complete timber failure 1

[stallion0002]

I want to start of saying I am a steel guy and as such, not entirely familiar with timber design. Here is the problem...

I am designing timber cribbing to absorb the impact of a 14.5kip load free falling about 50ft without effecting the structure below.

I am looking for some guidence on how much impact force timber can absorb in bending. I am not looking for the allowable or the ultimate strength, so the NDS is no help. We are not looking for the modulus of rupture as this only gives the stress at the instant of failure. We intentionally want the timbers to bend, crack, completely break in half, etc. As the layers of cribbing break, the energy of the impact force will be dissipated.

 

[MiketheEngineer]

Well NDS is kind of the gospel in this arena. The assumed safety factor is probably somewhere between 1.6 and 2.0

Impact loads have been discussed dozens of times in these forums - search it.

The BIGGEST factor is how far OR how long it takes to stop the load.

That gives you the de-acceleration number.

Anecdotal testing has shown me that wood cribbing "bounces" and "jumps" around a lot. No better term that I can think of. This EATS energy. And the cribbing/deck gives far better performance than one would think.

ONLY actual testing will prove it out.

Why do you think Ford, Chevy, BMW, etc wreck dozens of cars a year to prove their safety?? If this could be exactly computed, they would never waste the money....

 

[JAE]

ONLY actual testing will prove it out.

That's what I would say also.

 

[msquared48]

What's the difference between "actual testing", and "testing"?

I have never tested an actual...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MikeHalloran]

>>>I am designing timber cribbing to absorb the impact of a 14.5kip load free falling about 50ft without effecting the structure below.<<<

Scary.

You couldn't put in 50 ft of cribbing and remove it a little at a time while lowering the load a few inches at a time using jacks?



Mike Halloran
Pembroke Pines, FL, USA

 

[stallion0002]

This is the coutnerweight takeup for a conveyor system. The takeup has to have 50ft of clear travel based on the different loading cases and the expansion of the belting under those cases. So in a worst case senerio, the counterweight would be at the top at the time of failure.

This is a case of a construction project manager 'just doing it' in the field with no consulting of the engineers. Now that the client has questioned it, the PM is looking to us to bail him out. Personally, I don't think it's going to happen. We are also looking into a shock absorbing/braking system and will probably recommend that they remove the timbers, install the braking system, and beef up the structure accordingly.

 

[woodman88]

Wood is graded/tested to a 5th percentile value and this value is expected to be exceeded by 95% of the pieces in a grade, class or species. For the holding value testing of Metal Connector Plates used to make Wood Trusses, I had to go though stacks of 2x4's and measure the length, width, thickness, moisture content and weight of each piece to determine those pieces that were within a certain percentage of the species assigned specific gravity for the use in the testing. Depending on how actuate you need to be, you may need to do the same.
The following may be of some help to you.
The Wood Handbook—Wood as an engineering material. Forest Products Laboratory. 2010. Chapter 5 states
“Impact bending—In the impact bending test, a hammer of given weight is dropped upon a beam from successively increased heights until rupture occurs or the beam deflects 152 mm (6 in.) or more. The height of the maximum drop, or the drop that causes failure, is a comparative value that represents the ability of wood to absorb shocks that cause stresses beyond the proportional limit.”
and list values for difference species for Impact bending and Modulus of rupture. I have attached this chapter here. This publication is also available as a free pdf on-line at

http://www.fpl.fs.fed.us.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[hokie66]

Your approach is the best. It would be irresponsible to allow free fall of a counterweight of that size. Look into elevator braking systems.

 

[dhengr]

Stallion0002:
My goodness, there must be a more elegant way to tackle this problem than wooden cribbing which must span some distance but absorb this falling load. What is the structure being protected, how is the cribbing supported, on what and what is its structural configuration? All of these types of things would come into play in this impact analysis. Look for some sort of a safety cable or net attached to the counterweight which absorbs energy and slows the fall to a stop. Or a safety cable system attached to the structure above which automatically adjusts with counterweight movement but limits its fall to only a foot or so which can be designed around at each adjustment incriment. Does the counterweight move straight up and down or on some regular path, where it might follow a track of some sort, and be on some sort of sled with wheels on the track. Then Hokie’s elevator braking system makes sense.

 

[csd72]

Sounds like a good situation for the application of energy methods.

Kinetic enery creates deformation energy

 

[MiketheEngineer]

I've tested a lot of actuals - some have an "ly" attached.

It actually broke
She was actually beautiful
Actually, she was ugly
He was actually quite drunk.

Actually, does seem redundant... or as Jake says on 2.5 Men - redumbnant.

 

[Compositepro]

Energy methods should be used for analysis. A wood beam will absorb energy in bending according to its stiffness and deflection. When it breaks the energy will be permanently absorbed rather than reflected back as a spring would do. There needs to be gaps between beams so they break progressively.

Drums or bulk bags filled with sand or water may work better.

 

[Bobber1]

Vertically mounted traffic barriers/energy absorbers? I've seen big energy absorbers mounted on the backs of highway maintenance trucks used to protect the workers.

 

[JoshPlumSE]

This doesn't sound to me like the ability to absorb force or moment. Rather it sounds more like energy disipation.... almost akin to blast loading.

I have not been involved in blast loading at all, but the general concept is there will be a large force impacting the structure over a very short period of time. Therefore, you want to design some "disposable" cladding or walls or such to fail in a manner which disipates the blast energy and protects the rest of the structure.

If this sounds at all like what you need, then you might check out some references related to blast loading or blast resistance.

 

[bootlegend]

stallion,

What kind of structure is below the conveyor? Do you have a sketch of the layout?

Assuming your takeup pulley is on a vertical slide assembly, you may need an arrestor system like elevators have.

That or rig up a pulley/cable system that allows the counterweight to move horizontally instead of vertical.

 

[MikeHalloran]

I'd be a lot more comfortable with a design that deformed steel plastically, e.g. plunge a round rod into a smaller square tube, or something like that, than timber in any form.

Primary reason: Here in Florida, the timber could be hollowed out by termites after installation and before use, and you wouldn't know it. ... until it didn't absorb anywhere near the design intent energy.

Maybe you have wood-eating bugs too?




Mike Halloran
Pembroke Pines, FL, USA

 

[chicopee]

The elevator brake system mentioned above is a good way to stop the counterweights from free falling. The original elevator braking systems used timber rails in which wedges would split these rails all the way down as the elevator cabs were detached from their roping.