Impact Loads 3

[LGMF Engineer]

I'm dealing with a wall in a psych ward that needs to be designed for ASTM E695 Hard body impact and soft body impact of 175 ft-lbs and 480 ft-lbs respectively. If you look up the E695 test, they are basically dropping a weighted bag on a pendulum and having it hit the wall. My question is this. I've got a LGMF engineer who submitted calcs using the 480 ft-lbs as an applied moment. This doesn't make sense to me. Should this be used as Energy (Kinetic) and resolved using physics with the force imparted on the wall = 2 * Energy / the stopping distance? I don't think you can swap this energy for a moment and call it good. Agree or disagree?

 

[phamENG]

Agreed. Impact is referring to energy and it is not the same as moment.

 

[phamENG]

I'd be interested to know what Hard Body and Soft Body refer to. Off the cuff I'd guess it refers to their deformation characteristics - hard body being essentially a non-deformable body (all energy absorption must take place within the wall) while the soft body will deform, and you have to work out how the two will share the energy dissipation.

I've never had to do a "service level impact" design before, so I'm not exactly sure how you'd do it. With extreme events, we'd usually simplify as a SDOF, size the member based on set plastic performance criteria, and design the connections for the max reaction corresponding to the lower bound plastic failure mechanism.

Maybe use the area under the stress-strain curve up to a stress = Fy/FOS?

 

[LGMF Engineer]

Hard body refers to a small object thrown at a door. Anything a patient can pick up and toss. The soft body impact is them throwing themselves or another person against the door/wall.

 

[JAE]

Soft Body:

Hard Body:

 

[LGMF Engineer]

Very helpful.

 

[3DDave]

Anything a patient can pick up and toss.

Based on what a friend who worked (past tense) in a psych ward related, that means people as well. She had a patient that did not like the word "No" and was slammed against a wall like a rag doll a half-second after she said it. She quit after she asked for a second person to be with her for protection and was refused.

So, yes, it's the exchange of kinetic energy into the wall with elastic energy stored in the wall or absorbed if there is absorption to be had - hence the padding in padded rooms.

Hopefully they never get a loose chair or desk.

 

[PropertyGuy67]

Hard body impact modeling is for things like battering rams and projectiles. Soft body modeling is for things like prisoners trying to run through a wall.

The numbers you're faced with (175 and 480) tell me these wall assemblies will be pretty stout, Cat 4 (extreme duty). Cat 5 is a security-level assembly, like a jail corridor.

But bottom line, a Cat 4 assembly is a pretty basic assembly: metal studs and abuse-resistant panels. Most gypsum wall board companies have their own tested assemblies. Pick a manufacturer, research their abuse-resistant products, and spec their assemblies for the job. You're reinventing the wheel if you go down the road of doing calcs on the wall studs and panels. There are product assemblies out there that already meet 175/480.

 

[JAE]

LGMF Engineer - sorry (sincerely) - I just couldn't resist.

A little while ago there was a discussion about impact forces (different situation) in this thread (thread507-444175) and our good member WARose responded to the question this way:

[red]...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. [/red]

This might get you where you want to go.

 

[IRstuff]

ft-lb(f) is an expression of energy, which is how the a pendulum operates; by adjusting the weight of the bag and the height from which it starts, you get a potential energy which is converted to kinetic energy at the bottom of the pendulum swing. Note that E695 only refers to impact bag height, so a potential energy calculation ensues.

Note that E695 describes a harder body than shown above; the bag is filled with 3.5-mm or smaller metal shot.

TTFN (ta ta for now)
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[ozPE]

The area under the force-displacement line is equal to the energy=KE=PE
You can reach a result with this.

 

[hydtools]

Look at how the Charpy impact testing is done. The weight of the dropped mass multiplied by the drop height equals the potential energy that gets to be converted to the kinetic energy at impact.

Ted