Adobe   Stone   Brick   Concrete Block             Unreinforced Brick masonry       Confined Brick masonry       Reinforced Brick masonry       Glossary, notations and useful knowledge       References       The basic rules to be followed and various requirements to be satisfied for masonry construction are specified in the codes of practice for structural masonry construction. This portion of the guide is based on the European structural design codes - EC6 and EC8. Eurocode 6 specifies the rules and provisions for structural masonry. Additional provisions to be considered for masonry construction in earthquake prone areas are outlined in Eurocode 8. In this portion of the website are given specifications and recommendations for an improved type of masonry construction- reinforced masonry. The information aims at improving plain and confined brick masonry. Non-Engineered Reinforced masonry applied to low-rise housing is quite common in many parts of India, Asia and Latin America. Reinfoced masonry is popular as well for engineered masonry construction. It is wide spread in North America, Australia and New Zealand ,UK and some other areas. However this type of construction requires good quality control due to the amount of hidden work and use of concrete infill - grout and isolated reinforcement.The following main points should be covered for an earthquake resistant construction : Materials for reinforced masonry construction Definition of reinforced masonry construction systems Mechanical properties for verification of masonry walls Planning and layout Simple Houses Details for seismic resistance Seismic resistance verification of masonry buildings Materials for reinforced masonry construction   Masonry units EC6 gives specifications regarding the use of following masonry units: Fired clay units Fired clay lightweight units Calcium silicate units Concrete block units Lightweight concrete block units Autoclaved aerated concrete units The properties of masonry units should comply with the requirements of relevant European standards (EN 771-1-6). Masonry units are classified into the following types: solid, perforated unit, hollow unit, cellular unit, horizontally perforated unit- see Figure 1. Figure 1- Types of masonry units (7)   Solid masonry units are either units without recesses or units with recesses that are filled with mortar during construction, or units with up to 25% by volume of vertical holes. Considering the total volume of holes, volume of any hole, area of any hole, as well as combined thickness of webs and shells, EC6 provides the following classification- Table 1:   Criteria 1 2a 2b 3 Volume of holes (% of the gross volume)1 <=25 >25-45 for clay units, >25-50 for concrete aggregate units >45-55 for clay units, >50-60 for concrete aggregate units2 <=70 Volume of any hole(% of the gross volume) <=12.5 <=12.5 for clay units, <=25 for concrete aggregate units <=12.5 for clay units, <=25 for concrete aggregate units2 Limited by (see below) Area of any hole Limited by volume (see above) Limited by volume (see above) Limited by volume (see above) <=2800mm2 except units with a single hole when the hole should be <=18000mm2 Combined thickness (% of the overall width)3 >=37.5 >=30 >=20 No requirement Notes: 1. Holes may consist of formed vertical holes through the unit or frogs or recesses. 2. If there is national experience, based on tests, that confirms that the safety of the masonry is not reduced unacceptably when a higher proportion of holes is incorporated, the limit of 55% for clay units and 60% for concrete aggregate units may be increased for masonry units that are used in the country with national experience. 3.The combined thickness is the thickness of webs and shells, measured horizontally across the unit at right angles to the face of the wall Table 1- EC 6 requirements for the grouping of masonry units   This classification is employed to select the corection factor K in cases where the characteristic compressive strength fk and shear strength fvk of the masonry are calculated on the basis of empirical formulae correlating normalised compressive strength of masonry units fb and mortar fm. EC 8 provides further requirements for hollow units used for earthquake resistant masonry construction as listed: The units have less than 50% holes(in % of gross volume) Minimum thickness of shells is 15mm The vertical webs in hollow and cellular units extend over the entire horizontal length of the unit In the relevant European standards (EN 771-1-6) are given minimum mean values of compressive strength of masonry units to be used for masonry walls: Clay units: min fb=2.5 MPa Calcium silicate units: min fb=5.0 MPa Concrete units: min fb=1.8 MPa Autoclaved aerated concrete units: min fb=1.8 MPa Manufactured stone units: min fb=15 MPa According to the EC 8, the minimum normalised compressive strength of masonry unit, normal to the bed face, is fb=2.5 MPa. In the case of hollow clay units and concrete block units it is recommended that the minimum compressive strength is 7.5 MPa, especially for reinforced masonry walls construction. EC 6 suggests the use of normalised compressive strength fb for design. This is the mean value determined by testing of at least ten equivalent, air dried, 100 mm by 100 mm specimens cut from the masonry unit. In the case where the strength is obtained by testing full sized units, the mean value of strength is multiplied by the shape factor d, which takes into account the actual dimensions of the unit. In case the compressive stength of masonry is specified as characteristic strength, it should be first converted to the mean equivalent using a conversion factor based on the coefficient of variation, and than multiplied by the shape factor d (Table 2). Shape factor d for conversion of mean value of unit's strength to normalised value (EC6)   Height [mm] Least horizontal dimension [mm] 50 100 150 200 >250 50 0.85 0.75 0.70 - - 65 0.95 0.85 0.75 0.70 0.65 100 1.15 1.00 0.90 0.80 0.75 150 1.30 1.20 1.10 1.00 0.95 200 1.45 1.35 1.25 1.15 1.10 >250 1.55 1.45 1.35 1.25 1.15 Table 2- Shape factor for conversion of mean value of unit's strength to normalised value (4)   Mortar According to the specification used in EC 6, several types of mortar can be used for masonry walls: general purpose mortar, used in joints with thickness greater than 3mm and produced with dense aggregate thin layer mortar, which is designed for use in masonry with nominal thickness of joints 1-3mm lightweight mortar, which is made using perlite, expanded clay, expanded shale etc. Lightweight mortars typically have a dry hardened density lower than 1500kg/m3. In Table 3 below are shown typical composition of prescribed general purpose mortar mixes and expected mean compressive strength.   