IEBC Substantial Structural Alteration

[Labs763]

Hello all,

I am looking for opinions on how a substantial structural alteration limit (30% of total floor and roof area) applies with multi-story buildings. In my case, I have a two story CMU building - the ground floor is a slab on grade, second floor is wood joists with steel girders supported by the perimeter CMU walls and interior steel columns that terminate below second floor, and the roof is wood trusses. The architect would like create a foyer with no second story ceiling; so cut an opening in the second story to create an open area up to the roof.

For easy calculations, say rectangle building, each floor is 1000 sq ft, so a total of 3000 sq ft. So 30% is 900 sq ft.
1) 900sq ft would let me remove basically the entire second floor without triggering a substantial struct. alteration. (Of course I still have to retrofit the walls since they clear span two floors now, but I don't think that kicks me into substantial) If this building was three floors, I could remove the entire floor. That must be incorrect.
2) If the floor is removed, the interior columns are not doing anything and can be removed. But, do the columns/walls supporting the removed second floor also count as part of the area. So would we then divide that by half (just for arguments sake, I guess it would be by trib area) - 450 sq ft floor area can be removed from the second story?

Thank you for your insight.

 

[GC_Hopi]

I dont have the IEBC open but I thought there was a provision about changing the lateral system and about percent increase of the demand to capacity in any element. I think you changes would trigger those provisions.

 

[phamENG]

Where is the area thing coming from? Structural provisions in the IEBC are based on demand/capacity ratios as GC_Hopi mentioned. For gravity, it's a 5% increase and lateral it's 10%. You'll certainly be increasing demand/capacity ratios for several elements by more than 10% by removing a chunk of the diaphragm. Gravity is less likely to be an issue.

 

[bones206]

I'm not sure the first floor slab-on-grade would count towards the area calculation, since it is not supported by any structural element.

I'm not clear on exactly what your proposed alteration looks like, but I would think that the substantial structural alteration threshold would be triggered if an element was altered or removed that currently supports a tributary area of 600 sq ft. I.e. 0.30 x (1,000 sq ft 2nd floor + 1,000 sq ft roof).

 

[phamENG]

ah...found it. Looks like it only applies to a level 3 alteration - Virginia deleted that part of the code so I rarely need to use it.

 

[Labs763]

Thanks all. Yes I will be following the 5%/10% requirements, I am just trying to avoid the level 3 alteration. I think bones206 makes a logical point. If this was structural framing over a crawlspace, maybe the ground floor would count, but if we ignore ground floor and use the second floor and roof, that does reduce the total area (as he noted). I still don't quite follow what happens in say a 10 story building with a huge total area, but the 5%/10% rules would still apply and probably cover everything (and engineering judgement).

 

[GC_Hopi]

For tall structures, IEBC has provisions on the height that get triggered. I think the height provisions are for non-structural stuff like egress. That's outside of this discussion.

 

[lexpatrie]

This categorization of the work feels a lot more like an architect or building official decision.....

 

[Labs763]

@lexpatrie It is in the structural section of the IEBC for Alteration Level 3. A building official can obviously weigh in/override, but this is definitely structural. Thats why they reference significant structural alterations.

 

[bones206]

I agree. Since the structural work area is based on tributary areas, a structural engineer should make that determination.