safety grate design

huangyhg

safety grate design
i have been asked to design a safety grate for fall protection.  the grates are over a hopper which holds biosolids (75% water & 25% biosolids).  the biosolids have the consistency of mashed potatoes. the biosolids are 60-pcf and are dumped in 25-ton loads at a rate of 25-ton/minute. i know how to design the grates for the fall protection loads but i'm not sure how to figure the loading due to the flow of the biosolids through the grates?  i plan to space the bar grates at about 10-inches on center to allow the material to flow easily through the grates.  however, how do i determine if material build-up due to bridging may occur?
thanks in advance   
can you provide opening on the grate to allow un-restrict dump? can you design a grate to support the build-up from grate to the point of dumping? more information are required to look into this matter. the 10" bar spacing seems too large for a person to step on.
the grate spacing of 10" is what the plant wants for operation purposes.  i will determine the actual spacing based on loading design and osha safety requirements.  the gratings will be removable (for maintenance on hoppers) but the plant wants them in place when material is dumped.  material is dumped from dump trucks (about 5-ft drop height).  i'm looking for information to help me determine material loading.  for a thick but flowable material like i have, are there resources available which give me equations based on material density, viscosity, drag friction, or whatever?
fyi, aisi has a publication, useful information on the design of bins and silos. of which i have a old copy.  there is a lot of discussion of arching in this publication, though it looks very complicated.  i have never really delved into this topic.  it may be a basis for doing some type of calculations short of performing field test, which migh be best for your situation.
also, i should meantion that i dont think osha walking and working surfaces standard would appy here. you just don't want someone to pass through. maybe there are some other applicable osha requirements.
actual loads for industrial structures are really hard to define. suggest that you do not try to find an "exact" answer for operating conditions (material flowing correctly thru the grating).
there are at least three good reasons to be conservative:
1. someday the grating may plug (from out of spec material, etc.) and load will increase.
2. the grating will be eroded by material flow over time, reducing it's crossection.
3. the day will come when the plant will ask you something like this: "we want to lay steel plates over the grating and drive a motorized man-lift onto the plates to paint the overhead steel. is that ok?"
suggest that you use engineering judgment to assume a conservative uniform static depth of material and size the grating to safely support that load.
thanks everyone for your comments and suggestions.