NBCC 2015 Seismic Requirements - Mixing design procedures?

[EngDM]

I have a bit of a weird case, and I'm hoping someone has put some thought or could provide an outside opinion on the following discussion.

We are retained quite often to design the foundation for a pre-engineered building and act as EOR. This typically involves being provided stamped and sealed reactions by the supplier and running the combinations to size the load path from anchors down to piles. However, there are some instances where we don't get these reactions until way later in design, so we try our best to get a preliminary feel to provide something to the client for pricing. The problem with getting the reactions later is we can't verify their design procedure to try and match.

For a site that is compliant with the requirements of 4.1.8.1.(2) and can be designed with the simplified method, which design rationale would you choose if you had to design a foundation/anchors and lateral system? How do you even begin to choose?

If I start by designing to the simplified method, and the pre-engineered guys designed to equivalent static method for extra ductility to get lighter loads and a more cost-effective structure, does that then require me to satisfy the additional clauses that were otherwise eliminated by 4.1.8.1.(1)? Notably, Table 4.1.8.1.18 has a list of items that need seismic anchoring, that are not mentioned in the simplified method; would I then need to go back and have these miscellaneous structural elements designed for a seismic anchorage?

On the other hand, if I determine loads based on equivalent static, how would I even know what ductility to use without insight into the pre-eng design? If they came back having used simplified method, my preliminary loads would be much smaller.

 

[jayrod12]

I don't have an answer for you, but this is a super interesting question that I can 100% see coming up.

Forgive the question, I'm away from a code and you seem to have done a bit of digging already, but are the extra requirements you mention in 4.1.8.1.18 for the lower seismic loading change what you would be designing for using the simplified method with higher loads?

Based on some of the other analyses I've seen, the reduction in seismic loads by doing the more onerous analysis is equal to, or exceeds though additional clause load increases for the Anchorage etc. But maybe that doesn't work in all cases.

 

[EngDM]

Quick correction since I can't edit my original post ---> 4.1.8.1.18 4.1.8.18

Forgive the question, I'm away from a code and you seem to have done a bit of digging already, but are the extra requirements you mention in 4.1.8.18 for the lower seismic loading change what you would be designing for using the simplified method with higher loads?

4.1.8.1.18 is the section that addresses elements of structures, non-structural components and equipment. There is a large table that list things such as masonry veneer connections, diaphragms and a plethora of other things that should be designed for seismic loading; a lot of which are also mentioned in the simplified method 4.1.8.1. However, there are things in the table such as suspended ceilings and light fixtures that aren't addressed in the simplified method. So depending on the method used, it either makes you design these items for seismic or it doesn't. I had an architect ask me for this project whether or not they need to, and I didn't really have an answer since it appears to me that multiple design methods may be used.

Based on some of the other analyses I've seen, the reduction in seismic loads by doing the more onerous analysis is equal to, or exceeds though additional clause load increases for the Anchorage etc. But maybe that doesn't work in all cases.

I think the reduction in loads for more involved analysis methods is because there is less left to chance or to accidental loading. But in this case, its either you design for it or you don't. Kind of a weird one.

 

[canwesteng]

You are anchoring misc components from the PEMB? I'm used to these just being basic warehouses. If you go to 4.1.8.16, you're going to need the RdRo=1 forces anyway for the foundation, so some of this is moot, but if you are permitted to use the simplified method, then conventional construction would also be allowed, so that is the SFRS they will use as it is cheaper. If you're in simplified method territory, wind is almost certainly going to govern over seismic also.

 

[EngDM]

You are anchoring misc components from the PEMB? I'm used to these just being basic warehouses. If you go to 4.1.8.16, you're going to need the RdRo=1 forces anyway for the foundation, so some of this is moot, but if you are permitted to use the simplified method, then conventional construction would also be allowed, so that is the SFRS they will use as it is cheaper. If you're in simplified method territory, wind is almost certainly going to govern over seismic also.

Yea wind does govern over seismic in this case, but that doesn't mean the elements don't need to be checked. The architect is just wondering if their miscellaneous components like ceiling tiles and walk in freezers need to be designed for seismic as per 4.1.8.18, if at all. According to 4.1.8.1 they don't, but if the PEMB building is Equivalent static, then it would make them design these miscellaneous items for it, correct? This is my conundrum, I provided loads per simplified method but I am not sure what pre-eng will do.

 

[canwesteng]

For the elements, you go to 4.1.8.18.2 and determine if they need to be attached for seismic forces, independent of the building analysis check. The weights contribute to the seismic mass either way though, so it shouldn't affect foundations. I don't think 4.1.8.1 is meant to let you get out of that.

 

[Struct123ure]

If I start by designing to the simplified method, and the pre-engineered guys designed to equivalent static method for extra ductility to get lighter loads and a more cost-effective structure, does that then require me to satisfy the additional clauses that were otherwise eliminated by 4.1.8.1.(1)? Notably, Table 4.1.8.1.18 has a list of items that need seismic anchoring, that are not mentioned in the simplified method; would I then need to go back and have these miscellaneous structural elements designed for a seismic anchorage?

