unreinforced brick masonry

Upload: rakesh-paraliya

Post on 01-Jun-2018

248 views

Category:

Documents


1 download

TRANSCRIPT

  • 8/9/2019 Unreinforced Brick Masonry

    1/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 1

    Criteria 1 2a 2b 3

    Volume of holes(% of the gross

    volume)125-45 forclay units,

    >25-50 for concreteaggregate units

    >45-55 forclay units,

    >50-60 for concrete

    aggregate units2

  • 8/9/2019 Unreinforced Brick Masonry

    2/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 2

     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 

    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

    Mortar typeMean

    compresivestrength

    Approximate composition in parts of volume

    Cement Hydrated lime Sand

    M2 2.5 MPa 1 1.25-2.50

    2.25-3 timesceme nt 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

    Tab le 1- EC 6 requirem ents for the grouping of masonry units

     

    This classification is e mployed to se lect the corection factor K in case s where the characteristic compressive strength f k and shea

    strength f vk of the masonry are calculated on the bas is of em pirical formulae correlating norma lised compressive strength of 

    masonry units f b and m ortar f m.

    EC 8 provides further requireme nts for hollow units used for ea rthquak e resistant m asonry construction as listed:

    The units have less than 50% holes(in % of gross volume)Minimum thickness of she lls is 15mmThe 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 tobe used for masonry walls:

    Clay units: min f b=2.5 MPa

    Calcium silicate units: min f b=5.0 MPa

    Concrete units: min f b=1.8 MPa

    Autoclaved a erated concrete units: min f b=1.8 MPa

    According to the EC 8, the minimum normalised compressive strength of masonry unit, normal to the be d face, is f b=2.5 MPa.

    In the case of hollow clay units and concrete block units it is recomm ende d that the minimum com pressive strength is 7.5 MPa,espe cially for reinforced m asonry walls construction. EC 6 suggests the use o f normalised compressive s trength f b 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 themasonry unit.In the case where the strength is obtained by testing full sized un its, the m ean value of strength is multiplied by the shapefactor d, which take s into account the actual dimensions o f the unit. In case the compress ive stength of masonry is specified ascharacteristic strength, it should be first converted to the me an equivalent us ing a conversion fa ctor base d on the coefficient of variation, and than mu ltiplied by the shape factor d.

    Tab le 2, below displays shape factor d values.

     

    Table 2- Shape factor for conversion of mean value of unit's strength to normalised value (4)

     

    Mortar

    According to the specification use d in EC 6, se veral types of mortar can be us ed for masonry walls:

    General purpose mo rtar, used in joints with thickness greater than 3mm and produced with dense aggrega teThin layer mo rtar, which is designed for use in masonry with nomina l thickne ss of joints 1-3mm

    Lightweight m ortar, which is made using pe rlite, expande d clay, expa nded shale etc. Lightweight mortars typically have adry hardened density lower than 1500k g/m3.

    In Table 3 below are shown typical composition of prescribed general purpose mortar mixes and expected mean compressivestrength.

     

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    3/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 3

    Tab le 3- Typical prescribed composition and s trength of ge neral purpose m ortars (39)

     

    Mortars to be used in masonry construction in ea rthquake regions should comp ly with EC 8. According to this standard for theconstruction of plain a nd confined masonry, the minimum com pressive strength of m ortar f m is set to 5 MPa.

    Mechanical properties of m ortar are determined by testing mortar prisms 40x40x160m m (EN1015-11). The compress ivestrength of the m ortar is calculated after averaging the strength values of six specimens . The thickness of bed a nd hea d jointsis recommended to be in the range 8-15mm and all head joints should be fully filled with mortar.

