28-day compressive strength of concrete, psi Volume changes occur in lightweight concrete as in normal-weight concrete, but lightweight concrete is stabler when exposed to heat. Drying shrinkage causes the most undesirable volume changes, because it produces cracks in the surfaces of the concrete. The primary cause of drying-shrinkage cracks is excessive water in the mix. The water has two effects. First, it increases the W/C and weakens the concrete. Second, the additional water beyond that needed for hydration of the cement creates an excessive number of bleed channels to the exposed surfaces. When the cement paste undergoes normal drying shrinkage, these channels cannot provide any resistance to ingress of aggressive chemicals. Creep is an important concern for lightweight concrete, as it is for normal-weight concrete, especially for prestressed concrete. (Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI 211.2, and Guide for Structural Lightweight Aggregate Concrete, 4.17.3 Heavyweight Concrete Concretes made with heavyweight aggregates are used for shielding and structural purposes in construction of nuclear reactors and other structures exposed to highintensity radiation (see Art. 4.12). Heavyweight aggregates are used where heavyweight is needed, such as ships ballast and encasement of underwater pipes, and for making shielding concretes because absorption of such radiation is proportional to density, and consequently, these aggregates have greater capacity for absorption than those ordinarily used for normal concrete. With such aggregates, concrete weighing up to about 385 lb / ft3 can be produced. Concrete made with limonite or magnetite can develop densities of 210 to 224 lb/ft3 and compressive strengths of 3200 to 5700 psi. With barite, concrete may weigh 230 lb / ft3 and have a strength of 6000 psi. With steel punchings and sheared bars as coarse aggregate and steel shot as fine aggregate, densities of 250 to 288 lb/ft3 and strengths of about 5600 psi can be attained. Generally, grading of aggregates and mix proportions are similar to those used for normal concrete. The properties of heavyweight concrete are similar to those of normal-weight concrete. Mixing and placing operations, however, are more difficult than those for normal-weight concrete, because of segregation. Good grading, high cement content, low W/C, and air entrainment should be employed to prevent segregation. Sometimes, heavyweight aggregates are grouted in place to avoid segregation. Heavyweight concretes usually do not have good resistance to weathering or abrasion. (Recommended Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, ACI 211.1.) 4.17.4 High-Performance Concretes These concretes either have a high design strength (more than 6000 psi for normalweight concrete and 5000 psi for lightweight concrete) or will be subjected to severe service environments. The differences between high-performance concretes and normal-weight concretes is that the former have lower W/C and smaller maximum aggregate size. ACI 318 specifies the W/C and compressive strengths for concrete in severe exposures and the maximum chloride-ion content of concrete. Highperformance concrete is defined by either durability or strength-performance characteristics. Durability characteristics are resistance to freeze-thaw, scaling, abrasion, and chloride permeability. The strength characteristics have been defined in four grades as shown in Table 4.6. (See also Art. 4.17.1) High-strength, portland-cement concretes generally incorporate in the mix fly ash, silica fume, or superplasticizers, or a combination of these admixtures. A retarder is often beneficial in controlling early hydration. The W/C may be as small as 0.25. The maximum size of aggregate should generally be limited to 1/2 in. With superplasticizers, relatively high strengths can be achieved at early ages, such as 7-day strengths of normal concrete in 3 days and 28-day strengths in 7 days. Compressive strengths exceeding 10,000 psi can be achieved in 90 days. Aside from reduction in W/C, the use of superplasticizers in production of highstrength concretes does not require significant changes in mix proportioning. An increase in the range of sand content of about 5%, however, may help avoid a harsh mix. Curing is very important, because strength gain halts when water is no longer available for hydration. Also, it is important that proper quantities of air-entraining admixtures be determined by trial. Some air loss may result when melamine- or TABLE 4.6 High-Performance Concrete Strength Characteristics Characteristic Grade 1 Grade 2 Grade 3 Grade 4 Compressive strength,