I would say obviously yes.
Take a step back from the code and just look at it from a public point of view. You are EOR and the loads that the building takes need to be taken into the ground, for that to happen it need to go through your foundation design.
From a business point of view, if this happens tell your client that it will cost extra due to the complex nature of the SFRS. Client should ask building manufacturer to use conventional construction.

I don't know where you are located, but from my experience earthquake loads govern strength design, while serviceability is governed by wind design because of the stringent deflection requirements. Both need to be checked and designed.

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[EngDM]

I would say obviously yes.
Take a step back from the code and just look at it from a public point of view. You are EOR and the loads that the building takes need to be taken into the ground, for that to happen it need to go through your foundation design.
From a business point of view, if this happens tell your client that it will cost extra due to the complex nature of the SFRS. Client should ask building manufacturer to use conventional construction.

I don't know where you are located, but from my experience earthquake loads govern strength design, while serviceability is governed by wind design because of the stringent deflection requirements. Both need to be checked and designed.

​

Seismic rarely governs for either deflection or strength for me, but wind doesn't have as many requirements as far as miscellaneuous, especially for interior.

 

[EngDM]

For the elements, you go to 4.1.8.18.2 and determine if they need to be attached for seismic forces, independent of the building analysis check. The weights contribute to the seismic mass either way though, so it shouldn't affect foundations. I don't think 4.1.8.1 is meant to let you get out of that.

The code literally says you don't need to design for 4.1.8.2 to 4.1.8.22 if you satisfy 4.1.8.1.(2). Yes the weights contribute to seismic mass, that is not being debated. I am talking about designing for instance a ceiling tile fastener seismic loads.

 

[canwesteng]

I've never treated it like that, since that clause is giving forces for the SFRS, but I can understand how it's read like that. It doesn't seem to make much difference anyway, since it tells you to design for the building components you would have to design for in 4.1.8.18, and doesn't mention the non structural components you can ignore in low seismic design categories.

 

[skeletron]

I agree with @canwesteng approaches described:

1. Your building is probably governed by wind. Foundations will likely come out in the wash, so design to RdRo = 1.0 and move on.
2. Design OFC's to Article 4.1.8.18. I interpret the clauses as being specifically written for primary building system and then Article 4.1.8.18 is for parts and portions. There is Sentence 4.1.8.1.(13) that mentions an alternate formula for parapet walls, cantilever walls, exterior ornamentation and appendages, etc. but the intent is pretty much "take a percentage of the weight and apply it to the component". I think in your case if Sa(0.2) = 0.16 (...max case), then your ceiling restraint wire would be designed for 49lbs and the anchor shear/tension component would be 87lbs. I view this as pretty much a non-issue. Standard 12ga wire would work and you can probably get away with an eyelag to be quite honest.

 

[EngDM]

I agree with @canwesteng approaches described:

1. Your building is probably governed by wind. Foundations will likely come out in the wash, so design to RdRo = 1.0 and move on.
2. Design OFC's to Article 4.1.8.18. I interpret the clauses as being specifically written for primary building system and then Article 4.1.8.18 is for parts and portions. There is Sentence 4.1.8.1.(13) that mentions an alternate formula for parapet walls, cantilever walls, exterior ornamentation and appendages, etc. but the intent is pretty much "take a percentage of the weight and apply it to the component". I think in your case if Sa(0.2) = 0.16 (...max case), then your ceiling restraint wire would be designed for 49lbs and the anchor shear/tension component would be 87lbs. I view this as pretty much a non-issue. Standard 12ga wire would work and you can probably get away with an eyelag to be quite honest.

1. Simplified method has Rs, not RdRo. Are you suggesting to just use equivalent static and move on?
2. For an ACT ceiling, you would design the wire for lateral load originating from the hung ceiling? The 12ga wire would have to be quite strong if the ceiling is at 12' and the deck is at say, 20'.

I've never treated it like that, since that clause is giving forces for the SFRS, but I can understand how it's read like that. It doesn't seem to make much difference anyway, since it tells you to design for the building components you would have to design for in 4.1.8.18, and doesn't mention the non structural components you can ignore in low seismic design categories.

4.1.8.1.(13) doesn't say that though; it doesn't stipulate any other requirements or give reference to 4.1.8.18.

I'm not designing the element, I was asked by a client if they had to follow 4.1.8.18 and strictly by the letter of the code it doesn't seem that they do. This is a super low seismic area, such that it is what CSA S16 calls SC0.

Back to my original point, if we are doing pre-eng foundations are we to then instruct the pre-eng building supplier on which design method to use so that loads are consistent when provided to a pile supplier?

Going in a different direction, if someone comes along and does a reno later on and discovers that they can use simplified method, but the original base building was done to equivalent static, is this permissible?