Mortar type Mean compresive strength Approximate composition in parts of volume Cement Hydrated lime Sand M2 2.5 MPa 1 1.25-2.50 2.25-3 times cement and lime M5 5 MPa 1 0.50-1.25 M10 10 MPa 1 0.25-0.50 M20 20 MPa 1 0-0.25 Table 3- Typical prescribed composition and strength of general purpose mortars (39)   Mortars to be used in masonry construction in earthquake regions should comply with EC 8. According to this standard for the construction of plain and confined masonry, the minimum compressive strength of mortar fm is set to 5 MPa. When mortar is to be used for reinforced masonry the minimum compressive strength is 10 MPa since the rebars are embedded in mortar. The bond strength is specified as a function of type of rebar and type of mortar. The recommended values of characteristic bond strength fbok are specified in Table 4 below:   Mortar M5-M9 M10-M14 M15-M19 M20 fbok for plain bars [MPa] 0.7 1.2 1.4 1.5 fbok for high- bond bars [MPa] 1 1.5 2.0 2.5 Table 4- Characteristic anchorage bond strength of reinforcement in mortar (4)   Mechanical properties of mortar are determined by testing mortar prisms 40x40x160mm (EN1015-11). The compressive strength of the mortar is calculated after averaging the strength values of six specimens. The thickness of bed and head joints is recommended to be in the range 8-15mm and all head joints should be fully filled with mortar.   Concrete infill In the case of reinforced masonry construction care should be taken to ensure the properties of the concrete infill (or grout). According to specifications provided in EC 6, the maximum aggregate size should not exceed 10mm where the least dimension of the void is 50mm and the rebars cover is between 15 and 25mm. The maximum aggregate size should not exceed 20mm where the least dimension of the void is 100mm and the rebars cover is more than 25mm. The values of characteristic compressive strength of the concrete infill fck and the values of characteristic shear strength of concrete infill fcvk are specified in table 5 below:   Strength class of concrete C12/15 C16/20 C20/25 C25/30 or stronger fck [MPa] 12 16 20 25 fcvk [MPa] 0.27 0.33 0.39 0.45 Table 5 Characteristic compressive fck and shear strength fcvk of concrete infill (4)   Reinforcing steel Steel bars are used as reinforcement in the case of reinforced masonry (Figure 2). Figure 2- Typical ladder and truss type of prefabricated bed joint reinforcement   According to EC 6, reinforcing steel may be assumed to possess adequate elongation ductility, if the following requirements are satisfied: for high dutility class: euk > 5% and (ft/fy)k > 1.08 for normal dutility class: euk > 2.5% and (ft/fy)k > 1.05, where: e uk= the characteristic value of the unit elongation at max tensile stress, ft= tensile strength of rebar steel, fy= yield strength of rebar steel, (ft/fy)k = the characteristic value of ft/fy In the case where high bond rebars with diameter less than 6mm is used it should not be considered as having high ductility. When prefabricated ladder-type or truss-type bed joint reinforcement is used it should be considered as having normal ducility.   Definition of reinforced masonry construction systems To beginning of document   As discussed in the Confined masonry document, the confinement of plain masonry walls greatly improves both the strength and the ductility. However as research and experience from past earthquakes have shown that to fully employ the resistance and energy dissipation capacity of masonry, the plain masonry has to be reinforced. A construction system where steel reinforcement is embedded in the mortar joints of masonry or placed in holes and after filled with concrete or grout is called Reinforced masonry. There are various practices and techniques to achieve reinforced masonry. According to the ways in which reinforcement is arranged, reinforced masonry can be classified into three types: Reinforced hollow unit masonry Reinforced grouted cavity masonry Reinforced pocket type walls The most common type is the reinforced hollow unit masonry. Units from group 2a and 2b are used for this purpose. This construction type is discussed in the Concrete block reinforced masonry section. The second type of reinforced masonry walls- the reinforced grouted cavity masonry is the recognised earthquake resistant type of masonry. It consists of a cavity masonry wall constructed from group 1 masonry units. Into the cavity is placed a steel mesh providing the vertical and horizontal rebars. In order to achieve integrity of the wall the two leaves are connected by means of standard wall ties or rebars. The size and number of connecting ties are determined according to design calculations. However at least 4f6 rebar links or an equivalent wall ties per m2 of wall area should be provided. After completition of the reinforcement details the cavity is grouted or infilled with concrete. The leaves are usually one masonry unit thickness (about 100 mm) and the size of the cavity is 60 to 100 mm wide. The concreting of the cavity can be done in steps after construction of each course or in one operation after the masonry walls in the whole storey have been laid. Before grouting the cavity, all mortar droppings on foundations or rebars should be removed from the previous grout stop to ensure proper bonding. To achieve satisfactory grouting vent holes should be formed in the wall to allow for the air to escape and facilitate removing away mortar debris at the bottom of each grout stop. The vent holes are formed as work proceeds and these can be in the form of open mortar joints or masonry units left out. Reinforced grouted cavity masonry wall construction is shown on Figure 3.   