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 "rehabilitation" and strengthening after the earthquake is called "retrofit". As already discussed on this website 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 stone masonry houses Details for improving the structural integrity of stone masonry houses The discussion is structured around the following main points Main criteria for repair and strengthening Evaluation of building damage Verification of seismic resistance for existing masonry houses Details for strengthening the structural components of stone masonry houses Details for improving the structural integrity of stone masonry houses Strengthening of foundations 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 damage mechanism. 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 [13] and [14] 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 [10]. The component strength and load patterns are initially assumed using the guides [13], [14] 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 [16], 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 [2]. The ambient vibration technique was used. Results are shown in Table 1:   Description of building Number of storeys Dimensions in plan Height 1st natural frequency E - W N - S Torsion - - [m] x [m] [m] [Hz] [Hz] [Hz] Stone, historical 2 29.3 x 13.0 7.1 3.4 4.2 5.8 Stone, historical 4 18.6 x 25.3 13 3.8 4.6 4.6 Hollow block, pre-1963 11 22.4 x 16.1 29.7 2.2 2.2 2.8 Concrete- filled block, pre-1963 13 18.6 x 17.9 37.7 1.8 2.2 2.6 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 [4], [18]. 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 [2] 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 [2], Table 2:   Mechanical property Stone masonry Brick masonry 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 stone 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, 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 additional elements are being added to the structure their foundations should be carefully detailed and constructed In this section of the document will be discussed the strengthening of stone masonry structural walls. Some of the techniques described here in detail are specific to a certain type of stone masonry, others are applicable for most types of stone masonry 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: Construction of reinforced cement or reinforced concrete jacket on one or both sides of stone walls Grouting Partial reconstruction of stone masonry walls Construction of RC tie-columns for partially collapsed stone walls   Construction of reinforced cement or reinforced concrete jacket on one or both sides of stone 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. Strengthening by jacketing of rubble or cut, dimensioned stone masonry walls is a simple and effective way to increase walls shear resistance and lateral stiffness. However instead of approximately 30 mm thick reinforced cement coatings normally applicable for brick walls, a reinforced concrete jacket is constructed with thickness of min 50 mm. Normally for this purpose reinforced concrete is poured into forms. If the thickness of coating is less than 80 mm the concrete can be applied by shotcreting. In this instance the coating is applied in two layers. After completing the first layer of shotcrete the steel mesh is installed and then is applied the second layer. The steps for reinforced concrete jacketing, applied to rubble or cut-stone stone masonry walls, as described in [2] are : In the case of plastered stone masonry, plaster is removed Loose stones are removed and any crack or voids are sealed by injecting cement grout or mortar Through holes are drilled for f8 mm rebars anchors 9/m2 - every 0.5 m Alternatively in cases of thick walls or only one-sided jackets shear connectors can be formed with reinforced concrete. For this purpose stones are removed from the walls in a regular pattern and reinforcement cages are placed in the voids spliced 400 mm to the main reinforcing mesh - see Figure 1.   Figure 1- Detail of a shear connector for bonding the RC coating to stone masonry wall (8)   The surface of the wall is treated with water jet The reinforcing mesh- 250 x 250 mm, f10 mm bars is fixed to the cross bars in position The formwork is erected and the concrete is poured into 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 is used: 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)   Figure 2- Mechanism of action of vertical and horizontal reinforcement of a masonry wall failing in shear (22)   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 2. 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. Conctruction of RC jacket to a stone masonry wall is shown on Figure 3.   