Designing a center lattice tower for a temporary grain storage system - calculating grain pressures?

[teemunney]

I have been stumped on this for a few days, and so have my colleagues, so I figured why not give asking you all a shot.

I am attempting to design a “center pile tower” for a temporary grain storage unit. A schematic in elevation view can be seen here. Grain is transported up the conveyor inside the catwalk and dumped into the chute inside the middle of the 80’ tall lattice tower, where it eventually comes out through one of the five openings , forming a pile which is covered by a tarp. The capacity of this unit is about 1.5 million bushels, to give you an idea (a bushel is about 1.22 cubic feet, something like that).

The tower is completely open for the bottom 20 feet; the rest of the tower is cladded with 14 gauge steel for the remaining height (except for where there are spouts).

Calculating the wind load was pretty straightforward using chapters 29 and 30 of the ASCE 7-10. Calculating the grain pressure on the tower, however, is where I am stumped. Or at least I think we are being ridiculously conservative. We wanted to investigate different cases to figure out the worst-case load on the tower (empty, ¼ the way full of grain, halfway full, ¾ of the way full, and completely full).

At first, we decided to take the worst case, being all the pressure on just one side of the tower, which would occur if this pile is full and will be emptied out on one side when a door is opened. My model would not converge unless ridiculous sizes were used.

We then decided to see what would happen if we loaded the tower with those loads shown in the last link, except they would act equal and opposite on all four sides, effectively “crushing” the tower. This time, my model converged except for when the storage pile is completely full—the P-delta instabilities likely are due to the angles buckling. Turning the P-delta shears off yields a very overstressed model.

I have taken the equivalent fluid density of grain as 22pcf per this document , and the point loads as 22 times the depth times the area of the bay, then divide in half for a point load on each leg. For example, at a depth of 60 feet (or height of 20 feet), P=22* 60 * 3.7 *5.5/2=13.4kips. Which as I said, is ridiculously conservative given there have been no catastrophic failures of towers built in the past and this load is seen on an annual basis. And I've been scouring the Internet to see what is out there on the subject, but there isn't much.

Thoughts on this anyone? I know my loads are off by a factor of god-knows-how much, but I have little idea on how to prove it.

Thanks, everyone.

 

[chrislaope]

Could you please attach schematic drawings again? we can not see anything through any of your link.

 

[TehMightyEngineer]

Links work for me.

Maine EIT, Civil/Structural.

 

[dhengr]

Teemunney
The material can’t pile up much higher on one side or another than the angle of repose of the material will allow this pile to grow, be that against individual members or some form of cladding . Of course, there could be some extenuating circumstances, these must be taken into account, and your discharge control system must control most of these. It would seem wise not to clad most of the tower, except for the vert. cutes themselves, because of wind loads and these lateral material loads, let er flow and equalize. Aren’t there also extending/retracting cute systems where the cute telescopes back on/into itself as the pile grows?

 

[Canuck67]

The National Farm Building Code of Canada (1995) has several things that might assist. There are tables bulk densities of many different agricultural materials. Also, there are formulae for lateral design pressures of tower silos. Not sure if this is exactly what your are looking for, but hopefully will have some useful information.

 

[Splitrings]

Below are a couple of references that may be of some use:

ASABE D252.1 "Tower Silos: Unit Weight of Silage and Silo Capacities"
ASABE EP545 "Loads Exerted by Free Flowing Grain on Shallow Storage Structures"

 

[EdStainless]

You don't get as much pressure as you expect because the pile is not constrained.
It is really a thick fluid. The angle of repose (really internal friction) keeps it from applying too much lateral pressure.
Put a wall around it and things change a lot.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[paddingtongreen]

@EdStainless, I think that anything below a line being the angle of repose, drawn from the ground at the outer edge of the pile, upward to the tower, would be considered to be constrained.

I think the material can be unbalanced by a strong wind, I'm thinking of the scouring effects of wind on fallen snow. If my paragraph above is correct, scouring could cause a significant imbalance.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[slipstick50]

We (agricultural engineers in our firm) have long term and considerable experience with grain storage piles and with towers, columns and structures in the grain pile.

You are correct that wind loading is pretty straight forward.

What you will find is that grain settlement/drag down loads are huge and will bend down and or tear tower diagonals and horizontal off the tower columns if designs don't consider the grain settlement and drag during filling and storage time.

Another issue is what happens if the pile's center tower has a center unloading gate to a tunnel below... The grain loads when empty through the tower center are huge.

Also, if the pile is unloaded from the outside in working across the pile to the pile's center, sliding grain loading on the tower can be huge too and hard on diagonals and horizontals, bow columns, etc.

Most of the damages and failures we see and do repair designs are towers which were too lightly designed and constructed of expected loads the designer didn't know about or consider.

Rod

 

[slipstick50]

PS... this post should probably be in the Agricultural section of the forums to reach more people with experience with grain storage structures.. I just happened to see the post..
Rod

 

[slipstick50]

The design load on the tower's diagonals and horizontals is basically 50 or 52 PCF times the grain's depth at any given location. Remember the design load will be the working load and that settlement and consolidation of grain over time will significantly increase the loads on the diagonals and horizontals... The EFD of 22 is really low....... ration of later to vertical pressure for grain as an equivalent fluid is about 0.5 so .5 x 52 PCF gieve 26 PCF EFD... need to add in Factor of Safety (factor of ignorance) and probably use 30 PCF EFD for horizontal loads and 52 PCF for vertical loads.. Look into deep bin and shallow bin theory, Janssen's formula, etc.

Rod

 

[teemunney]

@slipstick50 So the load on a horizontal member 5' long and 10' deep, for example would be 52pcf * 10' * 5'? 2.6kips per foot? That still sounds ridiculously high. I could be off somewhere here, though.

 

[teemunney]

Also, just curious--what member sizes for columns/diagonals/horizontals do you typically see with these?

 

[Agent666]

The lateral pressure should peak and won't increase linearly with depth as it would for a soil, look at using Janssen formulation as someone else mentioned for 'silo' loading.

 

[slipstick50]

horizontal member 5' long x .33' wide,,,,,, for design 52 PCD x 10' =520 psf... on the 5' long 4" member....... in the grain,,,,, so wind design easy.... grain loads not so easy,,, design load is the working load......... we work on repairs designs regularly where designers under predict what grain drag and settlement/compaction loads are.... and horizontal and diagonal members tear loose and/bend... pulling columns in to compensate...... look at "K" bracing going to horizontal centers..... horizontal x braces to help hold tower square too....... 5". 6", 8" pipe columns depending on weight and depths of grain.... 3" 4" and 5" horizontals and diagonals.... r

 

[slipstick50]

Piles are not deep grain storage but rather shallow... so janssen's does not apply.. the semi fluid nature of grain is tricky and designs have to work with a huge number of variables.... difficult to design safely and conservatively..... Seriously, look at shallow design theory for grain... grain depth times design bulk density.... for vertical loading on tower members.. then for horizontal loads shallow loads.. basically L/V of .5 x 52 PCF . grain depth for the lateral loads..... r......