Figure 3- Reinforced grouted cavity masonry construction   The third type of reinforced masonry walls-the reinforced pocket type walls is common for engineered structural masonry construction. Vertical wall reinforcement can be placed in vertical ducts( pockets) formed between solid or hollow masonry units. This is the case when so called "quetta bond"( a brick and a half wall thickness bond) is constructed. In "quetta bond" close spacing of vertical rebars is possible. The reinforced pocket type masonry aslo allows for forming reinforced masonry columns, where ducts of bigger size can accommodate multiple bars as well as stirrups for concrete infill or grout confinement. For this type of reinforced masonry the vertical rebars are placed into position ideally before the laying of masonry units. Horizontal reinforcement is placed in the bed joints at vertical spacing maximum 600 mm. The vertical reinforced ducts are filled with concrete or grout as the costruction of the wall progresses. Proper planning is necessary to ensure rebar splices lengths, anchoring lengths, correct cover and keeping the concrete infill or grout surface of each grout stop clean from mortar debris. Reinforced pocket cavity masonry wall construction is shown on Figure 4   Figure 4- Reinforced pocket cavity masonry construction   In order to achieve durability of the reinforced wall it is essential to ensure rebar protection against corrosion or fire damage. For this purpose is required that the reinforcement has sufficient concrete/grout cover. For unprotected steel in dry, humid or aggressive environment the cover should be respectively 20 ,25 and 40 mm thick. For all three types of reinforced masonry to be constructed in seismic regions reinforcement specifications are provided in EC 8. According to this code the minimum percentage of horizontal reinforcement, referred to as the gross area of the section should be min 0.05%. The min percentage for vertical reinforcement is not specified, however according to EC 8 are required rebars with cross-sectional area min 400 mm2 placed at free edges of walls and at every wall intersection. Reinforced with rebars zones of the masonry wall should be max 4 m apart. Limitation of the size of horizontal rebars is required to achieve good embedment in the mortar. It is recommended that rebar diameter is max 6 mm when placed in standard 10 mm bed joint. The effectiveness of the reinforcement however strongly depends on the type and quality of the masonry ie. masonry units and mortar. When subject to seismic load the bond between the rebars and mortar deteriorates. Consequently high tensile stresses and yielding in rebars cannot be develop preventing ductile behaviour and energy dissipation. For certain hollow masonry units premature crushing of face shells under cyclic lateral load may occur even in cases where the compressive strength of the units is good. In order to achieve a ductile behaviour of masonry is necessary that the shear strength of the wall is greater than the bending strength to ensure bending failure. Therefore increased amount of vertical reinforcement at the edges of wall may not improve the resistance of the wall particularly with weak masonry units. Thus the minimum percentage of reinforcement, either vertical or horizontal, depends on the strength of the masonry units. The maximum percentage of reinforcement should also be limited based on the strength of the masonry units and mortar such that a ductile bending failure is possible. The requirements for anchoring and lapping of reinforcement are similar to those specified for reinforced concrete structures. All reinforcement should be anchored to allow for the stresses in the bar to develop. On way to achieve economic anchorage is to terminate the rebar past the point where it is no longer required. This is called straight anchorage. According to EC 8 straight anchorage is not allowed for rebars with diameter more than 8 mm. Ec 6 provides the following formulae to calculate the anchorage length lb: lb = (f/4)*(fyk/cs)*(cM/fbok) where the meaning of symbols in the above equations are as follows: f - the diameter of reinforcing bar, fyk - the characteristic strength of reinforcing steel, fbok - the characteristic anchorage bond strength, cs, cM - the partial safety factors When anchorage is achieved by hook ending the anchorage length for rebars in tension can be reduced by 30%. Lapping of rebars is necessary to facilitate construction and progress of the works. The provision of laps should be considered by the designer. When lapping bars through staggering care is needed to avoid rebars congestion which can result in poor workmanship. The required lap length is determined from the formulae discussed above. In the equation however the diameter of the smaller of the two bars participates. Depending on the detail the lap length provided should be equal to : lb for bars in compression and for bars in tension where less than 30% of the bars in the section are lapped, and where the clear distance between the lapped bars in transverse direction is not less than 10f and the mortar or concrete cover is not les than 5f. 1.4lb for bars in tension where either 30% or more of the bars at the section are lapped, or if the clear distance between lapped bars in transverse direction is less than 10f , or the mortar or concrete cover is less than 5f. 2lb for bars in tension where both 30% or more of the rebars at the section are lapped, and the clear distance between the lapped bars in a transverse direction is less than 10f or the mortar or concrete cover is less than 5f. Typical anchorages of reinforcing bars are shown on Figure 5. Figure 5- Typical anchorages of reinforcing bars (4)     Mechanical properties for verification of masonry walls To beginning of document   This part of the document explains the mechanical properties of masonry for verification of masonry walls. This section is included in cases where engineered building is required.   Earthquake resistance of masonry walls In the event of an earthquake, apart from the existing gravity loads, horizontal racking loads are imposed on walls. However, the unreinforced masonry behaves as a brittle material. Hence if the stress state within the wall exceeds masonry strength, brittle failure occurs, followed by possible collapse of the wall and the building. Therefore unreinforced masonry walls are vulnerable to earthquakes, and should be confined and/or reinforced whenever possible. Masonry walls resisting in-plane loads usually exhibit the following three modes of failure (see Figure 6): Sliding shear- a wall with poor shear strength, loaded predominantly with horizontal forces can exhibit this failure mechanism. Aspect ratio for such walls is usually 1:1 or less (1:1.5) Shear- a wall loaded with significant vertical load as well as horizontal forces can fail in shear. This is the most common mode of failure. Aspect ratio for such walls is usually about 1:1. Shear failure can also occur for panels with bigger aspect ratio ie. 2:1, in cases of big vertical load. Bending- this type of failure can occur if walls are with improved shear resistance. For bigger aspect ratios ie. 2:1 bending failure can occur due to small vertical loads, rather than high shear resistance. In this mode of failure the masonry panel can rock like a rigid body (in cases of low vertical loads).   