Calculating dead loads for seismic design
Calculating dead loads for seismic design
(OP)
Haven't done much seismic work yet. If I'm doing a facility that has animal confinement (chickens that are free to move around in the building.......not caged) and I'm trying to calculate the total weight of my structure to come up with the Seismic Base Shear, do I need to include the animal weight?
Am I thinking about this correctly in that unless the chickens were rigidly attached to the structure that their weight is inconsequential? In other words, as the building shakes, they lose their footing and they haven't contributed to the mass of the structure that just got racked. Now.............I think I understand that if a chicken was up against a rigid surface and the movement of the structure immediately pushed against that bird that her weight WOULD have to be included, right? I know I'm talking a little silly here perhaps, but in reality isn't that the case? To be conservative I would say that every chicken would be tight against a rigid pen or wall or something, but that load condition is unrealistic. Therefore, could an assumption be made that a certain percentage of the bird weight be added to the total structure weight? Or do I even need to consider the bird weight at all??
Thanks.
Am I thinking about this correctly in that unless the chickens were rigidly attached to the structure that their weight is inconsequential? In other words, as the building shakes, they lose their footing and they haven't contributed to the mass of the structure that just got racked. Now.............I think I understand that if a chicken was up against a rigid surface and the movement of the structure immediately pushed against that bird that her weight WOULD have to be included, right? I know I'm talking a little silly here perhaps, but in reality isn't that the case? To be conservative I would say that every chicken would be tight against a rigid pen or wall or something, but that load condition is unrealistic. Therefore, could an assumption be made that a certain percentage of the bird weight be added to the total structure weight? Or do I even need to consider the bird weight at all??
Thanks.






RE: Calculating dead loads for seismic design
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
Thanks for the quick response.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
So I'm understanding that the seismic levels we're designing for in ASCE 7 are for the maximum load, at which point the birds are knocked off their feet and probably don't contribute to the weight of the building. I'm sorry if most of you think this is a stupid discussion, but bear with me as I seek to understand. So what if the seismic event is 50% of design. Birds don't get knocked off their feet, but they're also not rigidly attached, so I'm thinking that they don't contribute to the weight of the structure. Let's say the birds are all laying down (not sure if birds do that or not) and the building shakes........they would roll around...............again they aren't glued to the structure. Am I not thinking correctly regarding the only way the birds should contribute to the weight of the building is if they are wedged against something vertical?
Thanks for humoring me.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
I vote we get some chickens and put them on a shake table. This will be the most entertaining engineering report ever.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
Hahahahaha, I actually found some empirical evidence of chickens during a seismic event, they were even sitting down and still remained relatively stationary: https://www.youtube.com/watch?v=6iRqC5l7tX4
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
1) With storage loads, is the 25% based on the probability of full load being present or the assumed vibrational participation of the stored mass? I lean towards the former.
2) If heavy storage loads are seismic mass, why aren't ordinary live loads? A far as I can tell, the only difference is scale. Seismically, is there a philosophical distinction between a box of solenoids and me huddled up in the fetal position on the floor?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Calculating dead loads for seismic design
I would say that books may move with the building but chickens will not, chickens will be startled just like people and stand or otherwise not generally be moving with the building.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
Mike McCann, PE, SE (WA)
RE: Calculating dead loads for seismic design
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Calculating dead loads for seismic design
My gut tells that to take the "ag-exempt" approach is a little overboard, but I will be challenged with that thought from those who are wanting to keep costs down. I guess that's an age-old problem.
RE: Calculating dead loads for seismic design
On the "Exempt" question, it sounds to me like the exemption is for items where you don't mind too much if they collapse or are damaged in a seismic event. If there's a substantial investment in the property, I would be inclined not to use the exemption. I wonder if that would be addressed by insurance requirements anywhere?
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
Each room is the same as the next in this chicken hotel though I suppose there could be some ventilation differences that could make an area preferable to another. Good point.
RE: Calculating dead loads for seismic design
I'll change my opinion, I would agree that the chickens should be considered a storage load (25% LL and all that), but only if the cages are bolted down sufficiently that the cages themselves can contribute to the seismic weight.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
A sketch might help. I don't quite understand what you are asking.
On another note. What I don't get about storage loads and seismic weight is that you need to include the weight from storage loads in your seismic load..... but when you are figuring 0.6D+0.7E you are not allowed to take 25% of the weight to resist these loads? At least, that is always the way I have been taught to approach the problem.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
The only thing I am not sure about is what R factor would you use for that system? Maybe you would be forced to use R=3.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
I'm not 100% sure, but I believe this is because the earthquake acceleration is both horizontal and vertical. Thus, you may be helped or hurt by that 25% as they're assuming that 25% may be bouncing around in the helpful or hurtful direction (and probably both).
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Calculating dead loads for seismic design
Dave
Thaidavid
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
1) Your chicken racks cantilever up from the ground. In this scenario, there would be no seismic load on the building proper.
2) Your chicken racks are laterally restrained at the top by the roof deck, directly or indirectly. In this case, you may have a rather complicated seismic situation on your hands as our normal procedures are predicated up the lion's share of the seismic mass being located at the diaphragm levels rather than between the diaphragm levels.
How does the total weight of your roof system compare to the total weight of your occupied chicken racking assembly?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Calculating dead loads for seismic design
Perhaps what I ought to do is brace the mid-height of this mass by an exterior guy wire as well. In other words, have one brace connected to my roof diaphragm and one brace at the center of my cage mass. When I think about it that way it starts to make more sense. That would actually give me a "diaphragm level" at the center (mid-height) of my loaded rack assembly.
My roof system weighs 8% of what the dead load of the racking assembly weighs. Put birds in there at 100% load and the roof becomes 5% of the occupied racking assembly. The weight clearly is in the birds and racking assembly........not the shell of the structure.
Do I really have any other choice but to put another brace in at mid-height as I suggested?
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
There are other ways but, before we really get into it, is this a real option? I had imagine that there would be human travel lanes around the outside of the chicken storage that would make it tough to connect the racking diaphragms to external guy wire bracing. If anything, I thought that the chicken racking might have internal cross bracing etc.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
1) The seismic period of the building will elongate as it would include the flexibility of the racking.
2) The racking rightfully deserves to be designed as a part of the seismic LFRS.
3) ELF procedures determine base shear based on the assumed SDOF acceleration at the center of seismic mass. In a one story building, that location is usually more or less at the roof deck. That's not the case for your poultry affordable housing project. As a result, I think that you need to scale your base shear up by something to the tune of h_eave / h_c.m.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Calculating dead loads for seismic design
RE: Calculating dead loads for seismic design
"The purpose of the earthquake provisions herein is primarily to safeguard against major structural failures and loss of life and not to limit damage or maintain function"
I don,t think the authors of the code were really considering animals/chickens as loss of life, even though the farmers etc would not b too happy