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Existing building alteration change in snow load?

Existing building alteration change in snow load?

Existing building alteration change in snow load?

(OP)
I have a client who recently purchased a building and would like to make some alterations to the building by way of removing some interior columns which are in his way. The building is a metal building with rigid frames and Z purlins on the roof and was suspected to have been built in the 70’s. We visited the project last week and took our preliminary measurements of the structure in preparation for the column removal. The building is in great shape with little to no rust inside with exception of the column bases which is really good since the building has been vacant and unheated for the last 4-5 years. Once we got back to the office we then discovered a problem.

The problem revolves around the roof purlins. The purlins are constructed in such a way that they are simple span. Upon calculation we discovered that the roof purlins are good for 35-40psf of snow load. However, the building code today (in a best case scenario) requires the building to hold 56psf as it is in a high snow area (Pg=100psf). We have informed the owner of our findings and we have begun a debate in our office of what is going to be required to remove the columns and be in compliance with the building code (IBC 2009). We have two differing opinions:

1) Chapter 34 says that as long as the alterations do not involve the roof purlins, we are neither increasing nor decreasing the load on the purlins therefore we do not need to upgrade them to the current building code requirements.
2) The roof purlins are so under designed and the alterations are so large (the columns being removed effect about 50% of the area of the roof) that the roof needs to be upgraded to comply with the current building code.

From a strict code interpretation, which opinion is correct?

RE: Existing building alteration change in snow load?

I would say if you aren't touching the roof purlins then they can stay as is. However any new beams or columns should be designed to the new code level loading even though the purlins could never actually transfer that load to the beams and columns on paper.

RE: Existing building alteration change in snow load?

(OP)
jayrod,

All new members will comply with the current building code in both plans.

We have also ask the owner, in a nice way, if dumping large amounts of money into a building with a roof system that is suspect is a wise economical decision. Especially since the building may some day end up with expensive pieces of equipment inside. Either way, they are stuck with this building.

RE: Existing building alteration change in snow load?

if they're all simple span, maybe you can reinforce them over the frames to make them continuous and gain a little extra capacity. not sure if that gets you up to modern code requirements, but certainly cheaper than reinforcing the entire length.

RE: Existing building alteration change in snow load?

(OP)
Canpro

Not possible with this purlin configuration.

RE: Existing building alteration change in snow load?

I would inform the owner that the roof purlins don't meet the current code. Warn him of the consequences but that's as far as I would go.

The fact that the code level loads changed don't mean the purlins can't do their job. They've obviously been performing adequately for the current lifespan of the building.

RE: Existing building alteration change in snow load?

Agreed with jayrod12.

RE: Existing building alteration change in snow load?

Quote (jayrod12)

They've obviously been performing adequately for the current lifespan of the building.

That is...until when the statistical variation in snow loads over time creates a condition where the actual snow event loading exceeds the capacity of the purlins...right?

I agree with jayrod12 to notify the owner...good response in my view.

But I've always been frustrated by contractors who look at me when I suggest something doesn't meet code with the reply, "well its held up for 20 years, it ain't goin' nowhere."
While long term durability is fine and reassuring, the fact remains that according to our best research and understanding of snow load in a particular location, the required level of safety may not be met.

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RE: Existing building alteration change in snow load?

I agree with the comments above that you proceed with designing the new members to current code and notifying the Owner of the current status re capacity of the purlins. However, I would tack on a recommendation that "while not currently required by Code, we would recommend you consider upgrading the current purlins to meet Code either at the time this project is completed, or start budgeting for it in the future".

This of course assumes the purlins meet the code requirements at the time of initial construction. If they do not, then you better be sure the Owner is notified and upgrades take place.

RE: Existing building alteration change in snow load?

Obviously either way you need to notify the owner.

Do the purlins at least work for the loads required in the 70's? If yes I think you can simply tell the owner what Jayrod said. But if it never worked, I am not sure that is sufficient.

Best case by your numbers the purlins are 40% overstressed. I don't have it handy, but I think the IEBC considers that condition dangerous. Unfortunately you are now on the hook for knowledge of the situation. Jurisdictions vary, are you required to report this to the Bldg Dept?

RE: Existing building alteration change in snow load?

I am inclined to agree with JAE....usually the structural engineer is the ONLY individual on the project qualified to ascertain the structural adequacy/safety of the structure and is the reason you were hired by the owner....it just does not make any sense to me to design any new steel for the current code loads and ignore the adequacy of the purlins....the most common form of failure are roof collapses due to snow ....I personally anticipate more frequent extreme weather events due to the ongoing climate change....the old saying that if an engineer makes changes to an existing bldg he ends up responsible for the whole bldg wheather he likes it or not.....

RE: Existing building alteration change in snow load?

(OP)
I'm not sure if you guys missed it in the original post, but the new owner of the building has been informed of the problem and we are trying to work with him on a solution that meets his needs. One option is to not make any changes to the structure at all. We do have other problems onsite that are related to the building being unheated for the past 5 years in temperatures that go well below 0 for weeks on end.

The question proposed in the OP is one in relation to the Chapter 34 of the IEBC and whether or not the existing purlin system needs to be updated to code requirements or not if the owner decides to remove columns. I'm not sure how we are now off on a slight tangent.

RE: Existing building alteration change in snow load?

SteelPE - why did the snow loads increase so much? In my neck of the woods the snow requirements have actually lowered a bit compared with the 1970's level of snow.

IBC 2006 - 3403.1 - "...Portions of the structure not altered and not affected by the alteration are not required to comply with the code requirements for a new structure."
See also 3403.2.1

IBC 2012 - 3401.4.1 - "...Materials already in use in a building in compliance with requirements or approvals in effect at the time of their erection or installation shall be permitted to remain in use unless determined by the building official to be unsafe per Section 116." The commentary for this states that the building code is not retroactive.

Section 116 is simply putting authority on the building official to determine if a building is unsafe. Based on the above it appears that if you notify the owner that new snow criteria suggests that the purlins are possibly more at risk than previously thought during their original design, and the building official believes that the new snow loading isn't that onerous to require retroactive measures on the whole roof, you should be OK to proceed with leaving the purlins as is. If you didn't do any work at all, the owner would still have theoretically insufficient purlins.

However, I think I might also write a letter to the owner (and writing in layman terms) describing to them how the code snow has changed over the years and to be cognizant that the roof should be carefully monitored for build-up of snow.


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RE: Existing building alteration change in snow load?

Do the Z purlins have long laps or short laps? If they have long laps, there is some fixity that increases their capacity.

Regarding the direction to take from here- I agree with JAE's post. Let the owner know the risk so he accepts the liability- not you.

RE: Existing building alteration change in snow load?

I interpret the IBC to say that the members have to meet the Code specified loading from when it was built. If the purlins meet the 1970's snow load requirements I would say you are ok. If they never worked, then you are not ok and it needs to be fixed.

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