Importance Factors Before The IBC 3

[zurch1818]

I'm hoping to pick the brain of some more experienced and seasoned engineers. I'm working on a six story college campus building in Wisconsin that has had a 50' tower that was constructed on top of it. The tower itself spans to two sets of exterior concrete columns that seem to be part of the lateral system on their based upon their rectangular shape orientation and the beams that spans between them. Under the current code (IBC 2015), I would classify this as a Risk Category III structure and would use corresponding windspeeds for the MWRS analysis.

When this tower was originally constructed (late 90's), the IBC wasn't the law of the land. Since that is before my time of practicing engineering, I'm clueless of how this analysis should have been conducted. I don't even know what Wisconsin's building code was at that time. I'm just curious from a historical standpoint, were there any provisions in the code that would have required this structure to be analyzed to more load than just an average building (say Risk Category II under today's code)?

 

[bones206]

BOCA codes had different wind design pressure maps based on occupancy group.

 

[zurch1818]

I think I found the old Wisconsin code (here...see page 85)...but I don't see any reference to importance factors. It's just a straight pressure based on the height. Does anyone see anything different? I'm also curious if other building codes (BOCA, UBC, and SBC) had adjustments for Risk Categories or if the IBC was the first code to do so.

 

[JAE]

The 1991 UBC used "Occupancy Categories" rather than "Risk Categories".
Essentially the same thing but the nomenclature was changed to avoid conflicts with other language in the IBC.

See the attached table from the 1991 UBC. This shows the various required importance factors used.
The code you linked to appears to be an administrative code of some sort. My guess is that Wisconsin used the UBC based on the prevalent codes used in the old geographical regions (BOCA in the NE, SBC in the southeast, and UBC everywhere else).

The applicable code was probably derived from the city adoption where the buildng was located. You might try contacting the city to see if they can tell you what codes were in effect in the late 90's.

 

[dold]

I don't have an answer to your question directly but...

My first step when working with older buildings is to file a FOIA (sometimes FOIL) request with the jurisdiction where the building is located. Sometimes they have nothing, sometimes they have a treasure trove of drawings and as-builts and inspection reports and permits. Depending on the age of the building it's likely that codes and loads may be listed.

Usually word it as such on their request form: "Requesting any and all information, documents, construction documents/drawings, specifications, permits, etc., related to the property/structure located at _______." Sometimes I'll include parcel IDs, coordinates, etc because they might not know what "old sears building" or whatever is.

If they have such documents they are required to provide them to you. Sometimes there is a small fee. Sometimes you have to get written permission of the property owner. Sometimes you have to get written permission from the design professional of record who sealed the drawings (some places in California). But you can always go to the records department and view the documents in person without such permission.

 

[JStephen]

I'm thinking any wind loads from that era would also be allowable-stress-design/ service basis, rather than ultimate basis.

 

[EdStainless]

Which campus is it?
I'll ask a friend but at some date buildings in the UW system were all required to be built to a specific code.
Even if that was a lot more strict than the local requirements.

 

[JAE]

One other thing that is important to realize if you are trying to compare older UBC wind designs to current IBC wind.
The UBC used a "fastest mile" wind speed vs. the current 3 second gust speed.

For the middle part of the US, this equated to a 70 mph wind vs. a 90 mph under the 3 second gust (both service level).
The wind pressures from these two types of wind speed were roughly equivalent.

At a later date (ASCE 7-10 I think) changed the wind speeds further by incorporating the risk categories (importance factors) and load factors into the wind maps themselves.
Essentially making the wind maps ultimate (vs. earlier service load based maps).