IBC 2006 Chapter 5

Building Height and Areas After determining occupancies, the code analysis determines what height and areas are allowable for the occupancy classification given various types of construction. Building economics and utility generally dictate that buildings be built using the least costly and complicated type of construction that will meet the code criteria.

There are two design choices that impact the use of Table 503.

1 – When the design of a given occupancy must provide enough area to contain the known uses according to the program of the building.

2 – When speculative buildings are built for economic gain, hence maximizing the allowable height and area using the most economic construction type.

Iterative process. (iterate = repeat, make charge, assertion, objection) repeatedly.

The designer uses an iterative process by making assumptions regarding construction type and analyze the relative cost and return before determining the final choice.

Chapter 5 is organized upon basic criteria that are modified to allow tradeoffs.

Session 5 | Codes | Gary Parker

Definitions are cross-referenced from the list of definitions of Chapter 2. Height and area limitation used in Chapter 5 of IBC should not be confused with those used in planning and zoning regulations. Do not apply IBC criteria to planning issues or vice-versa.

Building Area is the area included within surrounding exterior walls. Interpreted as meaning to the outside face of exterior wall.

Vent shafts and courts are excluded from BA taking into account the wall thickness of the shaft.

Area included within the horizontal projection of a roof or a floor above, even if not included by surrounding walls, are included in BA.

Definitions

Basement is the portion of a building that is partly or completely below Grade Plane.

If the finished surface of the floor above is

>6’ above grade plane

or above ground level for >50% of the total building perimeter,

or if it is >12’ above grade at any point,

the the basement is considered to be a ‘story above grade plane’

Definitions

Building Height is the ‘vertical distance from the grade plane to the average height of the highest roof surface.

Story height for the topmost floor takes into account the volume of the upper floor but not projections above the roof (chimneys). It is measured to the top of sloping roof rafters or, to the top of ceiling joists.

Parapets are excluded from height definition, but pitched rafters are included.

For sloping sites, the Grade Plane is defined according to the smallest lowest points within the area between the bldg. and the property line, or between the bldg. and a point 6’ from the bldg.

Grade Plane = average grade levels

Definitions

Mezzanine is an intermediate floor level inserted between larger floor plates.

The area of the mezzanine is restricted to 1/3 of the floor area of the space where it is located.

Note that the definition language allows for multiple levels of mezzanines.

Definitions

Table 503

Exceptions to Table 503

§503.1.2 exempts low-hazard high-bay spaces, such as mills or foundries from the height and building are limitations of T 503 to recognize special functional requirements and low occupant loads.

§503.1.4 states that Type I bldgs., permitted to be UL in height and area, do not require the mitigations for increasing height as per Section 507. However, certain Group H have height and area limitations even for Type I buildings.

The open parking garage is not included in T 503 and has no cross-reference to a separate set of height and area criteria. The criteria for this type of construction are included in Table 406.3.5

Exceptions to Table 503

§503.1 allows one or more fire walls to divide a single structure into a number of ‘separate buildings’.

§503.1.1 allows a bldg. to have a single level basement, limited in area to that allowable for one-story bldg., that does not contribute to the overall bldg. area. Hence, it is possible for the basement to be larger than the first floor as long as it does not exceed the allowable area for a one-story bldg.

Exceptions to Table 503

§503.1.3 states that multiple bldgs. on a single site may be considered as separate bldgs., or as a single bldg. for determining building areas.

For a group of bldgs. to be considered as a single bldg., the group must meet the aggregate area limitations based on the most restrictive occupancy. Means of egress must also be examined.

Example. Areas of A + B must not exceed the single bldg. area of the more restrictive occupancy.

Exceptions to Table 503

§506.4 Area Determination. The total allowable bldg. area is permitted to be 3 times the max area permitted by floor. Two story bldgs. are limited in 2 times the max area permitted by floor.