Figure 6- Failure modes for masonry walls subject to in-plane loads   Lateral resistance and ductility of plain masonry walls can be improved by reinforcing the masonry with steel. Reinforcing bars can be placed horizontally in the bed joints and embedded with mortar. Vertical reinforcing bars can be placed in hollow block masonry channels. The contribution of vertical and horizontal reinforcement to the resistance of the wall, falling in shear, is shown on Figure 7. The shear strength of such reinforced wall depends on the tension capacity of horizontal steel, dowel action of vertical steel, arching of masonry and interlocking of crack surfaces.   Figure 7- Mechanism of action of vertical and horizontal reinforcement of a masonry wall failing in shear (11)   Mechanical properties In order to estimate the resistance of masonry walls, the following mechanical properties for the masonry needs to be determined: The compressive strength- f The shear strength- fv The bending strength- fx The stress-strain relationship, s-e Other essential mechanical characteristics of masonry: The tensile strength- ft, as an equivalent to shear strength- fv The modulus of elasticity- E The shear modulus- G The ductility factor- m The ductility factor is determined only for a specific structural element(specific proportions, boundary conditions etc). It cannot be determined for the masonry itself. Mechanical characteristics of masonry are determined by testing standard specimens of masonry wallets and walls according to code EN 1052.   Compressive strength Compressive strength is determined by testing masonry specimens of at least 1.5 units length and 3 units height or by testing walls of 1.0-1.8 m length and 2.4-2.7 m height. In cases where the masonry specimen is slender(height/thickness>20), lateral displacements at the mid height of the wall are measured. The slenderness can be taken into account using the measured value for this displacement d and the thickness of the wall t. Thus the measured compressive strength can be increased by the following factor: t/(t-d), provided the increase is not more than 15%. According to EN 1052-1 three identical specimens are tested and the results evaluated. In cases where the measured mean compressive strength f of masonry is different from the one of its constituents( masonry units and mortar) by 25% the value of f is modified. The characteristic compressive strength of masonry fk is determined as the smaller value of either fk=f/1.2 or fk=fmin. When verifying load bearing masonry and test data is not available, the characteristic compressive strength of plain masonry made with general purpose mortar may be calculated on the basis of normalised compressive strength of masonry units fb and compressive strength of mortar fm as follows: fk = K*(fb0.65)*(fm0.25) [MPa], and fm is less than 20 MPa or 2fb, whichever is the smaller. The value of constant K depends on the classification of masonry units into groups as per Table 1. Below are shown recommended values for K: 0.60 for group 1 masonry units in a wall without longitudinal mortar joint, 0.55 for group 2a masonry units in a wall without longitudinal mortar joint, 0.50 for group 2b masonry units in a wall without longitudinal mortar joint, and for group 1 masonry units in a wall with longitudinal mortar joint, 0.45 for group 2a masonry units in a wall with longitudinal mortar joint, 0.40 for group 2b masonry units in a wall with longitudinal mortar joint, and for group 3 masonry units Shear strength Shear strength of masonry is defined as a combination of initial shear strength under zero compressive load and increase in strength due to compressive stresses perpendicular to the shear plane. Initial shear strength at zero compressive stress is denoted with fvko. This property is determined according to EN 1052-3 by testing a triplet specimen such that only shear stresses develop in the mortar to masonry unit contact planes. A minimum of five triplets are tested. The minimum acceptable value of fvko is 0.03 MPa. The characteristic shear strength of plain masonry is then calculated as follows: fvk = fvko+0.4*sd, where sd is the design compressive stress perpendicular to the shear plane. The value of sd should be greater than 0.065fb and a limiting value specified in EC 6 depending on masonry unit's group and mortar quality. In Table 4, are shown typical values of initial shear strength at zero compression fvko and limiting values of characteristic shear strength fvk .   Masonry unit group Mortar fvko [MPa] Limiting fvk [MPa] 1 clay M10-M20 0.3 1.7 M2.5-M9 0.2 1.5 1 other M10-M20 0.2 1.7 M2.5-M9 0.15 1.5 2a clay M10-M20 0.3 1.4 M2.5-M9 0.2 1.2 2a other 2b clay M10-M20 0.2 1.4 M2.5-M9 0.15 1.2 Table 4- Shear strength at zero compression fvko and limiting values of characteristic shear strength fvk (4)   Another approach exists for determining the shear resistance of plain masonry walls, that lead to virtually same results. According to this approach, the shear failure of masonry wall, ie. diagonal cracking of the wall, is caused by the principal tensile stresses. The shear strength can be determined by reducing the masonry wall to a structural element from elastic, homogeneous and isotropic material, experiencing plane stress state. For this purpose are evaluated the principal compressive and tensile stresses, respectively that develop in the middle section of the wall. Thus the value of the principal tensile stresses, measured when the wall panel is loaded in shear at failure, defines the tensile strength, ft. The equations for principal compressive and the principal tensile stresses in plain masonry wall panel under vertical load- N, and lateral load- H, are : sc = SQRT((so/2)2+(b*t)2)+so/2 , st = SQRT((so/2)2+(b*t)2)-so/2 , And the plane of the principal stresses is defined as follows: fc = ft = 0.5*ARCTAN(2*t/so), where the meaning of symbols in the above equations are as follows: so = N/Aw - average compressive stress due to vertical load N, t = H/Aw - average shear stress due to lateral load H, Aw - the horizontal cross section area of the wall, b - the shear stress distribution factor, depending on the geometry of the wall and N/Hmax ratio. For a wall with geometrical aspect ratio height/length=1.5, b=1.5 . Hmax - the maximum resistance of masonry wall The principal tensile stress that develop in the wall at the moment of maximum resistance- Hmax is called the tensile strength of masonry: ft = st = SQRT((so/2)2+(b*tHmax)2)-so/2 , In the above equation ft is the tensile strength of masonry and tHmax- the average shear stress in the wall at the attained maximum resistance Hmax The lateral resistance Hs,w of a plain masonry wall panel, loaded in shear is evaluated by : Hs,w = Aw*(ft/b)*SQRT((so/ft)+1) When the resistance envelope is bilinear relationship, the above equation is multiplied by a factor of 0.9. If the design value of the shear resistance Hsd,w should be correlated with the design