Calculating dead loads for seismic design

[PostFrameSE]

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.

 

[TehMightyEngineer]

Unless you're bolting your chickens down then I would consider them a live load that is not a storage load and thus does not get added to the seismic weight.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Trenno]

Chickens will obviously provide increased damping...

 

[PostFrameSE]

Given the fact that the probability of the building loaded with chickens to it's capacity all of the time is very high there is SUBSTANTIAL weight added to the structure. You certainly could call it a live load, but unlike people in an office, these birds are there 24/7/360. For my own understanding..........if these birds were caged, confined to a cell that was attached to the structure, does the answer change?

Thanks for the quick response.

 

[SAIL3]

I would include their weight up to the point where the chickens still have contact with the ground...so the question is really what acceleration(g-force) would cause this loss of contact...

 

[PostFrameSE]

Maybe a clarifying point Sail3. These chickens are layered, or have the opportunity to be located, in a 23' tall building at nearly any point in that 23' of building height. There are open "cages" at roughly 4' levels that the birds can go in, lay eggs, get feed & water, etc, so I think the assumption can be made that the birds will be pretty evenly distributed heightwise in this 23' tall structure. Your comment about "contact with the ground" made me think that I wasn't clear enough in my original description.

 

[SAIL3]

same principle...if the cages are mechanically connected to the building or if the friction between the cage and it's support is sufficient to overcome the seismic force....not all seismic events are at the max level so what is the cut-off point of the level of intensity of the seismic event that would make the stated assumptions invalid.....

 

[PostFrameSE]

The cages are mechanically connected to the building but the birds are free to roam wherever they want, within a partitioned section of the building.

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.

 

[SAIL3]

it is not a stupid question and can be applied to say human occupancy....I have seen no evidence(there may be some out there) where a human being was knocked off it's feet during a seismic event and ,besides, chickens have clawed feet that give them the ability to cling to a support, if necessary,...with all these unknowns, I would come down on the conservative side...

 

[TehMightyEngineer]

It's also worth noting that chickens are relatively low density compared to a human. It wont take much for a chicken to be taken off it's feet or otherwise not be fully attached to a building by it's weight and friction.

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

http://www.americanconcrete.com

 

[PostFrameSE]

Ha! That would be good. I am talking about a project that is going to house upwards of 3,000,000 lbs of chickens in one building.

 

[TehMightyEngineer]

Right, but each chicken has to anchor itself and move with the building. When the building shifts suddenly with an acceleration at some proportion of gravity will each chickens little legs and low mass be able to restrain themselves enough to move with the structure.

Hahahahaha, I actually found some empirical evidence of chickens during a seismic event, they were even sitting down and still remained relatively stationary: [URL unfurl="true"]https://www.youtube.com/watch?v=6iRqC5l7tX4[/url]

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

This isn't a silly question at all. A nearly identical issue comes up in the design of libraries: should the books be seismic mass? See this thread for more information: Link. Two things that I've never been clear on:

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.

 

[TehMightyEngineer]

My understanding to (2) relates to how well you will be seismically isolated. If you're in the fetal position you may slide on the floor a bit but may contribute some to the seismic weight. However, most people will probably be standing or sitting and will be isolated from the seismic force (e.g. they rock or fall over when the building shifts). Thus, a person live load shouldn't add to the seismic weight but the box of solenoids which we can assume that 25% of the will move with the building.

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

http://www.americanconcrete.com

 

[msquared48]

Would that not be compensated for in the live load with appropriate seismic factors used for anything over 100 psf?

Mike McCann, PE, SE (WA)

 

[KootK]

@TME: I think that it's prudent to recognize that the live loads that we use aren't meant to capture just the weight of people. They also include the weight of stationary, inanimate things like furniture. i.e. things that won't be staggering around in a panic during a seismic event. I'd argue that, in most spaces, live load is predominately inanimate mass.

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.

 

[PostFrameSE]

I appreciate all of the comments. Thanks. One last question......and maybe this should have been the first question. Does the building even need to be designed for seismic loads? ASCE 7-10 Section 11.1.2 says that "Agricultural storage structures that are intended only for incidental human occupancy" are exempt from needing designed from that standard. Would anybody interpret this such that an incidentally occupied chicken building could be exempt? I could see "grain storage" or "ag-equipment storage" much more readily fit that bill than a chicken barn. However, manure and egg belts take away the bad and the good and there is little human occupancy in the structure.

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. Would anybody vehemently argue that this should be exempt?

 

[JStephen]

One thing I find is that building codes are written for "normal" structures, and if you're dealing with some oddball project, you may need to include loads or combinations that are not addressed.
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?

 

[SAIL3]

the other question this raises is the distribution of the weight, if one includes it....the OP indicated that they were open cages and the chickens were free to roam....is there a situation in this environment where the chickens would tend to congregate/flock in one area ie. a feeding station, one part of the bldg more comfortable than the other, etc...

 

[PostFrameSE]

My understanding is that chickens are not very smart animals and they can "pile" and suffocate each other, perhaps if they got scared. If the design event is generally 30-45 seconds, the likelihood of them piling and causing adverse effects in that short of time seems fairly minimal.

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.