     

    Definition of brick masonry construction systemsTo beginning of document

     

    Brick masonry houses are structures d efined by vertical and horizontal eleme nts, respe ctively walls and floors. Since the ma inservice loads a re applied on the floo rs the s eism ic forces will be mainly concentrated at ea ch floor level. Floors should b e rigid itheir plane to distribute the seism ic load am ong the vertical wall elements in proportion to their stiffness . Such floors arereferred to as horizontal diaphragms. However diaphragms alone will be inadequate unless good connection between them andthe supporting walls exists.Whe n constructing RC slabs, casting of bond-be am s just below floor level is econom ic and efficient solution. Good floor to wallconnection can a lso be achieved by designing stee l ties between timber floor joists and suppo rting wall.

    In EC6 a re discussed the following types of m asonry walls, as shown on Figure 2:

    Single-leaf wall- defined as a wall without continous vertical joint or cavityDouble-lea f wall- defined as a wall constitued from two pa rallel leaves and a joint between them m ax 25 mm , filled withmo rtar. The leaves can be tied together with steel wall ties to a chieve solid wall cross sectionCavity wall- defined as a wall constructed of two parallel single-leaf walls, tied together with wall ties or bed jointreinforcement. One or both leaves can be loa d-bea ring. The cavity between the leaves can be filled, or partially-filled,with non-load be aring insulation ma terialGrouted cavity wall- defined as a wall like the cavity wall but the two lea ves are spaced m in 50 m m apa rt and are tiedsecurely in place with steel wall ties and bed joint reinforcement, and with a cavity filled with concrete.

    Figure 2- C ross section of a single lea f(half brick), single leaf(whole b rick), double leaf a nd cavity wall

     

    Unreinforced- ie. Plain clay brick masonry

    Unreinforced clay brick m asonry is a traditional form for construction of low-rise house sthat has been extensively practiced inalmost every part of the world. With the increased popularity and availab ility of reinforced concrete, im proved ma sonry forms of

    construction, like confined and reinforced masonry became more common for low-rise houses. However traditional houses withload-be aring system of unreinforced bu rnt clay brick walls are still being constructed in many areas o f Asia, Indian Subcontinen

    and Latin America. This type of housing can be vulnerable to the earthquake shaking unless all rules and recommendations inthis guide a re followed.

    Brick masonry should be constructed following sim ple ins tructions for quality workmanship:

    In dry and ho t climate, m asonry units should be soaked in water before the construction in order to prevent quick dryingand shrinkage of ceme nt based m ortarsMasonry units should be asse mbled together in overlapped fashion (see Figure 3 and Figure 4 ) so that the vertical joinare staggered from course to course. To ensure adequate bonding the units should overlap by a lenght equal to 0.4times the he ight of unit or 40 m m, whichever is the greater. At the corners and wall intersections the overlap shou ld bemin the width of the units.

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm#Rossen%20Rashkoffhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    4/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 4

    Figure 3 - Flemish bond for one brick thick wall

    Figure 4- English bond for one brick thick wall

     

    Same type of masonry units and mo rtar should be used for structural walls in the same storeyBracing walls should be constructed in the same time as the loa d-bea ring wallsThe thickness o f individual walls is k ept constant from storey to storeyIn cases where general purpose m ortar is going to be used, the m ortar joints thickne ss should be be tween 8 and 15mm.

    In se ismic zones, it is recommende d that the minimum thickness of load-bea ring walls is 240 m m.To e nsure stability of walls, the ratio of the e ffective wall height to wall thickness should be max 15 .

    Ope nings in pla in masonry walls sho uld be limited to ens ure load be aring capacity. Therefore the leng th of a s tructural wallshould be at leas t 1/2 of the greater clear height of the ope nings ad jacent to the wall.

     

    Mechanical properties for verification of masonry walls

    To beginning of document

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm#Rossen%20Rashkoffhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    5/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 5

     

    This pa rt of the do cument ex plains the m echanical properties o f masonry for verification of m asonry walls. This s ection isincluded in case s where eng ineered building isrequired.

