Adobe   Stone   Brick   Concrete Block             Earthquake resistant Stone masonry       Glossary, notations and useful knowledge       References       The basic rules to be followed and various requirements to be satisfied for stone masonry construction are specified in the codes of practice for structural masonry construction. This portion of the guides dedicated to masonry construction types 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 regions are outlined in Eurocode 8. The discussion in this document aims at achieving safe masonry houses constructed from stone. When constructing earthquake resistant stone masonry houses follow the information as provided in the four sections below : Materials for stone masonry construction Definition of stone masonry construction systems Planning and layout for stone masonry houses Details for seismic resistance Materials for stone masonry construction   Masonry units Masonry units manufactured from different kinds of natural stone(igneous rocks, limestone, slate etc) are still used for residential construction. According to standards such as EC6 and EC8, only the use of uniformly cut dimensioned stone units, i.e. square dressed stones is allowed. Stone masonry construction where the wall has two outer leaves of uncoursed stone and inner infill of rubble stones, i.e. rubble stone masonry, has been traditionally practised in many earthquake prone parts of the world. Such type of masonry is not earthquake resistant. In the relevant European standards (EN 771-1-6) are given minimum mean values of compressive strength of stone masonry units to be used for masonry walls. For manufactured stone units the required minimum mean values of compressive strength, fb is 15 MPa. 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, 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 block. 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 1 lists the shape factor d   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 1- 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. For stone masonry construction is used general purpose mortar, where the thickness of joints is greater than 3mm. This mortar is based on dense aggregate In the table 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 2- Typical prescribed composition and strength of general purpose mortars (12)   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. 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.   Definition of stone masonry construction systems To beginning of document   Stone masonry houses are structures defined by vertical and horizontal elements, respectively walls and floors. Since the main service loads are applied on the floors the seismic forces will be mainly concentrated at each floor level. Floors should be rigid in their plane to distribute the seismic load among the vertical wall elements in proportion to their stiffness. Such floors are referred to as horizontal diaphragms. However diaphragms alone will be inadequate unless good connection between them and the supporting walls exists. When the floor is constructed in-situ from reinforced concrete, casting of bond-beams just below floor level is economic and efficient solution. Good floor to wall connection can also be achieved by designing steel ties between timber floor joists and supporting wall   Unreinforced- ie. Plain stone masonry In different parts of the world different types of unreinforced masonry exist. Unreinforced stone masonry is still used for residential housing in many earthquake prone areas. In rural areas Plain masonry can be constructed as "rubble stone masonry" which is uncoursed double leaf masonry with a cavity in the middle filled with rubble stones binded with mortar. The two outer layers of stonework are not levelled(coursed) as the construction progresses. The stone units are not cut in regular dimensions and can have wedged shape. Sometimes field stones/boulders can be used without any additional shaping. The two outer layers of stonework are connected together through the weakly binded with mortar inner infill. In urban areas Plain masonry can be also constructed as "massive stone masonry" which is often coursed double leaf masonry. The two outer layers of stonework are levelled(coursed) as the construction progresses and follow a well established masonry bond. The stone units are cut in regular dimensions. To improve connection between cross walls better quality units are used for the bond in those areas. European codes EC 6 and EC 8 don't consider unreinforced stone masonry as being earthquake resistant. To ensure earthquake-resistance, such walls needs to be confined as well as provided with connecting- "through" stones. Also mortar type M2 should be used for construction, filling all the voids between the stones particularly the inner rubble infill portion of the wall. Stones passing through the entire wall thickness( connecting stones or through stones) should be provided. At least 1 per m2 of the vertical wall area. Dimensioned cut stones should be provided at corners and wall intersections, to ensure better interlocking. When bigger stone units are not available to suit the purposes of the "through stones" such units can be cast in concrete. Care should be taken to reinforce the concrete "through stones" with cage-like well tied together reinforcement. Masonry course must be levelled every 1.0 m of wall height in the zones of high seismicity and at each 2.0 m of wall height in the zones of moderate and low seismicity. Earthquake resistant stone masonry construction is shown on Figure 1. Figure 1- Construction of earthquake-resistant stone masonry wall (7)   Confining of the unreinforced walls, greatly improves their resistance and can be provided by means of timber elements or RC elements. RC horizontal confining elements ie. bond beams at plinth and at lintel and/or floor or roof level should be provided to improve wall to wall connections. In the cases where reinforced concrete cannot be produced timber horizontal confining members can be used. Timber members are usually assembled in ladder or truss-like pattern and placed in the horizontal joints. Vertical RC confining elements should be provided following the instruction for confined clay brick masonry. Masonry walls should be constructed following simple instructions for quality workmanship: In dry and hot climate, masonry units should be soaked in water before the construction in order to prevent quick drying and shrinkage of cement based mortars Masonry units should be assembled together in overlapped fashion (see details of masonry bonds in the plain brick guide) so that the vertical joints are staggered from course to course. To ensure adequate bonding the units should overlap by a lenght equal to 0.4 times the height of unit or 40 mm, whichever is the greater. At the corners and wall intersections the overlap should be min the width of the units. Only cut dimensioned units should be used at these locations Same type of masonry units and mortar should be used for structural walls in the same storey Bracing walls should be constructed in the same time as the load-bearing walls The thickness of individual walls is kept constant from storey to storey In cases where general purpose mortar is going to be used, the mortar joints thickness should be between 8 and 15 mm. EC 8 specifies that, in seismic zones, the load-bearing masonry wall thickness should be min 400 mm for the case of cut stone masonry. To ensure stability of walls, the ratio of the effective wall height to wall thickness should be max 10. To ensure load bearing capacity of masonry walls with openings the length of a structural wall should be at least 1/2 of the greater clear height of the openings adjacent to the wall in the case of cut stone masonry and 1/3 when the masonry is confined.   