Designing structures for intentional failure

[jltl0]

I'm designing a structure which, when impacted with sufficiently large force in one direction, should fail in a particular way (ie break along a particular line). I can think of some obvious ways to accomplish this, but I'd like to know if there are any bodies of work out there or similarly functioning products/structures which I can learn from first. If anyone has heard of similar applications, that would be very helpful -- I'm sure this is not uncommon in product design, etc, but I can't think of anything off the top of my head and my googling skills are failing me entirely.

Thanks a lot!

 

[rb1957]

called a structural fuse, they're used in undercarriage to make sure the undercarriage breaks off the plane before it breaks te plane.

usually a sharp notch, carefully machined to ensure the final net section is as required.

 

[CANPRO]

I've never dealt with this personally, but some buildings built in flood plains have break away walls that will allow the water pass through without the entire house being swept away (I only know as much about this as I've read on this site).

Also, the concept of weak beam - strong column comes to mind...but I don't think anything is intended to fail there.

Can't think of much more besides that...what is your application?

 

[charliealphabravo]

I too am interested in the OP. I don't think the way forward is always clear. So many material/loading standards/codes are based on the presumption that failure is always bad and so many of the coefficients/safety factors/testing values/load maps are conservatively selected to err on the "safe" side of any uncertainty. Sometimes these values are not rationally based, just committee consensus or spitballing.

How do you peel back all these assumptions when there is a certain load where a component ABSOLUTELY POSITIVELY MUST FAIL (not shouting)

 

[PicoStruc]

This is similar to the capacity design procedure in the canadian seismic codes - The weakest link = Structural fuse.

In seismic design, the fuse must be ductile and the other element of the structure must be designed for the capacity of the fuse and NOT for the actual computed load... that might be lower if using reduced seismic loads. !

 

[Ron]

This is sometimes done for towers in urban areas to limit the fall radius.

 

[GregLocock]

Very common feature in automotive, where progressive plastic collapse of the structure is the neatest way of absorbing impact energy. Sometimes stocky columns are designed to fail in compression, with built in concertina sections. Sometimes failure in bending is used.

It is also fairly common to see bolted joints that are designed to fail, so that the components they restrain (typically hard lumps) can move out of the way of the collision.

So far as I know there are no standards on this, but the common solutions are robust.

Cheers

Greg Locock


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

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[BAretired]

There are several methods given in the Canadian code to provide "Structural Integrity of Firewalls". One method is the so-called "Weak-link connection". The firewall is laterally supported by the floors each side. The following is taken from User's Guide, NBC 2005.

If a weak link is provided on each side of the firewall, the link on the fire side will break away while the link on the non fire side will not.

I have never used this particular method because I believe it is fundamentally flawed. There is no guarantee that the link on the fire side will fail before the link on the non fire side. If the non fire side link fails first, the firewall will be pulled down by the collapsing steel members on the fire side.

However, I would agree that, if all you want is a failure in a particular joint, a weak link will work just fine. Same goes for any other part of the structure. If you want something to fail, make sure it is weaker than every other part of the structure.

BA

 

[slickdeals]

As someone mentioned, FEMA has some literature on breakaway walls in areas susceptible to flood damage

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[cve60069]

Thanks for the tips and references on flood damage. My next project (hopefully) is in a flood zone.

 

[ACtrafficengr]

Large traffic signs are designed to fail when struck by a vehicle.

The base of the post is designed to break away or disconnect, and a cut is made through the leading flange and web of the I-beam just below the sign panel. The cut is spliced with a perforated plate that resist wind loads, but releases under impact. Alternately, the bolt "holes" in the plate are u-shaped open slots, and the bolt can slide out under impact.

 

[charliealphabravo]

called a structural fuse, they're used in undercarriage to make sure the undercarriage breaks off the plane before it breaks te plane. usually a sharp notch, carefully machined to ensure the final net section is as required.

Wouldn't the stress concentrations forming around a sharp notch result in a strength less than predicted by the net area? The breakaway bolts I have seen have a smooth narrow round neck but I don't recall a sharp notch.

Also to get back to the design theory...For break-away walls in the V-Zone and I recall basically using the ultimate strength of conventional materials with no explicit strength reduction factors, factored wind loads and unfactored flood loads. Most break-away walls are actually designed prescriptively (20 psf I think) to bypass all the mucking about, but there can still be complications if you want to use the engineered method so that the wall stands up under wind load but fails under flood load.

I don't mean to muddy the water, but again I don't think that design is a straight-forward process when you are using material and load standards for the opposite purpose for which they were originally intended. There are often conservative assumptions built into the formulas/coefficients that don't appear explicitly. Design for failure is further complicated by the fact that structural fasteners and anchors (Simpson, Hilti, etc) are approved by one of several methods that may be based on actual tested load, tested deflection, or and/or rational analysis.

 

[btrueblood]

Another example:

Burst discs in fluid systems, used to safely vent a runaway process (pressure excursion), or sometimes to trigger/start a process. There are many designs, some include scribed pattens on the disks. Knowing (by test) the exact strength of a given lot of diaphragm material is necessary to predict the burst pressure with accuracy, and the equations for rupture include bending and membrane stresses (i.e. large deflection solutions).