|The technical measures required to return a building to its pre-earthquake
state are defined as "Repair". When the technical measures and interventions in the structural system improve the
seismic resistance of the building by increasing its strength and ductility this is termed as "Strengthening".
In this text, strengthening a building before an earthquake is termed as "rehabilitation"
and strengthening after the earthquake is termed as "retrofit".|
As already discussed in this guide the performance of a building subject to an earthquake motions is governed by
the inter-connectivity between structural components as well as the individual component's strength, stiffness and ductility.
Thus the details provided for repair and strengthening will be separated in two categories:
- Details for strengthening the structural components of brick/block masonry houses
- Details for improving the structural integrity of brick/block masonry houses
|The discussion is structured around the following points :|
|Main criteria for repair and strengthening|
|Whether deciding to repair and/or strengthen a particular building is a matter of assessing a number of issues. |
|Evaluation of building state/damage is an essential step and is covered in detail below.|
|Whenever possible structural analysis to assess the structural resistance to expected lateral loads should be carried out. Again guidelines are presented below under separate heading.|
|The availability of adequate technology and implementation skills are factors that would define the repair technology.
In some cases the duration of construction works and its impact on the building purposes should be considered as well.
Last but not least is the economic criteria.|
Below is shown the basic economic criteria that can be considered:
- the estimated cost of repair KC
- the cost of replacement KR
- the estimated life of the building EL
- the age of the building in years AG
|The accepted criteria include that the repair cost does not exceed 80% of the remaining
value of the building. The remaining value of the building is determined by the cost of replacement, proportionally
reduced by the years of age related to the estimated total life term. Consequently the criteria is expressed as follows:|
|KC <= 0.80*KR*((EL-AG)/EL)|
|Obviously, from engineering point of view, the main criterion for repair and strengthening
of the building is based on the seismic resistance verification. In case where the calculated resistance of the
structure considering a suitable q factor is less than the imposed loads due to an earthquake of expected intensity, strengthening
is required. The seismic resistance analysis will also define the most stressed components from the structure. |
|Evaluation of building damage|
|To beginning of document|
|The objectives of the investigation and evaluation of building damage are listed below:
- To identify structural components and elements of the lateral-force-resisting system and their performance
- To observe and record damage to the components
- To distinguish, to the extent possible, between damage caused by the earthquake and damage that may have existed before
|The following understandings are required for satisfactory evaluation of building damage: |
|Individual structural components control global performance|
Codes of practise for structural engineering seismic analysis focus on the displacements of a building, rather
than forces as the primary parameter for the quantification of building performance. This approach defines the structure as an assembly of its individual
elements. The elastic stiffness, yield stress, failure mode and ductility for each component determines its performance which in turn
determines the building's displacements. During a strong earthquake the structural components that first reach
elastic limit tend to exhibit their own dynamic vibrations and can separate from each other towards ultimate state. Therefore the stiffness,
energy dissipation as well as fundamental period of vibration of the structure as a whole are variable during the earthquake.
Therefore the evaluation of earthquake building damage should focus on the component performance to understand the
|Component damage should be analysed in the context of component behaviour|
Earthquake damage observation and experimental tests as well as numerical simulations show significant redistributions of the
seismic load between the structural components. Depending on the geometric proprotions and material properties a component
can exhibit brittle or ductile post-elastic behaviour. The specific behaviour of each component determines its damage pattern.
Consequently cracks and other signs of damage, must be analysed based on the component behaviour. For example for a component
with tension shear behaviour crack size is critical. For a component dominated from bending and exhibiting "bending" cracks
the size of the crack is not as critical.
|Considerations based on the three-dimensional response of the building|
The interpretation of earthquake damage in masonry wall buildings and RC Infilled Frames can be complicated by the three-dimensional response of the buildings as follows:
- Global horizontal torsion of the building can affect the distribution of damage to vertical elements. In references  and  there are analysis techniques that take into account this effect. However, the magnitude of the actual torsional response may differ from the estimates (actual plus accidental torsion) conventionally used for design. Careful interpretation of the distribution of damage in the field is required to interpret the torsional behaviour.