= or

• Group B • 5 stories • 2 stories @ 28,500 sf per floor

• Type III-A construction @17,100 sf per floor +

• Max BH 5 stories or 65’ 3 stories @ 9,500 sf per floor

• Max area per fl. 28.500 sf

• Max BA 3x28.500 sf = 85,500 sf

Exceptions to Table 503

§506.1 allows the basic areas limited in Table 503 to be increased due to bldg. frontage and/or automatic sprinkler system protection. Thus, the total allowable area per floor is equal to the basic tabular area from Table 503 plus any increase due to frontage on a public way or open space, plus any increase from the installation of an automatic sprinkler §506.3.

= or

• Max BH 5 stories or 65’ • 5 stories • 2 stories @ 57,000 sf per floor

• 200% increase in allowable @ 34,200 sf per floor +

area pr fl. for automatic 3 stories @ 19,000 sf per floor

sprinkler (§506.3)

2 x 28,500 sf = 57,000 sf

• Max BA 3 x 57,000 sf = 171,000 sf

Exceptions to Table 503

§504 Height Modifications. Permits bldgs. that are provided with sprinklers to increase 20’ or 1 story in height. Residential bldgs. are limited to an increase 20’ and one story in height. Residential bldgs. protected by an NFPA 13R sprinkler system are limited to no more than 4 stories and overall height of 60’. These increases do not apply for Gropu H or Group I-2 of less than Type IIA construction.

Towers and steeples may project up to 20’ above allowable height for combustible constructions and to any height for non-combustible constructions. This provision presumes that these spaces are not used for habitation but only for operational or decorative functions.

Mezzanines

§505 considers mezzanines that meet the criteria of §502 to be part of the floor below them. If they are less than 1/3 of the area of the floor below they are not considered part of the overall bldg. area or as an additional story. However the area must be calculated for the fire provisions (702)

Typically, a 1000 sf floor space can have a 333 sf mezzanine. Type I and II buildings for special industrial occupancies get a bonus allowing the mezzanine to be up to 2/3 of the floor area.

The habitable height of mezzanines must be of a minimum of 7’ in and under the mezzanine.

Enclosed portions are not included when determining the size of rooms where mezzanines are located.

Mezzanines

Mezzanines are conceived by the IBC as being open and unobstructed to the floor below except for the rail-height edge. There are exception that regard the occupant load, way of furnishing and size.

Mezzanines are required to have 2 separate means of egress. This is typically except for cases with low occupant load or when common paths of egress are very short.

Area Modifications

§506 specifies when areas that are limited by Table 503 can be increased (sprinkler system) and (frontage on public way).

When a building fronts are on at least one side onto a public way or open space of at least 20’, it may be increased based on added width of the public ways and/or open space and the extent of the building perimeter surrounded by them. This increase is limited to 75% by Equation 5-2.

If = 100(F/P – 0.25) * W/30

= 100(250/250 – 0.25) 40/30

(W/30 cannot exceed 1 (506.2.1)

= 100*0.75 = 0.75

Area Modifications

Equation 5-1 combines the increase for frontage with the increase for sprinklers.

Aa = At + (At * If / 100 + At * Is / 100)

When sprinklers are installed, the area limited by Table 503, may be tripled for one-story bldgs. and doubled for multistory bldgs.

Unlimited Area Buildings

§507 allows bldgs. other than Type I construction to be of unlimited area when certain conditions are met.

Special Provisions

Group S-2 enclosed Parking Garage with Groups A, B, M or R above. The special provisions of §508.2 applies for enclosed parking garages built underneath other uses, hence a mixed use occupancy.

Group S-2 enclosed Parking Garage with S-2 Open Parking above. The special provisions of §508.3 applies similar criteria to the above provision.

If the enclosed parking garage is separated by fire-resistant barriers of defined ratings (3hrs rating with openings protected by 2hrs enclosures), then the building above is considered a separate bldg. for height and areas. Hence, the ground level is redefined affecting the allowances of the bldg. above. This gives more flexibility than Table 503 of mixed uses.

Singe bldg. for height limitation, separate bldgs. for allowable

area, # stories and types of construction.