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]

@KootK: Good point, also I imagine the cages will be bolted down and the chickens will weigh down the cages.

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

http://www.americanconcrete.com

 

[PostFrameSE]

One last question..........I hope. So I arrive at my Seismic Base Shear. This structure is actually supported with "guy wire" external bracing at periodic intervals near the eave. I'm supporting it there so that I can take load out of my roof diaphragm at periodic intervals. At the expense of showing my ignorance, I ask, "Can I apply that shear load up high to my external bracing points or do I need to do something different? Generally when I've designed structures for seismic I have more of a shearwall approach that is of relatively equal strength from top of wall to foundation. In this case I have my resistance up high with a lot of ductility between the eave and finish floor. Is that correct thinking? If the ground is moving, I guess so are my external bracing points (deadmen) and it would all act together. Thoughts?

 

[SteelPE]

PostFrameSE

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.

 

[PostFrameSE]

Here's a crude sketch if I did this correctly.

 

[SteelPE]

I would say that as long as you can get the load through collectors and into the bracing at the perimeter you should be fine. My old boss did something similar years ago on a large warehouse. The footings were large and he needed to pay special attention to the collectors but it will work.

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.

 

[SAIL3]

I would consider the total weight of the chickens as an operating load and therefore use 100%....this is a major load in the OP's project...if this was my project, my first instinct would be to use a 100% of the chicken weight...if this results in a unreasonable design and the possibility of saving meaningful costs exists, then , I would do a dilligent research on the subject of chicken behavior(if it exists)during a seismic event or if someone qualified presents a sound arguement in reducing this load, I would consider it..in the end, it would come down to my engineering judgement....like it or not, this is real engineering and an opportunity to apply one's engineering knowledge without allot of cookbook handholding...

 

[TehMightyEngineer]

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.

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

http://www.americanconcrete.com

 

[thaidavid40]

Don't unattached live loads tend to provide damping of seismic effects on structures (think: bridges)? Things tend to roll around in a responsive way to the earth motions, providing damping effects (like water sloshing in a tuned mass damper). I always thought the 25% participation factor was based on the fact that the live load was actually providing some damping effect, though not perfectly. Therefore, the 25% level of contribution was assumed for conservatism's sake. Have I been taught incorrectly here on this point? I would recommend using 100% of the attached mass (cages?), and 25% of the chickens' mass (well fed). ;-)
Dave

Thaidavid

 

[SAIL3]

my understanding for using 25% of the live load in a seismic event is based on the assumption that it is a low probability of having a full live load and a seismic event occuring at the same time, on the otherhand, an operating load is considered to be present the majority of the time, hence, the use of a 100% of the operating load....

 

[KootK]

When I look at the elevation sketch, I see a one story building with what is essentially racking inside. I that correct? If so, one of two scenarios seem likely:

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.

 

[PostFrameSE]

It is a one story building with multiple levels of cage racks. Rack legs are one-piece and actually run all of the way up to the roof from concrete floor, so the roof is supported by vertical members at 4' o.c. the length of the building and approximately 6' o.c. across the width of the building. Essentially my roof is supported by a floor-to-roof "column" or rack leg every 24 square feet. Chicken racks do NOT cantilever from the floor, they are truly a pinned connection there. Your #2 more accurately describes the scenario wherein the seismic mass is certainly between the diaphragm levels. You worded that well.

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?

 

[SteelPE]

So, your racks support the roof? Is that correct? That is kind of an odd setup (my familiarity with this type of structure ends at Napoleon Dynamite). Unless the chickens are squeezed into the cages such that they can't move, I wouldn't add their weight to the seismic weight of the building, however, I would include the racking system. The seismic load needs to be transferred to the ground in some way. Your rack columns may be able to distribute the load to the ground and to the roof through local bending, if not, then you will need to install a mid-height brace and diaphragm (probably going to be tough).

 

[PostFrameSE]

That is correct SteelPE. This is a new venture for us. Giving it a shot. That's why I'm being rather inquisitive here and trying to churn up as many issues as I can. I hope this discussion has been as fruitful for others as it has been for me. Thanks.

 

[KootK]

Do I really have any other choice but to put another brace in at mid-height as I suggested?

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.

 

[PostFrameSE]

Here's a better snip of the plan. Unfortunately there are belts and walkways and all kinds of things that are preventing any X-bracing or walls to be installed that could act as shearwalls to transfer load to the foundation. Where I've shown the two guy-wire connections, there is a continuous member from side-to-side of building so there is continuity there in that fashion. The racks only have X-bracing where shown, and it can't be anywhere else in that building section. Hence my thought that we need to tie off that mid-level continuous member.

 

[TLHS]

This thread is the best.

 

[KootK]

The extra guy wire may well be advisable if you can swing it. Either way though, your racks are going to have to span some distance vertically to distribute the seismic loads to the diaphragm level(s). The sketch below shows the seismic weirdness that comes of having the bulk of your mass between diaphragms. Some of the implication of that, in my opinion, include:

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.

 

[SAIL3]

to me the design concept raises a red flag...it has ,at least, one stability loop in there...the integrity of the roof depends on being supported by the stacks of cages while the stability/integrity of the cage stacks depend on the integrity of the roof...as Koot previously mentioned, I would have expected the roof to be independent of these stacks and provide lateral bracing/support @ the top of the stacks....this is not the first henhouse ever built..there must be some examples of various design concepts out there...

 

[rogerheadford]

To summarise, while all the Engineering theories for this "chicken" problem, I quote the PURPOSE of seismic assessment as provided originally in UBC code.
"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