With member size of particular importance of fire endurance of wood, building codes classify buildings with wood framing as heavy-timber construction, ordinary construction, or wood-frame construction. Heavy-timber construction is the type in which fire resistance is attained by placing requirements on the minimum size, thickness, or composition of all loadcarrying wood members; by avoidance of concealed spaces under floors and roofs; by use of approved connections, construction details, and adhesives; and by providing the required degree of fire resistance in exterior and interior walls. (For design procedures and typical connector details, see Design for Code Acceptance #2, Design of Fire-Resistive Exposed Wood Members, American Forest & Paper Association, Washington, D.C.) Ordinary construction has exterior masonry walls and wood-framing members of sizes smaller than heavy-timber sizes. Wood-frame construction has wood-framed walls and structural framing of sizes smaller than heavy-timber sizes. Depending on the occupancy of a building or hazard of operations within it, a building of frame or ordinary construction may have its members covered with fireresistive coverings. The fire endurance rating of protected wood-frame assemblies is measured in a standard fire endurance test, ASTM E119. Fire endurance ratings for various assembly designs can be found in listings that are published by recognized fire testing laboratories. These listings are updated on a regular basis. Alternatively, for many assemblies, a conservative estimate of the fire endurance rating can be obtained through a simplified calculation method (Design for Code Acceptance #4, Component Additive Method (CAM) for Calculating and Demonstrating Assembly Fire Endurance, American Forest & Paper Association.) This method estimates the overall fire endurance rating as the sum of fire endurance times assigned to individual components of an assembly. The interior finish on exposed surfaces of rooms, corridors, and stairways is important from the standpoint of its tendency to ignite and spread flame from one location to another. The fact that wood is combustible does not mean that it will spread flame at a hazardous rate. Code requirements to limit the flame-spread propensity of interior finishes are based on the flame-spread index (FSI) measured in the ASTM E84 tunnel test. Most wood products of sufficient thickness (1/4 in or greater) have an FSI between 76 and 200 (Class III or C). Some wood products have an FSI between 26 and 75 (Class II or B). Fire-retardant treatment can improve performance to an FSI of 25 or less (Class I or A). For an extensive list of flame spread indices of wood products, see Design for Code Acceptance #1, Flame Spread Performance of Wood Products, American Forest & Paper Association. In general, codes require Class I or A materials for enclosed vertical exits. Class II or B materials in exit access corridors, and Class III or C materials in other areas and rooms. Thus, wood products are accepted by the codes for a wide range of interior finish uses. Most codes exclude the exposed wood surfaces of heavy-timber structural members from flame-spread requirements, if the exposed area is sufficiently

1. Seismic Use Group According to the nature of Building Occupancy, each structure is assigned a Seismic Use Group (I, II, or III) and a corresponding Occupancy Importance (I) factor (I  1.0, 1.25, or 1.5). Seismic Use Group I structures are those not assigned to either Seismic Use Group II or III. Seismic Use Group II are structures whose failure would result in a substantial public hazard due to occupancy or use. Seismic Use Group III is assigned to structures for which failure would result in loss of essential facilities required for post-earthquake recovery and those containing substantial quantities of hazardous substances. 2. Site Class Based on the soil properties, the site of building is classified as A, B, C, D, E, or F to reflect the soil-structure interaction. Refer to IBC 2000 for Site Class definition. 3. Spectral Response Accelerations SS and S1 The spectral response seismic design maps reflect seismic hazards on the basis of contours. They provide the maximum considered earthquake spectral response acceleration at short period SS and at 1-second period S1. They are for Site Class B, with 5% of critical damping. Refer to the maps in IBC 2000. 4. Basic Seismic-Force-Resisting System Different types of structural system have different energy-absorbing characteristics. The response modification coefficient R in Table 5.9 is used to account for these characteristics. Systems with higher ductility have higher R values. With the above basic parameters available, the following design and analysis criteria