     

    Earthquake resistance of masonry walls

    In the e vent of an ea rthquak e, apa rt from the ex isting gravity loads, ho rizontal racking loads a re impose d on walls. However,the unreinforced m ason ry behaves as a brittle material. Hence if the stress state within the wall ex ceeds m asonry strength,brittle failure occurs, followed by possible collapse o f the wall and the building. The refore unreinforced mason ry walls a revulnerable to ea rthquak es, and should be confined and/or reinforced whene ver possible. Nevertheless, low-rise residential plain

    masonry construction limited to the spe cifications provided in this docume nt and including certain earthquake -resistant detailscan still be safe.

    Masonry walls resisting in-plane loads usually exhibit the following three modes o f failure:

    Sliding shea r- a wall with poor she ar strength, loaded predom inantly with horizontal forces can ex hibit this failureme chanism. Aspe ct 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 fa il in shea r. This is the most comm onmode o f failure. Aspect ratio for such walls is usually abo ut 1:1. Shea r failure can also occur for panels with bigge r aspecratio ie. 2:1 , in cases of big vertical load.Bending- this type o f failure can occur if walls a re with improved she ar resistance. For bigger aspect ratios ie . 2:1 be ndinfailure can occur due to sm all vertical loads, rather than high shea r resistance. In this m ode of failure the masonry panecan rock like a rigid body (in case s of low vertical loads).

    Failure m odes for masonry walls sub ject to in-plane loads are shown on Figure 5

     

    Figure 5- Fa ilure m odes for masonry walls subject to in-plane loads

     

    Mechanical properties

    In order to estimate the resistance of masonry walls, the following me chanical properties for the ma sonry needs to bedetermined:

    The compressive strength- f The shear strength- f vThe bending strength- f xThe stress-s train relationship, s-e

    Other essential m echanical characteristics of masonry:

    The tensile strength- f t, as an equivalent to shear strength- f vThe modulus of elasticity- EThe shea r modulus- GThe ductility factor- m

    The ductility factor is de termined only for a specific structural elem ent(specific proportions, boundary conditions etc). It cannotbe de termined for the masonry itself. Mechanical characteristics of m asonry are determined by testing standard specimens of masonry wallets and walls according to code EN 1052.

     

    Compressive strength

     

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    6/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 6

    Masonry

    unit groupMortar f 

    vko [MPa]

    Limiting f vk

    [MPa]

    1clay

    M10-M20 0.3 1.7

    M2.5-M9 0.2 1.5

    1other

    M10-M20 0.2 1.7

    M2.5-M9 0.15 1.5

    2aclay

    M10-M20 0.3 1.4

    M2.5-M9 0.2 1.2

    2a other2b clay

    M10-M20 0.2 1.4

    M2.5-M9 0.15 1.2

    Compress ve strengt s eterm ne y test ng masonry spec mens o at east 1.5 un ts engt an 3 un ts e g t or y test ngwalls of 1.0-1.8 m length and 2.4-2.7 m height.

    In cases where the masonry specimen is slender(height/thickne ss>20), lateral displacemen ts at the mid he ight of the wall aremeasured. The slenderness can be taken into account using the measured value for this displacement d and the thickness of the wall t. Thus the me asured compressive strength can be increased by the following factor:

    t/(t-d), provided the increase is no t more than 15%.

    According to EN 1052-1 three iden tical spe cime ns are tested a nd the results evaluated. In cases where the mea sured me ancompressive strength f of masonry is different from the one of its constituents( masonry units and mo rtar) by 25% the value off is modified. The characteristic compressive strength of ma sonry f k is determined as the smaller value of either f k=f/1.2 or

    f k=f min. Whe n verifying load bea ring masonry and test data is not available, the characteristic compressive strength o f plain

    masonry made with general purpose mortar may be calculated on the basis of normalised compressive strength of masonryunits f b and compressive strength of m ortar f m as follows:

    f k = K*(f b0.65)*(f m

    0.25) [MPa],

    and f m is less than 20 MPa or 2f b, whichever is the smaller. The value o f constant K depends o n the classification of m asonry

    units into groups a s per Tab le 1. Below are shown recomm ende d values for K:

