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As noted in Chapter 3, building designers almost invariably start a project with a given occupancy. After a building's occupancy classification is determined, the code-analysis task becomes one of determining what heights and areas are allowable for the occupancy classification given various types of construction. These interactive criteria are set forth in Table 503 of the IBC. Economics and utility generally dictate that buildings be built using the least costly and complicated type of construction that will meet the criteria set forth in the Code.
There are two types of design choices that typically impact the use of Table 503 for determining allowable heights and areas. The first is when the design of a given occupancy must provide enough area to contain the known uses. The second choice comes into play in speculative buildings built for economic gain. The goal in these facilities, such as office buildings or retail uses, is to maximize the allowable height and area using the most economic construction type.
When addressing the question of maximizing the economically viable building size, the designer must use an iterative process to maximize the space for a given building. One must make assumptions regarding construction type and analyze the relative cost and return for various construction types before the owner can make a decision. This iterative process may also be used to maximize the economic efficiency of a building where the program size requirements are the primary consideration. In either case, the goal is to achieve the maximum area with the minimum investment in construction materials.
The organization of Chapter 5 is based upon a set of basic criteria that are then modified by mitigating factors to allow increases or tradeoffs between heights, areas, construction types, fire protection and life safety systems. Upon first reading, Table 503 may seem very restrictive, but there are modifications contained throughout the rest of the chapter that give the designer greater flexibility.
The definitions in Chapter 5 of the Code are cross-referenced from the list of definitions in Chapter 2. As with other similar definitions, they apply throughout the code, not just within Chapter 5. The definitions have very specific code-related criteria that may be different from the colloquial meanings of the terms. They should be studied carefully for applicability when determining allowable heights and areas.
Height limitations for buildings contained in planning and zoning regulations do not necessarily use the same definitions or criteria for determining heights. The definitions in the code are developed to facilitate uniform application of regulations. Read the documents that pertain to the regulations in question. Do not ever apply building-code criteria to planning issues, or vice versa.
Building Area is usually considered to include the outside face of the exterior walls of a building. This is based on the language "area included within surrounding exterior walls." The word included implies that the wall is part of the area to be considered, as opposed to using the word enclosed, which would imply the area begins at the inside face of the exterior wall. Another real-estate term often used for this area is gross building area. The safest way to calculate building area is to measure from outside face of wall to outside face of wall. This generates the most building area, and is the conservative way to determine a building's area when nearing the upper limit of allowable area.
The definitions of Basement, Grade Plane and Building Height correlate and are based upon the relationship of building parts below or above the grade plane as it is defined. Note that building height, as measured in feet, takes roof profiles into account. Building heights are measured to the average height of the highest roof, thus allowing for pitched roofs, varying parapet heights and rooftop equipment enclosures. The height is calculated using different criteria than those used to determine the height of a building in stories.
HEIGHT AND ARE LIMITATIONS
We will first discuss the standards in Table 503 as general limitations. We will then examine the allowable modifications contained in Chapter 5 of the Code.
EXCEPTIONS TO TABLE 503
§ 503.1.2 exempts low-hazard, high-bay spaces, such as mills or foundries, from the height and building area limitations of Table 503. This recognizes the special process requirements for such uses as well as their limited occupant loads with little or no public access.
§ 503.1.4 states that Type I buildings, permitted to be of unlimited height and area per Table 503, do not require the mitigations for unlimited- area building imposed upon other construction types by other sections of this chapter. Note, however, that certain Group H occupancies have height and area restrictions even for Type I buildings.
Having set the allowable limits for basic heights and areas in Table 503, the rest of the chapter addresses modifications and exceptions to the basic criteria. It is essential that the designer read the entire chapter after making the initial determination of heights and areas. Only by reading the chapter can all of the factors affecting heights and areas be determined.
