Impact loading????

[SteelPE]

I am currently working of a hanging TV system that will hang above a bar inside a restaurant that I designed. During the initial design phase I was not responsible for this portion of the project, however, this has now changed. Unfortunately I had to include this weight in the design of my roof and nobody was willing to go on record as to what the weight actually was. So I did the best that I could and now it appears that I may be a little short. Originally I was figuring for an impact load on this system (don’t know why it just seems like something I should have done). Now I have to be a little more exact and the impact factor may need to go.

I was wondering if others would include an impact loading on this hanging system and if so, what impact factor would you use?

Why do these things never end up the weight I originally estimate???? I should just figure 2.5x the weight everyone says it is then I won’t have to worry about these things later on when they get larger than expected.

Code would technically be IBC 2003.

 

[Splitrings]

The first thing that came to mind was impact loading for a crane listed in ASCE 7-05 Section 4.10.2. It is 25% of the rated capacity. The older (1996) AASHTO Bridge Manual used a maximum of 30% for impact.

 

[SteelPE]

That was pretty much the first thing that came to my mind as well. However, this is only supporting televisions that hang above a bar (approx 19 of them) it’s not a crane or a bridge which has to support dynamic loading. I’m thinking that 25% is too conservative. Especially since I am beginning to have some problems with the larger than expected weight of the system.

 

[csd72]

Where is the impact coming from.

Any type of impact that would cause 2.5 times the dead load would tear the TV apart. Personally I would normally allow 20 to 50% extra to allow for the next model installed but would not allow for impact.

 

[SteelPE]

csd72

Maybe I was that clear. The final weight of the system is larger than I was originally lead to believe. For some reason I always have problems estimating the weight of these things and that I should figure out what my weight is (during the design phase of the structure) and then multiply that load by 2.5 in order to make sure I do not have the problem I am now (with something that may need field reinforcing).

I estimated the system at 4000# then I designed the roof for 5600# but now the system weighs 5400#.

Sorry for the confusion.

 

[paddingtongreen]

The only time I see impact is at installation and servicing, it is not like a working crane. What is the probability of it occurring at the same time as maximum roof load? After all, design wind and earthquake know not to happen simultaneously.

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

 

[csd72]

SteelPE,

I understood you but perhaps you did not understand me - where is the dynamic load coming from?

 

[TXStructural]

Structural response, or more precisely response of materials, to sudden loading is equivalent to twice a static load. (Timoshenko and others have deduced and tested this.) However, I'd always encourage an added point load of either 200 (or for sudden loading, 2 X 200) pounds to anything hanging low enough in a bar to be grabbed by a drunk patron.

I would design for actual weight plus one person in this instance.

As to the original design loading, the load factors in the codes are there precisely for the reason you find here: uncertainty of loading. As long as the actual loads do not exceed the design live load times the load factor, you are in compliance with code required loading. One great result of LRFD design is our ability to isolate how much load is acceptable. (ASD doesn't explicitly define how much "extra" loading is allowed.)

 

[EngineerTex]

It seems that the only impact load will be weight of an individual TV, wouldn't it? They're not going to hold all 19 of them on some sort of a truss and drop them simultaneously into the installation, will they? The impact loading should be the weight of 18 of the TV's plus the impact loading of one TV, wouldn't it? Or as TXStructural points, out -- the weight of 19 TV's plus a drunk acrobat.

Then again, I've only ever designed to ASD, so there may be something I'm missing here. Perhaps you're already taking that into account.

Engineering is not the science behind building. It is the science behind not building.

 

[GregLocock]

"Structural response, or more precisely response of materials, to sudden loading is equivalent to twice a static load. (Timoshenko and others have deduced and tested this.)"

Not reliably. A better, but more cumbersome, way of describing that situation is that the peak deflection of a linear single degree of freedom system to a load that would cause a deflection of x if applied slowly is 2x if applied rapidly.

Dropping a TV from 25 feet onto a TV stand will not merely cause twice the deflection caused by the static weight of the TV.



Cheers

Greg Locock


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[paddingtongreen]

Guys, the idea is suddenly applied loads, not loads dropped from a great height. The load held in position by an external force which is suddenly removed.

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

 

[GregLocock]

Agreed, but it is commonly misinterpreted.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BAretired]

I would not consider an impact factor for suspended TV sets. If you use LRFD, you could arguably classify the load as dead load and use a load factor of 1.25 although I believe most engineers would opt to call it live load with a factor of 1.5. If you are using ASD, it doesn't matter what you call it, but you have an ample safety factor if you design for the actual weight without impact factor.

BA

 

[SteelPE]

BA/csd72

I guess that I am the one that is a little confused then. I guess we agree to some degree, why would there be an impact factor if there are no moving loads. I just wasn’t sure if it was wise to not include some sore of impact factor.

paddingtongreen

Unfortunately the jurisdiction this system is in has a load combination of 1.0D + 1.0S + 1.0L. Therefore, according to the code officials, the probability of all loads occurring at the same time would be extremely high.

I still haven’t completely gone through the calculations yet. However, it is looking like I may have a lot more “fluff” in the design than I originally figured. It turns out that I picked the worst loading condition for the roof members and then designed all the roof members (in the area) for this loading condition.

