aci 313 overpressure factor

huangyhg

aci 313 overpressure factor
i am currently reviewing a design of an aggregate bin (for the storage of sand and gravel) using the aci 313-97 specification to calculate material pressures on the bin and hoppers.  this bin is an existing design that i've been told has not shown any problems in the past but my calculations indicate many of the stiffeners are undersized.  i am looking at this design now because i am planning on using the upper sections on a larger bin bottom for a new higher capacity bin.  over the years i've noticed that my designs have generally been heavier than what the "old timers" did and i'm starting to wonder if i am over designing.  on the other hand it is possible that existing designs have a very small factor of safety.
my question is whether or not the aci overpressure factor of 1.5 per section 4.4.2.2 is realistic for funnel flow bins of this type.  i've been told by a local consulting engineer that they have designed many bins based on only filling pressures and consider that good enough.  all recent research however clearly shows that emptying pressures are significantly larger than filling pressures.
if anyone has some ideas or comments i would appreciate the help.
thanks,
-mike
   

mike,
most everyone that i know uses either aci or gaylord.  (k) calculated per aci uses the at rest pressure and (k) per gaylord uses the active pressure.  their is some difference but it is generally fairly small.  gaylord uses different overpressure factors based on the zone verses aci using the constant cd = 1.5.  i think where their is a lot of confusion is when you are looking at shallow bunkers.  most shallow bunkers use just active pressures with no overpressure factors.  per aci their is not a difference between bunkers and silos, they are all designed the same.  i have noticed significant differences in design in these cases.  i have designed using both methods without a failure and i think aci is very conservative with shallow bunkers.  what i typically do on shallow bunkers is design for at rest pressures without an overpressure factor.  right or wrong, i don't know, but i have found this to be a balance.
hi aggman,
i have both aci 313-77 and 97 specs.  gaylord discusses the 77 spec which uses table 5-2 (table c.1 in aci) for values of cd.  i use the 97 spec since it is easier to use cd=1.5 throughout the depth than to work with several zones.  as you mentioned k is computed differently between the two specs.  table c.1 in the 77 spec states that values of cd are not adequate for mass flow bins so clearly cd is required for funnel flow bins and needs to be considered, so it seems clear that funnel flow bins need to be designed for overpressures.  i've been doing some research trying to find more info on funnel flow bins but without any success.
in my designs i've been using cd=1.5 (sometimes 1.35) for deep bins and the rankine pressure for shallow bins without the overpressure factor, which i think agrees with what you are doing.  but still i see existing designs that i wonder how they have held up without any problems.
thanks for the post.  i appreciate your comments.
regards,
-mike
from what i recall of the gaylord book from times past, they say "you can do it this way.  or you can do it this was as well."  you can make reasonable assumptions and get design loads, or make other, equally reasonable assumptions, and get altogether different design loads.
on small silos, you can resolve this by being very conservative, and it has minimal effect on the design.  with larger silos, this is not the case.
jstephen,
the chapter in gaylord on loads is an overview of several different methods to compute design loads and it is interesting (and confusing) to me how different the answers can be.  i use the aci method because with a simple overpressure factor it give results as good or better than the more complicated procedures.  in addition to that it is the only major code writing authority in the us that covers bin loads, a plus for me.
your comment..  don't assume that the "old timers" or "the other guys" always knew what they were doing.  is very true in my opinion.
thanks,
-mike
jstephen,
i am curious and if you don't mind me asking, do you design for filling pressures or emptying pressure, and what method would you recommend.
regards,
-mike