Nonseparated Office and Storage in a Sprinklered Type II-B Building
A single-story, 26,000-square-foot building of Type II-B construction is protected throughout by an NFPA 13 sprinkler system. The tenant program is 18,000 square feet of Group B office and 8,000 square feet of Group S-1 storage, with nothing between them but a nonrated partition. The building has 250 feet of claimed qualifying frontage on a 60-foot-wide public way; the building perimeter is 640 feet. The designer's worksheet lists allowable areas of 61,000 square feet for Group B and 74,000 square feet for Group S-1, each figured as though the whole building were that single occupancy in this construction type, including frontage and sprinkler increases. The narrative conclusion is: "Office area 18,000 is less than 61,000 allowed; storage 8,000 is less than 74,000 allowed; therefore the building complies." The owner wants to keep the plan nonseparated so that future tenant reconfiguration does not trigger a rated wall. A separate sheet shows that 60 feet of the claimed 250-foot frontage faces a fenced equipment yard controlled by the owner and 190 feet faces the public way.
1. Is the designer's compliance check the correct method for a nonseparated mixed-occupancy building? Show the correct method and apply it.
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No. The designer has tested each occupancy against its own single-occupancy allowable area. That is the separated-occupancy logic, and it is exactly the logic the nonseparated approach replaces. Comparing each area to its own large allowable area will pass almost any program and proves nothing.
Correct method. For nonseparated occupancies, the allowable area is checked with a sum of ratios: for each occupancy, divide its actual floor area by the allowable area that occupancy would have if it alone occupied the building, and require the ratios to sum to 1.0 or less. The allowable area in each denominator includes the frontage and sprinkler increases applicable to the building.
Applied. Office: 18,000 / 61,000 = 0.295. Storage: 8,000 / 74,000 = 0.108. Sum = 0.403. That is well under 1.0, so the building passes the area test - but for a completely different reason than the narrative gives, and with a much larger margin than the designer realizes. On a future expansion, the designer's method would keep approving floor area long after the sum of ratios had crossed 1.0.
Other consequences of choosing nonseparated. The building must then comply with the most restrictive requirements of the occupancies involved for everything else - egress, fire protection, interior finish, and height - not merely the area check. Some separations remain mandatory regardless: incidental uses, hazardous materials control areas, and Group H occupancies are not made nonseparated just because the applicant prefers open planning. The review should state these consequences in the approval so the owner understands what the nonseparated choice actually costs.
2. Check the frontage increase shown in the worksheet. What frontage actually qualifies, and how does the answer change if the yard does not?
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Formula. The frontage increase factor is computed as (F/P − 0.25) × W/30, where F is the qualifying frontage length, P is the building perimeter, and W is the width of the public way or open space, taken between 20 and 30 feet. Because the street is 60 feet wide, W is capped at 30 feet, so W/30 = 1.0 and the formula collapses to (F/P − 0.25).
As claimed. F/P = 250/640 = 0.391. 0.391 − 0.25 = 0.141, so a 14.1 percent frontage increase. That is the largest single driver of the 61,000 and 74,000 figures.
The yard. Frontage counts only where it abuts a public way or qualifying open space. A fenced equipment yard controlled by the owner is problematic: the space must meet the definition of open space, including public accessibility and permanent maintenance, and a fenced, owner-controlled storage yard generally fails that test. Answer the question with the definition in Section 202 applied to the actual site plan - do not accept the applicant's characterization.
If the yard does not qualify. F drops to 190 feet. F/P = 190/640 = 0.297. 0.297 − 0.25 = 0.047, a 4.7 percent increase instead of 14.1 percent. Both allowable areas fall by roughly nine percent. The sum-of-ratios test still passes at roughly 0.44, so the building is still approvable - but the reviewer must require the worksheet to be redone with the corrected F, because a 9 percent error in the denominator silently consumes the applicant's future margin.
Also check. Confirm that F/P actually exceeds 0.25. If qualifying frontage were under 160 feet, no increase would be available at all under this formula, and the plan would need a fresh analysis.
3. The owner asks what changes if a 1-hour separation is added between office and storage instead of keeping the nonseparated condition. Explain the trade-offs and what must be verified.
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What separation buys. Adding a rated separation converts the project to the separated-occupancy approach. Each occupancy is then evaluated as though it were a separate building for allowable area: each uses only its own floor area against its own allowable area, and the sum-of-ratios test disappears. For this project, 18,000 against roughly 56,000 and 8,000 against roughly 68,000 is comfortable, so separation is not being used here to cure an area problem - it is being used to buy flexibility.
What separation costs. A rated wall means rated doors, rated penetration protection, rated joints at the roof and exterior wall, and a sign or detail set that survives tenant churn. The owner's stated goal was avoiding a rated wall, so this reverses the original intent. The honest advice is: the nonseparated design already works on area, so a rated wall is buying nothing structural - it may, however, simplify future expansion if the storage side is likely to grow.
Verify, do not assume. Look up the required separation for Group B adjacent to Group S-1 in Table 508.4 in the locally adopted edition; the entry is commonly 1 hour but editions and local amendments differ. Check the footnote that permits a reduction where the building is sprinklered in accordance with Section 903.3.1.1, and confirm whether the local amendment retains it - if it applies, the "1-hour wall" may become an unrated wall that still must be smoke-sealed and detailed for the separated analysis. Verify that the wall is continuous from floor slab to the underside of the roof deck, and that the rated doors are on hold-opens tied to the alarm.
Return a written comparison. Give the owner both paths with the numbers so the decision is documented before the permit is issued rather than discovered at final inspection.