Note that one set of occupancies that occurs relatively often in practice is not addressed in Table 503 and has no cross-reference to a separate set of height and area criteria. This building type/occupancy group is the open parking garage. The height and area tables for this use are contained in Table 406.3.5. This use is also restricted to the types of construction listed in that table. It is worthwhile for code users to add a hand annotation to Table 503 to highlight this missing cross-reference.
§ 505 considers mezzanines that meet the definition in § 502 to be part of the floor below them. If they meet the criteria limiting their area to one-third of the floor below, then they are not considered part of the overall building area, or as an additional story. However, the area must be counted toward the overall fire area as defined in § 702.
§ 506 specifies when the areas limited by Table 503 may be increased. The two basic factors that permit increased building areas are frontage on a public way, as defined in § 1002.1, or a complying open space and provision of a sprinkler system.
§ 507 allows buildings of other than Type I construction to be of unlimited area when certain conditions are met. These exceptions overlay Table 503 and are summarized in the table below.
§ 508 is devoted entirely to exceptions to the provisions of Table 503 and the other sections in Chapter 5. These exceptions apply only when all of the conditions in the subsections are met. These conditions are based on specific combinations of occupancy groups and construction types. These special provisions were code responses developed over time to meet construction conditions found in the jurisdictions of the model codes that preceded the IBC. These special provisions will usually give the designer greater flexibility in meeting the requirements of the building program than will Table 503 alone. These conditions should be annotated in the table for reference by the designer to be certain that they are not overlooked when commencing design and code analysis.
Group S-2 Enclosed Parking Garage with Groups A, B, M or R Above
The special provisions of § 508.2 apply to a set of conditions where an enclosed parking garage is built with other uses above. This is a very common building type. It is a mixed-use facility where the uses occur above the parking provided for the uses. The special provisions applicable to this building type have been developed over the years in response to the proliferation of apartment buildings using wood-frame construction over a Type 1 concrete garage structure. As more of these mixed-construction structures came into being, they required provisions to make code interpretations more uniform.
Refer also to § 508.5 for parking provisions beneath Group R occupancies.
Group S-2 Enclosed Parking Garage with S-2 Open Parking Garage Above
§ 508.3 section uses the same principle as that for mixed-use buildings in IBC § 508.2. The criteria are different but basically allow an open garage to be built over an enclosed garage while treating the transition between them as a new ground plane when fire-resistance requirements are met.
Special Unlimited Height for Groups B, M and R
§ 508.4 allow Type I-B buildings, with the noted uses and provided with column and roof fire-resistance as for Type I-A buildings, to be of unlimited height. The code does not mention that stories are also unlimited, but the unlimited nature of the height and story count for Type I-A buildings in Table 503 would lead to this conclusion. Note that areas for such Type I-B buildings are unlimited in Table 503 without application of this special provision.
Special Provisions for Group R-2
§ 508.6 and 508.7, written to address apartment construction issues, will apply in most cases to apartment buildings. Where fire-resistance ratings are met, fire separations are provided, and/or buildings are separated by required distances, these sections allow greater design flexibility than noted in Table 503.
Open Parking Garage Beneath Groups A, I, B, M and R
§ 508.8 allows constructing these use groups above an open parking garage and treating them as separate buildings for allowable height and area purposes. The height and area of the open garage are regulated by § 406.3, and the heights and areas of the groups above the garage are regulated by Table 503. There is a restriction that heights in feet and stories for the part of the building above the garage be measured from the grade plane for the entire group of stacked uses. Also this section requires fire-resistance to be provided for the most restrictive of the uses. It also requires egress from the uses above the garage be separated by fire-resistance-rated assemblies of at least two hours.
Excerpted from Building Codes ILLUSTRATED by Francis D. K. Ching Steven R. Winkel Copyright © 2003 by John Wiley & Sons, Inc.. Excerpted by permission.
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|2||Navigating the Code||9|
|3||Use and Occupancy||19|
|4||Special Uses and Occupancies||31|
|5||Building Heights and Areas||43|
|6||Types of Construction||55|
|10||Means of Egress||135|
|17||Building Materials and Systems||319|
Posted February 25, 2008