Height & Area Requirements In The IBC

As noted in the textbook, the three previous US model code organizations have consolidated themselves into the International Code Council, and merged their model codes into the International Building Code. Recently, this author had reason to compare height and area limitations between one of the "legacy" model codes and the new International Building Code. It's no secret that the new code is at times less restrictive than its predecessors, but even knowing this in advance, the results were eye opening.

The following two charts are adapted from our design handbook, The Architect's Studio Companion. One portion of this book deals with the effects of the building code on building form by providing a series of charts that allow the designer to quickly determine allowable height and area for a project under consideration depending on such factors as applicable code, construction type, and related considerations. By comparing tables from an earlier edition of the Companion, that compiled results for the previous codes, with information from the latest edition, which refers to the new International Building Code, easy comparisons can be made.

Combustible Construction, UBC vs IBC
This first chart compares height and area limits for two codes, the "legacy" Uniform Building Code (UBC) and the International Building Code (IBC), for buildings with Occupancy Group B (Business) and constructed of combustible construction. The base chart (rendered in grey scale) represents data for the UBC. Information overlaid in blue represents data from the IBC.

Read the chart as follows:

• Various construction types and their level of fire protection are listed across the top rows of the chart. For example, "Wood light frame, 1-Hour" refers to stick built construction with 1-hour fire protection applied to the framing (typically in the form of fire-rated gypsum wallboard). Mill construction refers to buildings with noncombustible exterior wall construction and heavy timber framing for the interior structure. Noncombustible structures made from concrete or steel (included in the chart below) are listed simply according to the level of fire protection applied.
• Below the construction types, columns are further divided to distinguish between sprinklered ("Spr") and unsprinklered ("Unspr") buildings.
• Once a construction type, level of fire protection, and sprinkler condition have been established, allowable height and area can be read as follows. For maximum height in feet, read from the row directly below the sprinkler designation row. ("UH" on the chart below refers to unlimited height.)
• For maximum height in stories, proceed down a column until arriving at the solid horizontal line. Read across to the side of the chart to find the allowable stories permitted.
• To determine the maximum permitted area for the building, read the number directly below (or above, where space was limited) the solid line. This is the maximum allowable area for all floors of the building. ("UA" refers to unlimited area.)

Keeping in mind that data for the IBC is rendered in blue, a few facts are readily apparent:

1. For the cases illustrated here, more often than not, the IBC is moderately more generous in its allowable building height than the UBC.
2. In these cases the IBC is significantly more generous in its allowable building area than the UBC. This is particularly true for sprinklered buildings, where on average, the allowable area for all floors has increased by a factor of 7 times in comparison to allowable area for the UBC.

Noncombustible Construction, UBC vs IBC
This second example continues the comparison for noncombustible construction types, for the same Occupancy Group B.

The results are similar: The IBC is more often than notgenerous in allowable height than the UBC. But the IBC is significantly more generous in its allowable area than the UBC, particularly for cases where the building is sprinklered. (Though not illustrated here, the same trends are true for most other occupancy groups as well.)

Comments
Reportedly, at the time of the creation of the IBC, the approach taken to establishing height and area limitations was to take the least restrictive requirements from any of the three legacy codes and apply those requirements to the new code. As it was explained to this author by one representative of the International Code Council, this decision was based on the absence of any data indicating any one of the legacy code's limits was technically superior to the others.

For any design and construction professional interested in the topic of building safety, these are interesting times. A confluence of events have brought new attention to a number of questions that have always been part of this profession, but that often may be taken for granted by many of us. To name just a few: How is building safety measured? Who decides what constitutes a reasonably safe building? Who should be responsible for ensuring the safety of buildings? This site will continue to report on developments in these areas as they arise.

More Information
Category 01 Making Buildings on this site includes articles related to regulation of construction, building codes, and the general considerations of how buildings get built. Category wtc on this site includes articles related to building safety and in particular issues arising out of the collapse of the World Trade Center towers. Many of the articles in both of the categories link to other Web resources for further coverage of these topics.

For an introduction to building codes, Occupancy Groups, Construction Types, Height and Area restrictions, and related topics, see pages 4 - 10 of the textbook.