huangyhg

seismic design of silos considering fluctuating live loads
i am trying to find literature/code guidance on determining the effective seismic mass to use for a silo with a highly fluctuating live load.
there are three silos in a building feeding a boiler. currently the analysis is using 100% weight for all 3 silos in the building for the dynamic analysis as this will produce the largest horizontal forces. i don't believe this to necessarily be the case as there is variability within a day on the load configuration within/between the silos which may result in increased(due to changes in natl freq & inducing torsional loadings) or decreased (the contents absorbing energy)loading.
if anyone could point me in the right direction to determine what is currently being done in high seismic areas in this regard it would appreciated.
lms

section 9.14.7.3.10 of asce 7-02 titled "elevated tanks and vessels for liquids and granular materials" and in general the whole section 9.14.7.3 for "tanks and vessels" may contain some useful information.
i have always used the full weight for bin designs (cement and agg bins) because i haven't found a technical justification to do otherwise.
regards,
-mike
another consideration may be the "plant operating criteria" that the owner has described. the assumptions presented are often idealized, i.e. the plant operates on a regular schedule, without anything "unusual" happening. in reality, there are many times when equipment failure, unplanned outages, etc. may require to silos to be idle, possibly filled, for days, weeks, or more.
most design guides/codes will require that you consider 100% of the contents for seismic effects.
weff = 80% of stored mass
ref: structural engineering handbook,section 22 "reinforced concrete bunkers and silos", german gurfinkel, page 22-14, equation 10.
thanks so much for the input. in addition to the references noted, i found the following which provide some guidance which i thought i would share in case others have a similar problem.
aci 212 97, section which indicates "effective weight of the stored material shall be taken as 80 percent of the acutal weight...." 4.4.8 --
tung, kiremedjian, "a seismic reliability analysis method for structures with large live load fluctuations," proceedings of the fourth u.s. national conference on earthquake engineering, 1990.
i wonder if it is consistant and proper to mix and match different seismic design theories into one procedure, and if the 80%w would apply the the science behind the ibc specs. it certainly could i suppose. i've seen discussions about bulk solids with the point of view that the effects of bulk materials would be less due to damping, or more due to sloshing. i don't know the answer and that's why i am still using the 100%.
years ago i did contact the ubc about the k=3 used for bins and tanks. their response was the k=3 was for stored liquids because of sloshing and that for bulk materials k should be 2.5. it might be a good question to present to the icc.
regards,
-mike

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