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.

 

[TehMightyEngineer]

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

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

http://www.americanconcrete.com

 

[CBSE]

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.

 

[TehMightyEngineer]

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

http://www.americanconcrete.com

 

[racookpe1978]

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?

 

[dhengr]

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.

 

[CBSE]

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.

 

[TehMightyEngineer]

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

http://www.americanconcrete.com

 

[GregLocock]

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


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

http://eng-tips.com/market.cfm?

 

[dhengr]

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.

 

[racookpe1978]

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?

 

[CBSE]

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.