Masonry Structures

9.1 Introduction 9-1

9.2 Masonry in the United States 9-2

Fundamentals of Masonry in the United States • Modern Masonry Construction in the United States • Historical Structural Masonry in the United States

9.3 Fundamental Basis for Design of Masonry in the United States 9-8

Design Approaches for Modern U.S. Masonry

9.4 Masonry Design Codes Used in the United States 9-10

Introduction to Masonry Design Codes in the United States • Masonry Design Provisions of Modern Model Codes in the United States • Seismic Design Provisions for Masonry in the 2000 International Building Code • Future of Design Codes for Masonry in the United States 9.5 Seismic Retrofitting of Historical Masonry in the

United States 9-16

Observed Seismic Performance of Historical U.S. Masonry • Laboratory Performance of Historical U.S. Masonry • Basic Principles of Masonry Retrofitting • History of URM Retrofitting in the Los Angeles Area

Richard E. Klingner 9.6 Future CMkngw 9-19

Department of Civil Engineering, Performance-Based Seismic Design of Masonry Structures •

University of Texas, Increased Consistency of Masonry Design Provisions

Austin, TX References 9-19

9.1 Introduction

Masonry is traditionally defined as hand-placed units of natural or manufactured material, laid with mortar. In this chapter, the earthquake behavior and design of masonry structures is discussed, extending the traditional definition somewhat to include thin stone cladding.

Masonry makes up approximately 70% of the existing building inventory in the United States (TMS, 1989). U.S. masonry comprises Indian cliff dwellings, constructed of sandstone at Mesa Verde (Colorado); the adobe missions constructed by Spanish settlers in Florida, California, and the southwestern United States; bearing-wall buildings such as the 16-story Monadnock Building, completed in 1891 in Chicago; modern reinforced bearing-wall buildings; and many veneer applications. Clearly, the behavior and design of each type of masonry is distinct. In this chapter, fundamental applications and nomenclature of U.S. masonry are discussed, major construction categories are reviewed, historical seismic performance of masonry is presented, and principal design and retrofitting approaches are

noted. Its purpose is to give designers, constructors, and building officials a basic foundation for further study of the behavior and design of masonry.

9.2 Masonry in the United States

9.2.1 Fundamentals of Masonry in the United States

Masonry can be classified according to architectural or structural function. Each is discussed later in this chapter. Regardless of how it is classified, U.S. masonry uses basically the same materials: units, mortar, grout, and accessory materials. In this section, those materials are discussed, with reference to the national consensus specifications of the American Society for Testing and Materials (ASTM). Additional information is available at the web sites of associations such as the National Concrete Masonry Association (NCMA), the Brick Industry Association (BIA), and The Masonry Society (TMS).

9.2.1.1 Masonry Units

Of the more than 20 different classifications of masonry units commercially available in the United States, only the most widely used are discussed here.

9.2.1.1.1 Clay or Shale Masonry Units

The most common structural clay or shale masonry units are Building Brick and Facing Brick. The former are specified using ASTM C62 Building Brick (Solid Masonry Units Made from Clay or Shale). The latter, specifically intended for use when appearance is important, are specified using ASTM C216 Facing Brick (Solid Masonry Units Made from Clay or Shale). Units are usually cored rather than being completely solid. The net cross-sectional area of the unit must be at least 75% of the gross area, that is, the cores occupy less than 25% of the area of the unit.

Many different sizes and shapes of clay or shale masonry units are available, varying widely from region to region of the United States. One common size is probably the ''modular'' unit, which measures 7| in. (194 mm) long by 2j in. (57 mm) high by 3| in. (92 mm) deep. Using mortar joints | in. (9 mm) thick, this unit produces modules 8 in. (203 mm) wide by 2| in. (68 mm) high. That is, three courses of such units produce modules 8 in. (203 mm) wide by 8 in. high.

Clay or shale masonry units are sampled and tested using ASTM C67 (Methods of Sampling and Testing Brick and Structural Clay Tile). Specified properties include compressive strength and durability. Facing brick can have more restrictive dimensional tolerances and appearance requirements.

9.2.1.1.2 Concrete Masonry Units

The most common concrete masonry units are hollow load-bearing concrete masonry units, specified in ASTM C90 (Loadbearing Concrete Masonry Units). The units are typically made from low- or zero-slump concrete. In the eastern United States, these units are used for unreinforced inner wythes of cavity walls. In the western United States, these units are used for reinforced, fully grouted shear and bearing walls. The net area of the units is usually about 55 to 60% of their gross cross-sectional area. These units are commonly 15| in. (397 mm) long by 7| in. (194 mm) high by 7| in. (194 mm) thick. Using mortar joints 3 in. (9 mm) thick, this unit produces modules 8 in. (203 mm) wide by 8 in. high. These modules are compatible with those of the modular clay brick discussed above. Concrete masonry units are sampled and tested using ASTM C140 (Methods of Sampling and Testing Concrete Masonry Units). Specified properties include shrinkage, compressive strength, and absorption.

9.2.1.2 Mortar

Mortar holds units together, and also compensates for their dimensional tolerances. In the United States, mortar for unit masonry is specified using ASTM C270 (Specification for Mortar for Unit Masonry), which addresses three cementitious systems: portland cement-lime, masonry cement, and mortar cement. These cementitious systems are combined with sand and water to produce mortar.

