Beam to support impact load. 3

[CSFlanagan]

I'm designing a steel frame to protect employees walking under an overhead crane that carries a basket of steel. The frame I am designing is like a heavy duty canopy. It is 15' x 60' long and 13'-6" tall. It is not supporting any other loads, and will only see a load if the basket being carried by the crane above falls.
Maximum beam span is 30 feet. The basket carried by the crane weighs 5,000 pounds fully loaded. The bottom of the basket is only 12 inches above the top of the "canopy", so height of fall is only 1 foot.
My gut tells me to use an impact factor of 2.0, and design for a 10 kip load applied anywhere on the support frame. However, I'd like to have something more scientific to include in my calculations.
I've tried using assumptions of time for deceleration to determine the load. Using this method, the stiffer the beam that is selected the higher the force that gets applied. Essentially, an infinitely stiff beam creates an infinite force. Intuitively this cannot be correct, so I must be doing something wrong.
I've seen other posts, and they have asked for specifics. I hope I have provided enough information for a calculation.
What are your thoughts?

 

[WARose]

I think one of the best methods for figuring impact force is the method/formulas in Blodgett's 'Design of Welded Structures' (that takes into account stiffness). See Section 2.8.

You can typically come out of that with a much more reasonable force than from estimating impact times. With a 1' drop though.....you better be coming down on something squishy.

 

[CSFlanagan]

Thanks WARose. I just ordered the book.

 

[JStephen]

"I've tried using assumptions of time for deceleration to determine the load. Using this method, the stiffer the beam that is selected the higher the force that gets applied. Essentially, an infinitely stiff beam creates an infinite force. Intuitively this cannot be correct, so I must be doing something wrong."

And why can't that be correct? Did it let you derive a beam size?

One approach would be to set the potential energy of the elevated basket equal to the energy stored in the beam under full deflection, neglecting the weight of the beam (which may or may not be realistic, but simplifies things a lot). That should give you similar results to the approach above, but in a more straightforward way.

You could expand this idea to include plastic deformation in the beam, which would probably reduce the beam size considerably. That's assuming you don't plan to continually dribble the load on the roof.

I'd suggest hardhats and work boots for both figures under the beam as well.

 

[BAretired]

If the 5000# load falls through the roof, hard hats won't offer much protection.

BA

 

[CSFlanagan]

Well, if a steel beam only 1 foot below the basket won't stop it, then a hard hat 14 feet below will only make a greasy spot on the floor with whoever is under it.

This frame is not intended to be loaded like this in normal plant operations. However, if the crane fails, I'm sure the owner will not want to shut down operations while the frame is re-built. It would be less costly to design for inelastic deformation instead of elastic.

Attached is an example of a method I've used to try and calculate an Impact Factor (IF) to use static analysis for the impact load. The stiffer the beam, the smaller the deflection. Since deflection is in the denominator, this makes the IF go up quickly.

Other methods I have seen and tried have similar results when assuming deflection and deceleration. The stiffer the beam. the higher the load.

One paper I read seems to bear this out...
"The chief problem usually involves estimation of deformability. The assumption of a rigid impact is generally useless, since rigidity implies an instantaneous velocity change, therefore infinite acceleration and an infinite force. In real structures the deceleration is limited by elastic and plastic deformation, which in effect cushions the blow, and a major ‘trick’ is making a reasonable estimate the local compliance or stiffness at the point of impact."[/i]

 

[BAretired]

Would it help to add shock absorbers or a neoprene cushion to the underside of the copper basket?

Why is the basket made of copper?

BA

 

[robyengIT]

"Introduction to structural impact"

https://pdhonline.com/courses/s164/s164content.pdf

"converting dynamic impact events to equivalent static load in vehicle chassis"

http://publications.lib.chalmers.se/records/fulltext/251445/251445.pdf

Hope it can help

 

[BAretired]

The notion of allowing access to a space below a crane carrying a 5000# load seems like a very bad idea, particularly in view of the dearth of knowledge regarding impact loads. I expect OSHA would either disallow it or demand a very high safety factor on the design of the canopy.

