belt conveyor truss design

huangyhg

belt conveyor truss design
i had this post in the materials handling forum but did not get any response so i thought i would try it here.  i have been asked to design some standard truss parallel chord frames, with allowable spans for particular belt widths.  the trusses are to carry limestone and coal, and are to be designed to support all the bells and whistles, (walkways, hoods, idlers, pull-stops, and electrical).  my question is as follows.  normally i have engineered conveyors for a specific application.  this means that the tph and the pulley resultant loads are known.  for the standard truss designs these properties are not known.  i have found a previous engineers calcs for similar work and all that he did was design the truss for a uniform moment of 1/8 wl^2, due to the distributed load.  this was before computers were widly used in design offices.  while i would agree that would give you the worst case chord force, i am not sure that the loadings are really ideal.  he used a surcharged belt with 20psf on the walkway, but of course had no way to account for the compression in the truss due to the conveyor drives.  is this a good way to design the truss, since the likelyhood of a surcharged belt and a 20psf walkway load all happening at once is somewhat unlikely?  if not what would you recommend?  thanks in advance for your help.
what about the impact loadings?
boo1,
the rock will be minus 2", thus no impact loading for the rock is required, assuming that this is a typical span, and not being loaded somewhere in the span.  any impacts due to startup or variances in the drive would again be unknown.
designing with the moment equation that you have quoted should be ok ... if you consider all of the appropriate loads (dead, live, wind?, seismic?, etc.). as you and boo1 have discussed, there are dynamic loads to consider also. without specific information, i recomend that you use the impact loading requirement from aisc (part 5, page 5-29 in 9th edition, asd). this would be a 20% to 50% increase of the live loading (depending on your the type equipment).
also the 20psf live load for the walkway sounds light by today's standards, you will probably find numbers more like 60 psf. sometimes these walkways are used for more than foot traffic - the conveyor breaks down, mechanics have to haul heavy tools & repair parts over the walkway.
thank you all for your replies.  what i am most concerned about is the compression in the truss due to the drives.  i reviewed a few of my old designs and found that designing the truss as a surcharged belt induced more chord force, than the drives put into the conveyors.  from this i guess designing the truss this way, while not being really accurate, will yield a safe design.  i will also be performing the calculations with a computer model, verifying the results using the approximate moment equations.  as far as impact is concerned i have never allowed for impact until the particle size is 8" or larger.  cema has equations that they use for belt idler loading and do not allow for impact until the particle size meets or exceeds 8".  i allow for the same impact as cema does on the idlers, reasoning is that if the idlers can see it, then the truss obviously can also.  i agree that the walkway loading is a little light at 20psf.  i have always designed for 25psf, as the asce 7-98 specified for catwalks.  it appears that in the 02 version the catwalks are to be designed for 60 psf.  i don't know if i will end up designing for this or not.  my client has been doing this type of work for 25 years, and he definitly has his own idea about what the truss should look like.  for a typical 80' span, it appears that the 60psf would allow for 48, 200lb men on the span.  this seems like an unlikely amount of people, but what it does allow for is some buildup of material on the walkway.  i think i will design for the 60 and see what happens.  i think that the biggest problem with this type of design is that it is not really coded.  my client is basicly trying to stay competitive in a tight market.  i would agree with everything that you guys have said above, and i think the conveyors should be designed for these types of loads.  the problem is that when i do i will end up with a 5" angle chord, while his competition is using a 4" or even a 3" chord.  i have also done several structural reviews for clients using these   
i would consider additional analysis of the conveyor during running, starting, braking, drive belt loading, conveyer wt, product wt, accumulating product loads, vibration, wind.  
my experience (nickel, iron ore, copper and gold mining/processing plants) is that belt tension loads are typically resisted by the head-end, take-up and tail-end structures, which take the horizontal loads to ground in braced frames.
therefore the intermediate trusses are typically designed for a combination of gravity loads (self weight of steelwork, idlers, belt, piping; cabling; normal material load; flooded material load; walkway load) and wind loads.
an exception to this is a stockpile feed conveyor where there is no head-end structure (final truss span is cantilevered) and the belt tensions must be taken through the trusses to the take-up tower.
i expect that the standard trusses you are designing would not see belt tensions at all.
also, a flooded (or surcharged) belt load can occur simultaneously with a high walkway load.  consider the following scenario: belt is flooded, material has spilled onto the walkway, conveyor is stopped and operators work from the walkway to clear material from the walkway and belt.