    0.60 for group 1 masonry units in a wall without longitudinal mortar joint,0.55 fo r group 2a masonry units in a wall without longitudinal m ortar joint,0.50 fo r group 2b m asonry units in a wall without longitudinal m ortar joint, and for group 1 m asonry units in a wall withlongitudinal mo rtar joint,0.45 fo r group 2a masonry units in a wall with longitudinal m ortar joint,0.40 for group 2b masonry units in a wall with longitudinal mortar joint, and for group 3 masonry units

    Shear strengthShear strength of masonry is defined as a combination of initial shear strength under zero compressive load and increase instrength due to compressive stresse s perpend icular to the shear plane . Initial shea r strength at zero compressive stress isdeno ted with f vko. This property is determined according to EN 1052-3 by testing a triplet specimen such that on ly shea r stresse

    develop in the mortar to masonry unit contact planes. A minimum of five triplets are tested. The minimum acceptable value of f vko is 0.03 MPa. The characteristic shear strength of plain masonry is then calculated as follows:

    f vk = f vko+0.4*sd,

    where sd is the design compressive stress perpendicular to the shear plane. The value of sd should be greater than 0.065f b and

    a limiting value spe cified in EC 6 depe nding on m asonry unit's group and mo rtar quality. In Table 4, a re shown typical values oinitial shear strength at ze ro comp ression f vko and limiting values of characteristic shea r strength f vk .

     

    Table 4- Shear strength at zero compression f vko and limiting values of characteristic shear strength f vk (4 )

     

    Another approach exists for determining the shea r resistance o f plain m ason ry walls, that lead to virtually same results.According to this a pproach, the she ar failure of m asonry wall, ie. diagona l cracking of the wall, is cause d by the principal tensilestresses.The shear strength can be determined by reducing the masonry wall to a structural element from elastic, homogeneous andisotropic material, expe riencing plane s tress s tate. For this purpose a re evaluated the p rincipal compressive and tens ilestresse s, respectively that develop in the m iddle se ction of the wall. Thus the value of the principal tensile stresses , measuredwhen the wall panel is loaded in shear at failure, defines the tensile strength, f t. The e quations fo r principal compressive and

    the principal tensile s tresse s in plain masonry wall pane l under vertical load- N, and lateral load- H, a re :

    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 de fined as follows:

    fc = ft = 0.5*ARC TAN(2*t /so),

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    7/20

  • 8/9/2019 Unreinforced Brick Masonry

    8/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 8

    Figure 6- Ve rtical orientation of fa ilure plane a nd corresponding be nding strength normal to be d joints

    Figure 7- Ho rizontal o rientation of fa ilure plane a nd correspo nding bending strength parallel to bed joints

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    9/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 9

     

    Elastic properties

    The modulus of elasticity E of masonry can be determined after compression tests. The elastic modulus is defined as a secantmodulus 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=1000f k

    However in the calculated value o f E modulus m ay not be correct. Reliable E values are the one in the m argin:

    200f k 

  • 8/9/2019 Unreinforced Brick Masonry

    10/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 10

     

    The plan shape should be simple.Total dimension of projections, reentrant corners or recesses in one direction is limitedto 25% of the overall dimens ion of the building in the correspo nding direction -see Figure 9

    Figure 9- Examples of regular configuration of masonry houses in plan

     

    The length of a single po rtion of the building is limited to fou r times its width. In cases where longe r building is requiredanother house should be built separated from the first one at min 0.2 m -see Figure 10

    Figure 10- Irregula r configurations in plan should b e se parated in regular portions

     

    Vertical regularity is achieved by uniform distribution along the he ight of the building of stiffness and masses. Lack of vertical regula rity may lead to horizontal plane of weak ness /stress concentration and collapse .Mixed s tructural systems, such as a combination of m asonry structural walls in one level and R C frame in the nex t arenot allowed. For planning flex ibility is possible comb ined system consisting of RC columns and m asonry shea r walls. Forsuch configurations the masonry bearing walls should be reinforced and the RC members should be connected into RC