BA

 

[CSFlanagan]

BAretired - The concept is really no different than when they put scaffold protection over sidewalks during high-rise construction. The crane runs the length of the building (North-South) and there is a defined pedestrian walkway running East-West about 60 feet long.
A 5K load with a 3x - 5x Safety factor can easily be designed. The problem is finding the solution to what appears to be a complicated problem. Is it 2x? 3x? 15x?
The basket is not made of copper, it is made of steel. It is carrying drawn copper tubing coiled into the basket. I can't add shock absorbers or other materials because the basket is lowered into a manufacturing machine to unwind the tubing.

 

[CSFlanagan]

RobyengIT - I read through these earlier. The concept is to start with static loading, and use that to generate the Impact Factor (IF). I ran numbers on a W14x34 with a static deflection of 0.43". Plugging into the equations, and the IF goes higher and higher the bigger the beam that is selected. Hence "the stiffer the beam that is selected the higher the force that gets applied" issue I keep running into. Thanks for the information... I will keep plugging!

 

[SlideRuleEra]

CSFlanagan - Are pedestrians using the walkway while the load is passing over them?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

SRE... could be... I've seen this many times and have always been amazed at how flimsy the scaffold and planks are. I think the purpose is so the pedestrian cannot see what hits him/her.

Dik

 

[SlideRuleEra]

Thanks, Dik. I did not state my question clearly, will try again: Does the loaded basket pass directly over workers on the walkway?

The reason I ask is because of the following OSHA regulation:

Overhead and Gantry Cranes, §1910.179(3)(vi), "The employer shall require that the operator avoid carrying loads over people."

I can offer the OP a potential solution, which I have used (successfully) on small scale similar applications... but not for personnel protection. The ethics of the OP's project are troubling to me. I worked a career in bridge construction and heavy industry and am very aware of danger from overhead loads. Using E-mail, discussed the ethics of this thread with one of the most experience engineers on Eng-Tips and have decided to proceed cautiously.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dhengr]

CSFlanagan:
To add some confusion and complication to the actual math and physics problem, the real consideration is that you want to absorb a bunch of impact energy if an accidental loading does happen. And you would prefer not to destroy the primary stl. structure in the accidental loading. You really haven’t described your problem very well. I would want to know much more about the whole operating situation and the general plant layout, the various structural components, etc., before doing any design. Can the distance btwn. the top of the stl. bms. and the bottom of the basket be more than 1’? What is the structure of the underside of the basket made up of? Can you/they control the orientation of the basket and its height above the stl. frames pretty well? Does the loaded basket only cross the walkway at 2 or 3 discreet 10’ wide machine locations in the 60’ walkway width? The 60’ walkway width and the 30’ bm. span would seem to indicate that there are three of the moment frames show in your sketch, and 30’ beam spans (beams shown in x-section in your sketch), correct? You better pay close attention to the stability of this whole framing system, since you may have the gravity loading and some lateral basket loading in an accident. Then, you will have a number of basket locations which are worst conditions for various walkway framing members; single 30’ bm. with full loading at mid-span, multiple bms. sharing this load, basket over mid-pt. of moment frame, basket over one frame column, etc.

I’m not sure I see this as an ethical engineering problem. It seems that the client needs good, sound advice on a difficult operating dilemma, and that they are trying to come to a safe and reasonable design for the operating conditions. And, I would review your design solution with the local OSHA people. I think that the OSHA section that SRE cites is to cover general crane lifting operations in a plant or on a job site. Even these lifts should be planned w.r.t. various safety considerations, rigging, etc, but there are people milling allover the plant floor or job site, without an overhead protection system, and they should be protected and/or avoided by the crane operator and lift supervisor. There is plenty of dangerous equip. that people work around and under every day, but not without some special thought and planning or protection. Heck, we don’t keep people out from under the RR overpass every time a train passes over, but the bridge is designed for this, as the client is trying to do here for his situation.

