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Weight and Pressure of Firewood

Weight and Pressure of Firewood

Weight and Pressure of Firewood

(OP)
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.

RE: Weight and Pressure of Firewood

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

RE: Weight and Pressure of Firewood

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.

RE: Weight and Pressure of Firewood

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)


RE: Weight and Pressure of Firewood

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

RE: Weight and Pressure of Firewood

(OP)
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.

RE: Weight and Pressure of Firewood

(OP)
35 pcf

RE: Weight and Pressure of Firewood

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

RE: Weight and Pressure of Firewood

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.

RE: Weight and Pressure of Firewood

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_gtr19... ) might be a better resource.

RE: Weight and Pressure of Firewood

(OP)
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!

RE: Weight and Pressure of Firewood

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.

RE: Weight and Pressure of Firewood

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?

RE: Weight and Pressure of Firewood

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
www.americanconcrete.com

RE: Weight and Pressure of Firewood

(OP)
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.

RE: Weight and Pressure of Firewood

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....

RE: Weight and Pressure of Firewood

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
www.americanconcrete.com

RE: Weight and Pressure of Firewood

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

http://knechts.net/wp-content/uploads/2010/10/P326... - 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.

RE: Weight and Pressure of Firewood

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.

RE: Weight and Pressure of Firewood

Now you've done it Teguci, the formating of this thread is all screwed up now. :P

Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com

RE: Weight and Pressure of Firewood

(OP)
I wish my firewood stack at home looked like that!!!

I like the idea of the 3/8" thick walls. We will certainly look into the tapered walls as well.

RE: Weight and Pressure of Firewood

And, just because I suppose I should support my current industry; have you considered making these out of precast concrete? You could easily get some steel deck embeded into a precast concrete structure. Probably cheaper and more durable than a steel hopper, but this really depends on the final location and use.

Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com

RE: Weight and Pressure of Firewood

What is the expected "length" of the log segments, compared to the width and length (horizontal dimensions) of the hopper inlet? What is max and minimum expected diameters?

What is the purpose? "Store" the wood chunks for a while, or transfer them from conveyor rapidly to the inlet?

RE: Weight and Pressure of Firewood

CBSE:
You really need to give us much more detail to really make sense out of the wood dryer. It’s 10'x20' in plan and 30' high. Is the drying bin really 30' deep? Or is the bin some lessor depth (20' deep?), and the whole thing on 10' high legs , so you can unload it into a truck? How is it actually unloaded, through a hopper at the bottom, or through a side door with a front end loader? We know of a conveyor, but how far does it drop the wood to the floor of the bin? Can the conveyor move down the center of the bin, along the 20' length, to distribute the wood? Are these split pieces of hardwood, 20" long, or are they 16' long full logs?

You should know the fall line (trajectory) from the conveyor to the bin floor, a function of the conveyor speed, its wood feed rate, etc. The first foot or so in depth of wood pieces (first few chords) will impart an impact loading on the bin floor. After that, the impact loading will be partly absorbed and spread out by the depth of piled wood. My experience is that thrown split wood will stand at an angle of repose of 45̊ or more. There is considerable interlocking within the pile. Off the top of my head, I don’t know what to say about the lateral loads on the walls, it won’t be like a uniformly graded granular material. Obviously, the incoming wood impacting on one area of wall is another matter.

RE: Weight and Pressure of Firewood

(OP)
dhengr: Great questions!

This dryer is 30ft tall. According to the owner, he wants to fill it to the max with split firewood. The standard lengths he is going for is 18" and 16". He won't be doing any full size logs. The dryer is completely open at the bottom and the idea is that as the wood dryers, he will bring in a loader or something else to pull wood the wood out from the bottom. As he pulls wood out, he will put wood in from the conveyor up top.

The wood will drop from the conveyor to the bottom of the dryer. There will be a vertical "chute" of sorts at the top of the bin so that the wood falls vertically instead of spilling off the top and placing more wood on one side of the dryer than the other.

The main structure will be supported by steel columns at the bottom with big bollards surrounding them. We are going to build some concrete retaining wing walls so that the loader, or whatever else he is using cannot damage our structural steel.

I have done a lot of work in mills in the past, but they were whole log mills for plywood and dimensional lumber. From my experience, columns get absolutely destroyed without a lot of protective barrier.

As for the angle of repose, we have settled on around 35-40deg for this.

RE: Weight and Pressure of Firewood

Quote (CBSE)

From my experience, columns get absolutely destroyed without a lot of protective barrier.

This! Buckled columns, buckled columns everywhere.

Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com

RE: Weight and Pressure of Firewood

The idea of neatly stacked logs is cute, the reality is one log will up-end between two horizontal ones, and then the next load of wood pile drives it between its neighbours. Steel is cheap, rectification is not.

To reduce bridging have you considered tapering the shape out from a narrow top?



Cheers

Greg Locock


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RE: Weight and Pressure of Firewood

CBSE:
If the conveyor doesn’t move, I’d make the bin 15' sq. in plan (same plan area or bin volume) so I got fairly even wood distribution in plan and depth. Maybe see if you could find some good, used sheet piling, 35 or 40' long, and drive these 5 to 8' into the ground. Design it like a filled coffer dam cell; with a conc. ring beam/found. on the outside, 2' below grade, 4' + above grade, formed outside face and top, maybe 16" thick. Frame a couple side openings to fit the loaders, and continue the found. wall, as wing walls perpendicular to the wall plane, still 6' tall and 2' into the bin and 4-5' outside the bin. In effect, a chute which will steer the loader through the opening, without touching the stl. wall. Make a lifting stl, grillage door for the opening with the verts. on the inside layer and a couple horizs. and diags. as the outside layer, and with the verts matching the flutes in the sht. piling. These drop doors can be pulled up the outside of the bin, in side tracks, for access. How do you move dry air through, and up and out to dry the wood? You need air holes in the found. wall. A sloped roof on top, with center opening for the conveyor. Then move the conveyor straight back a few feet, and at the same time move a suction fan over the center roof opening.

The split wood is very likely to arch and lock-up, hang-up in place, at 10 or 15' high, and this will really be dangerous during the unloading process, from below. Also, you can’t just unload a portion and then refill, or you will have large volumes of wood that never get removed, in the corners and away from the doors.

RE: Weight and Pressure of Firewood

Well, that trapping and "kinking" of the sticks between each other to create a Lincoln-log "bridge" was the purpose of my question above.

But at the wood only 16 - 18" inches long across a 20 ft x 30 ft opening, pure bridging of several hundred sticks at once is less likely to occur than 4 foot long logs falling into a 6 or 8 foot opening.

The opening is 20 ft x 30 ft, right? Is that all "effective loading" or does only end of the machine go towards another processor or crusher?

RE: Weight and Pressure of Firewood

(OP)
The opening at this time is 20ft x 10ft. Conveyor conveys the cut wood into the bin and then the owner removes it and stacks it somewhere else. Still pursuing the tapered section.

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