     

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    11/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 11

    Design ground a cceleration ag < 0.2 [g] 0.2 - 0.3 [g] >= 0.3 [g]

    Unreinforced m asonryH [m] 12 9 6

    n 4 3 2

    Confined Maso nryH [m] 18 15 12

    n 6 5 4

    Reinforced masonryH [m] 24 21 18

    n 8 7 6

    Design ground a cceleration 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

      .loads transfer.The floors are rigid in their plane providing diaphragm action and interconnected with masonry walls. To this end thefloors should be constructed in a single plane. In case s where large ope nings are present in the floor, such as forstairways the contour of the ope ning should be strengthened with a bond beam. Also two-way slabs are preferred to oneway slabs, a s they d istribute the vertical g ravity loads more uniformly onto the masonry walls

    Plan dimensions and height or number of storeys

    Limitations concerning the dimensions of masonry wall houses have been set in most existing seismic codes. Currently EC 8limits the construction of un reinforced(plain) ma sonry houses located in se ismic zones withag => 0.3g to only two storey houses. However for improved ma sonry systems- like confined and reinforced masonry wall

    buildings which conform with the spe cifications for structural configuration and quality of materials, the d imensions of thebuilding are not limited by the code. In this case the dimensions of the house are determined by design calculations based onthe load bea ring capacity of the masonry. The building should be verified according to ultimate limit states.

    On the other hand based on the experience from past earthquake as well as the existing technologies for masonry housingconstruction it is recomm ende d that the height and num ber of storeys conform with Table 6. The reinforced grouted cavity walltype of engineered structural masonry is exempted from these limitations.

     

    Table 6- Re commended max imum building height H and numbe r of storeys n (14)

     

    Distance between masonry bearing walls and wall openings

    In EC 8 there is no requirement for maxim um distance between walls. However based on expe rience for different type of masonry houses it is recommended that the distance between walls conform to Table 7 :

     

    Table 7- Recommended maximum distance between structural walls (6)

     

    Another esse ntial factor is the s tructural wall continuity. This m eans that the size and configuration of open ings in walls shouldbe carefully planned. Guide lines for openings in structural m asonry walls are included in the relevant Indian Standa rds - 4326.(see Figure 11)

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    12/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 12

    Figure 11- Guide lines for openings in external walls (3)

     

    General recommendations regarding the configuration and size of openings should be observed:

    Openings should be located away from portions of the wall underneath be am supports ( of the floo r or roof structure)When possible openings should be located in the less loaded wallsOpenings should be vertically aligned from storey to stroreyThe top ends of openings in the storey should be horizontally alignedOpenings should not stop continuous RC bond beams (at lintel and/or roof level)Openings sho uld be located symme trically in the plan of the building so that not to get in the way of the uniformdistribution of strength and stiffness in two o rthogonal directions.

     

    Simple houses

    To beginning of document

     

    According to EC 8 certain class o f masonry housing can be exempt from seism ic resistance verification provided that the qua lityof materials and construction rules specified in the code are met. Such houses are named "simple buildings" (Figure 12 )

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm#Rossen%20Rashkoffhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    13/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 13

    Design groundacceleration 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

    Figure 12- Simple House and bond-beams along the attic line to support gable-end walls

     

    According to EC 8 sim ple buildings are regular buildings with an approximately rectangular plan. The ratio be tween the long toshorter side o f the house is no m ore to four and the projections o r recesse s from the rectangula r shape a re not grea ter than15% of the length of the s ide pa rallel to the direction of projection. Such houses have the following limitations regarding num beof storeys above ground- Table 8 :

     

    Tab le 8- Number of storeys above ground, allowed for simple buildings (6)

     

