Support for 2-hour shaft wall in Type II-B building

[Redmund]

It is typical to see fire rated shaft walls installed at the slab edge of a shaft opening, however what about a situation as follows:

• 4 story B occupancy office building.
• Type II-B construction (sprinklered).

The floor will be non-rated, however the shaft walls need to be 2-hour rated. The shaft walls need to be fire barriers, which in turn must have their fire rating carried down to the foundation per IBC 707.5.1. Would this require the floor framing that is considered as supporting the shaft walls to also be 2-hour rated, with the columns that support those floor beams also 2-hour rated? This could end up making a lot of floor framing and columns needing to be 2-hour rated in an otherwise Type II-B building. Alternatively, the shafts could be 2-hour self-supporting construction (likely concrete or CMU), but that is not typically done.

If a Type II-B building is allowed to have a non-rated floor, roof, structural frame and bearing walls which could collapse in less than an hour, why would the shafts still need to 2-hours?

 

[mtlogcabin]

It is non-combustible construction. Why would you assume the walls or floors would collapse in less than an hour in a sprinklered building?

 

[Blazer]

If the shaft sits on the edge of the floor slab then the portion of primary structure that supports that slab will require individual protection to achieve a 2-hour fire resistance rating. Secondary structure can be protected by membrane as stated by 704.4. The intent is similar to the question you bring up about collapse of structure around the rated shaft. "It will prevent the effectiveness of the assembly from being circumvented by a fire that threatens the supporting elements." as stated by the commentary to 707.5.1. Better to build a freestanding shaft.

 

[RLGA]

Alternatively, the shafts could be 2-hour self-supporting construction (likely concrete or CMU), but that is not typically done.

The projects I am involved in and those I see around the area being constructed, even in Type V construction, this method is typically done, especially with CMU. When the construction type has nonrated floor assemblies, this is the only method to provide full-height unsupported construction for shaft heights that exceed the limitations of metal shaft wall assemblies (usually around 28 feet).

If a Type II-B building is allowed to have a non-rated floor, roof, structural frame and bearing walls which could collapse in less than an hour, why would the shafts still need to 2-hours?

This is why rated assemblies cannot be supported by nonrated assemblies and construction. With rare exceptions (e.g. World Trade Center on 9/11/01), entire buildings do not collapse at once. However, if a portion of a building adjacent to a rated exit enclosure collapses, the occupants inside the enclosure will likely remain protected until reaching the exit discharge.

 

[Tim Mailloux]

It is typical to see fire rated shaft walls installed at the slab edge of a shaft opening, however what about a situation as follows:

• 4 story B occupancy office building.
• Type II-B construction (sprinklered).

The floor will be non-rated, however the shaft walls need to be 2-hour rated. The shaft walls need to be fire barriers, which in turn must have their fire rating carried down to the foundation per IBC 707.5.1. Would this require the floor framing that is considered as supporting the shaft walls to also be 2-hour rated, with the columns that support those floor beams also 2-hour rated? This could end up making a lot of floor framing and columns needing to be 2-hour rated in an otherwise Type II-B building. Alternatively, the shafts could be 2-hour self-supporting construction (likely concrete or CMU), but that is not typically done.

If a Type II-B building is allowed to have a non-rated floor, roof, structural frame and bearing walls which could collapse in less than an hour, why would the shafts still need to 2-hours?

if you can arrange the HVAC system so that your vertical duct risers connect no more than (3) stories, you would use fire fire dampers at the floor slabs and the ducts would no longer require rated shaft enclosures, as far as the code is concerned you have a duct penetration through non fire rated floor assembly which is covered under IBC section 717.6.3.

717.6.3 Option #3 states:
In floor assemblies composed of noncombustible materials, a shaft shall not be required where the duct connects not more than three stories, the annular space around the penetrating duct is protected with an approved noncombustible material that resists the free passage of flame and the products of combustion and a fire damper is installed at each floor line.

 

[mtlogcabin]

We recently had 3 Type VA apartment buildings go up that used CLT in the elevator and stairway shafts all self-supporting from the foundation to the top. I think we will see more CLT and perhaps a code change to allow it in Type I and II construction for shafts in the future. They still where required to install gyp board on the exterior of the shaft to get the proper fire rating.

 

[Redmund]

Thanks for all the replies, this is very helpful information.