Damage to Structures - Fundamentals of Earthquake Engineering - page 309

Figure B.24 Shear cracks along the joint bed ( left ) and through the masonry units ( right ) of load - bearing walls
observed during the 1999 North Athens (Greece) earthquake
B.2.2 Masonry Buildings
The majority of existing masonry building stock, especially in Europe, pre-dates the introduction of
seismic provisions and hence was designed to resist vertical loads only, leading to heavy damage or
collapse under horizontal seismic loading. For example, the 1976 Friuli earthquake in the north- east of
Italy ( M S = 6.5) caused extensive damage to traditional masonry buildings, historical monuments and
churches (Braga et al ., 1977). Indeed, the combination of heavy weight and high stiffness along with
the low tensile strength of the material renders masonry structures highly vulnerable to earthquakes.
Some of the damage patterns relative to RC structures presented in the previous paragraph have also
been found for masonry structures, e.g. soft storeys, and pounding. Masonry systems can be either
engineered or non-engineered and include primarily unreinforced, confi ned and reinforced masonry.
Systems built through different construction technologies exhibit different seismic response: unrein-
forced masonry exhibits non-ductile behaviour, while confi ned and especially reinforced masonry have
enhanced strength and ductility. Their typical failure modes as found in previous earthquakes can be
classifi ed in a uniform way. In general, three groups are considered:
(i) Failure in load- bearing walls;
(ii) Failure in non- bearing walls;
(iii) Failure of wall connections.
These failure modes are analysed below, together with examples observed during past
earthquakes.
(i) Failure in Load - Bearing Walls
The most common damage patterns in load-bearing walls can be summarized as follows:
• Diagonal cracking due to shear, either through the bed joint or through the masonry units (Figure
B.24). Diagonal (shear) cracking usually begins at the corners of openings and sometimes at the
centre of wall segments (also called ' piers ' ).
• Spandrel beams between adjacent openings may be affected by diagonal shear cracking, usually prior
to cracking of piers (Figure B.25). RC bands tying the structure together at fl oor and door levels can