- Damage to individual elements and components can be due to actions from either, or both, orthogonal directions. For example, a shear wall element acting parallel to one orthogonal direction may include a perpendicular return at either or both ends. Damage to the perpendicular return can be due to forces in either direction and must be carefully interpreted.
- Wall elements and components are subject to both in-plane and out-of-plane earthquake forces. Cracking or other damage due to out-of-plane forces can be misinterpreted as an in-plane effect. If cracks are located on only one side of a wall element ,they may be due to out of plane forces
|Identification of structural components|
The procedures for damage evaluation focus on the earthquake resistant components of the building. The identification of these components is important step towards the overall evaluation process. The identification is based on a combination of theoretical analysis and observation of the damage itself.
The governing failure mechanism for each element in the lateral-force-resisting system is of particular importance. This analysis consists of determining the relevant stiffness and ultimate strength (bending, shear, axial) of each component to anticipate the behaviour and geometry of the mechanism that would form as the element is displaced laterally by a monotonically increasing lateral load pattern.
Strucural wall component types are summarised in Table 2-1 and Figure 2-4 in . The component strength and load patterns are initially assumed using the guides ,  and design standards.
|Verification of seismic resistance for existing masonry houses|
|To beginning of document|
|Seismic resistance verification of existing buildings follows the same
procedure as for new masonry buildings. However the wide variety of existing masonry houses their deterioration and different
quality of materials and construction complicates greatly the procedures. Specific construction details relating
to structural components interconnection as well as irregularities of structural layout should be taken into account
in the computer modelling. Physical tests are needed in order to determine the material and mechanical properties of the masonry
as well as to indicate the dynamic characteristics and energy dissipation capacity of the masonry structural system. |
According to , non-linear time-history analysis is not recommended because of the lack of reliable
models of the hysteretic behaviour of masonry walls. According to the same code of practice seismic vulnerability
evaluation can be applied for undamaged masonry houses, for evaluating the need for seismic rehabilitation. It is important that
the seismic vulnerability evaluation procedure is calibrated to the observed damage in past earthquakes.
|As outlined above in order to estimate the dynamic characteristics and
the value of the behaviour factor q, physical tests were carried out on a typical masonry buildings .
The ambient vibration technique was used. Results are shown in Table 1:|
||1st natural frequency|
|E - W
||N - S
||[m] x [m]
||29.3 x 13.0
||18.6 x 25.3
||22.4 x 16.1
||18.6 x 17.9
Table 1- First natural frequency of vibration of existing masonry buildings (2)|
|The results presented confirm once again that the fundamental period of
vibration of an undamaged low-rise masonry buildings is between 0.1 and 0.4 s. Consequently the value of the design base shear can be calculated using
the maximum spectral value from the design spectrum, bo/q. |
|The value of the behaviour factor q depends on not only the masonry system ie.-
plain, confined or reinforced but also on the floors details and global interconnection between components.
In order to estimate the value of the q factor depending on the floor-to-wall detail, physical tests were carried out on a typical
two-storey stone and brick masonry houses , . The potential seismic resistance of a masonry house is
correlated with the cumulative seismic input energy which the models had resisted. According these tests twice the
amount of input energy was needed to cause the colapse of the tied floor-to-walls masonry building than of the buildings
without ties. |
The contribution of the tying of floors to walls detail was found to be greater for the brick masonry house.