    For a masonry house to comply with a simp le building a number of spe cifications a re given for the masonry walls. The structurawalls should be symetrically located in plan in two orthogonal directions. A minimum of two structural walls per orthogonaldirection. The length of each wall should be greater than 30% of the length of the building in the wall plane and the d istancebetween these walls should be m ax imum 75% of the size o f the building in the other direction. Furthermore for unreinforcedmasonry houses the walls in one direction sho uld be connected with transverse walls at intervals m axim um 6 .0 m. The m inimumcross sectional area of the structural walls is also specified in EC 8. At every floor, the area of the structural walls in twoorthogonal directions is provided as a percentage of the total floor area above the level considered. Table 9, be low gives theminimum horizontal structural wall cross-section :

     

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    14/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 14

    Design groundacceleration 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

    Tab le 9- Minimum horizontal structural wall cross-section, given a s % 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 b e m ax imum 20%. The difference in the ma ss of structural walls in two orthogona l directions from storey to storeyshould b e a s well ma ximum 20%. For such buildings it is also required that 75% o f the vertical load is carried from the structurawalls.

     

    Details for seismic resistance

    To beginning of document

     

    Concept

    The performance of the building subject to an ea rthquak e m otions is governed by the inter-connectivity of structuralcompone nts as well as the individual componen t's strength, stiffness a nd ductility. Thus the de tails to provide s eism ic resistanccan be classified in two categories:

    Details for complete load path

    Provide wall to wall connection ie. tying of wallsProvide means fo r walls to founda tions connectionProvide connection of bond beams to roof Provide connection of walls to bond beamsProvide stiff in their plane floo rs/roofs

    Details to improve structural components strength and ductility

    Improve the compressive strength of structural compo nentsImprove the bend ing strength of structural compone ntsImprove the shea r strength of structural components

    Improve the ductility,

    m of the structural compone nts

     

    Bond beams

    Bond-beams should b e constructed in-situ from reinforced concrete and cast simu ltaneous ly with the slab( in the case o f RCfloors). Bond-beams should be cast on top of all walls. The minimum bond beam's cross section is recommended to be150x250. The bigger dimension being the thickness of the wall. Typical examples of monolithic cast in-situ RC bond beams withRC s labs a re shown below on Figure 13.

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm#Rossen%20Rashkoffhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    15/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 15

    Figure 13- Details o f cast in-situ RC slabs with bond be ams

     

    Maximum vertical distance between bond-beams is 4 m. Bond-beams contribute to the lateral resistance in a number of ways:

    Improves the in-plane stiffness of floors to provide diaphragm actionTransfers the horizontal load from the d iaphragm to the structural wallsConnects the structural walls and provides o ut-of-plane supportForms confined m asonry shear walls in com bination with tie-columnsConnects the RC tie-columns

    In order to achieve satisfactory performance of bond-be am s a numbe r of structural mea sures shou ld be followed. EC8 specifiesthe following minimum requirements:

    Concrete of class 15 should be usedCross section size should be not less than 150x150 m m

    Four mild steel reba rs with total area 240 m m2

    To ensure integrity of the bond be am the longitudinal rebars at corners and wall intersections sho uld be sp liced a lengthof 60fTransverse reinforcemen t-stirrups rebars f6 @ 200 mm intervals

    Figure 14 illustrates bond be am reinforcement at corners

     

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    16/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 16

    Numbe r 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

    Figure 14- Detail of RC bond beam showing splicing of rebars at wall corners

     

    According to EC 8 the resistance of the RC bond-be am should no t be take n into consideration in the de sign calculations.Consequantly there is no mandatory design through calculation for the bond-beams. As was discussed in the confined masonrysection the design parameters are determined on empirical basis. In Table 10 the members reinforcement can be determinedbase d on the s eism icity of the location the num ber of stroreys and pos ition.

     

    Table 10 Recomme nded reinforcement of horizontal RC bond-beams (9)

     

    Floors and roofs

    Traditionally the m asonry buildings had a timber floor and roof. However, currently are predomina ntly used RC slabs for floors iresidential masonry construction. In EC 8 it is specified that the floor and roof structure can be constructed in timber orreinforced concrete, provided a diaphragm action can be achieved.