 

[dhengr]

SteelPE:

Let me see if I have this right; you were told 4000# for 19 discrete units, you designed for 5600#, and the system actually weighs 5400#, assumed identical units at about 285# each, hung at 19 different locations from the roof structure; that sounds like pretty good weight estimating to me. And, it was not “unfortunate” that you give some thought to those loads in your roof design, you did a good job, but how did you know the exact locations, and have they changed, or has the concentration of the loading changed? At first I thought you mis-spoke and meant TV station on the roof.

The design problem now is: a support system under each unit to carry 300(load factor)#, to a couple bar-joists or other discrete points without overloading those discrete points. Then a quick look at shear cap’y. if units are near a joist reaction, or extra moment cap’y. if the units is near mid-span. They aren’t all hung from one bar-joist panel point are they? However, the low TV/drunk suggestion is a legit. issue.

The scary thing here is that you might be concerned about only over estimating this D.L. by 1%, I think that’s a spectactularly good guess. But, at the same time, it is a spectactularly bad assumption on your part that your analysis and design are so accurate and close-to-the-bone that some 300# TV’s would cause a problem, as long as you now pay attention to the connection details, etc. at this stage.

I think it’s essential that you consider impact loading, but not for the same reason other’s are arguing for or against that. What with the realism of HD TV and all, imagine what a 300# lineman can do in the way of lateral loading, hell he can bench press that TV, and imagine the impact when he hits the ground in that TV. At the volume level they play those things these days, I think you have a potential high & low frequency vibration and fatigue problem on your hands. In fact, I hear, they are already working on a 3-D deflector system for around beer bottles and drink glasses so they won’t get blown right off the bar when 3-D TV make its appearance.

 

[SteelPE]

Dhenger,

Basically there were a few reasons why I ended up cutting the design so close. The first was the fact that they added 5 more televisions to the rack (all the televisions are suspended from a single rack that is hung from the ceiling). The second was the fact that I didn’t think the hanging system would weigh this much as it ended up (of course the hanging system may be a little conservative).

My drawings indicate that the joists mfr is to design the joist top chord (for local bending) of an additional concentrated load of 900# located anywhere along the joist. This is where I am getting pinched. This note was provided to the mfr in order to eliminate the need for field applied struts (this note also made it to the joist shop drawings because I checked). My new calculated load on the joist is actually 810# (a little to close for me).

Now, the calcs are finished and my end capacity of my joist is 25.5K while the required capacity is 14.5K. My allowable moment is 447’K and my required moment capacity is 239’K. The closest my allowable shear diagram gets to my required shear diagram is around 4K.

Therefore it appears the design is more than adequate with exception of the field applied struts which I will probably end up making them apply.

 

[dhengr]

I think you were a bit over zealous in applying an impact factor to the TV weight, even if the installer can drop it a few inches, your design should not be that tight. The ‘drunken jerk’ was a very good reminder of what we should all watch for in our designs. If they can do it, they might or will, so you should design for that. That design encompasses your TV support system and its discrete connection points, it does not carry through to the whole roof or all TV’s at once. Would you add an impact factor to ice or snow falling off a higher roof, of course you would; but falling or blowing snow has a small impact factor.

Paddington was pulling your leg with the ‘wind and earthquake know better than to act simultaneous’ comment. But, your 1.0D + 1.0S + 1.0L, makes sense too, if you think about it from the correct angle. On a deck or a roof you could have D + S + L at the same time; on the deck L is the drunks who are not trying to chin themselves on your TV supports going out to play in the snow on the deck; on the roof L might be wind or mech. loads, etc. This is in recognition that those loadings can occur together. But further, note that none of them are load factored at greater than one, in recognition that it is not very likely that they will all occur at the same time. Now look at the code and you see 1.2, 1.4 & 1.6 load factors on loads which we know do occur together and must be designed for if the structure is going to be safe in everyday use, or with somewhat smaller L.F’s. when we start combining loads which may happen together, but are less likely to all occur at max. level simultaneously. Much of this used to be taken care of under the ‘engineering judgement’ part of our job. Now the codes try to cover every conceivable condition and combination in a very formulaic manner and drown you in bookkeeping. There’s not need to think and understand any longer, if you can only fallow the recipe correctly.

Of course you are correct to design for the worst condition or load combination, so it is likely that your design has some fat in it. You can’t (or shouldn’t) design every beam or bar-joist in an area differently, for minor variations. The manuf’er. wouldn’t quote the job if he had a new set-up for every joist he built; the steel erector would go crazy installing them; and there would be a fair to middling chance that they would make a mistake, and you would get the lightest joist where the strongest joist should be. I would also try very hard to justify a bar-joist which was 5% overloaded, so that the other 30 joists in this particular bay would not all 20% over designed.

 

[SteelPE]

dhenger,

Thanks for your comments. This design is not a LFD design but is actually a WSD design. So the 1.0 factors are the maximum they can be in a WSD load combination.

Side note, I recently looked at a job for a building that my client wanted to raise the roof. Every joist on the job as a KSP joist and every girder had a special loading condition. For a 90,000 square foot building there were over 100 joist and girder loading conditions. It was so bad that you had to flip through 5 drawings to find the correct loading condition. Not 1 extra pound could be added to the roofing system. So your comments about differing joist sizes may not be accurate (of course I don’t know how the joists were actually designed and maybe the mfr decided to make them all one size).

That was not something that I would be comfortable with… but it was extremely economical.

 

[Bobber1]

Just get a bunch of those helium-filled mylar balloons with the restaurant's name on them and tie them to tops of the T.V.'s.

Or you could chrome plate the additional struts and make them look like rabbit ears. No one will ever know.