Therefore the proposed value of the q factor of 1.5 in Eurocode 8 may not be realistic for houses with flexible floors. According to  the beahaviour factor q should be taken equal to 1.0. The q factor can be taken equal to 1.5 for tied floor-to-walls masonry buildings with rigid floors.
|To determine the mechanical properties of existing masonry walls physical
tests are required either in a laboratory or in-situ. However in cases of "old" stone or brick masonry buildings with
significant deterioration of the masonry fabric in-situ tests or cutting samples and testing in a laboratory
are the only reliable methods. The load carrying capacity and elastic properties can be determined
either with non-destructive or destructive testing methods. Some specific mechanical properties of
existing masonry buildings are given in , Table 2: |
|Compressive strength, fk [MPa]
||0.3 - 0.9
||1.5 - 10.0|
|Tensile strength, ftk [MPa]
||0.08 - 0.21
||0.10 - 0.70|
|Elastic modulus, E [MPa]
||200 - 1000
||1500 - 3800|
|Shear modulus, G [MPa]
||70 - 90
||60 - 165 |
Table 2- Characteristic values of mechanical properties of existing masonry (2)|
|Details for strengthening the structural components of brick/block masonry houses|
|To beginning of document|
|When separate components of the structural system are strengthened care should be taken
to ensure uniform distribution of lateral stiffness in both directions in plan.
When constructing additional structural elements, for example new RC columns or RC shear wall between columns these should also allow for
uniform distribution of stiffness in plan and in elevation. Rules for horizontal and vertical regularity should be
followed when strengthening the original structure. Below are listed rules that should be followed to achieve satisfactory repair:
- The structural walls should be uniformly distributed in both directions in plan. The number of walls and their cross section
dimensions are based on the seismic resistance design.
- The structural walls should be bonded together and tied to ensure common action
- Floors should be capable of rigid diaphragm action
- Floors should be connected to structural walls by means of steel ties to provide restraint at top of walls against out-of-plane vibration
- The foundations must be strengthened as well to ensure adequate load transfer to the soils. In cases where RC shear walls are being added to the structure their foundations should be carefully detailed and constructed
|In this document is discussed strengthening of masonry structural brick/block walls.
Based on the initial state of the structure, the seismic resistance verification will indicate
the required degree of improving the walls strength. The specific repair or upgrade method of choice is
influenced from walls layout and type of masonry units and mortar. The following techniques will be discussed:
- Repair of cracked walls
- Repointing the joints of brick/block masonry with cement mortar
- Construction of reinforced cement or reinforced concrete jacket on one or both sides of walls
- Partial reconstruction of brick/block masonry walls
- Construction of RC tie-columns for confining of brick/block walls
|Repair of cracked walls|
|Depending on the size of the cracks different techniques and repair materials
are used. For brick/block masonry according to , cracks may be sealed with cement mortar, when crack width is between 5 and 10 mm
and the thickness of the wall is small. In cases where the wall is of normal thickness and the depth of the crack
doesn't allow sealing with mortar, cement grout, which contains admixtures against shrinkage should be injected.
For fine cracks ie. width between 0.3 - 3.0 mm. the use of epoxy resins is recommended. When the cracks are larger
than 10 mm, the cracked masonry should be reconstructed.
For crack width between 1.0 - 3.0 mm cement grout can be injected instead of epoxy resins.
The steps for cement grouting of cracks as described in  are :
- Removing of damaged plaster, if present
- At 0.3 - 0.6 m intervals are drilled holes along the crack
- The injection nozzles are fitted into the holes
- The surface of the masonry is cleaned and the cracks are sealed
- The nozzles are fixed to the wall with fast binding mortar
- Cracks are washed with water
- Cement grout mixture is composed of 9/10 of Portland cement and 1/10 of Pozzolana. When cracks width is between 5 - 10 mm fine sand can be added to the mix
- The pressure in the grout container is gradually increased up to 3 bar and is kept constant until the wall absorbs the grout
- At the end of grouting the presure is increased to about 4 bar in order to densify the injected mix and to drain trapped water
|Repair of heavily cracked brick masonry wall with RC coating is shown on Figure 1.|
Figure 1- Repair of heavily cracked brick masonry wall with RC coating (8)|
|The effect of cracks grouting has been tested in a laboratory. Acording to 
the original load-bearing capacity of masonry is recovered. However the wall stiffness usually cannot be recovered. Results from these tests are given in Table 3: |
|Ceramic block, B20
|Ceramic block, B20
|Light concrete block, B7.5
|Fly-ash block, B15
|Fly-ash block, B15
Table 3- Effect of injecting the cracks on tensile strength and shear modulus (2)|
|In cases of masonry walls with cracks wider than 10 mm, where reconstruction
is not feasible due to lack of masonry units with required dimensions or properties, RC jacket can be applied.
