Trying again

[Yikes]

My previous question did not get any responses - I'll try and simplify it here:

We know that dwelling units sharing a common wall require a minimum 1 hour dwelling unit separation rating, which would not be allowed to have openings.

But what if those units are separated by about 5', and are on the same lot and can be treated as a single building (503.1.2)? Assume type V-B construction. Can the 2 walls 5' apart be nonrated, and have nonrated windows?

 

[steveray]

Interesting...technically there is some type of hole there, but how about this:

503.1 General​

Diagram
Unless otherwise specifically modified in Chapter 4 and this chapter, building height, number of stories and building area shall not exceed the limits specified in Sections 504 and 506 based on the type of construction as determined by Section 602 and the occupancies as determined by Section 302 except as modified hereafter. Building height, number of stories and building area provisions shall be applied independently. For the purposes of determining area limitations, height limitations and type of construction, each portion of a building separated by one or more fire walls complying with Section 706 shall be considered to be a separate building.

503.1.2 Buildings on Same Lot​

Two or more buildings on the same lot shall be regulated as separate buildings or shall be considered as portions of one building where the building height, number of stories of each building and the aggregate building area of the buildings are within the limitations specified in Sections 504 and 506. The provisions of this code applicable to the aggregate building shall be applicable to each building.

420.2 Separation Walls​

Walls separating dwelling units in the same building, walls separating sleeping units in the same building and walls separating dwelling or sleeping units from other occupancies contiguous to them in the same building shall be constructed as fire partitions in accordance with Section 708.

And I guess nothing in 503 technically gives you a pass on FSD either.....

 

[classicT]

It is definitely a gray zone @Yikes.

Is the 5-ft of space between the units under a roof? If so, I imagine that it has an occupancy classification assigned to it; thereby, under 420.2 the wall providing separation between the unit and this contiguous space requires separation via fire partitions, protected openings, etc.

However, if this is exterior space and thereby not assigned an occupancy, a strict reading of the code would allow these to be constructed as exterior walls. Given that the principle intent of the separation walls (420.2) is fire compartmentalization (vs. fire resiliency), then the walls would not be required to be fire resistance rated.

2021 IBC - 420.2 Separation Walls​

Walls separating dwelling units in the same building, walls separating sleeping units in the same building and walls separating dwelling or sleeping units from other occupancies contiguous to them in the same building shall be constructed as fire partitions in accordance with Section 708.

 

[Yikes]

The area between the 2 units is open to the sky.

I assume the "fire separation distance" in Table 602 refers to either the distance between 2 structures being treated separately, or between a structure and a property line. Neither apply in this case.

 

[steveray]

I would still argue this:

 

[classicT]

I would still argue this:

View attachment 12829

I see why you would, and I will agree that it is a valid path for a BO to take.

That said, the definition of FSD says to measure to very specific items. Lot line, nor CL of a street/alley/public way apply. You may suggest that an imaginary lot line be provided between the two units; however, under 503.1.2, you are treating these as a single building. Meaning there are no "two" buildings on the lot to measure between.

[BF] FIRE SEPARATION DISTANCE. The distance measured from the building face to one of the following:

1. The closest interior lot line.
2. To the centerline of a street, an alley or public way.
3. To an imaginary line between two buildings on the lot.

The distance shall be measured at right angles from the face of the wall.

 

[classicT]

Another way to think about this is what if there was a balcony (enclosed) connecting these two units. Clearly, they would then be part of the same building. Would the exterior walls of the adjoining units (separated by 5-ft breezeway or similar) require separation?

 

[steveray]

Meaning there are no "two" buildings on the lot to measure

For purposes of Ch. 5 I would agree......Ch. 7...not so much....Granted...if they are sprinklered it's likely only a 30 min. unit separation if they were right next to each other which Shirley any exterior wall would meet....

 

[classicT]

For purposes of Ch. 5 I would agree......Ch. 7...not so much....Granted...if they are sprinklered it's likely only a 30 min. unit separation if they were right next to each other which Shirley any exterior wall would meet....

OK, I see your logic there.

