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.
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
Dave
Thaidavid
RE: Weight and Pressure of Firewood
RE: Weight and Pressure of Firewood
Mike McCann, PE, SE (WA)
RE: Weight and Pressure of Firewood
RE: Weight and Pressure of Firewood
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
RE: Weight and Pressure of Firewood
RE: Weight and Pressure of Firewood
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
RE: Weight and Pressure of Firewood
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
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
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
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Weight and Pressure of Firewood
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Weight and Pressure of Firewood
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
RE: Weight and Pressure of Firewood
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
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
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
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Weight and Pressure of Firewood
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
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Weight and Pressure of Firewood
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
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
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
This! Buckled columns, buckled columns everywhere.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Weight and Pressure of Firewood
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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Weight and Pressure of Firewood
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
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