seismic action, in the above equation take part the characteristic value of tensile strength and a material partial safety factor : Hsd,w = Aw*(ftk/cM*b)*SQRT((sdcM/ftk)+1) There is currently no standard testing procedure for evaluating the shear strength fv or tensile strength ft. One possibility is to use monotonic diagonal compression test. Another test is subjecting the wall panel to monotonic or cyclic racking load. The effect of compressive stresses in the masonry is taken into account in these tests. Table 5 shows values of characteristic tensile strength of masonry -ftk correlated with values for the initial shear strength at zero compressive stress- fvko   Unit [MPa] Group Mortar [MPa] Strength [MPa] ftk fvko 10 1 - clay 0.5 0.04 0.10 15 1 - clay 2.5 0.18 0.20 7.5 2a - clay 2 0.30 0.10 15 2a - clay 2.5 0.12 0.20 15 2a - clay 5 0.18 0.20 7.5 2a - other 5 0.27 0.15 7.5 2a - other 5 0.27 0.15 7.5 2b - clay 3 0.10 0.20 Table 5- Correlation between experimental characteristic tensile strength ftk and initial shear strength fvk0 of masonry (14)   By analysing test results it has been established that the ratio between the tensile and compressive strength of any type of masonry varies in the following margins: 0.03fk <= ftk <= 0.09fk   Bending strength In cases where the masonry needs to be verified for out-of-plane loads the bending strength is the governing factor. The bending strength parallel to bed joints (see Figure 9) is denoteed with fx1 and the bending strength perpendicular to bed joints (see Figure 8) is denoted with fx2. According to EC 6 the value of fx1 should be taken as zero when evaluating seismic resistance. Figure 8- Vertical orientation of failure plane and corresponding bending strength normal to bed joints Figure 9- Horizontal orientation of failure plane and corresponding bending strength parallel to bed joints   Elastic properties The modulus of elasticity E of masonry can be determined after compression tests. The elastic modulus is defined as a secant modulus at service load condition. This load level corresponds to 1/3 of the maximum vertical load. When determined by testing E modulus value is not available the following equation may be used : E=1000fk However in the calculated value of E modulus may not be correct. Reliable E values are the one in the margin: 200fk <= E <= 2000fk Theoretically and as specified in EC 6 the G modulus is evaluated as being 40% of the E modulus. In reality the values of shear modulus G are much lower. Reliable G values are the one in the margin: 1000ftk <= G <= 2700ftk The discrepancy between experimental and predicted values for the mechanical properties of masonry can be explained with the composite nature of the material. There are wide variety of not only masonry units but also mortars and various composition of the masonry wall itself. Therefore the testing of masonry is essential step in seismic resistance verification of masonry houses.   Planning and layout To beginning of document   Surveys of earthquake damaged residential masonry wall houses and analysis of the causes of damage indicated that, well tied buildings with well defined, continuous load path to the foundations perform much better in earthquakes than building lacking such features. Well defined, continuous load path can be achieved with regular structural layout and uniformity both in plan and elevation. The degree of symmetry is also found to have a significant influence on earthquake resistance. Damage can be five to ten times worse in irregular buildings compared to regular ones. Thus satisfactory seismic behaviour can be guaranteed by following the requirements for regular and uniform layout both in plan and elevation, interconnectivity between structural members and strength of materials. To summarise an earthquake resistant structural form for masonry wall structure is the one which is: Regular both in plan and elevation i.e. uniform and symmetrical Redundant - capable of providing adequate resistance even after a failure of a structural member With rigid floors interconnected with walls that ensure diaphragm action Stable foundation should be provided able to transmit the maximum seismic loads from the superstructure to the foundation soil Masonry buildings with horizontal irreguliarities and lack of symmetry may have considerable eccentricity between the mass centre and stiffness centre giving rise to damaging coupled lateral/torsional response. Horizontal irregularities in the form of extensions, projections etc. may cause stress concentration and local failures since these extensions are prone to vibrate separetely from the rest of the structure. On the other hand vertical irregularity in masonry building may cause stress concentration at a horizontal plane that can lead to total collapse. In order to achieve satisfactory redundancy at least to lines of load bearing walls are required in each principal direction of the building. Lack of rigid floors will prevent proportionate load transfer onto walls at each floor level as well as will not provide out of plane restraint. Not supported masonry walls at floor level tends to separate at corners and/or fail out of their plane, causing collapse of floor or roof. According to EC 8 the following general criteria for structural regularity in plan and elevation should be considered: The building structure is approximately symmetrical along each principal axis in plan, for both stiffness and mass distribution. A sufficient number of load bearing walls with approximately the same stiffness, should be provided in both principal direction of the building -see Figure 10 Figure 10- Structural walls distribution in plan   The plan shape should be simple.Total dimension of projections, reentrant corners or recesses in one direction is limited to 25% of the overall dimension of the building in the corresponding direction -see Figure 11 Figure 11- Examples of regular configuration of masonry houses in plan   The length of a single portion of the building is limited to four times its width. In cases where longer building is required, a separation joint is necessary. The separation should be min 50 mm - Figure 12 Figure 12- Irregular configurations in plan should be separated in regular portions   Vertical regularity is achieved by uniform distribution along the height of the building of stiffness and masses. Lack of vertical regularity may lead to horizontal plane of weakness/stress concentration and collapse. Mixed structural systems, such as a combination of masonry structural walls in one level and RC frame in the next are not allowed. For planning flexibility is possible combined system consisting of RC columns and masonry shear