    Apart from developing diaphragm a ction and transfe r of the seism ic forces onto the walls the floors and roof should support thewalls out of their plane, ie. a ll structural walls should be restrained at floor/roof le vel. In the case of RC slab the connection isprovided by constructing RC bo nd beam onto the s tructural walls. In the case o f a timbe r joist floor the floor joists shou ld be tieto the walls by means of s teel ties. The anchoring of the timbe r floor joists to m asonry walls may be mo re difficult to achieve.Twisted stee l anchors anchored in the m asonry can be used to tie the joists to the walls. Timber joists can be directly anchoredto the RC bond-be am in the case when s teel ties are placed into po sition in the formwork and cast together with the bo nd-beam. De tails for anchorage o f timber floor joists to walls with steel ties, a s well as steel wall ties, u sed for tying together thewalls of existen t masonry buildings, to improve wall-to-wall conection. To find out more about this technique visit Repair andStrengthen ing of brick/block masonry house s. 

    The construction of monolithic RC slabs is recommended. The slabs are cast together with the bond beams. Floor systems madof prefabricated RC elements and cast in situ topping are not recommended.

    Whe n floors are constructed in timber special detailing is required both to e nsure diaph ragm action and to restraint the walls

    http://www.staff.city.ac.uk/earthquakes/Repairstrengthening/RSBrickMasonry.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    17/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 17

    out-of-plane . Solid strutting inline o r stagge red can be incorporated be tween joists in addition to nailing of boarding to stiffenthe floor. The boarding can be from plywood sheets and can be nailed to joists at the top and/or the bottom surface dependingon access. O ther than plywood timbe r planks can also be na iled to joists to form continous boarding as shown on Figure 15.

  • 8/9/2019 Unreinforced Brick Masonry

    18/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 18

    Figure 15- Stiffening of timber floors by nailing boards or plank s

     

    Com mon roof systems constructed in timber for low-rise m asonry housing are the joist-rafter roof and the truss roof. The joist-rafter roof system tends to spread and overturn masonry walls. The refore a collar beam a ttached to rafters is required. Toensu re diaphragm a ction bracing and blocking shou ld be constructed both in the plane of the joists and in the plane of therafters in two o thogona l directions. Only the perimeter joists a nd rafters may be included in bracing and b locking. Vertical crossbracing in the longitudinal ridge p lane( perpendiculiar to the joists) is a lso required. To achieve a satisfactory restraint on thewalls the ceiling joists should be anchored to the provided RC roof bond beam by means of stee l strap pla ced in position in thebond-be am 's formwork before casting of the bond-beam . See Figure 16

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    19/20

    5/13/13 Unreinforced Brick Masonry

    www.staff.city.ac.uk/earthquakes/MasonryBrick/PlainBrickMasonry.htm 19

    Figure 16- Timber roof anchorage to bond beam

     

    RC roofs can be also constructed. They can be both flat RC slabs or sloped s ystems cast together with the roof bond beam.These roofs can provide d iaphragm action and wall restraint however their mass is much highe r. In order to reduce se ismic loadlight roofs a re favoured. Light roof cover( tiles) sho uld be used preferably.

     

    Lintels and cantilever elements

    Lintels are load-be aring elem ents which support the weight of the wall and floo r above opening. Lintels can be made from in-situ reinforced concrete, timbe r and reinforced masonry. In seism ic zone s cast in-situ RC lintels are recomm ende d. If thedistance between the top of the opening to the top of the floor above is less than 600 mm the lintel can be cast simultaneouslywith the bond beam and floor slab a s shown on Figure 17. In cases where the distance is bigge r the lintels can be castsepa rately(Figure 17) a nd care should be taken to bond the R C lintels to the masonry of the adjoining wall through horizontalrebars.

    Figure 17- Requirements for lintels in seismic zones (9)

     

    Where the area of the ope ning 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 aminimum of 250 m m. The lintel width should be equal to the wall thickness and should not be less than 150 m m.

