705.3 Buildings on the same lot exception question

[blugosi]

Par 705.3 of the 2021 edition of IBC states:

705.3 Buildings on the same lot. For the purposes of determining
the required wall and opening protection, projections
and roof-covering requirements, buildings on the same lot
shall be assumed to have an imaginary line between them.

And then there is exception #1:

Two or more buildings on the same lot shall be
either regulated as separate buildings or shall be
considered as portions of one building if the
aggregate area of such buildings is within the
limits specified in Chapter 5 for a single building.
Where the buildings contain different occupancy
groups or are of different types of construction,
the area shall be that allowed for the most restrictive
occupancy or construction.

I was under the impression that this exception is applicable to maximum areas only, but the current wording makes me wonder.

If I want to build a small business occupancy building (2000 square feet or so) next to an existing larger building on the same lot and at a distance of 3 feet, not exceeding the aggregate area limit for the existing building, can I consider them as portions of the same building instead of separate buildings?

The reason I am asking is that the existing building never had any fire resistance requirement for its non bearing external walls and they are almost 80% tinted glass. Can I use this exception and ignore the separation requirement of Table 705.5 maybe?

 

[RLGA]

Yes you can, but you need to look at Section 503.1.2.

However, if the existing building is of Type IA construction, then your 2,000 sq. ft. building must also be of Type IA construction.

Further, if your 2,000 sq. ft. building is nonsprinklered and the existing building is sprinklered, then the allowable area for the aggregate of both buildings must be based on the nonsprinklered values.

Finally, if the area of the existing building was based on the provisions of Section 507 for unlimited area buildings, the 2,000 sq. ft. building must conform to the same conditions, including the 60-foot open area around the perimeter which would now include the new building.

I am sure there are other provisions of Chapter 5 that would come into play, but I cannot think of any others at the moment.

 

[blugosi]

Yes you can, but you need to look at Section 503.1.2.

However, if the existing building is of Type IA construction, then your 2,000 sq. ft. building must also be of Type IA construction.

Further, if your 2,000 sq. ft. building is nonsprinklered and the existing building is sprinklered, then the allowable area for the aggregate of both buildings must be based on the nonsprinklered values.

Finally, if the area of the existing building was based on the provisions of Section 507 for unlimited area buildings, the 2,000 sq. ft. building must conform to the same conditions, including the 60-foot open area around the perimeter which would now include the new building.

I am sure there are other provisions of Chapter 5 that would come into play, but I cannot think of any others at the moment.

Thank you!

The existing building is occupancy A-3, Type IB construction, 5 story fully sprinklered. Area factor is unlimited so I have no problem with area limits even if there were no sprinklers.

It still feels weird that if two buildings are in the same lot there are no requirements for external wall fire resistance even if they are close to each other.

NFPA 5000 in comparison demands 1 hour fire resistance if two buildings are less than 10 ft apart.

 

[RLGA]

Thank you!

The existing building is occupancy A-3, Type IB construction, 5 story fully sprinklered. Area factor is unlimited so I have no problem with area limits even if there were no sprinklers.

It still feels weird that if two buildings are in the same lot there are no requirements for external wall fire resistance even if they are close to each other.

NFPA 5000 in comparison demands 1 hour fire resistance if two buildings are less than 10 ft apart.

To illustrate the reasoning behind Sections 503.1.2 and 705.3, refer to the diagrams below. Both plans show what could be considered separate buildings: Plan 1 shows detached structures, and Plan 2 shows attached structures with a fire wall at the connection creating a court. Each building can have a construction type and occupancy groups that are not influenced by the other building's construction type or occupancy groups.

To be considered separate structures for Plan 1, an imaginary lot line is required to determine the fire separation distance for opening and exterior wall protection. The imaginary lot line cannot be placed in a location that would make the existing building (assume Building A) non-compliant with the building code. For Plan 2 to be considered separated structures, one of two options can be used per Section 706.5.1, of which #2 is the imaginary lot line applied similarly as described for Plan 1.

However, both situations can be considered portions of a single structure. Plan 2 would not require a fire wall, and both would require that the aggregate area of both buildings not exceed the allowable area based on the most restrictive construction type or occupancy group. The distance between the two buildings is irrelevant unless the court in Plan 2 is used as an egress court; thus, Section 1029 would come into play. Even if Plan 2 was constructed as one building with no fire wall, no one would question the distance between the opposing building faces at the court.

Therefore, if Plan 2 is acceptable as a single building without consideration of the distance between exterior walls, then Plan 1 should be equally acceptable as portions of a single building.