I've been asked to evaluate an existing flat roof structure (reinforced concrete) of a building built in the 1970's for the installation of replacement skylights. The new skylights that the owner wants are double glazed and between that, the new flashing needed, and a canopy shading mechanism, the load on the structure in the vicinity of the lights will increase by around 20 psf. The area of the lights is around 2,900 sf which is less than 10% of the total roof area. The question has been raised as to whether the current IBC or ACSE-7 (snow load) loading requirements need to be considered in the analysis. The original design allowed for a 30psf live load and in fact the structure has a little more capacity than that. Under the code requirements that were in effect at the time (BOCA) the roof structure will be ok for the new loads with even a little to spare. It's possible that it might pass even with the higher snow loads of ASCE-7-02, and then again it may not, but before I launch into the new code analysis, I first want to determine if it's even necessary. What do you think? If I do check it against the current code and it fails, then that would mean no new skylights for the owner I suppose and I wouldn't want to say that if it were not really necessary. Your views would be appreciated.
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- Thread starter jcoffey
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DaveVikingPE
Structural
Snow loads aren't live loads, right? So run the snow load analysis.
The existing roof *should* be adequate to handle the new loads, right? It's not to make sure, then. A flat roof snow load analysis using ASCE 7-02 shouldn't take more than an hour. Plus, the loads might not necessarily be higher. Did the ground snow load change?
Things to consider: do the new skylights project above the flat roof? If so, by how much? Will they cause drifts? What about impacts to drainage, rain-on-snow-surcharge (5 psf), etc., etc., all the good structural engineering snow load things we worry about...
Run the analysis and charge accordingly. Be safe rather than sorry, and all the other stuff we like to safe about ourselves practicing due dilligence and such.
The existing roof *should* be adequate to handle the new loads, right? It's not to make sure, then. A flat roof snow load analysis using ASCE 7-02 shouldn't take more than an hour. Plus, the loads might not necessarily be higher. Did the ground snow load change?
Things to consider: do the new skylights project above the flat roof? If so, by how much? Will they cause drifts? What about impacts to drainage, rain-on-snow-surcharge (5 psf), etc., etc., all the good structural engineering snow load things we worry about...
Run the analysis and charge accordingly. Be safe rather than sorry, and all the other stuff we like to safe about ourselves practicing due dilligence and such.
- Thread starter
- #3
Yes, ground snow loads here in MD are higher now. The skylights will project around 18" above the roof slab, plus there are a number of penthouses up there too, so it's not a case of a simple flat roof. I was leaning in the direction of saying the new code will apply. I'll run the numbers and see what it shows. Thanks.
jcoffey,
I believe that the interpretation by most governing agencies (such as city building departments that are responsible for ensuring code compliance) is that only the current code is to be used for new designs and design checks.
So if a building was designed based upon a 1970's era code, and you now want to do something different to it, you should check it against the current applicable code for that area, probably an IBC code that references out to the ASCE 7 document.
I believe that the interpretation by most governing agencies (such as city building departments that are responsible for ensuring code compliance) is that only the current code is to be used for new designs and design checks.
So if a building was designed based upon a 1970's era code, and you now want to do something different to it, you should check it against the current applicable code for that area, probably an IBC code that references out to the ASCE 7 document.
- Thread starter
- #5
Thanks. Yes, the code language regarding Alterations, which is what this is, says, "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". As I read this though it seems that since the existing roof beams will be affected they, and thus the whole roof, are now subject to the new higher IBC loads. I was just wondering what others here would think. I'm sure this must come up regularly in renovation projects.
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1
- #6
JAE:
My understanding (in my local area) is that if you are making a change to a building that was built in the 70's, if that change (using the original code from the 70's) does not require that the structure be reinforced then it is OK, however, if the structure needs to be reinforced then the reinforcement and affected structure needs to meet the current governing code.
My understanding (in my local area) is that if you are making a change to a building that was built in the 70's, if that change (using the original code from the 70's) does not require that the structure be reinforced then it is OK, however, if the structure needs to be reinforced then the reinforcement and affected structure needs to meet the current governing code.
The international existing building code would be a good reference. Depending on what type of work you are doing, how much you are doing, how much load you are adding, and the general type of importance of the building it will specify wether or not you are "required" to bring elements into alignment with the current building code.
akastud
akastud
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