Before applying the jacket the cracks should be sealed with cement mortar or grouted according to the above procedure.
The jacket is applied as a concrete cover reinforced with light welded mesh f4 - 6 mm over the cracks- see the section for coating of walls. |
|Repointing the joints of brick/block masonry with cement mortar|
|In cases of brick or block masonry walls where the masonry was left unplastered
the mortar can considerably deteriorate with time especially if exposed to severe weather conditions. In addition to that
if the brick or block masonry walls built with poor quality mortar where the bed-joints are relatively leveled by replacing
part of the original mortar the mechanical properties of the walls can be improved. This technique is termed repointing
and should be carried out at only one face of the wall at a time. |
The existing mortar can be removed using high-pressure
water jet with pressure about 40 bar. The depth of mortar removal can
be max 1/3 of the wall thickness, due to the eccentricity of the compression forces in the wall, and should be verified by calculation.
After mortar removal the surface should be prepared by cleaning watering. In cases where high-pressure
water jet was used surface preparation for repair is not necessary. When the joints in the brickwork are about 10 mm
it is possible to embed rebars in the joints to improve the dutility and strength of the masonry. The rebars should be anchored at the ends of the
wall panels by bending around the corner joints or in the case of confined masonry in the tie-columns. The new cement mortar is
then installed. After curing of the mortar on the repaired side the procedure is repeated on the other face of the wall.
A detail for wall repointing is shown on Figure 2 below.
Figure 2- Repair and strengthening of brick masonry by repointing|
|Construction of reinforced cement or reinforced concrete jacket on one or both sides of walls|
|This method is mostly used in the instances where strengthening of the
wall is necessary for rehabilitation purposes or when the wall was severely cracked and damaged in the event of an earthquake.
This method is applicable to most types of masonry walls and consists of applying reinforced cement or concrete coatings
onto one or both faces of a wall. However application of coating on both sides is strongly recommended. The reinforcing material can be steel rebars mesh, ferro-cement as well as carbon fibres or other carbon
composites. The steps for reinforced cement coating, applied to brickwork or blockwork, as described in  are :
- In cases where the masonry is plastered, plaster should be removed
- Mortar is removed from the bed joints to a depth of 15 mm
- The cracks in the wall are grouted acoording to the procedure outlined above
- The wall surface is cleaned, moistened with water and spattered with cement milk
- Then the first layer is applied in the form of cement mortar with thickness 10 - 15 mm. The stength of the cement mix should be around 20 MPa
- The reinforcing mesh f4 - 6 mm at 100 - 150 mm intervals is placed
- Holes are drilled through the wall for cross ties, 4 - 6 steel bars per m2
- The steel bars( anchors) f6 mm are threaded through the holes and then grouted with cement or epoxied
- The reinforcing mesh is connected to the anchors. Minimum laps of the reinforcing mesh to be 300 mm
- The second layer of cement is applied with thickness 10 - 15 mm
|Detail showing application of RC coating to brick masonry wall is shown on Figure 3.|
Figure 3- Applying RC coating to brick masonry wall (8)|
|For reinforcing the cement coating apart from steel bars, can be used
ferro-cement, and carbon fibre or polyester fibre composites. According to experimental studies
lateral resistance of brick walls can be approved with composites reinforcement to the same extent as
with steel reinforcement. Performance of jacketed brick and block masonry walls has been tested both in
laboratory and in-situ. The importance in the detail was found to be the cross-anchoring of the jackets through the wall.
Table 4 displays test results for both brick and block masonry and different types of coating reinforcement.
