Weight and Pressure of Firewood

[CBSE]

I have a decently unique project that we are trying to figure out loading conditions for. We are designing the structural "shell" for a firewood dryer. It is rectangular in nature, about 30ft tall, 10ft wide, and 20ft long. It will all be built out of steel.

Question: Anyone have any ideas on how to estimate the lateral forces from the firewood stacked in this thing? The firewood gets dropped in from a conveyor at the top and then just piles up inside.

I first thought about using the equivalent weight as compare to water, assuming douglas fir and a specific gravity of 0.50. This seams conservative though because there are an abundant amount of internal frictional forces at work here. The total weight of wood that this can hold is around 120,000 lb. This equates to about 20pcf of material inside. Would that be a reasonable volumetric weight to assume? It seams big though when calculating the pressure at the bottom of the box...600 psf.

 

[thaidavid40]

It sounds like your greatest lateral load would probably come from the action of dropping the logs into the hopper, assuming that there is no vibration or consolidation going on afterward.
Dave

Thaidavid

 

[oldestguy]

This akin to loads in an agricultural silo. I wouldn't be surprised to find a full hopper will hang up on the sides. It is common for corn silage to hang up and then a dangerous condition develops. Also having been heating with wood for many years and having had stacks of stored wood, there can be considerable "binding" together. Perhaps the drying, accompanied by shrinkage that the hang up may not happen. I'd look to making the hopper tapered, wider at the bottom to avoid, or minimize, the potential of hang up. As to pressure laterally, as with soil, maybe "at rest" at 50 percent of vertical. 20 pcf seems quite light. Run some simple tests with a bathroom scale.

 

[msquared48]

I figure DF and HF at 35 pcf. I think a dead weight of 30 pcf is not unreasonable and conservative considering the voids.

Mike McCann, PE, SE (WA)

 

[oldestguy]

When it comes to pressures in a silo there are many references. Do a Google search.

 

[CBSE]

I looked up silo loads but there's so much more going on with firewood in terms of stacking and linking together.

We are doing smooth walls on the inside so in theory nothing should hang up, unless a bridge of some sorts in the wood happens.

If I assume 35 off and then apply 50% of that for horizontal pressures seams reasonable.

 

[CBSE]

35 pcf

 

[oldestguy]

CBSE: I suppose the friction coef. between wood and steel is zero??? Wishful thinking can do you in.

 

[KootK]

Assuming super smooth cylindrical logs stacked as non-randomly as possible, I get lateral at 50% vertical as well. I would assume that a full bin considered statically would dwarf the impact of small groups of logs getting tossed in dynamically.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I can't help with the lateral pressure. But, I wanted to comment on the density. Don't forget that G=0.5 for oven-dry Douglas Fir - Larch. The weight of the green wood will be higher. Also, you should confirm the species of wood. Some hardwoods are denser than lumber typically used for construction. The NDS has specific gravities for some lumber in table 12.3.3A. The NDS supplement section 3.1 has an equation for calculating density based on moisture content. But FPL's Wood Handbook (

http://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf

) might be a better resource.

 

[CBSE]

Oldest guy: im sure things will get sticky at some point. But we are trying to mitigate the effect with the smooth walls.

We will explore the pyramid type shape as well again, the owner did not like the idea originally.

Thank you all for the input!

 

[KootK]

Crap. I think that my own model above might actually predict 0.25 x H x PCF.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ussuri]

A couple of other queries.

If the firewood dropped from a conveyor it will not be neatly arranged as per KootK sketch?

If the firewood is also pre split then it could be all manner of shapes?

 

[TehMightyEngineer]

Having done some engineering for a groundwood pulp mill and seen them load large logs into hoppers exactly as you described the steel was polished smooth from the logs and beat to hell and back. Make the walls of this robust as the impact of logs is going to be quite large.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

Pictures!

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[CBSE]

Thanks for the pictures!

We were initially going with 1/4" steel plate on the inside, and that would be supported by 6" vertical steel channel on the outside at 24" o.c. We currently have it designed to look kind of like (3) storage containers stacked on top of each other. I think we may explore tightening up the spacing of the vertical channel. I will post a pdf today sometime of what we are looking at doing.

 

[SAIL3]

agree with TME...the sides may start out smooth,but as stated, will get dented from random concentrated loads...so I would also make the pl a min of 3/8" to try and address this...changing external stiffener spacing would not directly prevent this denting....

 

[TehMightyEngineer]

Going off of memory I believe the plate for the pulp mill hoppers was 1/2" thick or so. This worked well but there were still places where the logs had dented or punched through the plate. Overall they were durable enough and actually the concrete floor was the reason we were there.

I'd say SAIL has a good idea, somewhere around 3/8" to 1/2" would be my rough guess of the thickness you'll want to target.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Teguci]

I'd think a friction angle of 45 degs would be appropriate (K0 = 0.30).

http://knechts.net/wp-content/uploads/2010/10/P3261096.jpg

- link to picture with logs piled at +/- 45 degs

No idea as far as density.

I'd think a flexible structure system would last better than a rigid system. I'd lean towards a thicker shell with fewer stiffeners.

 

[ornerynorsk]

From experience (if the design will allow), I would recommend the bin be built with the walls slightly tapered, gaining in inside dimension toward the direction of egress, to prevent clumping and jamming of the wood chunks. Just my 2 cents.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.