Reinforced brick masonry

7/30/2019 Reinforced brick masonry

1/21

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


2/21

hole should be

=37.5 >=30 >=20 No requirement

Notes:1. Holes may consist of form ed vertical holes through the unit or frogs or recesses.2. I f t here is national experience, based on tests, that confirm s that t he safety of t he masonry is not reducedunacceptably when a higher proportion of holes is incorporated, the limit of 5 5% for clay units and 60% forconcrete aggregate units may be increased for m asonry units t hat are used in the country with nationalexperience.

3.The combined thickness is the thickness of webs and shells, measured horizontally across the unit at rightangles to t he face of t he 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

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 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 to be used formasonry 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 reinforcedmasonry 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 befirst 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)

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 aggregatethin layer mortar, which is designed for use in masonry with nominal thickness of joints 1-3mmlightweight 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.

orced Brick Masonry http: //www.staff.ci ty.ac.uk/earthquakes/MasonryBrick/Reinforced

1 1/28/2010


3/21

M2 2.5 MPa 1 1.25-2.50

2.25-3 timescement 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

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

Strength classof concrete

C12/15 C16/20 C20/25C25/30 orstronger

fck [MPa] 12 16 20 25

fcvk [MPa] 0.27 0.33 0.39 0.45

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 ofplain and confined masonry, the minimum compressive strength of mortar f

mis 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 ofrebar and type of mortar. The recommended values of characteristic bond strength fbok are specified in Table 4 below:

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 themortar is calculated after averaging the strength values of six specimens. The thickness of bed and head joints is recommended to be in therange 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 tospecifications provided in EC 6, the maximum aggregate size should not exceed 10mm where the least dimension of the void is 50mm and therebars cover is between 15 and 25mm. The maximum aggregate size should not exceed 20mm where the least dimension of the void is100mm 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:

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


1 1/28/2010


4/21

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:euk= 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 prefabricatedladder-type or truss-type bed joint reinforcement is used it should be considered as having normal ducility.

Definition of reinforced masonry construction systemsTo 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 ofmasonry, 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 orgrout is called Reinforced masonry. There are various practices and techniques to achieve reinforced masonry. According to the ways in whichreinforcement is arranged, reinforced masonry can be classified into three types:

Reinforced hollow unit masonryReinforced grouted cavity masonryReinforced 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 isdiscussed 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. Itconsists of a cavity masonry wall constructed from group 1 masonry units. Into the cavity is placed a steel mesh providing the vertical andhorizontal 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 4 f6 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 thecavity 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 properbonding. To achieve satisfactory grouting vent holes should be formed in the wall to allow for the air to escape and facilitate removing awaymortar 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 mortarjoints or masonry units left out.Reinforced grouted cavity masonry wall construction is shown on Figure 3.

Figure 3- Reinforced grouted cavity masonry construction


1 1/28/2010


5/21

The third type of reinforced masonry walls-the reinforced pocket type walls is common for engineered structural masonry construction. Verticalwall 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 reinforcedpocket type masonry aslo allows for forming reinforced masonry columns, where ducts of bigger size can accommodate multiple bars as well asstirrups 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. Horizontalreinforcement is placed in the bed joints at vertical spacing maximum 600 mm. The vertical reinforced ducts are filled with concrete or grout asthe costruction of the wall progresses. Proper planning is necessary to ensure rebar splices lengths, anchoring lengths, correct cover and keepingthe 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 isrequired that the reinforcement has sufficient concrete/grout cover. For unprotected steel in dry, humid or aggressive environment the covershould 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 thiscode 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 6mm 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. Whensubject to seismic load the bond between the rebars and mortar deteriorates. Consequently high tensile stresses and yielding in rebars cannot bedevelop preventing ductile behaviour and energy dissipation. For certain hollow masonry units premature crushing of face shells under cycliclateral 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 ensurebending failure. Therefore increased amount of vertical reinforcement at the edges of wall may not improve the resistance of the wall particularlywith weak masonry units. Thus the minimum percentage of reinforcement, either vertical or horizontal, depends on the strength of the masonryunits.

The maximum percentage of reinforcement should also be limited based on the strength of the masonry units and mortar such that a ductilebending failure is possible. The requirements for anchoring and lapping of reinforcement are similar to those specified for reinforced concretestructures. All reinforcement should be anchored to allow for the stresses in the bar to develop. On way to achieve economic anchorage is toterminate the rebar past the point where it is no longer required. This is called straight anchorage. According to EC 8 straight anchorage is notallowed for rebars with diameter more than 8 mm. Ec 6 provides the following formulae to calculate the anchorage length l b:

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,


1 1/28/2010


6/21

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 isdetermined from the formulae discussed above. In the equation however the diameter of the smaller of the two bars participates. Depending onthe 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 10 fand the mortar or concrete cover is not les than 5 f.