    Cantilever structural elements in masonry houses like balconies and various forms of overhangs are vulnerable in an event of an e arthquake . Thes e po rtions o f the structure a re iinherently flexible in vertical direction( ou t-of-plane) a nd a re prone tovibrate se parately from the rest of the structure during an ea rthquak e. In order to reduce vertical motion o f balconies,overhangs and other cantilever eleme nts the following limitations are set:

    1.20 m for cantilever slabs cast continuously with the floor slabs, and0.50 m for cantilever slabs a nchored into the bond-beams without the continuity with the floor slab

    Design o f bigger cantilevers is p ossible however a rigorous analysis is required accounting for the vertical componen t of theseism ic motion. According to EC 8 when verifying a portion o f the s tructure on the vertical componen t of se ismic mo tion a partiamodel is adequate including the cantilever element and taking into account the stiffness of the adjacent elements to ensurerealistic bounda ry conditions. According to EC 8 the response spectrum as defined in previous se ction is applicable bu t with thefollowing corrections:

    For periods T < 0.15s the o rdinates o f the spectrum a re multiplied by 0.7For periods 0.15s < T < 0.5s a linearly interpolated value be tween 0.7 and 0.5For periods T > 0.5s the ordinates o f the spe ctrum are m ultiplied by 0.5

     

    Non-load bearing elements

    Failures of non-loa d bearing elem ents, such as partition walls, chimneys, m asonry veneer, architectural details, etc, can causecasualties and structural damage. In order to prevent failure and fall-downs of masonry non-structural e leme nts their out-of-plane stability to seim ic loads should be verified by calculation according to EC8.

    http://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htmhttp://www.staff.city.ac.uk/earthquakes/MasonryBrick/References.htm

  • 8/9/2019 Unreinforced Brick Masonry

    20/20

    5/13/13 Unreinforced Brick Masonry

    Pa rtition walls are made of mo st types o f masonry units including solid ones. The usual partition walls thickne ss is about 100mm and they can be plain or reinforced. The reinforcing can be by means of rebars f4 to f6 placed in the m asonry bed jointsevery 500 mm. T he partition walls a re usua lly confined in vertical direction by the floors through cemen t base d m ortar joints. Inhorizontal direction the partitions a re confined from RC tie-columns o r structural walls through s teel a nchors or just bond .

    Whe n constructing timber ridged roof, the triangular area formed by the sloping e nds o f the roof can be filled with maso nryforming a gable end wall. Out-of-plane failures of gable end walls are common during strong earthquakes and therefore requirespecial consideration. It is recomm ende d that masonry gable end walls and attics higher than 0.5 m are anchored to theuppermost floor bond-beams. The gable e nd walls should be confined by a bo nd beam running along the roof line. In caseswhere the height of the gable end wall is more than 4 m, intermidiate bond-beams should be added not more than 2m apart.As discussed in the confined masonry section the maximum distance between vertical confining elements is 4 m.

    For architectural purposes external solid walls can be constructed as faced o r veneered walls. The faced wall is built with differen

    masonry units bonded together to achieve common action under loading. Veneered walls has facing attached, but not bonded tthe backing leaf. The load applied to veneered wall is assumed to be carried by the backing leaf only which is designed on thebasis o f no structural contribution from the venee r. The venee r can be anchored by means o f steel ties to the backing masonrywall. No spe cific requireme nts can be found in EC 8 however its stability can be verified using the fo rmulaes 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 chimneyis not built of reinforced m asonry an effective so lution might be to deconstruct it and complete it in reinforced m asonry orreplace it altogether with a lighter metal chimney. In the case of reinforced m asonry chimney the rebars should be anchored intthe top floor. Architectural details, like cornices, vertical or horizontal cantiliver projections, etc., should be reinforced andanchored into the ma in RC s trucure. The out-of-plane beha viour should be verified by calculation according to the guidanceprovided for partition walls.