Farm Building Code - Tower Silos Vs. Storage of Grains

[efFeb]

Good Afternoon,
I am working with the farm building code and am having some difficulty with how it is laid out. There is a clause (cl. 2.2.1.11) that relates to tower silos, and another (2.2.1.14) that relates to storage for dry grains. If I have a tower silo for storage of dry grains, which would be appropriate to use?
Thanks so much,

 

[jayrod12]

I assume based on your location that you're talking about the Canadian Farm Building Code.

If so, please note the following update noted on NRC-CAN website regarding the applicability of the code in today's world.

Update

The farm building requirements related to fire protection, structural design and dangerous goods are being discussed for publication in the 2020 editions of the NBC and NFC. The fire and structural requirements in the National Farm Building Code have not been reviewed or revised since the 1995 edition and are increasingly in conflict with the latest edition of the Codes.

I didn't even think the farm building code was still applicable.

We don't have a digital one so I'd have to look to see if we have a hard copy at the office tomorrow.

 

[ryaneng]

I believe you would have to investigate both conditions to determine which governs for your silo. In Appendix A, A-2.2.1.14.(1) says this: Maximum lateral pressures on walls are produced by small values of (mu), which occur with very dry grain. Maximum friction loads on the walls are produced by larger values of (mu), which occur with higher moisture contents. The designer must investigate the worst combination of loadings by selecting appropriate values from tables A-2.2.1.11.B and A-2.2.1.11.C.

The way I read it, the tower silo section is purely dealing with storage of whole plant silages. If you're confident that only dry material will be stored, you can use values from the dry grains section. But you may want to check using the tower silos equations assuming some degree of moisture content.

@jayrod12 the farm building code is still applicable as far as I can tell. In the BCBC section 1.1.1.1.(4): Farm buildings shall conform to the requirements in the National Farm Building Code of Canada 1995.

This will all change when NBCC 2020 starts being used, as the farm code is updated and included in there. Unfortunately all the silo/grain storage info is in the commentary which hasn't been released yet.

 

[EngDM]

Depending where in the prairies he is, I know Manitoba is still using NBCC 2010, so even if an updated version is published for 2020, I'm not sure how it would get implemented. It's bad enough having to use an outdated steel code as it is, I hope they adopt something new soon.

 

[jayrod12]

To be fair, we don't design farm style buildings unless it is insurance repairs, and when it is we design it to meet part 4/part 9 of the code as applicable. We don't use the farm building code unless we're trying to justify an existing condition to remain.

I had said I didn't think it was still applicable, but when I went to the NRC website I did see that it was in fact still around. But the fact it hasn't been updated in 27 years gives me pause. The farm buildings built 27-30 years ago are the ones we have to fix the most often. It's not the 100 year old barns.

 

[TLHS]

Everyone agrees that the farm code is a bit of a shitshow, because yeah it feels wrong to be using a code that old. I think I've made too big of a deal of it in the past, though, and the structural part of it is fine. The farm code says it's a list of modifications of the NBCC and the NBCC governs otherwise. Technically it refers to the 1995 NBCC (it would be kind of amusing to chase codes from 2015 NBCC -> Farm Code -> 1995 NBCC as a justification to design to NBCC 1995) but I don't think anyone would mind if you used the latest code in a reasonable way.

I think you can basically meet the intent of the farm code for low human occupancy structures (except for the exceptions they give) by using

-0.8 importance factor and typical snow load design
-0.8 importance factor and typical wind load design OR 1/10 year wind and normal importance factor
-0.8 importance factor and live loads as per NBCC or farm code depending on the load
-no seismic

The wind is likely up for argument, but I think that's the intent, and both of those are likely valid (i.e. it's not the greater or lower of the two, I think they're probably both valid reads). I also wondered if the importance factor was intended to extend out to the Farm Code listed live loads, but it kind of makes sense since one is specified load level and the importance factor is basically a reduction of the structural safety factor.

None of this is all that out of line with the NBCC except for the seismic exclusion. It's basically the same as designing for any low importance structure in the NBCC.

Other than that, from a structural standpoint, it's mostly just a list of additional live loads that should be used and the clauses for storage and silo loads. It's not really all that contentious in my mind.

FYI, the farm code is available free like the other model codes:

https://nrc-publications.canada.ca/eng/view/ft/?id=8d848d71-3fc2-4e1b-b0c1-c45d15a79a46

Jayrod, I wonder how much of the fact that you're fixing 30 year old farm structures and not older ones is just survivorship bias. The old stuff that's still around has made it this long, and so it's the better constructed stuff of it's era. The poorly built part of the 20-30 year old stock of structures, on the other hand, is hitting end of life for that kind of construction.

 

[ryaneng]

@jayrod12, I didn't mean any disrespect, just wanted to clarify that it is apparently still in effect. I agree that the age of it gives me pause, but we have used it on a number of projects, and I would tend to agree with TLHS that it's likely survivorship bias that is bringing the younger buildings across your desk more often.

@TLHS, I'm not sure I agree with your factoring of loads. In 1995 NBCC, they allow for a 0.8 reduction factor for Low Importance buildings which applies to Live, Snow, Wind. The farm code than makes some modifications to how to calculate those loads. I would modify your summary as follows:

-0.8 importance factor for snow in additional to modified Cs for slippery roofs.
-0.8 importance factor for wind with 1/10 q values (update q values according to latest building code).
-0.8 importance factor for live loads as per NBCC or farm code.
-No seismic...until NBCC 2020 is enacted in your jurisdiction. (And even then most of the prairies are exempt).

 

[efFeb]

Thanks very much everyone for your input!