code, you need to follow the load path.
RE: Elevator Rail Stop Analysis
If you have ideas to improve the analysis, I'd love to hear, otherwise, I don't know what else to say to your comment.
wannabeSE: I did see those in the ASME code. That rope gripper device is used in-service, so it makes sense that it would be in there. I spent a half-hour searching through the testing section to look for something that could have been called something different than "rail stops" - no luck. Again, the testing portion of the code doesn't seem to go into a ton of detail into the means and methods of testing - mainly the required outcome of them. Even a simple Google search of 'elevator rail stops' seemed to give me little to work with.
Thanks to everybody else for your comments.
In Russia building design you!
RE: Elevator Rail Stop Analysis
RE: Elevator Rail Stop Analysis
Attached is one of the photos he sent (And, yes he sent a photo of a photo on a cell phone). Maybe it will look familiar to somebody. This is everything I had to work with.
In Russia building design you!