Type III exterior to interior

[Mick70]

Working on a type III building with open corridors. Type III buildings require all exterior walls to be 2-hour. Typically in a type V building we have a material transition from exterior to interior at 6’-0” from the exterior wall.
Is there a required distance (defining line) where the building code considered exterior is now interior. Where do you terminate the 2-hour exterior wall?

 

[RLGA]

Just to clarify,Table 601 only requires exterior bearing walls to have a 2-hour rating. Nonbearing walls are only subject to the fire-resistance ratings based on fire separation distance per Table 602.

When the wall no longer separates interior conditioned space from the exterior, then it is no longer an exterior wall. The IBC has definitions for exterior wall and exterior wall envelope.

 

[Mick70]

With an open end corridor that is using passive ventilation and not being used as a mechanical conditioned space, where does the code draw the line from exterior / interior.
All the other type III-A buildings I have done have always been enclosed. This one being the first to have open walkways and corridors I am concerned where the BD will make the 2-hour termination point.

 

[RLGA]

An open corridor is exterior unconditioned space, thus the walls along the corridor are exterior walls.

 

[Mick70]

Even if it is open on each end?
In that case, they would required to be 2-hours with fire retardant wood and plywood.
Corridor floor assembly still 1-hour

 

[RLGA]

Even if it is open on each end?
In that case, they would required to be 2-hours with fire retardant wood and plywood.
Corridor floor assembly still 1-hour

Yes. Table 601 says “exterior walls,” not “exterior perimeter walls.”

 

[RLGA]

If the corridor walls are not load-bearing, then they do not need to have the 2-hour fire-resistance rating. If the walls need to be load-bearing, then there are alternatives.

Since the structural frame for Type IIIB requires no rating and the structural frame for Type IIIA requires a 1-hour rating, you can use columns and beams to support the loads in lieu of walls. Thus, the exterior walls are no longer load-bearing and are not required to have the 2-hour rating.

For Type IIIB construction, no additional protection is required for the columns and beams.

For Type IIIA construction, the columns will need individual protection per Section 704.2 for the 1-hour rating. However, the beams supporting no more than two stories of nonload-bearing walls (i.e., a 3-story building) do not need individual protection per Section 704.3, so the 1-hour rating for the corridor walls (if required per Table 1020.1) would be sufficient for the beam fire-resistance protection.

Another option is to consider heavy timber framing for the structural frame of the corridor walls. This may require a code modification, since you'd be falling short of full compliance with Type IV construction, so, for portions of the building, you'd be using the inherent fire-resistance of heavy timber (roughly 1-hour+ equivalent) in lieu of tested assemblies. This would require 8-inch columns supporting floors and 6-inch columns supporting a roof. Beams supporting floors would need to be a minimum of 6 inches wide by 10 inches deep, and beams supporting a roof would need to be a minimum of 6 inches in width by 8 inches in depth.

 

[Francis Vineyard]

An open corridor is exterior unconditioned space, thus the walls along the corridor are exterior walls.

OPEN-ENDED CORRIDOR. An interior corridor that is open on each end and connects to an exterior stairway or ramp at each end with no intervening doors or separation from the corridor.

 

[ADAguy]

Almost as complicated as "how do you measure net leasable square footage?" Centerline of glass vs wall, (-) structural supports, shafts, (?).

 

[RLGA]

OPEN-ENDED CORRIDOR. An interior corridor that is open on each end and connects to an exterior stairway or ramp at each end with no intervening doors or separation from the corridor.

I agree, but the walls are still exterior walls and are part of the exterior wall envelope:

EXTERIOR WALL ENVELOPE: A system or assembly of exterior wall components, including exterior wall finish materials, that provides protection of the building structural members, including framing and sheathing materials, and conditioned space, from the detrimental effects of the exterior environment.​

Albeit, the detrimental effects in this situation are less than if fully exposed for typical exterior walls (i.e., UV, rain, snow, etc.), but moisture vapor transmission and thermal transmission are still conditions that must be addressed within an open-ended corridor; thus, the corridor walls are still exterior walls, and Table 601 does not make the distinction between one type of exterior wall from another.

What I have stated is my interpretation from a more conservative perspective--I'm not saying that the BO will not use or accept another interpretation.