But take that logic and apply to this: Two S-1 buildings. Each 2,000sf. They are 10-ft apart. Applicant proposes to treat them as a single building. Do you require the imaginary lot line and rated exterior walls where the FSD is 5-ft for each building?

Or think about an existing building where they want to do a small addition. Instead of connecting the existing and new building, they build them a foot apart. Buildings collectively can be treated as one (as if it were an addition), but in reality, they have a gap between them (avoids forces being transmitted b/w them).

How would you treat these two scenarios? All three should be treated the same, right?

 

[steveray]

OK, I see your logic there.

But take that logic and apply to this: Two S-1 buildings. Each 2,000sf. They are 10-ft apart. Applicant proposes to treat them as a single building. Do you require the imaginary lot line and rated exterior walls where the FSD is 5-ft for each building?

Or think about an existing building where they want to do a small addition. Instead of connecting the existing and new building, they build them a foot apart. Buildings collectively can be treated as one (as if it were an addition), but in reality, they have a gap between them (avoids forces being transmitted b/w them).

How would you treat these two scenarios? All three should be treated the same, right?

Kind of...other than the R's require the unit separation that can't /shouldn't be ignored....2027 IBC here we go....Oh wait we already missed it...

 

[classicT]

Kind of...other than the R's require the unit separation that can't /shouldn't be ignored....2027 IBC here we go....Oh wait we already missed it...

I just don't like picking-and-choosing how I make my interpretations. I want to be consistent.

I'd have real heartache in applying the requirement for rated exterior walls by establishing FSD between buildings in the two separate conditions I proposed (in post #9).

 

[steveray]

I just don't like picking-and-choosing how I make my interpretations. I want to be consistent.

I'd have real heartache in applying the requirement for rated exterior walls by establishing FSD between buildings in the two separate conditions I proposed (in post #9).

I get it....And I have a hard time giving a rating away for dwelling units an inch apart where the rating would be required when they touch.....

 

[classicT]

I get it....And I have a hard time giving a rating away for dwelling units an inch apart where the rating would be required when they touch.....

True and fair.

I suppose one thought would be that given a fire partition only provides compartmentalization of the fire and smoke, if the space between the two units was open, any smoke and heat would generally go vertical, minimizing the exposure (risk) to the adjoining unit.

 

[steveray]

True and fair.

I suppose one thought would be that given a fire partition only provides compartmentalization of the fire and smoke, if the space between the two units was open, any smoke and heat would generally go vertical, minimizing the exposure (risk) to the adjoining unit.

Except it could be lighting their adjacent house on fire while they sleep and they may never know....

 

[classicT]

Except it could be lighting their adjacent house on fire while they sleep and they may never know....

Good point. Although, easily addressed via interconnected fire alarm system.

Side note - I appreciate the back and forth. Thanks @steveray. @Yikes, hopefully our side quest is still beneficial towards your thinking.

 

[Yikes]

@Yikes, hopefully our side quest is still beneficial towards your thinking.

Yes, it has been very helpful. A similar real-world scenario is bay windows on different dwelling units facing each other (<180 degrees), less than 5' apart, like this:

 

[Paul Sweet]

The bay window townhouses were probably built before there were any building codes.

Since Fire Separation Distance is measured to an imaginary line between buildings, a 5 ft. FSD would require a 10 ft. distance for windows facing each other. However, if the buildings were only 5 ft. apart you could have windows in one building, but not the other, by placing the imaginary line at the face of the building with no windows.

 

[steveray]

Good point. Although, easily addressed via interconnected fire alarm system.

Side note - I appreciate the back and forth. Thanks @steveray. @Yikes, hopefully our side quest is still beneficial towards your thinking.

Likewise...nice to discuss/ debate with smart people instead of the normal remedial teaching stuff....

My opinion is that these structures normally would be built under the IRC and not the IBC and maybe that is why there is a "hole" in the code.....In which case you could go with R302.whatever for FSD >5' 1 hour but at 5' would be zero rating...

 

[Jay Smith]

It seems like if you have two dwelling units that are in buildings considered to be one building, the two walls with windows facing each other have not satisfied the requirement for a separation between dwelling units. That doesn’t change at 1 inch away or at 5 feet away. It changes when you apply the concept of fire separation distance.