walls. For such configurations the masonry bearing walls should be reinforced and the RC members should be connected into RC floors forming frames. The vertical reinforcement of the masonry shear wall should be anchored into the floor to ensure loads transfer. The floors are rigid in their plane providing diaphragm action and interconnected with masonry walls. To this end the floors should be constructed in a single plane. In cases where large openings are present in the floor, such as for stairways the contour of the opening should be strengthened with a bond beam. Also two-way slabs are preferred to one-way slabs, as they distribute the vertical gravity loads more uniformly onto the masonry walls Plan dimensions and height or number of storeys Currently EC 8 limits the construction of reinforced brick masonry houses located in seismic zones with high seismic risk ie. ag => 0.3g to six storey houses. In the same time for reinforced brick masonry wall buildings which conform with the specifications for structural configuration and quality of materials, the dimensions of the building are not limited by the code. In this case the dimensions of the building are determined by design calculations based on the load bearing capacity of the masonry. The building should be verified according to ultimate limit states. Height and number of storeys should conform with Table 8. The reinforced grouted cavity wall type of engineered structural masonry is exempted from these limitations.   Design ground acceleration ag < 0.2 [g] 0.2 - 0.3 [g] >= 0.3 [g] Unreinforced masonry H [m] 12 9 6 n 4 3 2 Confined Masonry H [m] 18 15 12 n 6 5 4 Reinforced masonry H [m] 24 21 18 n 8 7 6 Table 8- Recommended maximum building height H and number of storeys n (14)   Distance between masonry bearing walls and wall openings In EC 8 there is no requirement for maximum distance between walls. However based on experience for different type of masonry houses it is recommended that the distance between walls conform to Table 9 :   Design ground acceleration ag < 0.2 [g] 0.2 - 0.3 [g] >= 0.3 [g] Unreinforced masonry [m] 10 8 6 Confined Masonry [m] 15 12 8 Reinforced masonry [m] 15 12 8 Table 9- Recommended maximum distance between structural walls (6)   Another essential factor is the structural wall continuity. This means that the size and configuration of openings in walls should be carefully planned. The following recommendations regarding the configuration and size of openings should be observed: Openings should be vertically aligned from storey to strorey The top ends of openings in the storey should be horizontally aligned Openings should not stop continuous RC bond beams (at lintel and/or roof level) Openings should be located symmetrically in the plan of the building so that not to get in the way of the uniform distribution of strength and stiffness in two orthogonal directions.   Simple houses To beginning of document   According to EC 8 certain class of masonry housing can be exempt from seismic resistance verification provided that the quality of materials and construction rules specified in the code are met. Such houses are named "simple buildings". According to EC 8 simple buildings are regular buildings with an approximately rectangular plan. The ratio between the long to shorter side of the house is no more to four and the projections or recesses from the rectangular shape are not greater than 15% of the length of the side parallel to the direction of projection. Such houses have the following limitations regarding number of storeys above ground (Table 10):   Design ground acceleration ag < 0.2 [g] 0.2 - 0.3 [g] >= 0.3 [g] Unreinforced masonry 3 2 1 Confined Masonry 4 3 2 Reinforced masonry 5 4 3 Table 10- Number of storeys above ground, allowed for simple buildings (6)   For a masonry house to comply with a simple building a number of specifications are given for the masonry walls. The structural walls should be symetrically located in plan in two orthogonal directions. A minimum of two structural walls per orthogonal direction. The length of each wall should be greater than 30% of the length of the building in the wall plane and the distance between these walls should be maximum 75% of the size of the building in the other direction. The minimum cross sectional area of the structural walls is also specified in EC 8. At every floor, the area of the structural walls in two orthogonal directions is provided as a percentage of the total floor area above the level considered. Table 11 below gives the minimum horizontal structural wall cross-section :   Design ground acceleration ag < 0.2 [g] 0.2 - 0.3 [g] >= 0.3 [g] Unreinforced masonry 3 5 6 Confined Masonry 2 4 5 Reinforced masonry 2 4 5 Table 11- Minimum horizontal structural wall cross-section, given as % of the total floor area above the level considered (6)   To enforce reguliarity, the difference in structural walls cross-sectional area in two orthogonal directions from storey to storey should be maximum 20%. The difference in the mass of structural walls in two orthogonal directions from storey to storey should be as well maximum 20%. For such buildings it is also required that 75% of the vertical load is carried from the structural walls.   Details for seismic resistance To beginning of document   Concept The performance of the building subject to an earthquake motions is governed by the inter-connectivity of structural components as well as the individual component's strength, stiffness and ductility. Thus the details to provide seismic resistance can be classified in two categories: Details for complete load path Provide wall to wall connection ie. tying of walls Provide means for walls to foundations connection Provide connection of bond beams to roof Provide connection of walls to bond beams Provide stiff in their plane floors/roofs Details to improve structural components strength and ductility Improve the compressive strength of structural components Improve the bending strength of structural components Improve the shear strength of structural components Improve the ductility, m of the structural components   Bond beams Bond-beams should be constructed in-situ from reinforced concrete and cast simultaneously with the floor slab. Bond-beams should be cast on top of all structural walls at every floor level. The minimum bond beam's cross section is recommended to be 150x250. The bigger dimension being the thickness of the wall. Typical examples of monolithic cast in-situ RC bond beams with RC slabs are shown below on Figure 13. Figure 13- Details of cast in-situ RC slabs with bond beams   Maximum vertical distance between bond-beams is 4 m. Bond-beams are constructed because: