Protection Deck Design

Ryan_Berry

Ryan_Berry

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Good evening to all the engineers out there.

I am currently working on a job whereby there is a load of just 35kg falling from a height of 2m. I have calculated the impact force (average) based on the velocity just before impact and the kinetic energy. The calculations for this are as follows:

Mass = 35kg
h = 2m
Velocity before impact = √2gh = √2 x 9.81 x 2.000m = 6.26m/s
Kinetic energy just before impact = 0.5 x 35 x 6.26² = 686J

The value above does not satisfy the conditions for determining the impact force as we don't know how far the 'object' has travelled upon impact. Thus, I have calculated the deflection of a single tube (which we are using as protection as punching shear on boards is an issue and doubling them up is not possible as they will not act compositely in punching shear) which is given by:

Deflection upon impact = PL³/48EI = (0.686 x 10^3 x 1.166³ / 48 x 210 x 10^6 x 13.8 x 10^-8) = 0.781mm

I have then used the work-energy principle to determine an average impact force of 878361N = 878.361kN. As we all know this is a massive load and under such load a tube will fail.

In order for the system to work I will require the object to travel 70mm i.e. the tubes will need to deflect 70mm which if I do this the tube will fail under the applied load of approximately 10kN.

Has anyone had a situation similar to this before and can shed any light on a suitable solution as I have thought of the following items but there are issues with all of them:

- Netting over the handrails as a catenary cable. This would not be up to much as the 'sharp' objects would potentially tear the debris netting reducing some of the velocity / impact but not by much. Plus this is extremely difficult to prove as we don't quite know what will fall on the debris net to start with.
- Polystyrene blocks - with a sufficient depth say 200mm the impact force would be dissipated. This is a strong favourite. ??
- A sacrifical level of boards on the top chord of the beam with beam lacings on the bottom. The boards will fail and bottom level trapping the items. A bit sceptical with this one.

I look forward to any ideas you have.

Regards,
Ryan
 
Ryan_Berry said:
Good evening to all the engineers out there.

I am currently working on a job whereby there is a load of just 35kg falling from a height of 2m. I have calculated the impact force (average) based on the velocity just before impact and the kinetic energy. The calculations for this are as follows:

Mass = 35kg
h = 2m
Velocity before impact = √2gh = √2 x 9.81 x 2.000m = 6.26m/s
Kinetic energy just before impact = 0.5 x 35 x 6.26² = 686J

The value above does not satisfy the conditions for determining the impact force as we don't know how far the 'object' has travelled upon impact. Thus, I have calculated the deflection of a single tube (which we are using as protection as punching shear on boards is an issue and doubling them up is not possible as they will not act compositely in punching shear) which is given by:

Deflection upon impact = PL³/48EI = (0.686 x 10^3 x 1.166³ / 48 x 210 x 10^6 x 13.8 x 10^-8) = 0.781mm

I have then used the work-energy principle to determine an average impact force of 878361N = 878.361kN. As we all know this is a massive load and under such load a tube will fail.

In order for the system to work I will require the object to travel 70mm i.e. the tubes will need to deflect 70mm which if I do this the tube will fail under the applied load of approximately 10kN.

Has anyone had a situation similar to this before and can shed any light on a suitable solution as I have thought of the following items but there are issues with all of them:

- Netting over the handrails as a catenary cable. This would not be up to much as the 'sharp' objects would potentially tear the debris netting reducing some of the velocity / impact but not by much. Plus this is extremely difficult to prove as we don't quite know what will fall on the debris net to start with.
- Polystyrene blocks - with a sufficient depth say 200mm the impact force would be dissipated. This is a strong favourite. ??
- A sacrifical level of boards on the top chord of the beam with beam lacings on the bottom. The boards will fail and bottom level trapping the items. A bit sceptical with this one.

I look forward to any ideas you have.

Regards,
Ryan
Click to expand...

I wish i stuck in at school, sorry Ryan well over my head and i'm 17 lifts up:D
 
Yep, lost me at the velocity at impact stage !

However, couple of solutions I have seen involved a double layer of boards sandwiching a double layer of either corrugated steel floor decking or Kingspan insulation boards, which in turn had a layer of plywood between. Don't know if it ever got tested 'in anger' though :))
 
Hi Ryan. I'm bored so I had a quick look at the forum and came across this. The answer is not simple but you have to look at the object and see how it lands, what happens to it's centre of gravity to determine the deceleration distance and where the energy goes to on impact. For instance when you drop a brick, it may well break on impact and a fair amount of energy is dissipated in doing that. If an 80 kg man jumps down onto the deck he won't go through it because he bends his knees as he lands. A 25kg tube landing dead vertical will go through the boards. Your 35kg weight is probably somewhere between the two but it is impossible to define how it will land. In practice a double boarded deck with a design capacity of 5 kN/m2 static seems to be able to catch things without collapsing although there may be some slip on the fittings and non elastic bending of the horizontal tubes.

