A lack of separation in a mixed use leads to a tough call

[Simonsays]

Trying to help a designer comply in a tough situation …

The applicable building code is the 2006 IBC for an existing two story wood frame mixed use building, mercantile on the first floor and two R-3 residences above. There is no existing separation, just a wood floor and wood joists. The project involves modifying the center support of the floor joists by installing a lightweight steel beam, as a column is in the way of proposed mercantile area changes.

Can’t use 3410 as a new unprotected steel beam will make the building less safe per 3410.2.4, IMHO. (It’s the same principle as with new openings in an existing unrated and unsprinklered corridor why you must install new fire-rated doors.)

Can’t substitute an engineered wood beam and use three layers of type X per Table 721.6.2(1) to get the two hour rating as that would violate 721.6.1.1.

The proposed lightweight beam violates 721.5.2.1.2 as its W/D ratio is less than the beam in the approved assembly (U L design N 501).

Is there another, easier, and code-complying path that I am failing to see?

 

[fatboy]

Make the existing column work in it's existing location? Serious, off the top off my head, I can't see anything, it looks you closed the code loop yourself. At home though, not looking in the book, maybe someone else can see something.

 

[Paul Sweet]

He can use a heavier steel beam than is structurally necessary to meet 721.5.2.1.2, or use a different UL assembly.

He can use a floor/ceiling assembly where the ceiling membrane provides a 2-hour resistance for wood framing (GA 5275 or similar).

He can use a heavy timber member. American Forest & Paper Association Technical Report #10 shows how to calculate the fire resistance of an oversized heavy timber member.

A code modification to ignore the 1-hour limit of 721.6.1.1 would be appropriate. The building would be evacuated within an hour, and the unrated construction will start falling down long before the protected beam does.

 

[Simonsays]

Thanks to both of you. Paul, I especially appreciate your fourth suggestion as I believe the one-hour calculated limit in the code unfortunately forces you towards either heavy timber or a heavier steel beam, located in the middle of an existing tinderbox.