Forms confined masonry shear walls in combination with tie-columns Improves the in-plane stiffness of floors to provide diaphragm action Transfers the horizontal load from the diaphragm to the structural walls Connects the structural walls together and provides out-of-plane support Connects the RC tie-columns EC8 specifies the following minimum requirements: Concrete of class 15 should be used Cross section size should be not less than 150x150 mm Four mild steel rebars with total area 240 mm2 To ensure integrity of the bond beam the longitudinal rebars at corners and wall intersections should be spliced a length of 60f Transverse reinforcement-stirrups rebars f6 @ 200 mm intervals Figure 14 illustrates bond beam reinforcement at corners   Figure 14- Detail of RC bond beam showing splicing of rebars at wall corners   According to EC 8 the resistance of the RC bond-beam should not be taken into consideration in the design calculations. Consequantly there is no mandatory design through calculation for the bond-beams. As was discussed in the confined masonry section the design parameters are determined on empirical basis. In Table 12 the members reinforcement can be determined based on the seismicity of the location the number of stroreys and position.   Number of storeys Position (storey) Low: < 0.2 [g] Moderate: 0.2 - 0.3 [g] High: >= 0.3 [g] 2 1-2 4 bars, f8 mm 4 bars, f10 mm 4 bars, f12 mm 4 1-2 4 bars, f10 mm 4 bars, f12 mm 4 bars, f14 mm 4 2-4 4 bars, f8 mm 4 bars, f10 mm 4 bars, f12 mm 6 1-2 4 bars, f12 mm 4 bars, f14 mm 4 bars, f16 mm 6 3-4 4 bars, f10 mm 4 bars, f12 mm 4 bars, f14 mm 6 5-6 4 bars, f8 mm 4 bars, f10 mm 4 bars, f12 mm Table 12 Recommended reinforcement of horizontal RC bond-beams (9)   Tie-columns Although the tie-columns and bond beams do not provide frame system adequate splicing and anchoring of rebars is required at all joints. Sixty rebar diameters splices are required according to EC 8. The cross-sectional area of rebars for tie-columns can be selected in dependence of seismicity of the location and number of storeys in the house. Such data is presented Table 5 of the Confined brick masonry document. Horizontal reinforcement placed in the bed joints (at vertical spacing of maximum 600 mm) should be anchored in the tie-columns as shown on Figure 15. Figure 15- Anchoring of bed joint reinforcement to a tie-column at a corner   Floors and roofs In EC 8 it is specified that the floor and roof structure can be constructed in timber or reinforced concrete, provided a diaphragm action can be achieved. When building reinforced masonry houses RC floor slabs cast in-situ are preferred. Apart from developing diaphragm action and transfer of the seismic forces onto the walls the floors and roof should support the walls out of their plane, ie. all structural walls should be restrained at floor/roof level. In the case of RC slab the connection is provided naturally by constructing RC bond beam onto the structural walls. In the case of a timber joist floor the floor joists should be tied to the walls by means of steel ties. The anchoring of the timber floor joists to masonry walls may be more difficult to achieve. Therefore the construction of monolithic RC slabs is recommended. Floor systems made of prefabricated RC elements and cast in situ topping are not recommended. Common roof systems constructed in timber for low-rise masonry housing are the joist-rafter roof and the truss roof. The joist-rafter roof system tends to spread and overturn masonry walls. Therefore a collar beam attached to rafters is required. To ensure diaphragm action bracing and blocking should be constructed both in the plane of the joists and in the plane of the rafters in two othogonal directions. Only the perimeter joists and rafters may be included in bracing and blocking. Vertical cross bracing in the longitudinal ridge plane( perpendiculiar to the joists) is also required. To achieve a satisfactory restraint on the walls the ceiling joists should be anchored to the provided RC roof bond beam by means of steel strap placed in position in the bond-beam's formwork before casting of the bond-beam. See Figure 16 Figure 16- Timber roof anchorage to bond beam   RC roofs can be also constructed. They can be both flat RC slabs or sloped systems cast together with the roof bond beam. These roofs can provide diaphragm action and wall restraint however their mass is much higher. In order to reduce seismic loads light roofs are favoured. Light roof cover( tiles) should be used preferably.   Lintels and cantilever elements Lintels are load-bearing elements which support the weight of the wall and floor above opening. Lintels can be made from in-situ reinforced concrete, timber and reinforced masonry. In seismic zones cast in-situ RC lintels are recommended. If the distance between the top of the opening to the top of the floor above is less than 600 mm the lintel can be cast simultaneously with the bond beam and floor slab as shown on Figure 17. In cases where the distance is bigger the lintels can be cast separately (Figure 17) and care should be taken to bond the RC lintels to the masonry of the adjoining wall through horizontal rebars. Figure 17- Requirements for lintels in seismic zones (9)   Where the area of the opening is more than 2.5 m2, tie-columns are required on both sides of opening. The reinforcement of lintels should be anchored into the rc tie-columns. It is also recommended that lintels should be embedded in the walls a minimum of 250 mm. The lintel width should be equal to the wall thickness and should not be less than 150 mm. Cantilever structural elements in masonry houses like balconies and various forms of overhangs are vulnerable in an event of an earthquake. These portions of the structure are iinherently flexible in vertical direction( out-of-plane) and are prone to vibrate separately from the rest of the structure during an earthquake. In order to reduce vertical motion of balconies, overhangs and other cantilever elements the following limitations are set: 1.20 m for cantilever slabs cast continuously with the floor slabs, and 0.50 m for cantilever slabs anchored into the bond-beams without the continuity with the floor slab Design of bigger cantilevers is possible however a rigorous analysis is required accounting for the vertical component of the seismic motion. According to EC 8 when verifying a portion of the structure on the vertical component of seismic motion a partial model is adequate including the cantilever element and taking into account the stiffness of the adjacent elements to ensure realistic boundary conditions. According to EC 8 the response spectrum as defined in previous section is applicable