This is not an easy problem when you (or your client) expect an exact, closed form, solution. That’s why much of what you have read suggests much testing and/or crash testing to start to hone-in on an inexact approx. solution. The Physics and Engineering Mechanics we learned in school aren’t wrong, it is just that we can’t nail down all the all-important variables. So, sometimes the best approach is to explain the complexity of the problem (and math complexities) at about a 9th grade level, or early high school physics level and then to bracket the problem with some reasonable assumptions to show a practical range of solutions. Don’t forget, when you look at your deflection to determine your ‘IF’, the beam will deflect due to the loading, but so will the moment frames, so your deflection is low by ~50%.

One simple energy absorbing solution is as follows: put 4x4 wooden timbers perpendicular to the 30’ beams, every 3-4’ o/c, for load distrib. and elevation ; on top of these put a second lay of 4x4’s centered over the 30’ bms.; spike this gridwork of 4x4’s together, and bolt to the stl. bms. Then, I would want the basket to have two bottom skids 9’ long, about 3-4’ apart and perpendicular to the 30’ long bms. I want these skids to break some timbers in the upper layer, over the timbers in the lower layer, thus absorbing energy and distributing/reducing the impact loading to the stl. structure below. Beyond your bracketing calcs., etc., you could fairly simply do some testing on this timber cribbing on a couple bms. to start to show some range of agreement with your calcs.

 

[271828]

From JStephen: "One approach would be to set the potential energy of the elevated basket equal to the energy stored in the beam under full deflection, neglecting the weight of the beam ..."

This is what I'd do. Set the potential energy equal to the strain energy.

For linearly elastic and flexural deformation only, the strain energy in the beam is 1/(2EI) times the integral of M(x)^2 over the length of the beam, where M(x) is the moment diagram when the beam is deflected downward maximally after the load is applied. M(x) will be a function of the applied point load. For a specific EI, back out this point load.

I'd be interested in knowing how this method compares to the others.

 

[StrucPEng]

I don't have extensive experience in this area but I have seen a few of these calculations in the past while working in the industrial sector and a few of my observations and experiences are as follows:

- You will need to either determine through conservation of energy or through limiting assumptions an acceptable deflection of the beams. Stopping the load dead with no deflection will as you noted develop a calculated impact force that is approaching infinity.

- When you chose a beam there is a difference between the static stress and the impact stress that it can accept. Impact stress appears to be a function of I/c^2 so as you increase the beam size (depth) you will be increasing the c more than I in general and receive little benefit. Choosing a beam that has a higher "I" with little change in c would do you better. (This is outlined pretty well in the Blodgett reference WARose mentioned previously.

As for the OSHA requirements and concerns, I agree that lifting over the head of any workers is usually something you want to avoid but there are times where that is not possible in an industrial setting and the implementation of a "safety cage" of sorts is a smart move I think.

Hope some of this helps.

Matt

 

[rb1957]

@CSFlanagan ... your results (stiffer beam creates higher impact forces) is completely consistent with the theory. Stiffer structures reduce the time duration of the impact, hence higher forces, but also smaller deflections. Cars are built with crumple zones for the same reason ... they make the collision longer and lower the deceleration felt by the passengers.

another day in paradise, or is paradise one day closer ?

 

[swearingen]

This is a very common problem in marine fender design - I suggest you look into the load/deflection curves on some arch-type fenders. A basic frame underneath with a surface grid above the arch fenders would work well.

The energy method is what you want to use - note that the area under the load/deflection curve of the fender is the energy that can be absorbed. Once you have that energy (easily calculated), you then get a real reaction directly from the charts that you can be confident in. Naturally, since it is over workers, an extra safety factor would be in order.

Also, someone mentioned lateral loads which should be looked at closely, even to the point of putting barriers at the edges to make sure the load doesn't roll/slide off of the edge while it's bouncing around coming to equilibrium. The arch fenders are very good in shear and you shouldn't have a problem with them, but you must make sure your structure is good as well.


-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[CSFlanagan]

Thank you all for your responses. Definitely information to consider from all posts.