17-01-2013, 03:33 PM
FOR LOW-RISE CONFINED MASONRY BUILDINGS
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Introduction
Scope and Objectives
The purpose of this document is to:
• Explain the mechanism of seismic response of confined masonry buildings for in- and out-ofplane
seismic effects and other relevant seismic response issues,
• Recommend prescriptive design provisions for low-rise buildings related to the wall layout and
density, and prescribe minimum size requirements for structural components of confined
masonry buildings (tie-columns, tie-beams, walls), reinforcement size and detailing, and
• Provide a summary of the seismic design provisions for confined masonry buildings from
relevant international codes.
This document is divided into three chapters. Chapter 1 provides an overview of confined masonry
construction and its components. It discusses the seismic performance of confined masonry
buildings in past earthquakes, and is based largely on the publication Earthquake-Resistant
Confined Masonry Construction (Brzev, 2008). Chapter 2 presents general requirements related to
confined masonry construction. Chapter 3 outlines a guideline for low-rise non-engineered confined
masonry buildings (up to two stories high). These buildings could be constructed without
engineered design performed by qualified engineers or architects, and thus no design calculations
or procedures are included. Many single-family dwellings are built in this manner.
Although this guide is focused on low-rise confined masonry buildings, medium-rise engineered
buildings of this type (up to five stories high) can be designed and built following the
recommendations of this document and other relevant international codes and standards. However,
note that additional analysis and design procedures and requirements for engineered confined
masonry buildings are outside the scope of this document.
It is expected that this guide will be a useful resource for design engineers and architects,
academics, code development organizations and non-governmental organizations in countries in
which design codes and standards do not contain seismic design provisions for confined masonry
construction. This document may also be a useful reference for design engineers and other
professionals in the countries where code design provisions for confined masonry construction are
currently in place.
What is Confined Masonry Construction?
Key Components of a Confined Masonry Building
Confined masonry construction consists of masonry walls and horizontal and vertical reinforced
concrete (RC) confining elements built on all four sides of a masonry wall panel, as shown in Figure
1. Vertical elements, called tie-columns, resemble columns in RC frame construction except that
they tend to be of far smaller cross-sectional dimensions. Most importantly, these RC members are
built after the masonry wall has been completed. Horizontal elements, called tie-beams, resemble
beams in RC frame construction but they are not intended to function as conventional beams since
confined masonry walls are load-bearing. Alternative terms, horizontal ties and vertical ties, are
sometimes used instead of tie-beams and tie-columns.
The key features of structural components of a confined masonry building are discussed below:
• Masonry walls transmit the gravity load from the slab(s) above down to the foundation (along
with the RC tie-columns). This document addresses confined masonry construction consisting
of masonry walls made of solid clay bricks, hollow clay tiles, or concrete blocks. The walls act
as bracing panels, which resist horizontal earthquake forces acting in-plane. The walls must be
confined by RC tie-beams and tie-columns and should not be penetrated by significant
openings to ensure satisfactory earthquake performance.
• Confining elements (RC tie-columns and RC tie-beams) are effective in improving stability and
integrity of masonry walls for in-plane and out-of-plane earthquake effects. These elements
prevent brittle seismic response of masonry walls and protect them from complete
disintegration even in major earthquakes. Confining elements, particularly tie-columns,
contribute to the overall building stability for gravity loads.
• Floor and roof slabs transmit both gravity and lateral loads to the walls. In an earthquake, floor
and roof slabs behave like horizontal beams and are called diaphragms. The roof slabs are
typically made of reinforced concrete (see Figure 1 a), but light-weight roofs made of timber or
light gage steel as shown in Figure 1 b are also used.
• Plinth band transmits the load from the walls down to the foundation. It also protects the ground
floor walls from excessive settlement in soft soil conditions and the moisture penetration into the
building.
• Foundation transmits the loads from the structure to the ground.
Confined Masonry and Similar Building Technologies
Confined masonry building technology is somewhat similar to both reinforced masonry and
reinforced concrete frame construction with infill walls. It should be noted, however, that differences
between these building technologies are significant in terms of construction sequence, complexity,
and seismic performance. Since features of confined masonry construction practice are not well
known on a global scale, a comparison of these building technologies is presented next.
RC Frames with Masonry Infill Walls and Confined Masonry: A Comparison
The appearance of a finished confined masonry construction and a RC frame infilled with masonry
wall panels may look alike, however these two construction systems are substantially different, as
illustrated in Figure 3 (note that Figure 3a shows features of RC frames with infills, while Figure 3b
shows confined masonry construction). The main differences are related to i) the construction
sequence, and ii) the manner in which these structures resist gravity and lateral loads.
The differences related to the construction sequence are as follows:
• In confined masonry construction, masonry walls are constructed first, one story at a time,
followed by the cast in-place RC tie-columns. Finally, RC tie-beams are constructed on top
of the walls, simultaneously with the floor/roof slab construction.
• In RC frame construction infilled with masonry wall panels, the frame is constructed first,
followed by the masonry wall construction.
Seismic Response of Confined Masonry Buildings
Performance of Confined Masonry Buildings in Past Earthquakes
Confined masonry construction has evolved through an informal process based on its satisfactory
performance in past earthquakes. The first reported use of confined masonry construction was in
the reconstruction of buildings destroyed by the 1908 Messina, Italy earthquake (M 7.2), which
killed over 70,000 people. Over the last 30 years, confined masonry construction has been
practiced in Mediterranean Europe (Italy, Slovenia, Serbia), Latin America (Mexico, Chile, Peru,
Colombia, Argentina, and other countries), the Middle East (Iran, Algeria, Morocco), South Asia
(Indonesia), and the Far East (China). It is important to note that confined masonry construction is
widely used in countries and regions of extremely high seismic hazard. Several examples of
confined masonry construction around the world, from Argentina, Chile, Iran, Peru, Serbia and
Slovenia, are featured in the World Housing Encyclopedia (EERI/IAEE, 2000).
Well built confined masonry buildings were able to survive the effects of major earthquakes without
collapse and in most cases without significant damage. Confined masonry tends to be quite
forgiving of minor design and construction flaws, as well as material deficiencies provided that the
buildings have regular floor plan and sufficient wall density. Poor seismic performance has been
noted only where gross construction errors, design flaws, or material deficiencies have been
introduced in the building design and construction process. Poor performance is usually associated
with insufficient amount of confined masonry walls in one or both plan directions, inadequate size of
the tie-columns.