As an example, I was on a job a couple of years back where a 50kg lump of stone had dropped about 30m onto a deck and the only damage was a cracked board and slightly bent beam. I wouldn't claim that any deck would be guaranteed to catch something with that much energy but that one did.

I have done some testing in the past by dropping various objects onto decks from greater heights than you have and have found that the basic theory is not good at predicting what happens. Could I suggest that you arrange to do testing with a representative weight yourself and see what happens? Record the results and attach to your basic calculations as an empirical assessment. Job done.

TG
 
Rayan

Have you considered a cable suspended shielding system with metal decking, the system comprises of vertical and horizontal steel cables and steel decking---the elasticity of the cables may absorb sufficient energy from impacts, the steel decking will also absorb impact energy, It's worth a look...
 

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Hi Ryan

Yes you need to place some sort of impact absorbent materials over the boards, such as polystyrene, but you need to then place a further layer of boards/ plywood over them to help spread the impact loads and stop them blowing away.

The other thing you need to ensure is that the material does not absorb water, or prevent water moving off the deck, as this may add to the loads on the deck.

I tested a concrete fence panel base falling 30m on onto two levels of 25kg sand bags and it worked fine.

If it is a scaffold tube falling a long way vertically it will be difficult to design anything to stop it except steel road plates, but they are heavy, and the trouble is the falling object will bounce off and outside the deck.

You will need to consider a double height handrail with plywood or heras fencing to stop the object bouncing out.

All the best.

Chris Eng
 
Ryan,

I have designed a few over the past few years for bricks falling 10m to a 21" tube falling 30m (end on).

The impact deflection is critical and so is the combined shear capacity of the layers in the deck, along with the orientation of the object at impact and it's resulting shear length.

I always assume that layers are sacrificial and only there to slow the object down in essence. As Chris says, you also need some compression layers which will deflect without shearing, Kingspan, CI sheet, sand bags are all options. All these combine to give an overall impact deflection and so you don't need to rely on a single tube as you mention above.

The last time I did this we also tested the deck (which I suggest for any protection deck) and the theoretical calculations for penetration depth etc worked out more or less exactly (within 95%).

I'm happy to have a chat with you about it if you want, contact details are at 483 Scaffold Design.

Good luck! B.
 
Hi chaps,

Thanks for all the advise chaps. I have come up with a system utlising the sacrifical deck which has been designed to deform plastically. The scaffold contractor will be testing this in due course. I have my fingers crossed.

Cheers again, it has helped me get to the bottom of the problem.

Ryan
 
Ryan_Berry said:
Hi chaps,

Thanks for all the advise chaps. I have come up with a system utlising the sacrifical deck which has been designed to deform plastically. The scaffold contractor will be testing this in due course. I have my fingers crossed.

Cheers again, it has helped me get to the bottom of the problem.

Ryan
Click to expand...

Hi Ryan

Glad you have solved the problem mate, well done. However, on a footnote mate. You welcome all design engineers out there for feedback. We the Majority of us Scaffolders, can and will give 'Critical friend feedback'.

As Chriseng will testament, sometimes you all get the muddy end of the stick, albeit, sometimes deserved. However, a bit time away from the risk of DSE and have a chat with the guys that do the jobs, would be good not just for you to 'Chill' but also could secure you x ammount of future work.

Just a thought Friend, not a Criticism.

Regards
Paddy.
 
Hi paddy,

I agree my friend and I always welcome advice from anyone willing to pass on their experience. This is how we learn scaffolding and would not know a thing about it without the experienced scaffolders passing on their knowledge. In this instance I asked the engineers specifically as it was aimed towards proving it mathematically and how it was justified.

Thanks to everyone who responded to the post.

Ryan
 
Hi Ryan, I read your original post but did'nt know the answer, however I was discussing it with a mate of mine in the pub, (where all the best jobs get done) He is a doctor of mathematics, I told him I would print of your question and he would have a look at it, He has done a lot of calc's for me free of course.
 
Hi paddy,

Is there any chance of getting a copy of the calcs when they have been done? It would be interesting to see how a pure mathematician would attack it. The engineer will always make a conservative decision a maths man is more exact.

Cheers mate.
 
imagesCAXHKM1Xcarol.jpg
Carol on the job.:D
 
Hi paddy,

Is there any chance of getting a copy of the calcs when they have been done? It would be interesting to see how a pure mathematician would attack it. The engineer will always make a conservative decision a maths man is more exact.

Cheers mate.

---------- Post added at 04:35 PM ---------- Previous post was at 04:33 PM ----------

Lol I like it

---------- Post added at 04:37 PM ---------- Previous post was at 04:35 PM ----------

Got a bit confused with the names there with being on iPhone.
 
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