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?
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?
RE: Existing building alteration change in snow load?
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?
RE: Existing building alteration change in snow load?
Not possible with this purlin configuration.
RE: Existing building alteration change in snow load?
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?
RE: Existing building alteration change in snow load?
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.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: Existing building alteration change in snow load?
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?
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?
RE: Existing building alteration change in snow load?
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?
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.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: Existing building alteration change in snow load?
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?