complete timber failure
complete timber failure
(OP)
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.
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.





RE: complete timber failure
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....
RE: complete timber failure
That's what I would say also.
RE: complete timber failure
I have never tested an actual...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: complete timber failure
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
RE: complete timber 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.
RE: complete timber failure
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 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.
RE: complete timber failure
RE: complete timber failure
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.
RE: complete timber failure
Kinetic enery creates deformation energy
RE: complete timber failure
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.
RE: complete timber failure
Drums or bulk bags filled with sand or water may work better.
RE: complete timber failure
RE: complete timber failure
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.
RE: complete timber failure
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.
RE: complete timber failure
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
RE: complete timber failure