Due to the substantial increase of the shear strength the bending strength of the walls with height/width ratio greater than 1.0
governed the failure mechanism. The effectiveness of the jacketing technique is greater for the weakly bonded walls. |
|In order to complete seismic resistance verification of masonry buildings when
strengthening the walls according to this technique, the following equation for determining the lateral stiffness
Ke,eq = Ke,w + Ke,coat ,
where the meaning of symbols is as follows:
Ke,eq = lateral stiffness of equivalent wall ,
Ke,w = lateral stiffness of original masonry wall ,
Ke,coat = lateral stiffness of RC coating
|Conservative estimation of the design shear resistance can be achieved
using the following equation:|
Hsd,eq = Crh*Arh*fyk/gs + Crv*Arv*fyk/gs ,
where the meaning of symbols is as follows:
He,eq = design shear resistance of equivalent wall ,
Crh = 0.9 (horizontal reinforcement capacity reduction factor) ,
Crv = 0.2 (vertical reinforcement capacity reduction factor),
Arh = area of horizontal rebars cross-section ,
Arv = area of vertical rebars cross-section ,
fyk= yield stress of reinforcing steel,
gs = 1.0 (partial safety factor for steel)
|In the above equation the design shear resistance of the jacketed wall is
calculated based on the tensile capacity of the horizontal mesh rebars and the dowel capacity of vertical mesh rebars.
The dowel mechanism is created from the vertical rebars resisting bending at the moment of shear failure.
The mechanism of action of vertical and horizontal reinforcement of a masonry wall failing in shear is represented on Figure 4.|
Figure 4- Mechanism of action of vertical and horizontal reinforcement of a masonry wall failing in shear (22)|
|The shear capacity of the masonry and the concrete of the jacket is not taken into account. The bending strength of
jacketed walls can be conservatively calculated by neglecting the contribution of the masonry and analysing
the reinforced concrete jackets as RC shear wall. The effect of reinforced coating on the lateral resistance of walls is shown below in Table 4: |
|Type of masonry
|C. block, B7.5
Table 4- Effect of reinforced coating on the lateral resistance of a wall (2)|
|Partial reconstruction of brick/block masonry walls|
|For brick and block masonry walls where crack width is more than 10 mm the damaged
masonry should be reconstructed as outlined in . The damaged part of the wall should be demolished
and reconstructed. The new masonry should be from masonry units the same type and dimensions as the existing ones.
Cement mortar should be used for the reconstruction. Steel masonry connectors can be embedded in mortar to improve
the connection between new and existing masonry as well as the bonding between the wall leaves.
When reconstructing masonry walls around openings, possible measure is to increase the strength and stiffness of those zones.
This can be achieved through construction of reinforced masonry or reinforced concrete window frames. The downside is however the subsequent unpredictable behaviour.|
|Construction of RC tie-columns for confining of brick/block walls|
|In the case of seismic rehabilitation and retrofit
of unreinforced brick/block masonry the building's integrity can be improved by improving
its structural system. In the past years, both in rural and urban areas of the world, a large number of unreinforced masonry
buildings with RC slabs have been constructed. In the cases where RC bond-beams are provided
under the floor slab the masonry of such houses can be effectively confined by casting
tie-columns at corners, wall intersections and on both sides of large openings.
Construction of new tie-column at T junction in brick masonry wall is shown on Figure 5.|
Figure 5- Construction of new tie-column at T junction in brick masonry wall|
lends itself quite naturally in the case of retrofit of earthquake damaged houses. The possibly
cracked masonry at corners and wall intersections is deconstructed and the surfaces of the
adjacent walls prepared by spraying with water. |
In order to ensure confinement of masonry
panel, the reinforcement of the new RC column should be anchored into the bond beam or welded
and the column should be footed on the already existing strip footing. If foundation doesn't exist
a footing should be cast and incorporated in the foundation of the building. The size and distribution of
longitudinal and shear reinforcement are shown on Figure 5. Normally the depth of the tie-column
is determined to fit into the wall and the other dimension is similar but not less than 0.2 m.
Care should be taken to distribute these elements uniformly in plan and elevation.
In cases when the brick/block masonry at corners and wall intersections zones is undamaged
or deconstructing it is not possible tie-columns are replaced with installation of isolated rebars.