Portland cement-lime mortar consists of portland cement and other hydraulic cements, hydrated mason's lime, sand, and water. Masonry cement mortar consists of masonry cement, sand, and water. The contents of masonry cement and mortar cement, specified under ASTM C91 and ASTM C1329, respectively, vary from manufacturer to manufacturer, and are not disclosed. They typically include portland cement and other hydraulic cements, finely ground limestone, and air-entraining and water-retention admixtures. Mortar cement has a minimum specified tensile bond strength and a lower maximum air content than masonry cement. Model codes prohibit the use of masonry cement in seismic design categories C and higher. Portland cement-lime mortars and mortar cement mortars are not restricted in this respect.

Within each cementitious system, masonry mortar is also classified according to type. Types are designated as M, S, N, O, and K (derived from every other letter of the phrase ''MaSoN wOrK''). These designations refer to the proportion of portland cement in the mixture. Type M has the most, S less, and so on. Higher proportions of portland cement result in faster strength gain, higher compressive strength, and higher tensile bond strength; they also result in lower long-term deformability. Mortar types S and N are typically specified.

Within each cementitious system, mortar can be specified by proportion or by property, with the former being the default. For example, Type S portland cement-lime mortar, specified by proportion, consists of one volume of portland cement, 1 volume of hydrated mason's lime, about 4| volumes of masons' sand, and sufficient water for good workability. Type S masonry cement mortar or mortar cement mortar is made with one volume of masonry cement or mortar cement, respectively, three volumes of mason's sand, and sufficient water for good workability.

Masonry grout is essentially fluid concrete, used to fill spaces in masonry and to surround reinforcement and connectors. It is specified using ASTM C476 (Grout for Masonry). Grout for masonry is composed of portland cement and other hydraulic cements, sand, and (in the case of coarse grout) pea gravel. It is permitted to contain a small amount of hydrated mason's lime, but usually does not. It is permitted to be specified by proportion or by property, with the former being the default. A coarse grout specified by proportion would typically contain one volume of portland cement or other hydraulic cements, about three volumes of mason's sand, and about two volumes of pea gravel.

Masonry grout is placed with a slump of at least 8 in. (203 mm), so that it will flow freely into the cells of the masonry. Because of its high water-cement (w/c) ratio at the time of grouting, masonry grout undergoes considerable plastic shrinkage as the excess water is absorbed by the surrounding units. To prevent the formation of voids due to this process, the grout is consolidated during placement and reconsolidated after initial plastic shrinkage. Grouting admixtures, which contain plasticizers and water-retention agents, are also useful in the grouting process.

If grout is specified by property (compressive strength), the compressive strength must be verified using permeable molds, duplicating the loss of water and decreased w/c ratio that the grout would experience in actual use.

9.2.1.4 Accessory Materials

Accessory materials for masonry consist of reinforcement, connectors, sealants, flashing, coatings, and vapor barriers. In this section, each is briefly reviewed.

Reinforcement consists of deformed reinforcing bars or joint reinforcement. Deformed reinforcing bars are placed vertically in the cells of hollow units, horizontally in courses of bond-beam units, or vertically and horizontally between wythes of solid units. Model codes require that it be surrounded by grout. Joint reinforcement is placed in the bed (horizontal) joints of masonry and is surrounded by mortar.

Connectors are used to connect the wythes of a masonry wall (ties), to connect a masonry wall to a frame (anchors), or to connect something else to a masonry wall (fasteners).

Sealants are used to prevent the passage of water at places where gaps are intentionally left in masonry walls. Three basic kinds of gaps (joints) are used: expansion joints are used in brick masonry to accommodate expansion, control joints are used in concrete masonry to conceal cracking due to shrinkage, and construction joints are placed between different sections of a structure.

Flashing is a flexible waterproof barrier, intended to permit water that has penetrated the outer wythe to re-exit the wall. It is placed at the bottom of each story level (on shelf angles or foundations), over window and door lintels, and under window and door sills. Flashing should be lapped, and ends of flashing should be defined by end dams (flashing turned up at ends). Directly above the level of the flashing, weepholes should be provided at 24-in. spacing. Flashing is made of metal, polyvinyl chloride (PVC), or rubberized plastic (EPDM). Metallic flashing lasts much longer than plastic flashing. Nonmetallic flashings are subject to tearing. Modern EPDM self-adhering flashing is a good compromise between durability and ease of installation.

9.2.1.5 Masonry Nomenclature by Architectural Function

The architectural functions of masonry include acting as a building envelope to resist liquid water. Masonry walls are classified in terms of this function into barrier walls and drainage walls. Barrier walls act by a combination of thickness, coatings, and integral water-repellent admixtures. Drainage walls act by the above, plus drainage details. Examples of each are shown in Figure 9.1 and Figure 9.2. In drainage walls, an outer wythe (thickness of masonry) is separated from an inner wythe of masonry or from a backup system by a cavity with drainage details.

9.2.1.6 Masonry Nomenclature by Structural Function

From the viewpoint of structural function, U.S. masonry can be broadly classified as nonload-bearing and load-bearing. The former resists gravity loads from self-weight alone, and possibly out-of-plane wind loads or seismic forces from its own mass only. The latter may resist gravity and lateral loads from overlying floors or roof. Both classifications of masonry use the same materials.

Nonload-bearing masonry includes panel walls (an outer wythe of masonry connected to an inner wythe of masonry or a backup system), curtain walls (masonry spanning horizontally between columns), and interior partitions.

Load-bearing masonry walls resist out-of-plane loads by spanning as horizontal or vertical strips, in-plane gravity loads by acting as a shallow beam-column loaded perpendicular to the plane of the wall, and in-plane shear forces by acting as a deep beam-column loaded in the plane of the wall.

Single-wythe barrier wall

Composite wall (filled collar joint)

FIGURE 9.1 Examples of barrier walls. Copyright 2005 by CRC Press

Drainage wall (brick veneer over CMU backup)

Ties

Expansion joint with sealant

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