The Total operating weight of permanent equipment

[amkd]

Guys, As per ASCE_7-05, clause 12.7.2 "Effective Seismic Weight", The Total operating weight of permanent equipment should be considered in the earthquake analysis. My question is, in case those permanent equipment have both static weight and continues dynamic loads, should i consider those dynamic loads as part of the "Total operating weight" or neglect the dynamic and consider only the static weight of the equipment?.

 

[RobertHale]

The real driver of that provision is the mass of the equipment at the time of a seismic event. I think the reason they use total operating weight is to ensure you do not penalize your building unduly nor that you "miss" mass that will likely be in place during an event. To demonstrate the difference: suppose you have a tank and pump in your building, for overflow reasons the tank has a 1000 gallon capacity but in the normal mode of operation the take will only have 500 gallons in it. In this case the seismic analysis should have the weight of the tank, the pump, and the 500 gallons of liquid. You should design the gravity system for the 1000 gallons of liquid but the probability that the tank will overflow at exactly the same time as the earth starts to shake is very low. I would suspect in most cases the interaction between the building response to a dynamic force of equipment and the building response to ground motion is negligible, so you can ignore the equipment dynamic forces in the seismic analysis.

 

[amkd]

Thanks RobertHale, appreciate your help.
In fact i am thinking the same.. However, i want to be 100% sure that the interaction between the building response to a dynamic force of equipment and the building response to ground motion is negligible. i would appreciate if you can refer me to a book or code that discuss this matter.

 

[MotorCity]

The operating weight is simply that....the weight. Dynamic loads are loads generated by the equipment....not the weight of the equipment.

If the equipment requires liquid to operate, the weight of that liquid should be included in the operating weight.

The reason that dynamic loads from equipment are not included is due to a couple of things.

Low probability. This can be seen in various load combinations. The odds of the design seismic event occurring simultaneously with maximum equipment dynamic loads is probably extremely low. (You could make a case for a certain percentage of the dynamic load in addition to seismic load.)

The other reason is that considering dynamic loads could actually reduce the effects that seismic motion has on the structure. Depending on the nature of the dynamic loads, they may add damping or affect the period of the structure.

 

[thaidavid40]

I'm not sure about the type of dynamic equipment you are dealing with, but most industrial equipment which generates dynamic loads is vibrationally isolated from its supports, and likely wouldn't contribute to any seismic load increase. Larger, intermittent, unintended loads, like strip cobble in a roll-forming line, are so infrequent they would almost never (in a million years?!) coincide.

Thaidavid

 

[weab]

We have had similar considerations. Consider a tank or bin that normally runs low but occasionally must run high. In our case, the conveyor below a bin keeps the bin low. However during some sort of temporary shut down, trucks are allowed to continue filling the bin. The bin is sized for a certain time length of shut down. In our case this amount of time may be perhaps an hour until the bin is filled.

Does it matter if this happens only once a year? once a month? once a week? or several times a day? At some point we had to acknowledge that this is a normal operation load. If so, then the whole contents must be considered.

Personally, I would like to find a reference which can define the difference between such an upset case and an operating load.