The reinforcing bars are placed in vertical channels cut into the brickwork. This bars are anchored
to the rest of the wall by means of stirrups- see Figure 6.
Figure 6- Detail for confining of masonry walls using vertical steel ties (2)|
|Details for improving the structural integrity of brick/block masonry houses|
|To beginning of document|
|In this section of this document will be discussed details for improving the structural integrity ie. the interconnection between structural components of brick/block masonry houses.
These techniques are applicable for most types of brick/block masonry walls.
The following methods are discussed:
- Tying of walls with steel ties
- Tying of walls with RC bandage at floor/roof level
- Anchoring of floors and roofs to walls
- Stiffening of floors and roofs in their plane
- Reinforcing and/or stitching the wall intersections
|Tying of walls with steel ties|
|Steel ties have long been used for preventing lateral instability
of perimeter walls as well as for improving the integrity of corners by installing ties in
proximity to the buildings' corners. The method of tying of walls with steel ties was first tested
in Slovenia and then applied to earthquake damaged stone masonry buildings in Slovenia and Italy.|
|The specific techniques applied for tying of walls of brick/block masonry houses are the same as
the ones covered in the document titled Repair and Strengthening of Stone masonry houses.
|Tying of walls with RC bandage at floor level|
|Broad horizontal bandages at lintel or roof level are also recommended .
The steel mesh should be fixed to the brick/block wall by means of flat steel ties at 400mm
horizontal and 400mm vertical intervals. The steel ties should be embedded in the masonry wall through purposely drilled holes. After completion of
the wall a base of cement:sand mortar, mixed in 1:3 proportion and thickness 15mm, for the steel mesh should be installed. The welded steel mesh, minimum f5 @ 50x50 pitches, should be attached to the already cast strip of mortar by means of the embedded ties. Finally a second mortar layer of 15mm thickness
is installed to complete the reinforcing bandages. See Figure 13 and Figure 14 in the
Improved Adobe masonry document for specifications.
|Anchoring of floors and roofs to walls|
|The techniques for anchoring of floors and roofs to brick/block walls
are similar to the ones covered in the document titled Repair and Strengthening of Stone masonry houses.
|Stiffening of floors and roofs in their plane|
|The techniques for stiffening of floors and roofs in their plane
described in the Repair and Strengthening of Stone masonry houses
document are applicable here as well. |
|However in the case of brick/block masonry when casting bond-beams, the latter should be anchored to the walls
by means of shear connectors. The installation of perimeter beams will enforce tying of walls and
anchoring of the walls to horizontal diaphragms.|
|Reinforcing and/or stitching the wall intersections|
|The techniques for Reinforcing and/or stitching the wall intersections
described in the Repair and Strengthening of Stone masonry houses
document are applicable here as well. |
|Strengthening of foundations|
|To beginning of document|
|Measures regarding strengthening of foundations are usually taken
as part of seismic retrofit of a building. Geothechnical advice is required and specialised solutions
in cases where masonry building has been damaged due to soil failure. Soft clay sites or loose medium-densed sand
which is waterlogged may potentially liquefy and should be avoided.
In cases where no soil failure was observed foundations still may need to be strengthened when
introducing new vertical structural members like tie-columns or shear walls. Interventions to the foundation
system are also required due to deterioration of structural materials with time as well as improve the
integrity of the building.|
Existing old masonry buildings are often without foundations. The vertical loads are transferred to the soil
directly through the basement walls. In such cases construction of RC strip foundations under the basement walls
can be applied. Depending on access limitations or ownership boundaries, the new strip foundations can be constructed
by stitching to the sides of the existing basement walls or foundations. Before strengthening the existing foundations,
the walls are first consolidated by grouting. As discused in , hydrophobic
additives may be added to cement grout in order to prevent the propagation of humidity.
|Typically RC strip foundations are constructed under the walls for strengthening of basement walls
or foundations. Minimum diameter for longitudinal rebars is f16. Minimum transverse reinforcement
is to be f12 links at 0.2 m centres. All rebars must have concrete cover of min 50 mm. |