19-03-2012, 10:25 AM
STONE MASONRY CONSTRUCTION
BACKGROUND
Stone masonry is a traditional form of construction practiced for centuries in the regions
where stone is locally available. It is still found in old historic centers, often in buildings
of cultural and historical significance, and in developing countries where it represents
affordable and cost-effective housing construction. This construction type is present in
earthquake-prone regions of the world, such as Mediterranean Europe and North Africa,
the Middle East, India, Nepal, and other parts of Asia. The World Housing Encyclopedia
contains nine reports describing stone masonry housing construction practices in Greece
(WHE Report 16), Italy (WHE Report 28), India (WHE Report 18 and 80), Nepal (WHE Report
47 and 74), Palestinian Territories (WHE Report 49), Slovenia (WHE Report 58), and Algeria
(WHE Report 75).
STONE MASONRY BUILDINGS
Houses of this construction type are found both in urban and rural areas. There are broad
variations in their shape and the number of stories. Houses in rural areas are generally
smaller in size and have smaller openings since they are typically used by a single family.
Buildings in urban areas are often of mixed use, that is, with a commercial ground floor
and multifamily residential area above. Houses in the countryside are built as stand-alone
structures, while the neighboring houses in old town centers often share a common wall.
Rural one-story stone masonry houses with timber frames and thick flat roofs for thermal
insulation (“khan”) are widely spread throughout India. In the mountainous and hilly
regions of Nepal there are two types of stone masonry houses: a traditional ovalshaped
house and a rectangular-shaped house, both typically two stories high. In hilly
Mediterranean countries the number of stories varies between two in rural areas and
five in towns. Typically, stone masonry houses are used by the lower and middle classes,
however in historical urban centers they are often inhabited by the upper classes. Urban
masonry buildings are characterized with several internal renovations and updates in the
course of their useful life.
The main lateral and gravity load-resisting system consists of stone masonry structural
walls. The walls are generally uniformly distributed in both orthogonal directions with a
wall thickness ranging from 400 mm to 700 mm. The wall density (area of walls in one
direction versus total plan area) ranges from 5% to 25%. Stone masonry apartment
buildings in Algeria have an extremely low level of wall density (5% to 6%) despite having
up to five stories with a story height of 3.5 m.
Structural walls are made of the following:
• Rubble stone in mud/lime mortar or even without mortar (Greece, WHE Report 16;
Italy, WHE Report 28; Nepal, WHE Reports 47 and 74)
• Massive stone masonry in lime/cement mortar (Algeria, WHE Report 75; India,
WHE Report 80)
• Two exterior wythes of larger stones with rubble infill in mud/lime mortar, often
without through stones that should connect the exterior wythes (India,
Italy, Nepal, Slovenia)
• Two-wythe stone masonry walls filled with plain concrete (Palestinian Territories,
WHE Report 49)
The houses are built by local builders or by owners themselves, without any formal
training. The quality of construction in urban areas is generally superior to that found in
rural areas.
Stone Masonry Construction
Structural walls are supported either by stone masonry strip footings or there are no
footings at all. Floor structures in towns and historic centers are vaulted brick masonry
at the ground floor level and timber joists at the upper floor levels. Timber joists are
usually placed on walls without any physical connection. The original floor structures
in historic buildings have typically been replaced either by a precast joist system or by
solid reinforced concrete slabs especially in Italy (WHE Report 28) and Slovenia (WHE
Report 58). Reinforced concrete (RC) slabs are used for floor structures in the multistory
stone masonry buildings that are still being built in India. In the Palestinian Territories (WHE
Report 49), floor and flat-roof structures in the newer composite (stone and concrete)
masonry construction are solid RC slabs.
EARTHQUAKE PERFORMANCE
The most important factors affecting the seismic performance of these buildings are:
The strength of the stone and mortar
The quality of construction
The density and distribution of structural walls
Wall intersections and floor/roof-wall connections
Stone masonry construction generally shows very poor seismic performance. Poor quality
of mortar is the main reason for the low tensile strength of rubble stone masonry. Timber
floor and roof structures are usually not heavy and therefore do not induce large seismic
forces. However, typical timber floor structures are made of timber joists that are not
properly connected to structural walls. These structures are rather flexible and are not
able to act as rigid diaphragms. Due to their large thickness, stone masonry walls are
rather heavy and induce significant seismic forces.
Delamination and disintegration of the masonry are damage patterns typical for walls
built with two exterior wythes and rubble infill in weak mud mortar with many air voids.
Out-of-plane failure can occur when the connections between the exterior and interior
walls are inadequate. When the connections between the perpendicular walls are
strong, the wall shear capacity can be exhausted, thus causing typical shear cracks to
develop.
The key seismic deficiencies of the two-wythe stone masonry buildings in the Palestinian
Territories (WHE Report 49) are poor connections between the walls and slabs, as well as
a very poor bond between the concrete and stones in masonry walls. These deficiencies
result in the disintegration of wall wythes and collapse of the walls.
In the case of a one-story house in India with exterior masonry walls and an inner timber
frame supporting a heavy mud overlay, the structure could be unstable due to poor
post-to-beam connections. The presence of frames may be beneficial, however, if it
prevents the exterior walls from collapsing inwards (India, WHE Report 18).
Most vulnerable stone masonry houses in Nepal (WHE Reports 47 and 74) suffered severe
damage and the loss of integrity during past earthquakes even when the buildings were
located at a large distance from the epicenter.
The percentage of stone masonry buildings that collapsed or were damaged beyond
repair in recent earthquakes depends on the general quality of construction and the
earthquake intensity.
Figure 7: Typical damage during the 1998 Bovec
Figure 6: Typical collapse earthquake in Slovenia (WHE Report 58)
of a corner (WHE Report
16, Greece)
The percentage of collapsed masonry buildings in a few recent earthquakes is listed
below:
IMPROVED SEISMIC RESISTANCE OF NEW CONSTRUCTION
The main provisions to improve seismic resistance of stone masonry building are:
• Enhancing the building integrity by tying the walls together and ensuring that the
floor-to-wall connections are sound
• Achieving increased masonry strength by using cement mortar and throughstones
The integrity of a building can be increased by providing ring beams at the floor and roof
levels and by installing knee-braces to reinforce post-to-beam frame connections. This is
particularly true in the case of stone masonry houses in India (WHE Report 18).