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 10for 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 wallsTo 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 whereengineered 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 unreinforcedmasonry behaves as a brittle material. Hence if the stress state within the wall exceeds masonry strength, brittle failure occurs, followed bypossible collapse of the wall and the building. Therefore unreinforced masonry walls are vulnerable to earthquakes, and should be confined and/orreinforced 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. Aspectratio 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 canoccur 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).


1 1/28/2010


7/21

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 placedhorizontally in the bed joints and embedded with mortar. Vertical reinforcing bars can be placed in hollow block masonry channels. Thecontribution of vertical and horizontal reinforcement to the resistance of the wall, falling in shear, is shown on Figure 7. The shear strength ofsuch reinforced wall depends on the tension capacity of horizontal steel, dowel action of vertical steel, arching of masonry and interlocking ofcrack 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- fThe shear strength- fv

The bending strength- fxThe stress-strain relationship, s-e

Other essential mechanical characteristics of masonry:

The tensile strength- ft, as an equivalent to shear strength- fvThe modulus of elasticity- EThe shear modulus- GThe ductility factor- m

The ductility factor is determined only for a specific structural element(specific proportions, boundary conditions etc). It cannot be determined forthe masonry itself. Mechanical characteristics of masonry are determined by testing standard specimens of masonry wallets and walls accordingto code EN 1052.


1 1/28/2010


8/21

Masonryunit group Mortar fvko [MPa] Limiting fvk

[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

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 mlength 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. Theslenderness 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 compressivestrength f of masonry is different from the one of its constituents( masonry units and mortar) by 25% the value of f is modified. Thecharacteristic 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 becalculated on the basis of normalised compressive strength of masonry units fb and compressive strength of mortar fm as follows:

fk = K*(fb0.65)*(fm

0.25) [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 longitudinalmortar 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 tocompressive 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 contactplanes. 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 ofsd 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 zerocompression fvko and limiting values of characteristic shear strength fvk .

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 themiddle section of the wall. Thus the value of the principal tensile stresses, measured when the wall panel is loaded in shear at failure, defines thetensile 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:


1 1/28/2010


9/21

Unit[MPa]

GroupMortar[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

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 shearresistance 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. Theeffect 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

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 thefollowing margins:

0.03fk


10/21

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


21 1/28/2010


11/21

The modulus of elasticity E of masonry can be determined after compression tests. The elastic modulus is defined as a secant modulus at serviceload 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


12/21

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

Unreinforced masonryH [m] 12 9 6

n 4 3 2

Confined MasonryH [m] 18 15 12

n 6 5 4

Reinforced masonry H [m] 24 21 18

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 jo intis 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 regularitymay 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. Forplanning flexibility is possible combined system consisting of RC columns and masonry shear walls. For such configurations the masonrybearing walls should be reinforced and the RC members should be connected into RC floors forming frames. The vertical reinforcement ofthe 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 beconstructed in a single plane. In cases where large openings are present in the floor, such as for stairways the contour of the openingshould be strengthened with a bond beam. Also two-way slabs are preferred to one-way slabs, as they distribute the vertical gravityloads 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 o f materials, the dimensions o f the building are not limited by the code. In this case the dimensions o f the building are determined bydesign 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 isexempted from these limitations.


21 1/28/2010


13/21

n 8 7 6

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

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

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

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 isrecommended that the distance between walls conform to Table 9 :

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 carefullyplanned.

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

Openings 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 should be located symmetrically in the plan of the building so that not to get in the way of the uniform distribution of strengthand stiffness in two orthogonal directions.

Simple housesTo 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 andconstruction rules specified in the code are met. Such houses are named "simple buildings". According to EC 8 simple buildings are regularbuildings with an approximately rectangular plan. The ratio between the long to shorter side of the house is no more to four and the projectionsor recesses from the rectangular shape are not greater than 15% of the length of the side parallel to the direction of projection. Such houseshave the following limitations regarding number of storeys above ground (Table 10):

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 besymetrically located in plan in two orthogonal directions. A minimum of two structural walls per orthogonal direction. The length of each wallshould 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 thesize 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, thearea of the structural walls in two orthogonal directions is provided as a percentage of the total floor area above the level considered. Table 11below gives the minimum horizontal structural wall cross-section :

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 bemaximum 20%. The difference in the mass of structural walls in two orthogonal directions from storey to storey should be as well maximum20%. For such buildings it is also required that 75% of the vertical load is carried from the structural walls.


