Failure prevention for projectile impact

[strainstress]

Hi,

We have to design a grate which can survive a hit from a plank (85lbs) moving at 133 mph. The grate is 240 inch long supported at it two ends.

I am using the approach as described in Section 3.0 and 4.0 of the reference:

http://www.osti.gov/scitech/servlets/purl/4242097

The elastic deflection I obtained for this case is 11 inches and maximum dynamic deflection is 35.4 inches giving a ductility ratio of 35.4/11 = 3.2

35" of deflection seems a bit excessive to me. The reference says maximum ductility allowed is 20 as long we ensure there is no local buckling. The local buckling will not occur as there as cross member which will prevent the local lateral torsional buckling on the bearing bar.

See attached for the calculation. In your experience do you think the grate can take that hit, deflect by 35 inches and still be able to prevent the plank from piercing through the grate ?

Thanks

StrainStress

 

[WARose]

With that kind of deflection, I’d be more worried about the grate coming off its supports (and falling through into the gap it’s spanning) as much as anything else. As far as local penetration goes, you ought to be able to verify that with Roark’s handbook.

In addition, with that kind of deflection I’d be worried about the bending capacity of the grate…..usually with that much deflection, you’ve gone beyond the capacity of most structural members (even considering plastic hinging)……of course, on the flip side is: typically (with a simply supported member) something goes wrong with the connection before anything else happens (which goes back to my original point).

 

[strainstress]

WARose : So in your view, what limit should I place on ductility. Is maximum ductility = 1 a conservative pick ?

 

[EdStainless]

Check the geometry first, how much of the ends are supported? How much can it defect before the ends slide off of the supports? That is the practical buckling limit.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[WARose]

So in your view, what limit should I place on ductility. Is maximum ductility = 1 a conservative pick ?

First off, I apologize for the belated reply: after you replied I went home and I am having all kinds of computer problems here. But to answer your question: yes, using a ductility ratio of 1 is very conservative. And is typically used where reuse without repair is desired after an impact event.


One reference I have (i.e. ‘Blast Effects on Buildings’, by: Mays & Smith, p.71-72), has recommendations for limiting both the ductility ratio and support rotation depending on the amount of damage you are willing to take. The damage categories are divided into two categories. Category 1 involves fairly minimal damage to the structure; Category 2 involves extensive plastic deformation and the need for subsequent repair after such an event. In table 5.2 they give these limiting ratios as follows:

Category 1- Support Rotation=2 degrees, Ductility Ratio=10 (whichever is less)
Category 2- Support Rotation= 12 degrees, Ductility Ratio=20 (whichever is less)


So as you can see, a lot depends on what you are willing to tolerate (in terms of damage). I think the methodology followed here is based on an army manual put out many years ago called ‘Structures to resist the effects of accidental explosions’ (TM 5-1300). I think it’s available free on-line.


Hope this helps.

 

[SAIL3]

are you assuming all of the impact energy is absorbed by the grating....one would expect considerable deformation of the wood plank....

 

[strainstress]

Sorry for the delayed response.

@WARose: I had looked at the TM5-1300 manual too. In my case we can replace the grating after the hit (it's not too costly anyway). So I will go with Category 2 and limit the edge rotation and ductility to less than 12 and 20 respectively. The deflection seems a bit excessive to me, but I am sure the manual TM5-1300 must have been written after lots of experimentation and validation, therefore as long as I meet the requirements of the manual, I can defend my calculations.

@SAIL3: Yes, I agree, the initial kinetic energy is equated to the strain energy of the grate. This adds another level of conservatism in my calculations. But for now, I guess I am fine with this assumption.

@EDStainless: The ends are supported. They cannot move unless they tear apart the supports.