Planning and layout for stone masonry houses To beginning of document   The experience from past earthquakes in terms of surveys of earthquake damage and its analysis shows that well tied buildings which have well defined, continuous load path to the foundation 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 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. The following general criteria for structural regularity in plan and elevation should be considered in the context of stone masonry houses: 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, as shown on Figure 2 Figure 2- 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, as shown on Figure 3 Figure 3- 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, the house should be separated in two portions(houses), separated by a gap of at least 0.2m, as shown on Figure 4 Figure 4- Irregular configurations in plan should be separated in regular portions   Mixed structural systems, such as a combination of stone masonry structural walls in one level and RC frame in the next are not allowed. 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 Limitations concerning the dimensions of masonry wall houses, built in stone, have been set in most existing seismic codes. Currently EC 8 limits the construction of unreinforced(plain) masonry houses located in seismic zones with ag => 0.3g to only two storey houses. Therefore, unless improved system, like confined stone masonry is adopted, only of one, maximum two storey stone masonry houses should be constructed .   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 is maximum 6.0 m Another essential factor is the structural wall continuity. This means that the size and configuration of openings in walls should be carefully planned. Guidelines for openings in structural masonry walls are included in relevant Indian Standards - 4326. Figure 5 displays guidelines regerding openings location and size. Figure 5- Guidelines for openings in external walls (2)   Below are outlined general recommendations regarding the configuration and size of openings to be followed: Openings should be located away from portions of the wall underneath joists supports (of the floor or roof structure) When possible openings should be located in the less loaded walls Openings should be vertically aligned (in the case of a two-storey house) 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.   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 slab( in the case of RC floors). Bond-beams should be cast on top of all stone walls. 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 6. Figure 6- Details of cast in-situ RC slabs with bond beams   Bond-beams contribute to the lateral resistance of the house in a number of ways namely: 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 and provides out-of-plane support Forms confined masonry shear walls in combination with tie-columns Connects the RC tie-columns In order to achieve satisfactory performance of bond-beams a number of strutural measures are in palce. EC8 specifies the following minimum requirements : Concrete of class 15 should be used Cross section size should be not less than 150x150 mm Longitudinal reinforcement min 4 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 bar diameters(60f) Transverse reinforcement-stirrups rebars f6 @ 200 mm intervals Figure 7 illustrates bond beam reinforcement at corners   Figure 7- 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 is discussed in the confined masonry section the design parameters are determined on empirical basis. In Table 3 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 3- Recommended reinforcement of horizontal RC bond-beams (11)   Floors and roofs Traditionally the masonry buildings had a timber floor and roof. However, currently are predominantly used RC slabs for both floors and roofs in residential masonry construction. 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. 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 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. Twisted steel anchors anchored in the masonry can be used to tie the joists to the walls. Timber joists can be directly anchored to the RC bond-beam in the case when steel ties are placed into position in the formwork and cast together with the bond-beam. Figure 8 below displays typical distribution of joists' steel ties in plan of a stone masonry house. On the figure are also shown wall steel ties, which are often applied for tying together the walls of existent stone buildings, to improve wall-to-wall conection. To find out more and to review details for this technique check out Repair and Strengthening of stone masonry houses document. On Figure 9 is shown a detail for anchoring the timber floor joists to stone walls by means of steel ties.   Figure 8- Distribution of floor joists steel ties and wall steel ties in plan of a stone masonry house   Figure 9- Detail for anchoring of timber joists to stone masonry wall   The construction of monolithic RC slabs is recommended. The slab are cast simultaneously with the RC bond-beams. This represents an effective and simple solution. The thickness of the bond beams can be equal to the wall thickness and preferably not less than 250 mm. Floor systems made of prefabricated RC elements and topping cast in situ are not recommended. When floors are constructed in timber special detailing is required both to ensure diaphragm action and to restraint the walls out-of-plane. Solid strutting inline or staggered can be incorporated between joists in addition to nailing of boarding to stiffen the floor. The boarding can be from plywood sheets and can be nailed to joists at the top and/or the bottom surface depending on access. Other than plywood timber planks can also be nailed to joists to form continous boarding as shown on Figure 10. Figure 10- Stiffening of timber floors by nailing boards or planks   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 11 Figure 11- Timber roof anchorage to bond beam   RC roofs can also be 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 12. In cases where the distance is bigger the lintels can be cast separately(Figure 12) and care should be taken to bond the RC lintels to the masonry of the adjoining wall through horizontal rebars. Figure 12- Requirements for lintels in seismic zones (11)   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 seimic loads should be verified by calculation according to EC8. 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. As discussed in the confined masonry section the maximum distance between vertical confining elements is 4 m. 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. To beginning of document For information about this web site, click here. Purpose of this web site and disclaimer, click here. To find out How to use this web site, click here. The layout of this web site has been tested using Internet Explorer 6.0, Navigator 7.0 and Firefox 0.9.