21 1/28/2010


14/21

Details for seismic resistanceTo 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 theindividual 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 wallsProvide means for walls to foundations connectionProvide connection of bond beams to roo fProvide connection of walls to bond beamsProvide stiff in their plane floors/roofs

Details to improve structural components strength and ductility

Improve the compressive strength of structural componentsImprove the bending strength of structural componentsImprove 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 ontop of all structural walls at every floor level. The minimum bond beam's cross section is recommended to be 150x250. The bigger dimensionbeing 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-columnsImproves the in-plane stiffness of floors to provide diaphragm actionTransfers the horizontal load from the diaphragm to the structural wallsConnects the structural walls together and provides out-of-plane support


21 1/28/2010


15/21

Number ofstoreys

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






Connects the RC tie-columns

EC8 specifies the following minimum requirements:

Concrete of class 15 should be usedCross 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 60 f

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 isno mandatory design through calculation for the bond-beams. As was discussed in the confined masonry section the design parameters aredetermined on empirical basis. In Table 12 the members reinforcement can be determined based on the seismicity of the location the number ofstroreys and position.

Table 12 Recommended reinforcement o f 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. Sixtyrebar 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 thehouse. Such data is presented Table 5 of the Confined brick masonry document.


21 1/28/2010


16/21

Horizontal reinforcement placed in the bed joints (at vertical spacing of maximum 600 mm) should be anchored in the tie-columns as shown onFigure 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 beachieved. 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 theirplane, ie. all structural walls should be restrained at floor/roof level. In the case of RC slab the connection is provided naturally by constructing RCbond 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. Theanchoring 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 toppingare 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 roofsystem tends to spread and overturn masonry walls. Therefore a collar beam attached to rafters is required. To ensure diaphragm action bracingand blocking should be constructed both in the plane of the joists and in the plane of the rafters in two othogonal directions. Only the perimeterjoists and rafters may be included in bracing and blocking. Vertical cross bracing in the longitudinal ridge plane( perpendiculiar to the joists) is alsorequired. To achieve a satisfactory restraint on the walls the ceiling joists should be anchored to the provided RC roof bond beam by means ofsteel strap placed in position in the bond-beam's formwork before casting of the bond-beam. See Figure 16


21 1/28/2010


17/21

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 canprovide diaphragm action and wall restraint however their mass is much higher. In order to reduce seismic loads light roofs are favoured. Lightroof 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 reinforcedconcrete, timber and reinforced masonry. In seismic zones cast in-situ RC lintels are recommended. If the distance between the top of theopening 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 onFigure 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 tothe 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 beanchored into the rc tie-columns. It is also recommended that lintels should be embedded in the walls a minimum of 250 mm. The lintel widthshould 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 thestructure during an earthquake. In order to reduce vertical motion of balconies, overhangs and other cantilever elements the following limitationsare set:

1.20 m for cantilever slabs cast continuously with the floor slabs, and


21 1/28/2010


18/21

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 thecantilever element and taking into account the stiffness of the adjacent elements to ensure realistic boundary conditions. According to EC 8 theresponse 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 0.7For periods 0.15s < T < 0.5s a linearly interpolated value between 0.7 and 0.5For 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 andstructural damage. In order to prevent failure and fall-downs of masonry non-structural elements their out-of-plane stability to seismic loadsshould 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 canbe 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 RCtie-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 endwall. Out-of-plane failures of gable end walls are common during strong earthquakes and therefore require special consideration. It isrecommended that masonry gable end walls and attics higher than 0.5 m are anchored to the uppermost floor bond-beams. The gable end wallsshould 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, intermidiatebond-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.


21 1/28/2010


19/21

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 unitsbonded together to achieve common action under loading. Veneered walls has facing attached, but not bonded to the backing leaf. The loadapplied to veneered wall is assumed to be carried by the backing leaf only which is designed on the basis of no structural contribution from theveneer. The veneer can be anchored by means of steel ties to the backing masonry wall. No specific requirements can be found in EC 8 howeverits 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 ofreinforced 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 shouldbe verified by calculation according to the guidance provided for partition walls.

Seismic resistance verification of masonry buildingsTo beginning of document

This portion of the reinforced brick masonry section is included in cases where engineered building is required. No specific procedures and/oralgorithms for seismic resistance analysis and verification are outlined in EC 8. The following calculation procedures for seismic resistanceverification 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 thestructure) and portion of the characteristic value of variable load

The stiffness of individual structural walls in the sto rey under consideration is calculatedAnalysing a structural wall with rectangular cross section as an element being fixed at the floors the following formulae for the bendingstiffness 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


21 1/28/2010


20/21

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*(z i*Wi/Szj*Wj) [Force],

where the meaning of symbols is as follows:z i = 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


21 1/28/2010


21/21

The design resistance of wall sections is calculated and compared to the design action effects

To beginning of document


Documents

Reinforced brick masonry