Figure 3- Applying RC coating to stone masonry wall (8)   Grouting Grouting as a strengthening method can be effective for both existing and earthquake damaged stone masonry construction. Most types of stone masonry can be injected with grout although for well built cut dressed-stone coursed walls on lime-cement or cement mortar this technique may not increase the walls lateral resistance. However this technique is effective and widely used for all types of earthquake damaged masonry houses. Stone masonry walls constructed of two outer layers of undimensioned, uncut, and uncoursed stones with and infill consisting of small stones binded together with lime or clay mud mortar are common in many parts of the world. Due to the wall composition and often poor mortar quality the lateral resistance of such walls is low and is mainly achieved through the low wall slenderness - often about 6 or 7. Furthermore the outer and inner wall leaves can easily separate under lateral loading. The composition of rubble stone walls of exixsting houses can be improved by filling the voids with strong binding material, such as cement grout. The injected cement grout will bond the loose stones together achieving continium in the masonry structure. The injecting of cement grout technique has been applied on a large scale to brick masonry and later to earthquake damaged stone masonry buildings in former Yogoslavia. The grout mix consists of 90% Portland cement, PC-35 and 10% of Pozzolana, which is added in order to improve the consistency of the mixture. The ratio, water quantity to dry mix in volume varies between 1:1 and 1:0.9. The following procedures are outlined in [19]: The walls are moistened with water, Holes are drilled uniformly between the stones to a depth of at least half the wall thickness. Depending on the structure of the masonry at uniform intervals of 0.5 to 1.0 m Plastic or metalic injection tubes (nozzles) are inserted several centimeters into the wall. The nozzles are then fixed into position with fast binding mortar Grouting must start at the bottom of the wall and proceed to the top The grout is injected into the wall through the nozzles one at a time. The mix is injected into the first nozzle untill it begins to flow out of the adjacent nozzle. The pressure for injecting the material is 1 bar, maximum up to 2 bars After completing the process of injection at a nozzle its opening is sealed. According to experience so far the quantity of the dry mix necessary to thoroughly grout a stone masonry wall is about 50 - 150 kg/m3 masonry Where the masonry structure doesn't allow dense grouting the wall surfaces are treated with dry fast binding cement, to block leakages out of surface flaws and joints The injection hose is fixed to the next adjacent nozzle to proceed the grouting If there is lack of grouting equipment the effect of the gravity can be used when the building has more than one storey. The container with the grout can be placed on one of the upper storeys and pour the grout in the hose at that level. The improvement of the masonry wall properties has been verified both through lab tests and in-situ tests. The results of in-situ tests are summarised in Table 3 below:   Type of stone masonry Original Cement-grouted ftk [MPa] G [MPa] ftk [MPa] G [MPa] Two-leaf, rural, uncoursed stone 0.08 90 0.18 160 Two-leaf, urban, uncoursed stone 0.12 150 0.23 300 Compact, urban, uncoursed stone and brick 0.21 - 0.38 - Table 3- Characteristic values of mechanical properties for stone masonry before and after grouting (2)   The strengthened walls, using the grouting injection technique, not only show improved tensile strength but also substantially increased stiffness. This increase in stiffness should be included in the modelling of the building for the seismic verification calculations.   Partial reconstruction of stone masonry walls In EC 8 is specified that for brick and block masonry walls where crack width is more than 10 mm, the damaged portion of the wall should be reconstructed . In the case of cut dressed stone masonry (coursed or uncoursed) partial reconstruction is often the only option. If at least one of the wall leaves is stable the other can be reconstructed using similar type of stone with the same dimensions( thickness) and cement mortar. As the reconstruction progresses stones should be removed from the stable wall leave to allow the placement of connecting stones. These stones should be at 1 m distance apart. After construction the wall can be grouted to improve further the bonding between new and existing stone masonry.   Construction of RC tie-columns for partially collapsed stone walls In the case of rubble stone masonry, partial collapse and separation of walls at corners and T junctions, are frequently observed in post-earthquake surveys. When damage at the wall intersections has occured but the walls remained vertical, the surrounding portions of walls can be carefully demolished to allow casting of RC column between walls. Care should be taken to construct such additional columns symmetrically in plan. The casted tie-columns can be connected together by casting RC bond-beam on top of all walls.   Details for improving the structural integrity of stone 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 stone masonry houses. 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. 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. Position and distribution of steel wall ties in plan of a stone masonry house are shown on Figure 4. On Figure 5 the vertical position and distribution of steel wall ties are displayed.   