IBC Occupancy Classification vs. Live Load 1

[jtse]

Is there anything in IBC that explicitly links the Chapter 3 occupancy classifications to the Chapter 16 live loads?

I'm working on an old concrete building (2-way flat slab with drop panels) that has no existing drawings. It's currently office space and will be renovated, but will remain an office building. There are new layouts, including larger conference / meeting rooms, which the architect is classifying as "Assembly" areas on their life safety sheets per IBC Chapter 3. They want us to demonstrate the floor has capacity for 100 psf un-reducible live load to meet the 'assembly' requirements of Table 1607.1. Per the question above, is that truly intended/required by Code? Are we permitted to still classify it as 'office' with 50+15 live load to match its current use, in spite of the life safety designation?

As it's an existing building with no drawings, we're having difficulty analyzing accurately. We've done concrete testing including coring & crushing, exposing some bar, and GPR scanning. Unfortunately, the inaccuracy of GPR data for bar sizes and depths within the slab limits capacity in the analysis. We suggested keeping the classification as office to justify it as an existing building - successfully performing for many years, no change in load - but the architect is concerned by the difference in designations.

I suspect many/most engineers design office floors for 50+15 psf live load (or 80 psf to cover corridors) across the full building footprint, ignoring any areas an architect may be labeling as 'assembly' space. The current layout obviously has corridors and some larger conference rooms, but in different areas than in the proposed renovation. I strongly doubt the current layout was set to place corridors / meeting rooms in areas of higher structural capacity. Given that it's an office building now, has been for many years, and will remain one, saying it needs additional capacity now will be difficult to justify.

I have been unable to find anything in IBC, its commentary, or online addressing this subject. Has anyone come across any references that speak to the question?

We're also looking for other paths to solve this. We proposed posting load / occupancy limits in the areas of the building where the otherwise Code-required live load can't be demonstrated by analysis (i.e. large conference rooms). I don't know of a code provision that discusses this idea - does anyone else? We're also looking at load testing. I'm concerned about doing this with a flat 2-way slab with drop panels (punching shear) - has anyone done that before?

Thank you!

 

[phamENG]

I would say it's linked pretty closely. If it's a genuine Assembly area, the loading needs to match. Not sure if it's written in stone that way, but it's the way I do it.

That said, have they looked at IBC 303.1.2? Not sure how big these conference rooms are, but I don't think I've ever been in one larger than 750sf. They may be overly conservative in their classification.

Posting occupancy limits can be done, but it's generally at the discretion of the local AHJ.

Load testing? Of an entire existing building? Sounds fun, expensive, and counter productive. If it works, great, but if not you just cost the owner millions of dollars. If you ever convince somebody to do it, let me know. I want to watch!

 

[SlideRuleEra]

jtse - IMHO, you need to find out more about the building, more fundamental than the concrete testing, etc. performed to date.
When was the building designed/constructed?
Maybe some clues in public records.
With that info, you should be able to determine what code (if any) was in force at that location, at that time.
Also, an idea of which design method was in use. Ultimate strength? If it's really old, working stress?

If a load test is going to be performed, it should be based on sound, reasoned judgement of what to expect based on accumulated data, not "guesses".

We performed a full-scale ACI load test on our brand-new office building that had serious construction defects.
Retained a regional, respected consulting firm that we had worked with for years to determine if the the design was satisfactory. Answer (in a sealed, written report) "Yes, perform the load test".
Two weeks after load test completion, our senior VP gets an urgent call from the consulting firm's senior VP... "Don't test! We made a mistake! In addition to shoddy construction, the design is flawed! Load test will collapse the building!"
Fortunately, the building had not collapsed. Otherwise, two dozen workers performing the test, including me, would be dead.

Bottom Line - Be careful with a load test and don't assume anything you don't have to... but it may be the way you have to go.
Did not cost us millions of dollars, maybe $100,000.

http://www.SlideRuleEra.net

 

[DBronson]

jtse:

As I understand the code you are allowed to classify Assembly occupancies (such as conference rooms) as Group B occupancy if the space occupancy load is less than 50 persons or less than 750 sq ft. (See IBC 303.1.2.) Since the occupancy load for Group B is 100 sq ft/person a conference room of up to 4,900 sq ft would be considered Group B for occupancy purposes. Few conference rooms are this large. Where the architect is getting confused (I think) is using the occupant load for Assembly group A-3 (15 sq ft/person) as the determining factor in classifying the spaces for structural design. However, this number is only used for egress calculations. See the first sentence in 1004.1. As long as the conference rooms are clearly accessories to the Business occupancy and less than 4,900 sq ft I believe they can be designed with office live loads. It would, however, be prudent to discuss this with the Building official.

Regards,

DB

NB: My references are to the 2012 IBC - newer codes may have been renumbered. Also, in Group B occupancies I believe the newer codes
allow 150 sq ft/person.

 

[jtse]

Thanks everyone for the tips. I appreciate the feedback.

The building is a bit over 100 years old, so there is very little info to go on. Our concrete coring and strength tests yielded reasonable values, so those are useful. I think we're safe assuming fy = 33 ksi for our structural calculations based on what material references I could find, but there are a lot of unknowns about standard practice for design and detailing. Our preliminary GPR showed only one mat in a 9"± thick slab, at varying depths but mostly near the bottom. However, the report we received wasn't clear on the scanning locations. Has anyone ever heard of bars being draped in a slab of that age?

DBronson, I've been pushing the architect a bit on Chapter 3 of the IBC. A number of these rooms are less than 750 SF, so they would seem like obvious exceptions. There are a few larger ones that exceed that, so it may be a gray area. It doesn't matter presently, though, as I've been unable to convince them for any of them!

phamEng & SlideRuleEra, we're thinking about ways to rationalize strength using a limited load test to fill in the gaps. For example, our calcs indicate the punching shear and 1-way shear capacities are adequate, but we can't demonstrate moment strength. We are contemplating picking a column strip and loading it on opposite sides of a single column for a load equivalent to the design uniform live load, for maximum negative moment. This is all a hypothetical plan B, we're thinking another round of GPR and limited bar exposure may be done first (with me looking over their shoulder) to try and get better results.

 

[SlideRuleEra]

Has anyone ever heard of bars being draped in a slab of that age?

Absolutely,yes. Draping of floor reinforcement was popular circa 1920, see "Structure Magazine" Antiquated Structural Systems. With draped reinforcement, concrete compressive strength is not as important. And reinforcement may not be "bars", but mesh. If it is bars they may be square, twisted, or plain (not deformed).

For a building of that age, you need to be extra careful before considering a load test. Design would have been by an early version of working stress theory. There is a really good chance that shear reinforcement is inadequate by today's standards. Unlike a slab that lacks enough moment reinforcement, without proper shear reinforcement the slab can collapse with no warning under load.

http://www.SlideRuleEra.net