but with the following corrections: For periods T < 0.15s the ordinates of the spectrum are multiplied by 0.7 For periods 0.15s < T < 0.5s a linearly interpolated value between 0.7 and 0.5 For periods T > 0.5s the ordinates of the spectrum are multiplied by 0.5   Non-load bearing elements Failures of non-load bearing elements, such as partition walls, chimneys, masonry veneer, architectural details, etc, can cause casualties and structural damage. In order to prevent failure and fall-downs of masonry non-structural elements their out-of-plane stability to seismic loads should be verified by calculation according to EC 8. Partition walls are made of most types of masonry units including solid ones. The usual partition walls thickness is about 100 mm and they can be plain or reinforced. The reinforcing can be by means of rebars f4 to f6 placed in the masonry bed joints every 500 mm. The partition walls are usually confined in vertical direction by the floors through cement based mortar joints. In horizontal direction the partitions are confined from RC tie-columns or structural walls through steel anchors or just bond. When constructing timber ridged roof, the triangular area formed by the sloping ends of the roof can be filled with masonry forming a gable end wall. Out-of-plane failures of gable end walls are common during strong earthquakes and therefore require special consideration. It is recommended that masonry gable end walls and attics higher than 0.5 m are anchored to the uppermost floor bond-beams. The gable end walls should be confined by a bond beam running along the roof line. In cases where the height of the gable end wall is more than 4 m, intermidiate bond-beams should be added not more than 2m apart, see Figure 18. As discussed in the Confined masonry section the maximum distance between vertical confining elements is 4m. Figure 18- Provision of bond beams and tie-columns to secure gable end walls and attics   For architectural purposes external solid walls can be constructed as faced or veneered walls. The faced wall is built with different masonry units bonded together to achieve common action under loading. Veneered walls has facing attached, but not bonded to the backing leaf. The load applied to veneered wall is assumed to be carried by the backing leaf only which is designed on the basis of no structural contribution from the veneer. The veneer can be anchored by means of steel ties to the backing masonry wall. No specific requirements can be found in EC 8 however its stability can be verified using the formulaes applied to out-of-plane stability of partition walls. Heavy masonry chimneys and ventilation stacks represent a considerable hazard in the event of an earthquake. If the chimney is not built of reinforced masonry an effective solution might be to deconstruct it and complete it in reinforced masonry or replace it altogether with a lighter metal chimney. In the case of reinforced masonry chimney the rebars should be anchored into the top floor. Architectural details, like cornices, vertical or horizontal cantiliver projections, etc., should be reinforced and anchored into the main RC strucure. The out-of-plane behaviour should be verified by calculation according to the guidance provided for partition walls.   Seismic resistance verification of masonry buildings To beginning of document   This portion of the reinforced brick masonry section is included in cases where engineered building is required. No specific procedures and/or algorithms for seismic resistance analysis and verification are outlined in EC 8. The following calculation procedures for seismic resistance verification based on linear analysis are usually required: The weight, W=mass*9.81 at each floor level is calculated based on the characteristic value of permanent action( ie.self-weight of the structure) and portion of the characteristic value of variable load The stiffness of individual structural walls in the storey under consideration is calculated Analysing a structural wall with rectangular cross section as an element being fixed at the floors the following formulae for the bending stiffness is obtained: Ke = G*Aw/1.2*h(1+a'*(G/E)*(h/l)2) [Force/displacement], where the meaning of symbols is as follows: Ke = effective stiffness defined as being the ratio between the resistance and displacement of the wall at crack limit, h = the height of the wall, l = the width of the wall, G = the shear modulus of masonry infills, E = modulus of elasticity, Aw = the area of the cross section of the wall, a' = coefficient based on the location of the inflection point in the deformed shape of the wall. a'=0.83 in the case of fixed-ended wall and a'=3.33 in the case of a cantilever wall. Figure 19- Deformed shape of a fixed-ended wall subjected to lateral loading   The period of the fundamental mode of vibration is calculated. In most cases for low-rise masonry buildings the first period is between 0.1 and 0.4 s and calculation for it is not necessary From the design response spectrum, Sd(T), based on the soil parameter, S, the damping correction coefficient h( h=1 at 5% viscous damping) and first period T is determined the spectral value S The design base shear force is calculated from the equation: Fbd = Sd(T)*W [Force] , where the meaning of symbols is as follows: Sd(T) = the ordinate of the design response spectrum, W = the weight of the building The base shear is distributed vertically in proportion to the shape of the first vibration mode Fid = Fbd*(si*Wi/Ssj*Wj) [Force], where the meaning of symbols is as follows: Fid = the design horizontal seismic force acting at i-th storey, si = the displacement of mass mi in the first mode shape, sj = the displacement of mass mj in the first mode shape, Wi = the weight of mass of i-th storey, mi, Wj = the weight of mass of j-th storey, mj, Often is justified to approximate the shape of the fundamental mode of vibration with an inverse triangular distribution : Fid = Fbd*(zi*Wi/Szj*Wj) [Force], where the meaning of symbols is as follows: zi = the height of mass mi above the level of application of seismic loads, zj = the height of mass mj above the level of application of seismic loads, Figure 20- Vertical distribution of base shear   The storey shear is distributed horizontally in between the structural walls in proportion to their stiffness The design values of action effects are determined for each wall by combining the characteristic values of relevant actions The design resistance of wall sections is calculated and compared to the design action effects To beginning of document For information about this web site, click here. 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