Figure 4- Position and distribution of steel wall ties in plan of a stone masonry house Figure 5- Vertical position and distribution of steel wall ties for stone masonry house   The technique can be successfully applied for both rehabilitation and retrofit purposes. The steel ties can be made from ordinary reinforcing steel bars by threading them at the ends. The ties are fixed into position by being bolted to an anchor plate at each end. When installing the ties along longitudinal walls of considerable length stirrups should be placed through drilled holes in the wall to keep the ties in position. The anchor plates are made from steel and should be designed based on both structural and architectural considerations. The anchor plates can be rectangular, with an X or circular shape. For best performance ties should be installed on both sides of a wall just below the floor and bolted on a single plate. In the case of plastered masonry walls the plaster should be cut about 0.05 m deep on each side, to form a horizontal channel for placing the steel rebars. After completing the installation of the steel ties, all steel components should be protected against corrosion. Detail for tying of walls at T junction using steel ties is shown on Figure 6. On Figure 7, detail of anchoring of wall steel ties at a corner is displayed.   Figure 6- Detail for tying of walls at T junction using steel ties (3) Figure 7- Detail of anchoring of wall steel ties at a corner (3)   Based on experimental tests and experience, this technique has been evaluated and procedure for its design recommended [18]. The following recommendations are applicable for stone masonry houses with timber floors where the floor joists are anchored to perimeter walls by additional steel anchors. According to [18], the performance of ties is governed by: In case when steel ties are placed along the walls being transverse in respect to the seismic motion, the ties together with the upper part of wall perform like a bond-beam. Consequently ties together with a strip of the upper part of wall should be designed to sustain a bending moment, due to the out-of-plane wall vibration. According to this criteria both ties should be identical and designed just like longitudinal bond-beam reinforcement. In case when steel ties are placed along the walls being longitudinal in respect to the seismic motion, the ties take part in the global structural response by forming horizontal tensile( no compression) members at each floor level. A global truss system develops, where the compression forces carried from compression masonry diagonals are carried over from strorey to storey, thanks to the horizontal truss members ie. the steel ties. From the experimental studies[Seismic upgrading of old brick masonry urban houses], forces which develop in the longitudinal ties at ultimate state are close to the seismic shear force induced in the models. Consequently at least two design checks are required for each tie to estimate its min diameter. The biggest determined wall tie diameter from the design checks for all ties, in plan and elevation, should be used throughout the building. The minimum diameter of a single tie rebar can be determined by the formulae: Dmin = SQRT(4*Hu,seg/p*n*fy) , where the meaning of symbols is as follows: Dmin = the minimum tie diameter , Hu,seg = the ultimate seismic resistance of critical portion of the building , fy = the yield stress of reinforcing steel , n = the number of ties (rebars) , For more complicated buildings in plan is necessary to identify the critical portion of the structure. In this case this is the portion of the building with the highest lateral resistance capacity - Hu,seg.   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 masonry 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. 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 In the case of masonry buildings with timber floors tying the walls together at wall intersections, with steel ties, is not enough to ensure monolithic performance ,especially in the case where distance between transverse-cross walls is significant. To reduce uncoupled wall vibrations and prevent possible out-of-plane cracking or failure, the walls need to be anchored along their span with steel connectors to the floor joists. Figure 8 displays the position and distribution of floor joists steel ties as well as wall ties in plan.   Figure 8- Position and distribution of wall and floor joists steel ties in plan of a stone masonry house   A detail showing anchorage of timber floor joists to a stone masonry wall is shown on Figure 9.   Figure 9- Detail for anchoring of timber joists to stone masonry wall (2)   Another possible detail for anchoring timber floor joists to stone masonry walls is shown below (Figure 10):   Figure 10- Detail for anchoring of floor joists to masonry walls (3)   Furthermore in cases where the floor structure is weakly connected to walls this technique will prevent sliding mechanism and collapse of the floor. Depending on the structure of the timber floor and the distance between cross walls is determined the position of the steel connectors. Apart from connecting the floor joists to walls by means of steel connectors, additional diagonal ties can be installed. In the case of considerable distance between cross walls, and complications with anchoring of existing timber components to walls, a complete metal truss can be installed on top of the timber structure below the floor finish. The steel truss is placed so that its sides are in contact with at least three adjacent walls. Using steel anchor bolts the truss is connected to the walls - see Figure 11.   Figure 11- Stiffening of a large span timber floor by placement of steel truss (2)   Just like floor structures roofs should be able to provide diaphragm action and transfer the shear loads to supporting walls. Effective anchorage of the roof joists or lower chords of truss systems to the walls is achieved by casting a RC bond-beam on top of the walls at roof level together with vertically placed anchor bolts. The roof joists or lower chords of truss systems are then bolted to the bond beam through the anchor bolts.   Stiffening of floors and roofs in their plane In order to achieve distribution of seismic forces in respect to the walls stiffness the floors and/or roofs must be ideally stiff in their plane. In this case is said that the floors are acting like horizontal diaphragm. Semi-flexible or flexible floors/roofs distribute the seismic loads based on the floor to wall stiffness ratio or based on tributary wall areas. For retrofit purposes, especially when the timber floor and/or roof structure is damaged a straightforward solution is to replace it with cast in-situ reinforced concrete slab. Simultaneously with the slab are casted perimeter bond-beams on top of all walls. Installation of new RC slab is shown on Figure 12. If the perimeter bond-beams do not pass over the entire width of walls the slabs should be anchored to the walls using reinforced concrete anchors. The minimum allowed wall bearing for new RC slabs is 150mm.   Figure 12- New RC slab cast in-situ and its anchoring into the stone masonry walls (2)   In some cases the timber floor can be retained and just stiffened using thin reinforced concrete slab connected to a bond beam. For this solution the timber floor is used as a formwork for casting a thin RC slab. The steel reinforcement of the new slab needs to be securely nailed to the floor boarding. In the stone wall, stones are removed to allow casting of a bond beam together with the slab. Since the bond beam would only partially engage the walls, a through RC dowels are needed to anchor the bond beam to the walls - see Figure 13, below.   Figure 13- Construction of new thin RC slab and bond beam cast in-situ, anchored to stone masonry walls (3)   For rehabilitation purposes, when the quality of the timber floors and/or roofs is good, stiffening of the floors can be achieved by nailing timber planks perpendicular to the floor joists. Depending on access this can be done both on top and bottom faces of joists. In the case of nailing on only one face two layers of sheathing is applied, see Figure 14. Plywood sheets can also be used. Roof structures should be fully braced against lateral loads. In the case of retrofit of earthquake damaged building, damaged roofs can be removed and rebuild according to specifications for new construction.   Figure 14- Stiffening of timber floors by nailing boards or planks     Reinforcing and/or stitching the wall intersections Masonry wall intersections zones can be strengthened by constructing reinforced concrete jacket. In the case of post-earthquake repair/strengthening of masonry houses where the connection between intersecting walls has already been damaged stitching of the walls together can be done with steel elements called stitches. When using reinforced concrete the method consists of applying RC coating at corner and wall intersection zones. Often this method is applied together with the RC jacketing technique, discussed earlier. The same steps should be followed as in the method - construction of reinforced cement or RC jackets on one or both sides of wall. Additional rebar links should be provided, diagonally connecting the internal and external jacket across the corner. When jacketing wall intersections the rebar links are placed in X pattern across the corner. The links can be max 0.5 m appart. Details showing anchoring of RC jackets at corners and wall intersections are shown on Figure 15.   Figure 15- Details showing anchoring of RC jackets at corners and wall intersections (8)   If for some reason only the corners and wall intersections zones are being strengthened ensure that the lenght of the RC jacket on each side of wall is at least 3 times the wall thickness, to ensure sufficient anchorage. When using steel for stitching wall intersections are used steel strips. These should be with cross section of the order 40 x 4 mm and sufficient developing length of min 3 times the wall thickness. These anchor strips are welded or bolted to an anchor plate. To install the stitches the stones from the correspondent masonry course in the corner area are removed. Depending on the structure of the wall and its resistance stitches are placed in 0.5 - 0.75 m intervals. After the instalation of the steel strips the removed masonry units are placed back using rich cement mortar. Any cracking of masonry in the corner zone should be sealed and full grouting of the intersection zone should be completed. Strengthening of damaged corners and T junctions of masonry walls using steel stitching is shown below on Figure 16. Figure 16- Strengthening of corners and T junctions of masonry walls using steel stitching (8)   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 [19], 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.